Download - Light-dependent reactions of photosynthesis
© 2015 American Society of Plant Biologists
Tom Donald
Light-dependent reactions
of photosynthesis
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
Overview: Photosynthesis captures light
energy as reduced carbon
“Low energy”,
oxidized carbon
in carbon
dioxide
Energy input
from sunlight
“High
energy”,
reduced
carbon
Oxygen is
released as
a byproduct
6 CO2 + 6 H2O C6H12O6 + 6 O2
The first step is the capture of
light energy as ATP and
reducing power, NADPH
The second step is the transfer of
energy and reducing power from
ATP and NADPH to CO2, to produce
high-energy, reduced sugars
ATP NADPH
Light-dependent reactions
Light-independent reactions
© 2015 American Society of Plant Biologists
Photosynthesis is two sets of connected
reactions
Adapted from Kramer, D.M., and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol.155: 70–78.
Chloroplast 2 H2O O2 + 2 H+ + 2
ADP H+
ATP
The LIGHT reactions take place in
the thylakoid membranes
The CARBON-FIXING
reactions take place in
the chloroplast stroma
2 NADP+
2 NADPH
2 H+ e−
e−
© 2015 American Society of Plant Biologists
Light reactions (usually) take place in
thylakoid membranes
Reproduced with permission © Annual Reviews of Plant Biology Nickelsen, J. and Rengstl, B. (2013).Photosystem II assembly: From cyanobacteria to plants. Annu. Rev. Plant Biol. 64: 609-635.
Chloroplast Proplastid
Light-induced
differentiation
Gloeobacter violaceus,
the only cyanobacterium
without thylakoid
membranes
Chlamydomonas
reinhardtii, a model
green algae
Synechocystis spp. PCC6803,
a model cyanobacterium
PLANT
Prokaryotes Eukaryotes
© 2015 American Society of Plant Biologists
Light reactions produce O2, ATP and
NADPH
Adapted from Kramer, D.M., and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol.155: 70–78.
2 H2O O2 + 2 H+ + 2
ADP
H+
ATP
ATP synthase Photosystem II (PSII)
Photosystem
I (PSI)
Cytochrome
b6f complex The reactions
require several
large multi-protein
complexes: two
light harvesting
photosystems (PSI
and PSII), the
cytochrome b6f
complex, and ATP
synthase
2 NADP+
2 NADPH
2 H+ e−
e−
© 2015 American Society of Plant Biologists
Chlorophyll captures light energy to
initiate the light reactions
Buchanan, B.B., Gruissem, W. and Jones, R.L. (2015) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists.
First step of
photochemistry
Chlorophyll is
held in pigment-
protein
complexes in a
highly organized
manner
Chlorin ring
captures photons
Photon capture by
chlorophyll excites the
chlorophyll (Chl*). Chl*
can lose an electron to
become oxidized
chlorolphyll (Chl+)
Chl
Chl* e−
Photon
Chl+
H2O
e−
H+
O2
Chl+ is reduced by
stripping an electron
from water, releasing
oxygen and protons
© 2015 American Society of Plant Biologists
Excited chlorophyll can release energy in
several ways
Chl
Chl* Photon
Photochemistry (first step in photosynthesis)
Non-photochemical quenching (e.g., heat) Fluorescence
The fate of captured light energy and photosynthetic efficiency
depend on many factors including temperature, water availability,
nutrient availability, stress etc.
3Chl* Alternatively, it can convert to a damaging triplet state
© 2015 American Society of Plant Biologists
Heat, drought, & other stresses affect
photosynthetic efficiency
Light reactions
Carbon-fixing reactions
ATP NADPH ADP NADP+
The light reactions
and carbon-fixing
reactions are linked
through pools of
ATP/ADP and
NADPH/NADP+
For example….
• High temperature affects protein complex stability and thylakoid membrane fluidity
• Low temperatures slow enzyme-catalyzed reactions
• Drought causes stomata to close, lowering CO2 uptake and carbon-fixing reactions
• Nutrient deficiency or toxicity can affect electron transfer machinery
© 2015 American Society of Plant Biologists
Oxygenic photosynthesis requires TWO
photosystems
P680+
P680
En
erg
y
Redox p
ote
ntial eV
Strong
reductant
Strong
oxidant
−1.5
−1.0
−0.5
0.0
0.5
1.0
Oxidizers remove
electrons from other
species
Reductants donate
electrons to other
species
2 H2O
4 H+ O2
4
P700*
PSII
P680*
P700+
P700
PSI
P680+ is a very strong
oxidant – strong enough to
pull electrons from H2O
NADP+
e−
NADPH
P700* is a very strong
reductant – strong
enough to donate
electrons to NADP+
e−
© 2015 American Society of Plant Biologists
PSI & PSII are connected by an electron
transport chain
P680+
P680
En
erg
y
Redox p
ote
ntial eV
Strong
reductant
Strong
oxidant
−1.5
−1.0
−0.5
0.0
0.5
1.0
Oxidizers remove
electrons from other
species
Reductants donate
electrons to other
species
2 H2O e−
4 H+ O2
4
P700*
PSII
P680*
P700+
P700
PSI
NADP+
e−
NADPH
Cyt b6f
PQ
PC
The electron transport chain
generates proton-motive force that
drives ATP production
© 2015 American Society of Plant Biologists
This diagram is known as a Z-scheme
P680+
P680
En
erg
y
Redox p
ote
ntial eV
Strong
reductant
Strong
oxidant
−1.5
−1.0
−0.5
0.0
0.5
1.0
Oxidizers remove
electrons from other
species
Reductants donate
electrons to other
species
2 H2O e−
4 H+ O2
4
P700*
PSII
P680*
P700+
P700
PSI
NADP+
e−
NADPH
Cyt b6f
PQ
PC
The electron transport chain
generates proton-motive force that
drives ATP production
© 2015 American Society of Plant Biologists
PSI can function without PSII, but it
doesn’t produce oxygen or NADPH
P700*
P700+
P700
PSI
Cyt b6f
PQ
PC
The electron transport chain
generates proton-motive force that
drives ATP production
Cyclic electron transport:
• Involves PSI
• Does not involve PSII
• Involves the electron transport chain
• Results in ATP production
• Does not liberate O2
• Does not produce NADPH
© 2015 American Society of Plant Biologists
The photosystems are embedded in
thylakoid membranes
PSI
Cyt b6f
PC
2 NADP+
2 NADPH
2 H+ e−
2 H2O e−
4 H+ O2
4 PSII
PQ
Cytochrome b6f
(Cyt b6f) is a multiprotein
membrane-embedded
complex
Plastocyanin (PC) is a
small protein and
mobile electron carrier
Plastoquinone (PQ) is
a small molecule and
mobile electron carrier
Thylakoid
Membrane
LUMEN
STROMA
STROMA
LUMEN
Plastid
PQ
© 2015 American Society of Plant Biologists
Electrical and H+ gradients drive ATP
synthesis
ADP + Pi
PSI
Cyt b6f
2 NADP+
2 NADPH
2 H+ e−
PSII
H+
ATP
[H+]
Proton
gradient
from high
(in) to low
(out)
2 H2O e−
4 H+ O2
4
Thylakoid
Membrane
ATP
Synthase
© 2015 American Society of Plant Biologists
Products of the light-dependent reactions
drive carbon-fixation
ADP
H+
ATP
NADP+
NADPH
H+
Light-dependent reactions Carbon-fixing reactions
Adapted from: Buchanan, B.B., Gruissem, W. and Jones, R.L. (2000) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists.
Carboxylation
CO2
ATP
ADP + Pi
NADP+ + H+
NADPH
Energy
input
Reducing
power input
Glyceraldehyde 3-
phosphate (GAP)
For every 3 CO2 fixed, one
GAP is produced for
biosynthesis and energy
1 x GAP
Calvin-
Benson
Cycle
Each CO2 fixed
requires 3 ATP
and 2 NADPH Rubisco
Ribulose-1,5-
bisphosphate
Regeneration
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
Evolution and diversity of
photosynthesis
There are two types of reaction centers, Type I & Type II
Each type is found in various photosynthetic bacteria
Both types are found in cyanobacteria and chloroplasts
Type I are iron-sulfur
reaction centers
Type II are pheophytin-
quinone reaction centers
PSII
PSI
Note that
oxygenic
photosynthesis
requires Type I
& Type II
reaction centers
working in series O2
Most
photosynthetic
prokaryotes use
only a single
type of reaction
center and do
not release
oxygen
Reprinted from Allen, J.P. and Williams, J.C. (1998). Photosynthetic reaction centers. FEBS Letters. 438: 5-9 with permission from Elsevier.
© 2015 American Society of Plant Biologists
Type I & Type II reaction centers are
broadly distributed in prokaryotes
Reprinted from Macalady, J.L., Hamilton, T.L., Grettenberger, C.L., Jones, D.S., Tsao, L.E. and Burgos, W.D. (2013). Energy, ecology and the distribution of microbial life. Phil. Trans. Roy. Soc. B: 368: 20120383. by
permission of the Royal Society. See also Blankenship, R.E. (2010). Early evolution of photosynthesis. Plant Physiol. 154: 434–438;
Type I
Type II
Type II
Type I
Type I
Cyanobacteria,
chloroplasts
Type I + Type II
Several bacterial lineages
have some photosynthetic
members, indicating that
lateral gene transfer has
played an important role in the
evolution of photosynthesis.
© 2015 American Society of Plant Biologists
Prokaryotic photosynthetic diversity
Adapted from Raymond, J. (2008). Coloring in the tree of life. Trends Microbiology. 16: 41-43.
Phylum Colloquial name
Discovery Reaction center
Pigments
Cyanobacteria 1800s Type I + Type II Chl a,b,c,d
Proteobacteria Purple sulfur / nonsulfur bacteria
1800s Type II
BChl a,b
Chlorobi Green sulfur bacteria
1906 Type I BChl a,c, d, e; Chl a
Chloroflexi Filamentous anoxygenic phototrophs
1974 Type II
BChl a,c
Firmicutes Heliobacteria
1983 Type I BChl g; Chl a
Acidobacteria 2007 Type I BChl a,c
© 2015 American Society of Plant Biologists
Type I may be the ancestral reaction
center
With kind permission from Springer Science+Business Media Nelson, N. (2013). Evolution of photosystem I and the control of global enthalpy in an oxidizing world. Photosynth. Res. 116: 145-151.
