chapter 32 plant growth and development ap biology spring 2011
TRANSCRIPT
Chapter 32
Plant Growth and Development
AP BiologySpring 2011
Chapter 32.1
Overview of Plant Development
Seed Germination Germination: the resumption of growth after
a time of arrested development
Environmental Factors Influence Seed Germination Seasonal Rains: provide water amounts
necessary to swell and rupture the seed coat Water activates enzymes necessary to hydrolyze
the stored starch Starches are converted to sugars
Provides the energy for the meristems to initiate cell division
Oxygen is required, reaches embryo and aerobic respiration provides ATP needed for growth
Environmental Factors Influence Seed Germination Repeated cell divisions produce a seedling
with a primary root When the primary root breaks through the seed
coat germination is complete Seed dormancy and germination is climate
specific Occurs only when conditions are favorable for the
seedling to survive
Patterns of Early Growth Growth: an increase in the number, size, and
volume of cells Development: the emergence of specialized,
morphologically different body parts Patterns of germination, growth, and
development have a heritable basis dictated by a plant’s genes
Patterns of Early Growth Early cell divisions may result in unequal
distribution of cytoplasm Cytoplasmic differences trigger variable gene
expression, which may result in variations in hormone synthesis
Even though all cells have the same genes, it is the selective expression of those genes that results in cell differentiation
Patterns of Early Growth Plant growth and development starts with the
selective transcription and translation of genes
Ex. Page 543 Fig. 32.3 and 32.4 Pattern of growth and development of corn
(monocot) and bean plant (dicot)
Chapter 32.2
Plant Hormones and Other Signaling Molecules
Major Types of Plant Hormones Plant hormones have central roles in the
coordination of plant growth and development
Giberellins Acidic compounds synthesized in seeds and
young shoot tissues Promote stem elongation, germination and
starch hydrolysis Help induce flowering in some plants
Auxins Produced at apical meristems of roots and shoots,
coleoptiles in monocots Influence cell division and elongation either positively or
negatively depending on the tissue Cause leaves to grown in patterns, stems to bend toward
light, roots to grow down Auxins at shoot tips prevent lateral bud growth- apical
dominance Help prevent abscission where leaves,
flowers, or fruits drop from plant Abscission: dropping of leaves,
flowers, fruits
Cytokinins Stimulate cell division in root and shoot
meristems, where they are most abundant Can release lateral buds from apical
dominance and can stop leaves from aging prematurely
Used commercially to prolong the life of stored vegetables and cut flowers
Ethylene (a gas) Can promote or inhibit cell growth so that
tissues expand in the most suitable directions Induces fruit ripening Concentrations high when plant is stressed
Ex. Autumn or end of life cycle Induces abscission of leaves and fruits, and
sometimes death of whole plant
Abscisic Acid (ABA) Inhibits cell growth
When growing season ends, ABA overrides gibberellins, auxins, and cytokinins; causes photosynthetic products to be diverted from leaves to seeds
Helps prevent water loss (by promoting stomata closure) When plant is water stressed, root cells produce
more ABA which xylem move to leaves Promotes seed and bud dormancy
Other Signaling Molecules Brassinosteroids: help promote cell division
and elongation Stems stay short in their absence
Jasmonates: help other hormones control seed germination, root growth, and tissue defense responses to pathogens
FT protein: part of a signaling pathway that induces flower formation
Other Signaling Molecules Salicylic Acid: interacts with nitric oxide in
respose to attacks from pathogens Nitric Oxide: functions in plant defense
response Systemin: peptide that forms when insects
attack plant tissues; travels throughout the plant turning on genes for substances that interfere with the insect’s digestion
Commercial Uses Many synthetic and natural plant hormones
are used commercially Ethylene: makes fruits ripen quickly Gibberellin: promotes larger fruits Synthetic Auxins: spayed on unpollinated
flowers to produce seedles fruits Synthetic Auxin 2,4-D: used as herbicides
Accelerates the growth of eudicot weeds to a point that the plant cannot sustain it and the weeds die
Chapter 32.3
Mechanisms of Plant Hormone Action
Signal Transduction Plants have pathways of cell communication
Hormone Action in Germination Imbibed water stimulates cells of embryo to
release gibberellin Water moves giberellin to cells of aleurone (protein
storing layer) Water also activates protein digesting enzymes
In aleurone layer, hormone triggers transcription and translation of amylase genes to hydrolyze starch molecules Digests starch into transportable sugar
Amylase moves into endosperm’s starch rich cells Sugar monomers released from starch fuel aerobic
respiration ATP from aerobic respiration provides the energy
for growth of the primary root and shoot
Polar Transport of Auxin Auxin concentration gradients start forming
during early cell divisions of embryo sporophyte
Cells exposed to higher concentrations transcribe different genes than those exposed to lower concentrations
Help form plant parts (leaves) in expected patterns
Helps young cells elongate
Polar Transport of Auxin Auxin concentration highest at source: apical
meristem in a shoot (or coleoptile) Auxin transported down, toward shoot’s base
Polar transport takes place in parenchyma cells
Polar Transport of Auxin Auxin gives up hydrogen in each cell, which
alters cytoplasmic pH Membrane pumps activly transport H+
outside, which lowers pH of moist cell wall Enzymes in cell wall become active at lower
pH
Polar Transport of Auxin Enzymes cleave crosslink's between
microfibrils, which support the wall Water is diffusing into the cell, turgor pressure
builds against wall Microfibrils now free to move apart, wall is
free to expand Ta-dah….cell lengthens!
