BOT3015 - Plant Hormones: Growth Regulations

Ultra-violet B light, Blue/near UV, & Red/far red

The 3 colors of light that plants can sense.

Phytochromes

Photoreceptor proteins that can exists in 2 different states based on its photochromicity (wavelength it absorbs). Plays a key role in helping plants detect changes in day/night length.

Pr (Ground State) - Phytochromes

Absorbs red light (common in sunlight)

Pfr (Active State) - Phytochromes

Absorbs far-red light (common in shade). This also inhibits activity of Gibberellin. Plants growing in full sunlight will have morePfr and thus undergo less internode expansion. Plants growing in the shade will have less Pfr and thus undergo more internode expansion.

A fast process with Pr and Pfr

Exposure to red light changes Pr into Pfr & Exposure to far-red light changes Pfr into Pr

A slow process with Pr and Pfr

Darkness changes Pfr into Pr

Phytochrome - Germination

Some seeds cannot germinate in the darkness. Exposed to red light (for as little as 1 minute) promotes their germination. This can be reversed by exposure to far-red light. Seedling germinated in the dark form long, weak stems with a hook below the apical meristem (etiolation).

Etiolation

Plant morphological adaptations for growing in darkness.

De-etiolation

The changes in a plant that happens upon exposure to

red-light (sunlight)

Photoperiodism

Ability of organisms to control life processes based on changes in light. Changes in day/night length is the most

reliable environmental variable. Timing of germination, dormancy-induction and dormancy-breaking, and flowering

Long-Day Plants

Plants that begin flowering as days get longer (January-June in NH)

Short-Day Plants

Begin flowering as days get shorter (July-December in NH)

Photoperiod

The day length, or length of light in a 24 hour cycle, regardless of quantity

Nastic

Movements are non-directional responses to stimuli

Nyctinastic

Refers to circadian rhythm-based responses controlled by daily changes in light. This movement of leaves is caused

by ion transport the leads to changes in turgor pressure in cells.

Tropism

A directional growth in response to stimuli from the environment

Positive Tropism

Growth toward the stimuli

Negative Tropism

Growth away from the stimuli

Geotropism/Gravitropism

A growth response to gravity. Involves the unequal accumulation of auxin. Geotropism overrides hydrotropism.

Negative Geotropism

A plant's shoot grows from gravity

Positive Geotropism

A plant's roots grow down toward gravity

Statoliths

Specialized amyloplasts in cells of the stem cortex and root cap settle to the bottom of the cell under gravity. These amyloplasts enter in contact with the endoplasmic reticulum, releasing calcium ions.

Calcium Ions

These ions promote an accumulation of PIN (auxin transport) proteins

Hydrotropism

A growth response to moisture. Mechanisms remain largely unknown.

Thigmotropism

The growth that results from constant directional pressure. Tendrils have positive thigmotropism - upon touch, they will initiate coiling. Roots exhibit negative thigmotropism - upon

encountering an obstruction, roots bend around.

Thigmomorphogenesis

A slow developmental change in the shape of a plant subjected to continuous mechanical stress. This change is likely mediated by ethylene and jasmonic acid

Thigmonastic Movement

Refers to a touch response independent of the direction of stimulus

Jasmonic Acid

An important molecule in plant defense against herbivores. This acid mediates the response of Thigmotropism

Phototropins

Mediate phototropic responses (located in the apical meristem). They absorb blue light

Positive Phototropism

Shown in shoots

Negative Phototropism

Shown in roots