A proposed scenario for the initial evolution
of photosynthetic reaction centers
Gene duplication: PsA → PsaB
tim
e
pa
st
pre
se
nt
Chlorobi Cyanobacteria
Fe-S cluster as electron acceptor
Ancestral
reaction
center
Proteobacteria
Cyanobacteria Rhodopseudomonas
© 2015 American Society of Plant Biologists
Oxygenic photosynthesis evolved at
least 2.5 billion years ago
Adapted from Des Marais, D.J. (2000). When did photosynthesis emerge on Earth? Science. 289: 1703-1705 and Xiong, J. and Bauer, C.E. (2002). Complex evolution of photosynthesis. Annu. Rev. Plant Biol. 53: 503-521. NASA; Ruth Ellison
Eukaryotes
4 3 2 1 0 Billion years before present
Life
Phototropic bacteria?
Cyanobacteria
Algae
Plants
Mammals
Origin of
Earth
Stromatolites are nearly 3
billion years old and may
have been formed by oxygen-
producing cyanobacteria
Oxygenic
photosynthesis
© 2015 American Society of Plant Biologists
Eukaryotic photosynthesis is derived
from endosymbiosis
Reprinted with permission from Rumpho, M.E., Pelletreau, K.N., Moustafa, A. and Bhattacharya, D. (2011). The making of a photosynthetic animal. J. Exp. Biol. 214: 303-311.
A single primary
endosymbiotic event* about
1.5 billion years ago gave rise
to chloroplasts in
chlorophytes (green algae
and plants), red algae, and
glaucophytes *A second more recent event that
gave rise to Paulinella
chromatophora is described later
This lesson focuses on
photosynthesis as it
occurs in plants, green
algae and cyanobacteria
Ancestral
cyanobacterium
Secondary, tertiary
endosymbiosis
Brown algae,
diatoms,
dinoflagellates,
euglenoids….
Glaucophytes Rhodophytes Chlorophytes
Green plants
© 2015 American Society of Plant Biologists
Secondary
endosymbiosis
led to more
photosynthetic
eukaryotes
Reproduced with permission © Annual Reviews from Keeling, P.J. (2013). The number, speed, and impact of plastid endosymbioses in eukaryotic evolution. Annu. Rev. Plant Biol. 64: 583-607.
A huge variety of photosynthetic
euglenoids, diatoms, dinoflagellates
and other algae are products of
secondary (or tertiary) endosymbiosis;
the engulfment of a primary (or
secondary) endosymbiont
© 2015 American Society of Plant Biologists
1.5 billion years ago
Glaucophytes
Rhodophytes
(Red algae)
Chlorophytes
(Green algae) Plants
Plastid Primary eukaryotic host
Nucleus
Free-living
β-Cyanobacterium
Mitochondrion
0.06 billion years ago
Free-living
α-Cyanobacterium
Eukaryotic
amoeba host
Photosynthetic amoeba
Paulinella chromatophora
chromatophores
5 μm
(Gould, Nature 2012) (Gould, Nature 2012)
(Gould, Nature 2012)
Reprinted by permission from Macmillan Publishers Ltd: from Gould, S.B. (2012). Evolutionary genomics: Algae's complex origins. Nature. 492: 46-48. Reproduced with permission © Annual Reviews Gould, S.B., Waller, R. F., and McFadden, G. (2008).
Plastid evolution. Annu. Rev. Plant Biol. 59: 491 – 517; See also Mackiewicz, P., Bodył, A. and Gagat, P. (2012). Possible import routes of proteins into the cyanobacterial endosymbionts/plastids of Paulinella chromatophora. Theory in Biosciences. 131: 1-18.
A recent independent endosymbiotic
event: Paulinella chromatophora
© 2015 American Society of Plant Biologists
Summary: Evolution and diversity of
photosynthesis
Photosynthetic bacteria
Oxygenic (H2O as
electron donor)
Anoxygenic
Purple
nonsulfur
bacteria
Green
nonsulfur
bacteria
Purple
sulfur
bacteria
Green
sulfur
bacteria
Cyanobacteria
Photosynthetic
eukaryotes
Secondary, tertiary
endosymbiosis
Primary
endosymbiosis
Rhodophytes,
glaucophytes,
Viridiplantae (plants,
chlorophytes)
Brown algae,
diatoms,
dinoflagellates,
euglenoids….
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
Light and pigments
Gamma rays
X rays
IR
Visible
UV
Microwaves
Radio waves
1 nm 1 μm 1 mm 1 m Wavelength
500
nm
400
nm
600
nm Wavelength
Light travels at a fixed speed,
c (3 x 108 m/sec).
Frequency (ν) is inversely
proportional to wavelength (λ):
ν = c / λ Energy is proportional to frequency: E = hν and
inversely proportional to wavelength.
Therefore, shorter wavelength light (e.g., UV)
has higher energy than longer wavelength light
High energy
High frequency
Short wavelength
Low energy
Low frequency
Long wavelength
© 2015 American Society of Plant Biologists
Light that hits a leaf is mainly light in the
visible spectrum (400 – 700 nm)
NASA
300 400 500 600 700 800
Wavelength (nm)
Spectr
a o
f lig
ht hitting leaf
(μm
ol p
hoto
s /
m2-s
ec)
The sun emits light at a
range of different
wavelengths, but much of
the very short wavelength
light is absorbed by Earth’s
atmosphere
Ultraviolet
The earth’s
atmosphere blocks
much of the short
wavelength light
© 2015 American Society of Plant Biologists
Absorption spectra of photosynthetic
pigments
All chlorophyll-based
photosynthesis systems use
chlorophyll a
Different antenna systems use
different subsets of accessory
pigments which expand the range of
light absorbed
• Chlorophyll b is found in land
plants, green algae and
cyanobacteria
• Carotenoids are found in all
chlorophyll-based photosynthesis
systems
• Phycoerythrin are found in
cyanobacteria and non-green
algae, and phycocyanin in
cyanobacteria
300 400 500 600 700 800
Wavelength (nm)
Ab
so
rptio
n s
pe
ctr
a o
f
ph
oto
syn
thetic p
igm
en
ts
(no
rma
lize
d)
Chlorophyll a
Chlorophyll b
β-carotene
Phycoerythrin
Phycocyanin Acce
sso
ry
pig
me
nts
© 2015 American Society of Plant Biologists
Accessory pigments are in antenna
complexes next to reaction centers
Govindjee and Shevela, D. (2011). Adventures with cyanobacteria: a personal perspective. Frontiers in Plant Science. 2: 28; Reprinted by permission from Macmillan Publishers Ltd: Scholes, G.D., Fleming, G.R.,
Olaya-Castro, A. and van Grondelle, R. (2011). Lessons from nature about solar light harvesting. Nat. Chem. 3: 763-774.
In cyanobacteria,
accessory pigments
are arranged in
phycobilisomes Antenna pigments
transfer light
energy to the
reaction center
Antenna complex Antenna complex Photosystem
In green algae and
plants accessory
pigments are
embedded in the
thylakoid membranes
© 2015 American Society of Plant Biologists
Absorbance spectrum of photosynthesis
in a green plant
300 400 500 600 700 800 Wavelength (nm)
Absorbance
spectra
Chlorophyll a
Chlorophyll b
β -carotene
Absorbance
spectrum of a
green plant
The photosynthetic action
spectrum shows the rate of
photosynthesis that occurs
when a single wavelength
of light shines on a plant
A different action spectrum can be
measured in other organisms as a
function of their accessory pigments
Reaction center chlorophylls absorb maximally at 680
and 700 nm. Longer wavelength light does not have
sufficient energy to drive photosynthesis in plants
© 2015 American Society of Plant Biologists
Pigments are characterized by networks
of double bonds
300 400 500 600 700 800
Wavelength (nm)
Ab
so
rptio
n s
pe
ctr
a o
f
ph
oto
syn
thetic p
igm
en
ts Chlorophyll a
Chlorophyll b
β-carotene
Phycoerythrin
Phycocyanin Acce
sso
ry
pig
me
nts
Phycocyanobilin
(linear tetrapyrrole) β-carotene
Chlorophyll a
Tetrapyrrole ring
with Mg in center
© 2015 American Society of Plant Biologists
Bacteriochlorophylls are related but have
different absorption spectra
300 400 500 600 700 800
Wavelength (nm)
Spectr
a o
f lig
ht hitting leaf
(μm
ole
photo
ns /
m2-s
ec) Bacteriochlorophyll a
Bacteriochlorophyll b
Mg
Small changes in side chains are
sufficient to change the absorption
spectra of (bacterio)chlorophylls
© 2015 American Society of Plant Biologists
Chlorophyll biosynthesis
Rissler, H.M., Collakova, E., DellaPenna, D., Whelan, J. and Pogson, B.J. (2002). Chlorophyll biosynthesis. Expression of a second Chl I gene of magnesium chelatase in Arabidopsis supports only limited chlorophyll
synthesis. Plant Physiol. 128: 770-779; Yamazaki, S., Nomata, J. and Fujita, Y. (2006). Differential operation of dual protochlorophyllide reductases for chlorophyll biosynthesis in response to environmental oxygen
levels in the cyanobacterium Leptolyngbya boryana. Plant Physiol. 142: 911-922.
Protochlorophyllide Chlorophyllide
Protoporphyrin IX is a precursor
of Mg-containing chlorophyll and
Fe-containing heme
Chlorophyll
synthase
Attaches
phytyl tail
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
The light response curve and quantum
efficiency
Skillman, J.B. (2008). Quantum yield variation across the three pathways of photosynthesis: not yet out of the dark. J. Exp. Bot. 59: 1647-1661 by permission of Oxford University Press .
At low light intensities,
why is there is a
production (negative
consumption) of CO2?
At low light
intensities the
relationship
between CO2
consumption
and light
intensity is
linear. Why?
Why does the rate of
CO2 consumption level
off at higher light
intensities?
© 2015 American Society of Plant Biologists
Quantifying photosynthesis: The light
response curve
Skillman, J.B. (2008). Quantum yield variation across the three pathways of photosynthesis: not yet out of the dark. J. Exp. Bot. 59: 1647-1661 by permission of Oxford University Press .
