principles of biology biol 100c: introductory biology iii plant hormones & plant defenses dr. p....

Principles of Biology

BIOL 100C:BIOL 100C:Introductory Biology IIIIntroductory Biology IIIPlant Hormones & Plant DefensesPlant Hormones & Plant Defenses

Dr. P. NarguizianFall 2012

Phototropism

Phototropism is a phenomenon by which plants grow toward a light source

Phototropism occurs when the cells on the dark side of a plant stem elongate faster than those on the light side

Charles Darwin and his son Francis conducted experiments that showed that the shoot tips of plants controlled their ability to grow toward light

Peter Boysen-Jensen later conducted experiments that showed that chemical signals produced in shoot tips were responsible for phototropism

Light

Shadedside of shoot

Illuminatedside ofshoot

The Darwins’ experiment

– When plant tips were removed, plants did not grow toward light

– When plant tips were covered with an opaque cap, they did not grow toward light

– When plant tips were covered with a clear tip, they did grow toward light

Experiments on how plants turn toward light led to the discovery of a plant hormone

Jensen’s experiment

– When a gelatin block that allowed chemical diffusion was placed below the shoot tip, plants grew toward light

– When a mica block that prevented chemical diffusion was placed below the shoot tip, plants did not grow toward light

Experiments on how plants turn toward light led to the discovery of a plant hormone

Light

Control

1 2 3 4 5 6

Tipremoved

Tip coveredby opaquecap

Tip coveredby trans-parent cap

Basecovered by opaqueshield

Tip sepa-rated bygelatinblock

Tip separatedby mica

Boysen-Jensen (1913)Darwin and Darwin (1880)

Five major types of hormones regulate plant growth and development

A hormone is a chemical signal that is produced in one part of the body and transported to another, where it triggers responses in target cells

Binding of hormones to specific cellular receptors triggers a signal transduction pathway

Tiny amounts of hormone can have a big effect

All aspects of plant growth and development are affected by hormones

There are five classes of plant hormones and each class can have multiple effects on plant growth and development

Five major types of hormones regulate plant growth and development

Auxin stimulates the elongation of cells in young shoots

Indoleacetic acid (IAA) is a naturally occurring auxin that promotes seedling elongation

Auxin is produced in shoot apical meristems and transported downward through a plant

Copyright © 2009 Pearson Education, Inc.


Concentration of auxin and site of activity are important to auxin’s effects

– In moderate concentrations, auxin promotes cell elongation in stems

– In high concentrations, auxin reduces cell elongation in stems

– Auxins affects cell elongation in roots at lower concentrations


A hypothesis for the action of auxin

– Auxins stimulate plant cells to take up H+ ions, lowering pH

– Acidity causes separation of cross linkages in cellulose

– As the cell takes up water, the cell elongates because of weakening of the cellulose cell wall

– Auxins stimulate the plant to produce additional cell wall material

– As pH decreases, the larger cell wall restabilizes



Plasmamembrane

Cellwall

Vacuole

Cytoplasm

Proton pump(protein)

H+

1

H+

2

Cell wall

Cellulosemolecule

3

Cellelongation

H2O

Enzyme

Cellulose loosens;cell can elongate Cellulose

moleculeCross-linkingmolecule

Cytokinins stimulate cell division

Cytokinins promote cytokinesis, or cell division

Cytokinins

– Are produced in actively growing organs such as roots, embryos, and fruits

– Produced in roots move upward through the plant

– Retard aging in leaves and flowers


Cytokinins and auxins interact to control apical dominance

– Auxins inhibit axillary bud growth, reducing lateral branching

– Cytokinins counter the action of auxin by promoting axillary bud growth

– The ratio of auxins to cytokinins controls axillary bud growth

33.4 Cytokinins stimulate cell division


Terminal bud

No terminal bud

Gibberellins affect stem elongation and have numerous other effects

Gibberellins are plant hormones that promote stem elongation by increasing cell division and elongation

Gibberellins were named for a genus of fungi that produce the same chemical and cause “foolish seedling” disease

There are more than 100 distinct gibberellins produced primarily in roots and young leaves


Gibberellins also promote fruit development and seed germination

Gibberellins act antagonistically against another plant hormone called abscisic acid

Gibberellins affect stem elongation and have numerous other effects


Abscisic acid inhibits many plant processes

Abscisic acid (ABA) is a plant hormone that inhibits growth

High concentrations of ABA promote seed dormancy

– ABA must be removed for germination to occur

– The ratio of ABA to gibberellins controls germination


ABA also influences plant water relations

– Accumulation of ABA in wilted leaves promotes stomatal closure

– ABA produced in roots can signal low soil moisture conditions and triggers plants to conserve water by closing stomata

Abscisic acid inhibits many plant processes


Ethylene triggers fruit ripening and other aging processes

Ethylene is a gaseous by-product of natural gas combustion and a naturally occurring plant hormone

Plants produce ethylene in response to stresses such as mechanical pressure, injury, infection, and drought or flood


Ethylene promotes aging processes such as fruit ripening and natural cell death

– It is used commercially to ripen fruits

– Growers inhibit ethylene production using CO2 to inhibit ripening in stored fruit

Ethylene promotes leaf abscission in fall by breaking down cells at the base of the petiole

Ethylene triggers fruit ripening and other aging processes


1

3

2

GROWTH RESPONSES AND BIOLOGICAL RHYTHMS IN

PLANTS


Tropisms orient plant growth toward or away from environmental stimuli

Tropisms are responses that cause plants to grow in response to environmental stimuli

– Positive tropisms cause plants to grow toward a stimulus

– Negative tropisms cause plants to grow away from a stimulus

Plants respond to various environmental stimuli

– Phototropism—response to light

– Gravitropism—response to gravity

– Thigmotropism—response to touchCopyright © 2009 Pearson Education, Inc.

Light

Phototropism

Gra

vity

Gravitropism Thigmotropism

PLANT DEFENSES


EVOLUTION CONNECTION: Defenses against herbivores and infectious microbes have evolved in plants

Herbivores are organisms that feed on plants; many plant adaptations have evolved to defend against herbivores

– Production of distasteful or poisonous compounds

– Symbioses with organisms that defend plants


Plants have also evolved defenses against pathogens

– The epidermis is the first line of defense against infection

– Chemical defenses offer a way to fight pathogens that enter the plant

EVOLUTION CONNECTION: Defenses against herbivores and infectious microbes have evolved in plants


5 4

3

21

Wasplayseggs

Damage to plantand chemical incaterpillar saliva

Synthesisand releaseof chemicalattractants

Signaltransductionpathway

Plant cell

Recruitment of wasp

TALKING ABOUT SCIENCE: Plant biochemist Eloy Rodriguez studies how animals use defensive chemicals made by plants

Animals may “medicate” themselves with chemicals produced by plants

Scientists observe which plants animals eat for “medicinal” purposes and how much of each plant they eat


Observation of such animal behavior has led scientists to study how such chemicals might benefit humans

– Plant chemicals can kill animal parasites

– Some may be useful for treatment of tumors

TALKING ABOUT SCIENCE: Plant biochemist Eloy Rodriguez studies how animals use defensive chemicals made by plants


principles of biology biol 100c: introductory biology iii plant hormones & plant defenses dr. p....

Documents

plant cells

plant stem

lightwhen plant tips

cell elongation

classes of plant hormones

shoot tips of plants

elongation of cells

cellulose cell wallauxins