pH change also activates transcription factors, after auxin exposure, proteins that help cell assume its new shape are synthesized
Chapter 32.4
Adjusting the Direction and Rates of Growth
Response to Gravity Gravitroprism: growth response to gravity
Shoots grow up, roots grow down Auxin, with growth-inhibiting hormone:
may play a role in promoting or inhibiting growth in various regions of the plant
Statoliths: are unbound starch grains in plastids, respond to gravity and may trigger redistribution of auxin
Response to Light Phototropism: growth response to light Bending toward light is caused by elongation
of cells (auxin stimulation) on the side of the plant NOT exposed to light
Phototropins: pigments that absorb blue wavelengths of light and signal the redistribution of auxin that initiates the elongation of cells
Response to Contact Thigmotropism: shift in growth triggered by
physical contact with surrounding objects This response to auxin and ethylene is
prevalent in climbing vines and in the tendrils that support some plants Tendrils: new, modified leaves or stems
When cells at shoot tip touch stable object, cells on contact side stop elongating and cells on other side keep growing
Unequal rates of growth make vine or tendril curl around object
Response to Mechanical Stress Responses to the mechanical stress of strong
winds explain why plants grown at higher elevations are stubbier than those at lower elevations
Grazing animals, growing outside vs. greenhouse can also inhibit plant growth
Human intervention such as shaking can inhibit plant growth
Chapter 32.5
Seasonal Shifts in Growth
Seasonal Shifts Circadian Cycle: completed in 24 hour
period Photoperiodism: refers to biological
response to alternations in the length of darkness relative to daylight during a circadian cycle Ex. The number of hours plant spends in darkness
and daylight shifts with seasons
Seasonal Shifts Biological Clocks: internal mechanisms that
preset the time for recurring shifts in daily tasks or seasonal patterns of growth, development, and reproduction
Seasonal Shifts Phytochrome: blue-green pigment functions
as a receptor for red and far-red light Red light at sunrise causes phytochrome to shift
from its inactive form (Pr) to its active form (Pfr) Far-red light at sunset shifts to inactive form (Pr) Longer the nights, longer the interval when
phytochrome is inactive Pfr can induce gene transcription
Can bring about seed germination, shoot elongation, branching, leaf expansion, and flower, fruit and seed formation, then dormancy
Chapter 32.6
When to Flower?
Response to Hours of Darkness Flowering process is keyed to changes in day
length throughout the year Cue is length of darkness
Response to Hours of Darkness Short-day plants:
flower in early spring or fall Nights are longer than
some critical value Long-day plants:
flower in summer Nights are shorter than
some critical value Day-neutral plants:
flower whenever they are mature enough to do so
Response to Hours of Darkness Phytochrome is trigger for flowering Detection of photoperiod (alternations in
length of darkness relative to daylight) occurs in leaves, where hormones inhibit a shift from leaf growth to flower formation
Revisiting the Master Genes 3 groups of master genes A, B, C control
formation of floral structures from whorls of a floral shoot
In response to photoperiods of other environmental cues, leaf cells transcribe a flowering gene
mRNA transcript travels in phloem to as-yet undifferentiated floral buds, where they are translated into FT protein
This signaling molecule with a transcription factor turn on master genes that cause undetermined bud of meristematic tissue to develop into a flower
Vernalization Vernalization: low temperature stimulation
of flowering Unless certain biennials and perennials are
exposed to low temperatures, flowers will not form on their stems in spring
Chapter 32.7
Entering and Breaking Dormancy
Abscission and Senescence Abscission: the dropping of leaves, flowers,
fruits, other parts Senescence: sum total of the processes
leading to the death of plant parts or the whole plant
Abscission and Senescence Recurring cue is decrease in day length that
triggers a decrease in auxin production Cells in abscission zones produce ethylene,
which causes cells to deposit suberin in their walls
Simultaneously, enzymes digest cellulose and pectin in the middle lamella to weaken the abscission zone Lamella: cementing layer between plant cell walls
Bud Dormancy Dormancy occurs in autumn when days
shorten, and growth stops in many trees and non-woody perennials
It will not resume until spring
Bud Dormancy Strong cues for dormancy include short days,
cold nights, and dry, nitrogen deficient soil Requirement for multiple cues for dormancy
has great adaptive value in preventing plant growth on occasional warm autumn days only to be killed later by frost
Dormancy broken by milder temperatures, rains, and nutrients