Plants have mitochondria
and respire, consuming O2
and producing CO2. In the
light they are net CO2
consumers, but in the dark
production is greater than
consumption
At low light intensities,
photosynthesis is light
limited, so as more
photons are absorbed
more CO2 is fixed
As light intensity increases
above the light saturation
point, photosynthetic reaction
rate is determined by light-
independent reactions
This is the light compensation point:
The amount of light needed to balance photosynthetic CO2
consumption to respiratory CO2 production
© 2015 American Society of Plant Biologists
Quantum Yield: Moles CO2 fixed or O2
produced per moles photons
Note that the quantum
yield can only be
measured in light-
limiting conditions where
the relationship between
absorbed light and CO2
fixation is linear
In this study, the quantum
yield is 0.05 mol CO2 fixed
per mol absorbed photons
(the slope of the line)
What factors
influence
quantum yield?
Skillman, J.B. (2008). Quantum yield variation across the three pathways of photosynthesis: not yet out of the dark. J. Exp. Bot. 59: 1647-1661 by permission of Oxford University Press .
© 2015 American Society of Plant Biologists
Quantum Yield: Moles CO2 fixed or O2
produced per moles photons
Note that the quantum
yield can only be
measured in light-
limiting conditions where
the relationship between
absorbed light and CO2
fixation is linear
In this study, the quantum
yield is 0.05 mol CO2 fixed
per mol absorbed photons
(the slope of the line)
What factors
influence
quantum yield?
• Light absorbance by
photosynthetic vs
non-photosynthetic
pigments
• Balance in excitation
energy between PSI
and PSII
• (For CO2 yield,
activities of
downstream
processes)
• Temperature
Skillman, J.B. (2008). Quantum yield variation across the three pathways of photosynthesis: not yet out of the dark. J. Exp. Bot. 59: 1647-1661 by permission of Oxford University Press .
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
Plastids and chloroplasts: Essential
organelles for most plant cells
Kristian Peters; Louisa Howard, Dartmouth microscopy facility; and3k and caper437
Besides photosynthesis,
several metabolic pathways
occur in plastids including N
and S assimilation, and the
synthesis of secondary
metabolites, pigments, and
hormones
Envelope (double
membrane, resembles
prokaryotic membranes) with
specialized transporters
Stroma
Plant cell (outlined) with
many green chloroplasts A single chloroplast
Thylakoid membranes
© 2015 American Society of Plant Biologists
In plants, plastids divide by fission and
differentiate
Immage credit LadyofHats; see also Sakamoto W., Miyagishima S., and Jarvis P. (2008). Chloroplast Biogenesis: Control of Plastid
Development, Protein Import, Division and Inheritance. The Arabidopsis Book 6:e0110. doi:10.1199/tab.0110
Leaf
Flower,
fruit
Dark grown
photosynthetic
tissue
Seeds, embryonic,
meristems and
reproductive
tissues
Storage of starch, oils and proteins
© 2015 American Society of Plant Biologists
Chlamydomonas cells have a single large
chloroplast
These images show the thylakoids (green),
starch grains (brown) and a special region
called the pyrenoid (py) in which carbon-
fixing reactions take place
flagella
chloroplast
nucleus
py
Engel, B.D., Schaffer, M., Kuhn Cuellar, L., Villa, E., Plitzko, J.M. and Baumeister, W. (2015). Native architecture of the Chlamydomonas chloroplast revealed by in situ cryo-electron tomography. eLife. 4: e04889.
© 2015 American Society of Plant Biologists
Light induces conversion from etioplast
to chloroplast
Pribil, M., Labs, M. and Leister, D. (2014). Structure and dynamics of thylakoids in land plants. J. Exp. Bot. 65: 1955-1972 by permission of Oxford
University Press . Von Wettstein, D., Gough, S. and Kannangara, C.G. (1995). Chlorophyll biosynthesis. Plant Cell. 7: 1039-1057..
Prolamellar body
in etioplast
Transition to
lamellar layers
Primary lamellar
layers
Grana layers
DARK
LIGHT
© 2015 American Society of Plant Biologists
Land plants have distinctive grana stacks
in the thylakoids
Grana stacks
Membranes at the margins are non-
appressed (green) and those within the
grana stacks (red) are appressed. Different
complexes are found in appressed vs non-
appressed regions of the thylakoids
Grana
Louisa Howard, Austin, J.R., et al and Staehelin, L.A. (2006). Plastoglobules Are lipoprotein subcompartments of the chloroplast that are permanently coupled to thylakoid membranes and contain biosynthetic enzymes.
Plant Cell. 18: 1693-1703; see also Staehelin, L.A. (1976). Reversible particle movements associated with unstacking and restacking of chloroplast membranes in vitro. J. Cell Biol. 71: 136-158.
© 2015 American Society of Plant Biologists
Appressed and non-appressed
thylakoids have different functions
Daum, B., et al., (2010). Arrangement of Photosystem II and ATP synthase in chloroplast membranes of spinach and pea. Plant Cell. 22: 1299-1312; Nagy, G., et al. and Minagawa, J. (2014). Chloroplast remodeling during
state transitions in Chlamydomonas reinhardtii as revealed by noninvasive techniques in vivo. Proc. Natl. Acad. Sci. USA. 111: 5042-5047.
lumen
Grana
PSII packing into appressed membranes
State 1
ATP synthase
PSI-LHCI
supercomplex
Cyt b6f
LHCII Cyt b6f
PSII
PSII is mainly found in
appressed regions,
ATP synthase and PSI in
unappressed regions, and
Cyt b6f is distributed throughout
© 2015 American Society of Plant Biologists
Summary: Light, pigments, quantum
efficiency and chloroplasts
The first step of photosynthesis is light capture by
pigments in thylakoid membranes of chloroplasts.
β-carotene
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
Structure and function of photosynthetic
complexes
Photosystem II Cytochrome
b6f complex Photosystem I
Stroma (electro-
negative side)
Lumen (electro-
positive side)
Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochemistry and Photobiology. 84: 1349-1358.
© 2015 American Society of Plant Biologists
Linear electron transport involves three
complexes, PSII, Cyt b6f & PSI
H2O e−
H+
O2
Photosystem II
e−
e−
e−
e−
Cytochrome
b6f complex Photosystem I
Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochemistry and Photobiology. 84: 1349-1358.
© 2015 American Society of Plant Biologists
Structure and function of
Photosystem II – LHCII complex
Reprinted by permission from Calderone, V., Trabucco, M., Vujičić, A., Battistutta, R., Giacometti, G.M., Andreucci, F., Barbato, R. and Zanotti, G. (2003). Crystal structure of the PsbQ protein of Photosystem II from
higher plants. EMBO reports. 4: 900-905; Reprinted with permission © Annual Reviews Nickelsen, J. and Rengstl, B. (2013).Photosystem II assembly: From cyanobacteria to plants. Annu. Rev. Plant Biol. 64: 609 – 634.
Conserved reaction
center core
PSII is a multi-protein
complex that functions as a
dimer. This diagram
represents a monomer
Divergent extrinsic proteins
D1 D2
CP43 CP47
Dimer structure of PSII The conserved
reaction center
core is made of
up proteins D1
and D2, and
inner-antenna
proteins CP43
and CP47
The oxygen-evolving
Mn4CaO5 cluster is on the
luminal side and shielded
by the more divergent
extrinsic proteins
H2O
1/2 O2 + 2 H+
© 2015 American Society of Plant Biologists
Conserved cores, variable light
harvesting structures
Reprinted with permission © Annual Reviews Nickelsen, J. and Rengstl, B. (2013).Photosystem II assembly: From cyanobacteria to plants. Annu. Rev. Plant Biol. 64: 609 – 634.
The peripheral
antenna and light-
harvesting complex
(LHC) are different
between
cyanobacteria and
chloroplasts
LHCII
Same core
Cyanobacteria & red
algae harvest light
through peripheral
antenna systems
Plants and green algae
harvest light through
membrane-embedded light
harvesting complexes
© 2015 American Society of Plant Biologists
Proteins in PSII can be characterized by
SGC, SDS-PAGE and EM
Reprinted with permission from Caffarri, S., Kouřil, R., Kereïche, S., Boekema, E.J. and Croce, R. (2009). Functional architecture of higher plant
Photosystem II supercomplexes. EMBO J. 28: 3052-3063.
Protein complexes can
be separated by sucrose
gradient centrifugation
Hea
vie
r L
igh
ter
The subunit composition of each
band is determined by SDS-PAGE
EM can also reveal complex composition
© 2015 American Society of Plant Biologists
Proteins’ roles are to orient and position
pigment molecules Numerous
chlorophylls,
β carotenes and
other small
molecules are held in
position by PSII
proteins
Neveu,Curtis
© 2015 American Society of Plant Biologists
Electron transfer in PSII
(1) Light converts reaction center chlorophyll (P680)
to excited form P680*
(2) Electron leaves P680*, forming P680+ (photo-
oxidation, charge separation)
(3) The electron is transferred to Pheophytin
(Pheo), forming Pheo−
(4) Pheo− passes the electron to QA to produce QA−
(5) QA− passes the electron to QB to produce QB
−
P680 → P680* → P680+
Pheo → Pheo−
e- e−
(1) (2)
(3)
QA → QA−
e- e−
(4)
PSII
(4)
(3)
STROMA
LUMEN
(5)
QB → QB−
(5)
e- e−
© 2015 American Society of Plant Biologists
Plastoquinone/ plastoquinol is a carrier
of electrons and protons
See Cramer, W. A., Hasan, S.S., and Yamashita, E. (2011). The Q cycle of cytochrome bc complexes: A structure perspective. Biochim. Biophys. Acta - Bioenerg. 1807: 788–802.
Plastoquinone (PQ) at the QB site
is reduced to PQ2 − which picks
up to protons from the stroma to
form PQH2 (plastoquinol)
Plastoquinone (PQ)
at QB site of PSII
Plastoquinonol (PQH2)
2 e−
2 H+ Diffusion through lipid
bilayer to Cyt b6f
PQH2 diffuses
through the lipid
bilayer, carrying
protons and
electrons.
