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Hormones and Plant

responses to stimuli

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HOW PLANTS RESPOND TO

ENVIRONMENTAL STIMULI

Tropisms - plant growth toward or awayfrom a stimulus such as light or gravity.

Nastic Movements - response toenvironmental stimuli that are

independent of the direction of thestimulus. Pre-determined response.

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Phototropism ± ability to bend

towards light Auxins - responsible

for plants bending

towards light.

Auxins - move down

shaded side of the

stem and cause cellsto elongate

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Gravitropism (geotropism) ± plant

response to gravity Auxins ± responsible

for plant response to

gravity

Auxins ± move to

lowest side and cause

stem tissue toelongate ± stem

curves upwards

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PhototropismPhototropism

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Phototropisms

Phototropic responses involve bending of 

growing stems toward light sources.

 ± Individual leaves may also displayphototrophic responses.

auxin most likely involved

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Growth movementGrowth movement

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How Phytochrome Works

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Plants Respond to Gravity

Gravitropism is the response of a

plant to the earth¶s gravitational field.

 ± present at germination

auxins play primary role

 ± Four steps

gravity perceived by cell

signal formed that perceives gravity

signal transduced intra- and intercellularly

differential cell elongation

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GravitropismGravitropism

Increased auxin concentration on the lower side in stemscauses those cells to grow more than cells on the upper side.

 ± stem bends up against the force of gravity

negative gravitropism

Upper side of roots oriented horizontally grow more rapidly

than the lower side

 ± roots ultimately grow downward

positive gravitropism

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Taiz and Zeiger. 2002. Plant Physiology, 3rd Ed.

Auxin and GeotropismAuxin and Geotropism

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Increased auxin concentration on the lower 

side in stems causes those cells to grow

more than cells on the upper side

 ± Stem bends up against the force of gravity

Negative gravitropism

Upper side of roots orientated horizontally

grow more rapidly than the lower side

 ± Roots ultimately grow downward

Positive gravitropism

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Gravitropism = GeotropismGravitropism = Geotropism

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StatolithsStatoliths

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Plants Respond to Touch

Thigmotropism is directional growth

response to contact with an object.

 ± tendrils

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ThigmotropismThigmotropism

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Thigmomorphogenesis

 ± Changes in form of plant as a result of mechanical stress

 ± Mechanical stimulation activates signaltransduction, increases calcium, activatesgenes, which affects the cell wall

properties

 ± Strong wind = stockier trunk

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Pulvinus of Pulvinus of Mimosa pudicaMimosa pudica

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NYCTINASTY

sleep movements

prayer plant - lower 

leaves during the dayand raises leaves at

night

shamrock (Oxalis)

legumesCredit:(http://employees.csbsju.edu/ssa

upe/biol327/Lab/movie/movies.htm)

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Circadian Clocks

Circadian clocks are endogenoustimekeepers that keep plant responsessynchronized with the environment.

 ± circadian rhythm characteristics must continue to run in absence of external inputs

must be about 24 hours in duration

can be reset or entrained (to determine or modify the phase or 

period of <circadian rhythms entr ai ned by a light cycle>) can compensate for temperature differences

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Other Growth Responses

Thigmotropism - Other Objects Chemotropism - Chemicals

Traumatropism - Wounding

Thermotropism - Temperature Aerotropism - Oxygen

Skototropism - Dark

Geomagnetotropism - Magnetic Fields

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Hormones and

Signal Transduction

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Cells must communicate to coordinate their activities.

Cells may receive a variety of signals,

chemical signals, electromagnetic signals,and mechanical signals.

How plants respond to stimuli

The process by which a signal on a cell¶s

surface is converted into a specific cellular response is a signal-transduction

pathway.

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Signal Transduction Signals from outside a cell can be perceived,

sometimes by receptors that span the plasmamembrane.

After stimulation of such receptors, informationcan be relayed by a series of small molecules or proteins to the cell nucleus, where activation of specific transcription factors can stimulate new

gene expression programs.

Gene expression results in the production of new proteins that can function in the final

biological responses to the signal.

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Signal Transduction

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SignalSignal--transduction pathwaystransduction pathways

in plantsin plants

 Auxin interacts with calcium ions which in turn calmodulin, a protein, which

regulates many processes in plants, animals, and microbes.

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Loosening of cell wallLoosening of cell wall

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Signal Transduction Genes

The activated transcription factors

enter the nucleus and promote gene

expression.

Two classes of auxin-induced genes:

 ±Primary response, or EARLY

genes. ±Secondary response, or LATE

genes.

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Signal Transduction Genes

Primary (EARLY) Response Genes.

 ± These genes are present in cell at time of 

hormone exposure for fast response.

 ± Usually, they cannot be blocked by

inhibitors.

 ± Response time: a few minutes to several

hours. ± They regulate transcription of 2r genes.

 ± Involvement in cell-to-cell signaling.

 ± Involvement in stress adaptation.