© 2015 American Society of Plant Biologists
The oxygen-evolving complex (OEC)
resides on PSII luminal surface
Reprinted from Vogt, L., Vinyard, D.J., Khan, S. and Brudvig, G.W. (2015). Oxygen-evolving complex of Photosystem II: an analysis of
second-shell residues and hydrogen-bonding networks. Curr. Opin. Chem. Biol. 25: 152-158 with permission from Elsevier. Neveu,Curtis
The OEC’s catalytic center core is an inorganic
Mn4CaO5 cluster which performs the
mechanistically-challenging reaction of
removing four tightly-bound electrons and four
protons from water to form dioxygen
LUMEN
© 2015 American Society of Plant Biologists
PSII
Chl / Chl+
Pheo / Pheo−
PQA / PQA−
PQB / PQB−/ PQB
2−
H2O e−
H+
O2
e−
e−
e−
To Cyt c6f
Luminal side
Electron transport in PSII: Electrons
move from luminal to stromal side
Reprinted from Senge, M. O., Ryan, A. A., Letchford, K. A., MacGowan, S. A., & Mielke, T. (2014). Chlorophylls, symmetry, chirality, and photosynthesis. Symmetry. 6: 781-843
© 2015 American Society of Plant Biologists
In plants, LHCII assembles as trimers
Each monomer of LHCII
from spinach includes one
polypeptide, 14 chlorophylls
and four carotenoids
LHCII trimers
(green)
surrounding
PSII dimers
(grey)
The rate of energy
transfer from LHCII
to the reaction
center is regulated
in response to light
environment,
metabolic state etc.
Reprinted by permission from Macmillan Publishers Ltd Liu, Z., Yan, H., Wang, K., Kuang, T., Zhang, J., Gui, L., An, X. and Chang, W. (2004). Crystal structure of spinach major light-harvesting complex at 2.72 Å
resolution. Nature. 428: 287-292; see also Drop, B., Webber-Birungi, M., Yadav, S.K.N., Filipowicz-Szymanska, A., Fusetti, F., Boekema, E.J. and Croce, R. (2014). Light-harvesting complex II (LHCII) and its
supramolecular organization in Chlamydomonas reinhardtii. Biochimica et Biophysica Acta (BBA) - Bioenergetics. 1837: 63-72..
© 2015 American Society of Plant Biologists
Q cycle and Cytochrome b6f complex
Stroma (electro-
negative side)
Lumen (electro-
positive side)
PSII Reaction Center
Cytochrome
b6f complex
PSI Reaction Center
Mobile plastoquinone
(PQH2) carries electrons
from PSII to Cyt b6f
Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochem. Photobiol 84: 1349-1358.
© 2015 American Society of Plant Biologists
H+
Reprinted from Hasan, S.S., Yamashita, E., Baniulis, D. and Cramer, W.A. (2013). Quinone-dependent proton transfer pathways in the photosynthetic cytochrome b6f complex. Proc. Natl. Acad. Sci. USA 110: 4297-
4302; See also Tikhonov, A. (2013). pH-Dependent regulation of electron transport and ATP synthesis in chloroplasts. Photosynth. Res. 116: 511-534.. Stroebel, D., Choquet, Y., Popot, J.-L. and Picot, D. (2003). An
atypical haem in the cytochrome b6f complex. Nature. 426: 413-418.
Cyt b6f structure
(from cyanobacteria)
Stroma
Lumen
Lumen
Stroma
Electrons are transferred
from PQH2 through Cyt b6f
and ultimately passed to
plastocyanin (PC). This is
accompanied by the transport
of H+ from stroma to lumen
Cyt b6f is made
from eight
polypeptides
including Cyt
b6 and Cyt f
From
PSII
To
PSI
Redox centers include four
hemes and an Fe2S2 cluster
e−
e−
PQH2
PC
Cyt b6f
© 2015 American Society of Plant Biologists
Plastoquinone is a two-electron carrier
that delivers e− to Cyt b6f
Plastoquinone (PQ) is reduced to
plastoquinol (PQH2), then oxidized
to plastoquinone (it cycles). This
involves oxidation–reduction and
deprotonation–protonation, which
couples proton translocation and
electron transfer
PQ PQH2
© 2015 American Society of Plant Biologists
e−
Cyt b6f
First half Q cycle Second half Q cycle
PQH2
2 H+
e−
PC
PQ
PS
II
e−
PQH2
2 H+
e−
PC
PQ P
SII
e−
Cycle completion
e− PQ
e−
H+
PQH2
PQH2 delivers two protons to
lumen, one electron to PC.
PQ returns to PSII
Another PQH2 delivers two protons to lumen, one
electron to PC. PQ, two electrons and two
protons regenerate PQH2 which cycles again
Electrons & protons pass through Cyt b6f
through the Q cycle
© 2015 American Society of Plant Biologists
Structure and function of
Photosystem I – LHCI complex
Stroma (electro-
negative side)
Lumen (electro-
positive side)
PSII Reaction Center
Cytochrome
b6f complex
PSI Reaction Center
Electrons are passed from Cyt b6f
to PSI by plastocyanin (PC) or
sometimes in algae Cyt c6 Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochemistry and Photobiology. 84: 1349-1358.
© 2015 American Society of Plant Biologists
PSI is a large multi-protein, multi-pigment
complex
Å
Å
PSI is a complex of
17 protein subunits,
dominated by PsaA
and PsaB, and 178
prosthetic groups,
mainly chlorophyll
Reprinted from Amunts, A. and Nelson, N. (2009). Plant photosystem I design in the light of evolution. Structure. 17: 637-650 with permission from Elsevier.
© 2015 American Society of Plant Biologists
Structure and electron transport chain of
Photosystem I
Reprinted by permission from Macmillan Publishers Ltd Jordan, P., Fromme, P., Witt, H.T., Klukas, O., Saenger, W. and Krausz, N. (2001). Three-dimensional structure of cyanobacterial photosystem I at 2.5 Å resolution.
Nature. 411: 909-917, © 2015 David Goodsell & RCSB Protein Data Bank See also . Amunts, A., Drory, O. and Nelson, N. (2007). The structure of a plant photosystem I supercomplex at 3.4 Å resolution. Nature. 447: 58-63
Stroma
Lumen Chlorophyll
Ferredoxin
e−
To NADPH
Plastocyanin e−
e−
Phylloquinone
Fe4S4 (x3)
PSI complex showing
proteins and electron
transport chain
Electron transport chain
e−
© 2015 American Society of Plant Biologists
In plants, PSI is surrounded by a
crescent of LHCI complexes
Reprinted from Mazor, Y., Borovikova, A. and Nelson, N. (2015). The structure of plant photosystem I super-complex at 2.8 Å resolution. eLife. 4: e07433. Minagawa, J. (2013). Dynamic reorganization of
photosynthetic supercomplexes during environmental acclimation. Front. Plant Sci. 4: 513.
PSI core
LHCI
Proteins Pigments
Looking at the complex from
the lumen; the membrane is
in the plane of the slide
Chlamydomonas
Two rows of LHCI
Pea complex
Single row of LHCI
© 2015 American Society of Plant Biologists
Ferredoxin transfers electrons via
ferredoxin:NADP+ reductase (FNR)
Reprinted from Mulo, P. (2011). Chloroplast-targeted ferredoxin-NADP+ oxidoreductase (FNR): Structure, function and location. Biochim. Biophy. Acta (BBA) - Bioenergetics. 1807: 927-934 with permission from Elsevier.
From
PSI
e-
NADP+ NADPH
e− H+ + 2
Ferredoxin is
a small iron-
containing
Fe2S2 protein
Ferredoxin (Fd) Ferredoxin:
NADP+ reductase
Fe2S2
Reduced Fd can also pass
electrons to other enzymes
© 2015 American Society of Plant Biologists
Structure and function of ATP synthase
Daum, B., Nicastro, D., Austin, J., McIntosh, J.R. and Kühlbrandt, W. (2010). Arrangement of Photosystem II and ATP synthase in chloroplast membranes of spinach and pea. Plant Cell. 22: 1299-1312.
Thylakoid lumen
ADP + Pi
H+
ATP
H+
ATP
Synthase
H+
H+
H+ H+ H+
H+ H+
H+
H+
ATP couples the dissipation of the proton gradient to ATP synthesis
© 2015 American Society of Plant Biologists
ATP synthase is a multi-subunit rotary
machine
Reprinted by permission from Macmillan Publishers Ltd: from Abrahams, J.P., Leslie, A.G.W., Lutter, R. and Walker, J.E. (1994). Structure at 2.8 A resolution of F1-ATPase from bovine heart mitochondria. Nature. 370:
621-628.; Seelert, H., Poetsch, A., Dencher, N.A., Engel, A., Stahlberg, H. and Muller, D.J. (2000). Structural biology: Proton-powered turbine of a plant motor. Nature. 405: 418-419. See also Groth, G. and Pohl, E. (2001).
The structure of the chloroplast F1-ATPase at 3.2 Å resolution. J. Biol. Chem. 276: 1345-1352.
Rotary mechanism of ATP
synthase Video link
2 nm Stalk
The F1 subunit
has three α and
three β subunits,
and one each of
γ, δ and ε
F0 subunit: 10 – 15
copies of subunit c
assembled as a ring
within the thylakoid
membrane
Electron flow through F0 turns the
central stalk subunit generating
torque to energize ATP synthesis
© 2015 American Society of Plant Biologists
Summary: Structure and function of
photosynthetic complexes
ATP Synthase
The efforts of hundreds of scientists across several decades have
revealed detailed structures and mechanisms for each of the unusual,
important, and beautiful photosynthetic complexes, comprising dozens
of proteins and hundreds of pigments and other cofactors
Video link
Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex. Photochemistry and Photobiology. 84: 1349-1358.
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
Pathways of electron transport
Linear electron transport Cyclic electron transport Water-water cycle
• Electrons transferred
from H2O to NADPH
• Involves PSII, Cyc b6f,
and PSI
• Generates NADPH and
pmf for ATP synthesis
• Electrons cycle with no
net production of NADPH
• Involves Cyc b6f and PSI
• Generates pmf for ATP
synthesis but not NADPH
H2O
NADPH H+
H+
H+
H+
H2O
H+
H+
H2O
• Electrons transferred
from H2O to H2O
• Involves PSII, Cyc b6f,
and PSI
• Generates pmf for ATP
synthesis but not NADPH
© 2015 American Society of Plant Biologists
Pathways of electron transport
LET (diagrammed by the Z-scheme) is only one possible electron transport pathway. It is the only scheme
that results in NADPH, but other schemes can produce ATP and protect against photodamage
Cyclic electron transport
(CET) balances production of
ATP and NADPH and helps
alleviate photodamage
The Mehler reaction or
water-water cycle is
thought to contribute to
photoprotection
Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochem. Photobiol. 84: 1349-1358.
© 2015 American Society of Plant Biologists
LET: Flow of electrons from H2O to PSII
to Cyt b6f to PSI to NADPH
2 H2O e-
4 H+
O2
Photosystem II
e−
e−
e−
e−
Cytochrome
b6f complex Photosystem I
Plastoquinone
Plastocyanin
Ferredoxin
H2O PSII PQ Cyt b6f PC PSI Fd NADP+ NADPH
Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochem. Photobiol. 84: 1349-1358.
© 2015 American Society of Plant Biologists
Stoichiometry of electron transport
yields
Minagawa, J. (2013). Dynamic reorganization of photosynthetic supercomplexes during environmental acclimation. Front. Plant Sci. 4: 513.
1. Assuming a quantum
efficiency of 0.8, for
each 5 photons
captured by PSII, 4 e−
enter LEF from PSII…
3… and 4 H+ are
released by the
oxidation of H2O
4. The 4 e− from PSII lead
to 8 H+ released into the
lumen during the Q cycle
7. Therefore, at least 2 H+
must be contributed by
CEF to meet the
requirements of the
Calvin-Benson cycle
2… 4 e− are
required to
reduce
2 NADP+
+ 2 H+ to
NADPH…
5. The Calvin-Benson
cycle requires 3 ATP
for each 2 NADPH
6. ATP synthase
requires 14 H+ to
produce 3 ATP
© 2015 American Society of Plant Biologists
In cyclic electron transport, electrons
pass from PSI to Cyt b6f
As a result, protons
are moved into the
lumen (powering
ATP synthesis) but
electrons are not
passed to NADP+.
Thus, cyclic
electron transport
alters the ratio of
ATP to NADPH
produced as
compared to linear
electron transport.
It also alleviates
photoinhibition.
Reprinted from Shikanai, T. (2014). Central role of cyclic electron transport around photosystem I in the regulation of photosynthesis. Curr. Opin. Biotechnol. 26: 25-30 by permission from Elsevier..
© 2015 American Society of Plant Biologists
Reprinted from Shikanai, T. (2014). Central role of cyclic electron transport around photosystem I in the regulation of photosynthesis. Curr. Opin. Biotechnol. 26: 25-30 by permission from Elsevier..
There are two routes of cyclic electron
transport (CET)
(1) PSI Fd PQ Cyt b6f PC PSI
(2) PSI Fd NADP+ PQ Cyt b6f PC PSI
Scheme 1 requires PGR5
(PROTON GRADIENT
REGULATED5) and PGRL1
(PGR-LIKE1) Scheme 2 involves NAD(P)H
dehydrogenase (NDH)
© 2015 American Society of Plant Biologists
The water-water cycle is another form of
electron flow
2 H2O e-
4 H+
O2
Photosystem II
e−
e−
e−
Cytochrome
b6f complex Photosystem I
e−
O2 O2·− H2O2 H2O
SOD APX
SOD = superoxide
dismutase
APX = ascorbate
peroxidase
Like CET, the water-water cycle
produces ATP but not NADPH. It
usually is restricted to the early period
after the transition from dark to light
when reductant cannot be used
because the intermediates of the Calvin
Benson cycle have not built up.
H2O PSII PQ Cyt b6f PC PSI O2 H2O2 H2O
Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochem. Photobiol. 84: 1349-1358.
© 2015 American Society of Plant Biologists
Plastid terminal oxidase oxidizes
reduced PQH2 by reducing O2
Reprinted with permission from Rumeau, D., Peltier, G. and Cournac, L. (2007). Chlororespiration and cyclic electron flow around PSI during photosynthesis and plant stress response. Plant Cell Environ. 30: 1041-1051. See
also Nawrocki, W.J., Tourasse, N.J., Taly, A., Rappaport, F. and Wollman, F.-A. (2015). The plastid terminal oxidase: Its elusive function points to multiple contributions to plastid physiology. Annu. Rev. Plant Biol. 66: 49 -74.
Water-
water
cycle Chloro-
respiration
Chlororespiration
alleviates excitation
pressure on electron
transport
© 2015 American Society of Plant Biologists
Flavodiiron proteins provide
photoprotection in cyanobacteria
Reprinted with permission from Allahverdiyeva, Y., Suorsa, M., Tikkanen, M. and Aro, E.-M. (2015). Photoprotection of photosystems in fluctuating light intensities. J. Exp. Bot. 66: 2427-2436. Zhang, P., Eisenhut, M., Brandt,
A.-M., Carmel, D., Silén, H.M., Vass, I., Allahverdiyeva, Y., Salminen, T.A. and Aro, E.-M. (2012). Operon flv4-flv2 provides cyanobacterial photosystem II with flexibility of electron transfer. Plant Cell. 24: 1952-1971.
FLV1/FLV3 carry
out a Mehler-like
reaction that
regenerates NADP+
H2O
H+
H+
FLV1/FLV3
FNR
NADP+ NADPH
O2 H2O
FLV2/FLV4
FLV2/FLV4 function
in alternative
electron transfer
and alleviate PSII
excitation pressure
© 2015 American Society of Plant Biologists
Summary: Variations in photosynthetic
electron transport
In linear electron transport,
water (H2O) is the electron
donor and NADP+ the
electron acceptor.
In cyclic electron transport,
electrons transferred from
PSI are returned to PSI.
The water-water cycle is a
variation in which electrons
transferred from water are
passed to O2, reducing it to
H2O.
The relative contributions of each
are determined by metabolic
supply and demand
RUBISCO
3 GAP
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
Damage avoidance and repair:
Acclimations to light stress
Adapted from Li, Z., Wakao, S., Fischer, B.B. and Niyogi, K.K. (2009). Sensing and responding to excess light. Annu. Rev.Plant Biol. 60: 239-260.
Light intensity
Rate
of
ph
oto
syn
thesis
At low light
intensities, light
intensity is the
dominant factor
limiting
photosynthesis
At high light intensities,
excess light can damage
photosynthetic machinery
The metabolic and
physiological state of
the plant or cell
determines how much
light is “too much”
Optimal conditions
Stressed conditions
© 2015 American Society of Plant Biologists
Excess excitation energy can lead to
photo-oxidative damage
Reprinted by permission from Macmillan Publishers Ltd Demmig-Adams, B. and Adams, W.W. (2000). Photosynthesis: Harvesting sunlight safely. Nature. 403: 371-374.
Photon-excited chlorophyll forms
excited singlet chlorophyll 1Chl*
1Chl* can return to its ground
state through:
Photochemistry
Fluorescence
Dissipation
(e.g., heat, NPQ)
Alternatively, it can convert to the
excited triplet state 3Chl* which
can transfer energy to oxygen to
produce singlet oxygen with
subsequent damage due to
reactive oxygen species
P
F
D
NPQ
© 2015 American Society of Plant Biologists
There are protective strategies to avoid
high-light induced damage
Li, Z., Ahn, T.K., Avenson, T.J., Ballottari, M., Cruz, J.A., Kramer, D.M., Bassi, R., Fleming, G.R., Keasling, J.D. and Niyogi, K.K. (2009). Lutein accumulation in the absence of zeaxanthin restores nonphotochemical
quenching in the Arabidopsis thaliana npq1 mutant. Plant Cell. 21: 1798-1812. Havaux, M. and Niyogi, K.K. (1999). The violaxanthin cycle protects plants from photooxidative damage by more than one mechanism.
Proc. Natl. Acad. Sci. USA. 96: 8762-8767; Adapted from Li, Z., Wakao, S., Fischer, B.B. and Niyogi, K.K. (2009). Sensing and responding to excess light. Annu. Rev.Plant Biol. 60: 239-260
Decrease light incidence through
dynamic changes to antenna complex
Reaction
center
Antenna complex
Release excess
energy as heat or
fluorescence
Detoxify reactive oxygen side-
products of excess excitation energy
(e.g., antioxidant production)
lutein
A plant defective in energy dissipation
is susceptible to high-light induced
bleaching and death
zeaxanthin
© 2015 American Society of Plant Biologists
Movements to optimize light interception
Oikawa, K., Kasahara, M., Kiyosue, T., Kagawa, T., Suetsugu, N., Takahashi, F., Kanegae, T., Niwa, Y., Kadota, A. and Wada, M. (2003). CHLOROPLAST UNUSUAL POSITIONING1 is essential for proper
chloroplast positioning. Plant Cell. 15: 2805-2815. Donald Hoburn; Walter Siegmund; Kristian Peters
Lo
w lig
ht
Hig
h lig
ht
Top view Side view
Chloroplasts
move to sides
of cells to
decrease light
interception Selaginella
lepidophylla
Leaf reorientation
or curling are
adaptations to
minimize light
damage. Leaf
reorientation can
also decrease
heat damage
Arctostaphylos patula
Eucalyptus rossii
© 2015 American Society of Plant Biologists
Acclimation via stoichiometric changes
in complex abundance
Reprinted by permission from Walters, R.G. (2005). Towards an understanding of photosynthetic acclimation. J. Exp. Bot. 56: 435-447.
Low light: More
light harvesting
complexes (LHCII)
High light: Greater
photosynthetic
capacity (PSII etc)
Also more
photoprotection
Arabidopsis grown in low (black bars) or high (white bars).
Chloroplast components relative to total leaf chlorophyll
© 2015 American Society of Plant Biologists
Excess light energy is dissipated via non-
photochemical quenching
“Non-photochemical quenching”
encompasses several components:
qE = Energy-dependent quenching: The xanthophyll cycle
qT = State transition: Conformational changes in LHCII
qI = Photoinhhibition: Light-induced reduction in quantum
yield as a consequence of damage
Chl
1Chl*
Photon Non-photochemical
quenching (NPQ)
© 2015 American Society of Plant Biologists
Energy-dependent quenching (qE)
usually is dominant form of NPQ
PSII
[H+]
LHC PSII LHC
Unquenched:
High efficiency
transfer of light
energy to PSII
reaction center
[H+]
[H+]
[H+] [H+]
VDE Energy-dependent
quenching:
1. Lumen acidification
activates Violaxanthin
De-epoxidase
2. VDE converts
violaxanthin to zeaxanthin,
leading to light energy
dissipation in LHCII
© 2015 American Society of Plant Biologists
Xanthophyll cycle: reversible
interconversion of carotenoids
Violaxanthin
de-epoxidase
(VDE)
Zeaxanthin
epoxidase
(ZE)
High light / low luminal pH
induces VDE, which catalyzes
the conversion of violaxanthin
to zeaxanthin
In low light / high luminal pH,
the reaction is reversed by ZE
Hieber, A.D., Kawabata, O. and Yamamoto, H.Y. (2004). Significance of the lipid phase in the dynamics and functions of the xanthophyll cycle as revealed by PsbS overexpression in tobacco and in-
vitro de-epoxidation in monogalactosyldiacylglycerol micelles. Plant Cell Physiol 45: 92-102 by permission of Oxford University Press.
© 2015 American Society of Plant Biologists
Zeaxanthins promotes structural
changes & heat dissipation
Reprinted with permission from Ruban, A.V. (2015). Evolution under the sun: optimizing light harvesting in Photosynthesis. J. Exp. Bot. 66: 7 – 23; Müller-Moulé, P., Conklin, P.L. and Niyogi, K.K. (2002). Ascorbate
Deficiency Can Limit Violaxanthin De-Epoxidase Activity in Vivo. Plant Physiol. 128: 970-977.
Zeaxanthin accumulation (due to VDE
activation) causes a rearrangement of
LHCII and RCII, which decreases light
transfer to RCII
The structural changes
cause more light energy
to be dissipated as heat
© 2015 American Society of Plant Biologists
Zeaxanthin and lutein also have roles as
antioxidants and in photoprotection
Niyogi, K.K., Björkman, O. and Grossman, A.R. (1997). The roles of specific xanthophylls in photoprotection. Proc. Natl. Acad. Sci. USA 94: 14162-14167.
Chlamydomonas mutants deficient in
zeaxanthin and lutein production are
more susceptible to photo-oxidation
Interestingly, these two xanthophyll
pigments (obtained from dietary
sources) also protect human eyes from
phototoxic damage by accumulating in
the macula (orange color)
© 2015 American Society of Plant Biologists
The redox state of PQ pool contributes to
state transitions (qT)
Rosso, D., Bode, R., Li, W., Krol, M., Saccon, D., Wang, S., Schillaci, L.A., Rodermel, S.R., Maxwell, D.P. and Hüner, N.P.A. (2009). Photosynthetic redox imbalance governs leaf sectoring in the Arabidopsis thaliana
variegation mutants immutans, spotty, var1, and var2. Plant Cell. 21: 3473-3492.
When PSII, PSI and
downstream metabolism are
balanced, the PQ pools is
distributed between PQ
(oxidized) and PQH2 (reduced)
High light (or light that
favors PSII) or conditions
that decrease
downstream metabolism
lead to an over-reduction
of the PQ pool
© 2015 American Society of Plant Biologists
Reduced PQH2 activates LHCII kinase
and promotes state transition
Minagawa, J. (2013). Dynamic reorganization of photosynthetic supercomplexes during environmental acclimation. Front. Plant Sci. 4: 513.
Accumulation of PQH2
activates LHCII kinase
LHCII kinase
phosphorylates
LHCII. Some LHCII
relocates to PSI
© 2015 American Society of Plant Biologists
State 1
ATP synthase
State 2 PSI-LHCI
supercomplex
Cyt b6f
LHCII
trimer
(quenched)
LHCII
trimer
A current model indicates
that state transitions
balance PSII and PSI
mainly by quenching LHCII
energy transfer to PSII
LHCII
phosphorylation also
prevents light energy
from being passed to
PSII
Nagy, G., Ünnep, R., et al.. and Minagawa, J. (2014). Chloroplast remodeling during state transitions in Chlamydomonas reinhardtii as revealed by noninvasive
techniques in vivo. Proc. Natl. Acad. Sci. USA 111: 5042-5047. See also Ünlü, C., Drop, B., Croce, R. and van Amerongen, H. (2014). State transitions in
Chlamydomonas reinhardtii strongly modulate the functional size of Photosystem II but not of photosystem I. Proc. Natl. Acad. Sci. USA 111: 3460-3465.
© 2015 American Society of Plant Biologists
Photoinhibition (qI) is caused by light
damage to PSII
Reprinted by permission from Takahashi, S. and Murata, N. (2008). How do environmental stresses accelerate photoinhibition? Trends Plant Sci 178-182.
The D1 protein of
PSII is susceptible
to photodamage,
and when its rate
of damage
exceeds the rate
of repair,
photosynthesis is
inhibited.
© 2015 American Society of Plant Biologists
Damage and repair of PSII are stress and
environmentally sensitive
Reprinted by permission from Nath, K., Jajoo, A., Poudyal, R.S., Timilsina, R., Park, Y.S., Aro, E.-M., Nam, H.G. and Lee, C.H. (2013). Towards a critical understanding of the Photosystem II repair mechanism and its
regulation during stress conditions. FEBS Lett. 587: 3372-3381.
© 2015 American Society of Plant Biologists
Metabolic demand for NADPH and ATP
feed back into light harvesting
Reprinted with permission from Schöttler, M.A., Tóth, S.Z., Boulouis, A. and Kahlau, S. (2015). Photosynthetic complex stoichiometry dynamics in higher plants: biogenesis, function, and turnover of ATP synthase and
the cytochrome b6f complex. J. Exp. Bot. 66: 2373-2400.
When supply > demand,
elevated NADPH & ATP levels
feed back and induce
photoprotection (red arrows) NPQ
pH-induced promotes
conversion of violaxanthin (V)
into zeaxanthin (Z)
Photosynthetic
control of
plastoquinol
reoxidation
Metabolic imbalances, drought,
cold, pathogen infection and
other factors can decrease flux
through the Calvin-Benson cycle
© 2015 American Society of Plant Biologists
Timescales of high light responses
Seconds Minutes Hours Days
Electron transport affected
Δ pH
Reduced PQ pool
Protein degradation and PSII repair
Changes in antenna size
Increased xanthophyll pool Changes in PSI/ PSII stoichiometry
Photochemistry
Metabolism
Translatome
Transcriptome
Growth
Dark qE
qT
qI 10 – 200 s
10 – 60 s
10 – >30 min
10 – 60 min
NPQ kinetics
Adapted from Jahns, P. and Holzwarth, A.R. (2012). The role of the xanthophyll cycle and of lutein in photoprotection of Photosystem II. Biochim. Biophys. Acta Bioenergetics. 1817: 182-193. Dietz, K.-J. (2015). Efficient high light
acclimation involves rapid processes at multiple mechanistic levels. J. Exp. Bot. 66: 2401-2414. Erickson, E., Wakao, S. and Niyogi, K.K. (2015). Light stress and photoprotection in Chlamydomonas reinhardtii. Plant J. 82: 449-465.
© 2015 American Society of Plant Biologists
Light reactions
Carbon-fixing reactions
ATP NADPH ADP NADP+
High temperatures
increase membrane
permeability and
decrease proton-
motive force
Drought-induced
stomatal closure
prevents CO2 uptake
Cold temperature
slows enzyme-
catalyzed reactions
Nutrient deficiency
or toxicity affects
electron transport
Heat, drought, & other stresses affect
photosynthetic efficiency
© 2015 American Society of Plant Biologists
Retrograde signaling from plastid to
nucleus is critical for homeostasis
Reprinted with permission from Macmillan Publishing Ltd from Jarvis, P. and López-Juez, E. (2013). Biogenesis and homeostasis of chloroplasts and other plastids. Nat Rev Mol Cell Biol. 14: 787-802; see also Dietz, K.-J.
(2015). Efficient high light acclimation involves rapid processes at multiple mechanistic levels. J. Exp. Bot. 66: 2401-2414l Chi, W., Sun, X. and Zhang, L. (2013). Intracellular signaling from plastid to nucleus. Annu. Rev.
Plant Biol. 64: 559-582; Estavillo, G.M., Chan, K.X., Phua, S.Y. and Pogson, B.J. (2013). Reconsidering the nature and mode of action of metabolite retrograde signals from the chloroplast. Frontiers Plant Sci. 3: 300.
Retrograde signals
are biogenic (during
development) and
operational (during
function)
Possible signals include compounds
derived from heme biosynthesis,
ROS, nucleotide derivatives and
many more
© 2015 American Society of Plant Biologists
Reprinted from Takahashi, S. and Badger, M.R. (2011). Photoprotection in plants: A new light on Photosystem II damage. Trends Plant Sci. 16: 53–60. with permission from Elsevier
Summary of photosynthetic acclimation
mechanisms
Although the
photosynthetic
machinery is
susceptible to
photoinhibitory
damage, it also
has several
strategies for
photoprotection
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
Methods to monitor light reactions
NASA; Steveadcuk
• PSII activity can be measured by
chlorophyll fluorescence
• PSI activity can be measured by the
absorbance change at 810~830 nm
• Transthylakoid proton motive force
(pmf) and proton flux can be measured
by electrochromic shift (ECS)
Alternatively, photosynthesis can be
monitored spectroscopically
Photosynthesis can be monitored by O2
production and CO2 consumption
© 2015 American Society of Plant Biologists
Quantifying photosynthesis by gas
exchange measurements
Closed system
Light
Light on
O2
CO2
Light off
Co
nc
en
tra
tio
n
in c
ha
mb
er
Open system
Light
Light on
O2
CO2
Light off
Co
nc
en
tra
tio
n
in e
xit
ing
air
Air in Air out
Ambient [O2]
Ambient [CO2]
Gas exchange
in a leaf can be
measured in a
closed or open
(flow through)
system
[CO2] is
measured by
infrared gas
analysis (IRGA)
and [O2] by an
oxygen
electrode
© 2015 American Society of Plant Biologists
Principle of fluorescent measurements of
photosynthesis
PSII
chlorophyll
Incident
light
Fluorescence
(longer
wavelength)
Photochemistry
NPQ (heat)
When a chlorophyll a molecule absorbs light, it is excited
from its ground state to its singlet excited state (Chl*), which
can return to the ground state via one of three pathways:
(1) The excitation energy can be transferred to reaction
centers to drive photosynthesis (photochemistry, qP)
(2) Energy can be re-emitted as chlorophyll fluorescence
(3) It can be released as heat (non-photochemical
quenching, NPQ)
fluorescence
Chl
Chl* Photon
qP
NPQ
The practical application is
that fluorescence can be
quantified under various
conditions to measure
photosynthetic activities
© 2015 American Society of Plant Biologists
Reaction centers “close”, maximizing
fluorescence after pulse
Murchie, E.H. and Lawson, T. (2013). Chlorophyll fluorescence analysis: a guide to good practice and understanding some new applications. J. Exp. Bot. 64: 3983-3998 by permission of Oxford University Press.
P680 → P680* → P680+
Pheo → Pheo−
e- e−
QA → QA−
e- e−
QB → QB−
e- e−
Reaction
center
transiently
closed
A pulse of saturating light
leads to a transient pulse of
fluorescence as reaction
centers close (become
reduced) and light is emitted
as fluorescence
Reaction centers open with
measuring beam on, closed
after saturating pulse
© 2015 American Society of Plant Biologists
Determining max Fm, min Fo, and Fv:
Dark-acclimated tissues (no NPQ)
A very bright light
pulse is mainly
emitted as
fluorescence;
reaction centers
close until they
can be re-oxidized
PSII
chlorophyll
Saturating
pulse
Photochemistry
PSII
chlorophyll Very low
level light
Photochemistry In dim light
(measuring light),
the energy is
nearly all used for
photochemistry
Flu
ore
sc
en
ce
Fo
Fm
Fo
0
Fm
Saturating pulse
Measuring light on
Fv =
Fm
- F
o X
A saturating flash transiently closes reaction centers, resulting in maximum Chl fluorescence.
With measuring light only, the reaction centers are fully opened, resulting in minimum Chl fluorescence.
© 2015 American Society of Plant Biologists
Fv / Fm is an indicator of maximum
quantum yield of PSII
Fo = baseline
fluorescence, reaction
centers open
Fm = maximal
fluorescence when
reaction centers closed
(Fm− Fo) = Fv
= variable fluorescence
Fv / Fm = maximum
quantum efficiency of
PSII chemistry
(values less than ~0.8
indicate stress or
photoinhibition)
Murchie, E.H. and Lawson, T. (2013). Chlorophyll fluorescence analysis: a guide to good practice and understanding some new applications. J. Exp. Bot. 64: 3983-3998 by permission of Oxford University Press.
© 2015 American Society of Plant Biologists
Light-induced fluorescence is quenched
by qP + NPQ
F ' is steady state
fluorescence in the
light
NPQ and qP can be distinguished
by the difference in fluorescence
when reaction centers are open or
closed (Fm′)
Due to NPQ
Due to photochemistry
(photochemical quenching, pQ)
Fq′ / Fm′ = ΦPSII
ΦPSII = quantum efficiency of PSII
Fm′ – F′ = Fq′
Murchie, E.H. and Lawson, T. (2013). Chlorophyll fluorescence analysis: a guide to good practice and understanding some new applications. J. Exp. Bot. 64: 3983-3998 by permission of Oxford University Press.
© 2015 American Society of Plant Biologists
Different NPQ components relax at
different rates
• Fm increases as NPQ
reverses
• Different components
of NPQ have different
relaxation kinetics
(e.g., qE reverses
more rapidly than qI.)
Murchie, E.H. and Lawson, T. (2013). Chlorophyll fluorescence analysis: a guide to good practice and understanding some new applications. J. Exp. Bot. 64: 3983-3998 by permission of Oxford University Press.
© 2015 American Society of Plant Biologists
With kind permission from Springer Science+Business Media from Zhang, R. and Sharkey, T. (2009). Photosynthetic electron transport and proton flux under moderate heat stress. Photosynthesis Research. 100: 29-43.
PSI redox status can be determined by
810nm absorbance
P700 oxidation ratio = A / B
P700+ under actinic light
Max P700+ induced by far red and flash
P700
e-
e- P700+
Reduced P700 does not
absorb 810 nm light, but
oxidized P700+ does
P700+ Fully
oxidized
P700 Fully
reduced
An increase in
reduction rate of
P700+ can indicate
an induction of cyclic
electron transport
© 2015 American Society of Plant Biologists
Trans-thylakoid pmf can be measured by
electrochromic shift
Witt, H.T. (1979). Energy conversion in the functional membrane of photosynthesis. Analysis by light pulse and electric pulse methods: The central role of the electric field. Biochim. Biophy. Acta Bioenergetics. 505: 355-427.
The proton-motive
force (electrical and
pH gradient) across
the thylakoid can be
measured by its
effect on pigment
absorbance spectra
Ab
sorb
an
ce
Wavelength
Without
electric
field
With
electric
field
Electrochromic shift (ECS) is
the change in light absorbing
properties undergone by
pigments when subjected to a
changing electric field
© 2015 American Society of Plant Biologists
Electrochromic shift (ECS) monitors pmf
and ATP synthase activity
-400 -200 0 200 400
0.000
0.001
0.002
0.003
-10 0 10 20 30 40 50
-0.005
0.000
0.005
0.010
0.015
Op
tic
al
de
ns
ity
, re
lati
ve
Op
tic
al
de
ns
ity
, re
lati
ve
Time, ms
ECS
light off
ECS
light on
pH
Time, s
light off
ECS
1/EC: proton conductance,
proportional to ATP synthase
activity. In other words, the faster
the ECS decays, the more active
ATP synthase With kind permission from Springer Science+Business Media from Zhang, R. and Sharkey, T. (2009). Photosynthetic electron transport and proton flux under moderate heat stress. Photosynth. Res. 100: 29-43; see also
Baker, N.R., Harbinson, J., and Kramer, D.M. (2007). Determining the limitations and regulation of photosynthetic energy transduction in leaves. Plant Cell Environ. 30: 1107–1125..
Light induces pmf,
therefore ECS
ΔECS: light-induced transthylakoid
pmf (proton motive force)
ECS: relaxation
time constant
© 2015 American Society of Plant Biologists
With kind permission from Springer Science+Business Media from Bailleul, B., Cardol, P., Breyton, C., and Finazzi, G. (2010). Electrochromism: a useful probe to study algal photosynthesis. Photosynth. Res. 106: 179–189.
The kinetics of ESC can separate
activities of different components
EC
S
Light pulse
VERY fast = charge separation through PSI & PSII
Fast = Cyt b6f electron flow
Slow decay = H+ flow
through ATP synthase
Inhibitors can
distinguish PSI
from PSII
PSII inhibitors
© 2015 American Society of Plant Biologists
Chlorophyll fluorescence: Imaging and
scaling up
Murchie, E.H. and Lawson, T. (2013). Chlorophyll fluorescence analysis: a guide to good practice and understanding some new applications. J. Exp. Bot. 64: 3983-3998; NASA; NASA/Caltech; Guanter, L., et al. (2014).
Global and time-resolved monitoring of crop photosynthesis with chlorophyll fluorescence. Proc. Natl. Acad. Sci. USA 111: E1327-E1333; Joiner, J., et al., (2011). First observations of global and seasonal terrestrial
chlorophyll fluorescence from space. Biogeosciences. 8: 637-651..
F’
Photosynthetic efficiency of
PSII photochemistry (F q′/F m′)
Fluorescence measurements
can be coupled to imaging
systems to measure
photosystem efficiency across
many size scales – within a
leaf or within a plant.
Technologies are
being developed to
monitor solar-induced
fluorescence from
space
© 2015 American Society of Plant Biologists
Summary: Spectrophotometric
measurements of photosynthesis
The light-absorbing and emitting properties of
the photosynthetic complexes allow their
activities to be monitored with great
precision. These methods have been
developed using intact leaves and cells, but
to some extent can be applied to field- and
global-level monitoring
Reprinted by permission from Baniulis, D., Yamashita, E., Zhang, H., Hasan, S.S. and Cramer, W.A. (2008). Structure–Function of the Cytochrome b6f Complex†. Photochem. Photobiol. 84: 1349-1358.
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
Optimizing and improving
photosynthesis
Crop plants grow in
very unnatural
conditions, different
than those in which
natural selection has
acted. Can
photosynthetic light-
dependent reactions
be turbo-charged for
higher yields?
© 2015 American Society of Plant Biologists
Harvest other light wavelengths?
Ort, D.R., et al. (2015). Redesigning photosynthesis to sustainably meet global food and bioenergy demand. Proc. Natl. Acad. Sci. USA 112: 8529–8536 See also Croce, R. and van Amerongen, H. (2014). Natural
strategies for photosynthetic light harvesting. Nat Chem Biol. 10: 492-501 Blankenship, R.E., et al. and Sayre, R.T. (2011). Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for
improvement. Science. 332: 805-809. .
A hypothetical system in which PSI
is replaced by a bacterial reaction
center that uses longer wavelength
light. (Note that the system would
have to be extensively engineered to
bridge the gap between water
oxidation and NADPH reduction in
the absence of PSI.)
Another suggestion is to
increase green-light
absorption by introducing
phycoerythrin from red algae
(who wants black leaves?)
Current oxygenic photosynthesis
Hypothetical long-wavelength oxygenic photosynthesis
© 2015 American Society of Plant Biologists
Can single-celled organisms be
optimized for culture conditions?
Reprinted from Kirst, H., Formighieri, C. and Melis, A. (2014). Maximizing photosynthetic efficiency and culture productivity in cyanobacteria upon minimizing the
phycobilisome light-harvesting antenna size. Biochim. Biophys. Acta - Bioenergetics. 1837: 1653-1664; IGV; Biotech
Light penetration into culture vessels is
a problem for algal biofuels production.
By engineering cyanobacteria with
truncated light antennas (phycocyanin
deletion mutants), productivity
increased (due to improved light
penetrance and less excess light
dissipated from surface cells).
© 2015 American Society of Plant Biologists
Can shading be decreased in field
conditions?
Ort, D.R., et al. (2015). Redesigning photosynthesis to sustainably meet global food and bioenergy demand. Proc. Natl. Acad. Sci. USA 112: 8529–8536.
In a typical canopy,
upper leaves
intercept too much
light and must
dissipate some as
heat, meanwhile
shading lower
leaves
In a “smart
canopy”, upper
leaves would have
smaller antenna
complexes and a
more vertical
orientation,
permitting more
light to reach lower
leaves for greater
overall efficiency
© 2015 American Society of Plant Biologists
Can photoprotection be optimized for
less photooxidative damage?
Murchie, E.H. and Niyogi, K.K. (2011). Manipulation of Photoprotection to Improve Plant Photosynthesis. Plant Physiol. 155: 86-92.
Enhance PSII repair
Increase NPQ responsiveness
Increase flow of electrons through CEF or water-water cycle
ROS production can induce damage that is metabolically costly to repair
Manipulating photoprotective
pathways can enhance stress
resistance and photosynthetic
productivity of crop plants.
What are the metabolic
costs of enhanced
photoprotection?
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
Applying photosynthetic insights
towards solar electricity and fuels
The energy that reaches the earth from the sun in
an hour is equivalent to “all the energy humankind
currently uses in a year”
Solar panels make electricity that is hard to
store
The goal of artificial
photosynthesis is to
develop cheap,
durable ways to
make fuels from
sunlight (like a leaf)
Barber, J. and Tran, P.D. (2013). From natural to artificial photosynthesis. J. Roy. Soc. Interface. 10: 20120984; Photos by Tom Donald
© 2015 American Society of Plant Biologists
Developing biomimetic systems for
photosynthesis
Cogdell, R.J., Gardiner, A.T., Molina, P.I. and Cronin, L. (2013). The use and misuse of photosynthesis in the quest for novel methods to harness
solar energy to make fuel. Phil. Trans. Royal Soc. A: Mathematical Physical Engineering Sci 371: 20110603 by permission of the Royal Society.
The four fundamental steps that
comprise photosynthesis can be
engineered using catalysts 1. Light harvesting
2. Charge
separation
3. Water
oxidation
4. Proton
reduction
© 2015 American Society of Plant Biologists
Semiconducting materials can be used
as photocatalysts
Barber, J. and Tran, P.D. (2013). From natural to artificial photosynthesis. J. Roy. Soc. Interface. 10: 20120984 by permission of the Royal Society.
Semiconducting
materials can
carry out charge
separation
They can be
augmented with
electrocatalysts
They can be wired
together as a Z-
scheme
© 2015 American Society of Plant Biologists
Hybrid and bio-inspired systems are
being explored for fuel production
Torella, J.P., Gagliardi, C.J., Chen, J.S., Bediako, D.K., Colon, B., Wray, J.C., Silver, P.A., and Nocera, D.G. (2015). Efficient solar-to-fuels production from a hybrid microbial–water-splitting catalyst system. Proc. Natl.
Acad. Sci. USA. 112: 2337- 2342. Reproduced from Duan, L., Tong, L., Xu, Y. and Sun, L. (2011). Visible light-driven water oxidation-from molecular catalysts to photoelectrochemical cells. Energy Environ. Sci. 4:
3296-3313 with permission of The Royal Society of Chemistry. .
A bioelectrochemical cell. Water-
splitting and proton-reducing
electrodes are immersed into a
culture of bacteria that convert H2
to isopropanol A photoelectrical cell that mimics the
electron transfer reactions of PSII
© 2015 American Society of Plant Biologists
Bacteriorhodopsin: non-chlorophyll
based phototrophic system
Bacteriorhodopsin uses light
energy to pump protons (or
other ions) which can be used
for ATP synthesis Structure of
bacteriorhodopsin
Retinal is the
chromophore
As an alternative to chlorophyll based
systems, bacteriorhodopsins are being
explored for solar-driven chemistry.
See for example Claassens, N.J., Volpers, M., Martins dos Santos, V.A.P., van der Oost, J., and de Vos, W.M. (2013). Potential of
proton-pumping rhodopsins: Engineering photosystems into microorganisms. Trends Biotechnol. 31: 633 – 642.
© 2015 American Society of Plant Biologists
Lesson outline
• Photosynthesis overview
• Evolution and diversity of photosynthesis
• Light and pigments
• The light response curve and quantum efficiency
• Plastids and chloroplasts
• Structure and function of photosynthetic complexes
• Pathways of electron transport
• Damage avoidance and repair: Acclimations to light
• Monitoring light reactions
• Optimizing and improving photosynthesis
• Artificial photosynthesis
• Photosynthetic fungi and animals
© 2015 American Society of Plant Biologists
Photosynthetic fungi and animals
This lesson focuses on photosynthesis
in cyanobacteria and the products of
primary endosymbiosis
Secondary, tertiary
endosymbiosis Brown algae,
diatoms,
dinoflagellates,
euglenoids….
Other, unrelated
eukaryotic organisms
can serve as hosts to
photosynthetic bacteria
and algae:
Fungi
• Lichen
Animals
• Corals
• Flatworms
• Sea slugs
• Salamanders
Reprinted with permission from Rumpho, M.E., Pelletreau, K.N., Moustafa, A. and Bhattacharya, D. (2011). The making of a photosynthetic animal. J. Exp. Biol. 214: 303-311.
© 2015 American Society of Plant Biologists
Photosynthetic fungi: Lichen
Photos by Vernon Ahmadjian courtesy of BSA; jdurant
Lichen are mutualistic associations
of fungi (blue) and green algae or
cyanobacteria (green).
The photosynthetic partner
(phycobiont) resides outside the fungal
cells (mycobiont). The phycobiont
provides the mycobiont with reduced
carbon in return for shelter, nutrients
and water.
Lichen have many forms
Algal cell Fungus
© 2015 American Society of Plant Biologists
Photosynthetic animals:
Reef-building corals
Credit: Todd LaJeunesse, Penn State University. Fournier, A. (2014). The story of symbiosis with zooxanthellae, or how they enable their host to thrive in a nutrient poor environment. BioSciences Master Reviews.
Weis, V.M. (2008). Cellular mechanisms of Cnidarian bleaching: stress causes the collapse of symbiosis. J. Exp. Biol. 211: 3059-3066. Allisonmlewis
Symbiodinium
dinoflagellates live
within the bodies of
corals and provide
them with fixed
carbon
Various species of
coral and their
dinoflagellates
Coral bleaching is
the loss of the
symbiont.
Bleaching is
caused by stress,
particularly heat,
and often leads to
coral death
Bleached
© 2015 American Society of Plant Biologists
Bailly, X., Laguerre, L., Correc, G., Dupont, S., Kurth, T., Pfannkuchen, A., Entzeroth, R., Probert, I., Vinogradov, S., Lechauve, C., Garet-Delmas, M.-J., Reichert, H. and Hartenstein, V. (2014). The chimerical and
multifaceted marine acoel Symsagittifera roscoffensis: from photosymbiosis to brain regeneration. Front. Microbiol. 5: 498.
Lif
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ycle
of
the p
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wo
rm S
ym
sag
itti
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ffen
sis
© 2015 American Society of Plant Biologists
Photosynthetic animals:
Spotted salamanders
Kerney, R., Kim, E., Hangarter, R.P., Heiss, A.A., Bishop, C.D. and Hall, B.K. (2011). Intracellular invasion of green algae in a salamander host. Proc. Natl. Acad. Sci. USA 108: 6497-6502. Camazine
Egg capsule
Spotted salamander
Ambystoma maculatum
The algae provides the
animal with oxygen and
photosynthate, and
benefits from the animal’s
nitrogenous waste
The single-celled algae
Oophila amblystomatis lives
inside the egg capsule of
spotted salamander
Recently, algae (red fluorescent
spots) were found within the
animal’s cells, the first evidence
for endosymbiosis in a vertebrate
© 2015 American Society of Plant Biologists
Photosynthetic animals:
Plastid-stealing sea slugs
Pelletreau, K.N., Bhattacharya, D., Price, D.C., Worful, J.M., Moustafa, A. and Rumpho, M.E. (2011). Sea slug kleptoplasty and plastid maintenance in a metazoan. Plant Physiol. 155: 1561-1565. Rumpho, M.E.,
Pelletreau, K.N., Moustafa, A. and Bhattacharya, D. (2011). The making of a photosynthetic animal. J. Exp. Biol. 214: 303-311.
Vaucheria litorea (algae)
Elysia
chlorotica
(sea slug)
The plastids ingested from
the algae stay viable for
several months within the
sea slug – this “plastid
stealing” is known as
kleptoplasty Chloroplasts in sea slug
Ingested chloroplasts are
stored in animal’s cells
© 2015 American Society of Plant Biologists
Summary: Light-dependent reactions are
billions of years old
The core chemical reactions
have remained essentially
the same for > 2.5 billion
years, since the days of the
first cyanobacteria
Core reactions are conserved,
but variation in tolerances and
capacitieis of each of the
reactions allows for dynamic
responses and adaptations to
variable light, temperature etc.
The Z-scheme is a
familiar way to
depict the light-
harvesting
reactions of
photosynthesis
Ruth Ellison
© 2015 American Society of Plant Biologists
Summary: Photosynthetic reactions are
variable and responsive
Variable
Light intensity,
wavelength,
angle, duration
Responses
Light harvesting
Chloroplast and leaf
movements, accumulation of
pigments and proteins
Variable Chloroplast chemistry
Transthylakoid pH/ pmf
PQ / PQH2
Energy dissipation and photon balancing
Photoprotection
qE, quenching in LHC
* *
qT, state transition
Variable Whole-plant physiology
environmental stress
Plastid terminal oxidase
Antioxidant production
Variable Light and stress-
induced reactive oxygen
species
Protecting plants from
photo-oxoidative damage,
particularly when they are
stressed, may be a fruitful
approach to improving
photosynthesis
© 2015 American Society of Plant Biologists
Summary: Photosynthesis research may
lead to better crops and fuels
Ort, D.R., et al. (2015). Redesigning photosynthesis to sustainably meet global food and bioenergy demand. Proc. Natl. Acad. Sci. USA 112: 8529–8536.; IGV Biotech; Caltech; Joint Center for Artificial Photosynthesis
© 2015 American Society of Plant Biologists
Photosynthesis has to be integrated with
stress & development
Tom Donald; Tom Donald; See Allahverdiyeva, Y., Battchikova, N., Brosché, M., Fujii, H., Kangasjärvi, S., Mulo, P., Mähönen, A.P., Nieminen, K., Overmyer, K., Salojärvi, J. and Wrzaczek, M. (2015). Integration of
photosynthesis, development and stress as an opportunity for plant biology. New Phytol. 208: 647–655; FEMA
We need to understand photosynthesis
within the context of a whole plant,
considering the effects of development,
stress, age, demand for photosynthate and
all other factors that affect it.
STRESS
DEVELOPMENT
PHOTOSYNTHESIS