Chapter 39:Control Systems in Plants

Question Do plants sense and respond to their

environment ? Yes - By adjusting their pattern of growth and

development.

In Dark In Light

Comment Plants can’t “move” away from a stimulus,

but can change their growth response. Result – plant bodies are more “flexible” in

morphology than animals.

Classical Example Phototropism - plant growth response to

unilateral light. Observation – plants bend or grow towards

the light.

Phototropism Experimenters

Darwins: late 1800's. Boysen & Jenson: early 1900's. F.W. Went: 1926

Went Experiments

Mechanism of Phototropism Cells on the dark side elongate faster than the

cells on the light side. The uneven growth rate causes the bending of

the stem toward the light.

Question What is the adaptive value of phototropism? It tilts the leaves toward the light source for

more efficient photosynthesis.

Cause of Phototropism Chemical messenger from the tip caused the

growth response in the stem. The distribution of the chemical changes in

the unequal light, resulting in unequal cell elongation.

Hormone Chemical signal produced in one location,

transported, has effect in another location. Phototropism is caused by a plant hormone.

Plant Hormones Are produced in small quantities. Effects may reflect balance between several

hormones.

Mechanism

Plant Hormones1. Auxins

2. Cytokinins

3. Gibberellins

4. Abscisic Acid

5. Ethylene

Auxins Named by Went in 1926. First plant hormone described. Ex: IAA (natural)

2,4-D (synthetic)

Major Functions Stimulates cell elongation. Fruit development. Apical Dominance. Tropism responses.

Apical Dominance

Where Produced Apical Meristems. Young leaves. Embryos.

Cytokinins Isolated from coconut "milk" (endosperm) in

the 1940’s. Named because they stimulate cell division. Ex: Zeatin

Major Effects Stimulates cell division. Delays senescence. Root growth and differentiation. Where Produced - roots

Auxin/Cytokinin Ratios Control shoot or root

differentiation in tissue cultures.

Gibberellins Found from the "Foolish Seedling" disease in

rice. Ex: GA3

70 types known

Foolish Seedlings

Major Effects Internode elongation. Seed/Bud germination. Flowering (some species). Fruit development.

Extra GA3

No GA3

Lack GA3

Have GA3

Where Produced Apical Meristems. Young leaves. Embryos.

Abscisic Acid Slows or inhibits plant growth. "Stress" hormone produced under unfavorable

conditions.

Major Effects Inhibits growth Seed/Bud dormancy. Stomata closure. Leaf drop – produces abscission layer.

Abscission Layer

Where Produced Leaves Stems Green fruit

Ethylene Gaseous hormone (fast diffusion rates). Often interacts with Auxin.

Major Effects Fruit ripening. Accelerates Senescence. Stem/Root Elongation (+ or -).

Where Produced Ripening fruits. Senescent tissue. Nodes.

New Hormones Oligosaccharins – short chains of sugars released

from the cell wall. Function:

Pathogen responses Cell differentiation Flowering

New Hormones Brassinosteroids – steroid hormones similar to

animal sex hormones. Function:

Needed for normal growth and development.

Commercial Applications of Plant Hormones Weed killers Seedless fruit Rooting of cuttings Tissue culture

Plant Movements1. Tropisms

2. Circadian Rhythms

Tropisms Growth responses in response to external

stimuli. + toward a stimulus - away from stimulus

Examples1. Phototropism

2. Gravitropism

Phototropism Response to light (blue).

Movie

Gravitropism Response to gravity. Stems are – gravitropic and roots are +

gravitropic.

Gravitropism - mechanism Statolith movement

may be the receptor for the stimulus.

Thigmotropism Response to touch. A series of 5 genes are involved. Ex: Tendrils

Climbing stems Wind direction response of stems.

Turgor Movements Movement caused by turgor pressure

differences in certain cells.

Types1. Rapid Leaf Movement

Ex: Mimosa2. Sleep Movements

Ex: Bean Leaves Prayer Plant

Day

Night

Sleep Movements

Mimosa Rapid Leaf Movement

Circadian Rhythms A physiological cycle about 24 hours long. Ex: Stomata opening

Sleep movements

Causes Synthesis of a transcription factor protein that

regulates is own manufacturing through feedback control.

Gene is believed to be common in most eukaryotic organisms.

Photoperiodism A physiological response to changing day

lengths. Used to detect and direct growth responses to

seasonal changes.

Advantages Match growth responses to proper season. Ex: Leaf drop in fall

Flowering

Flowering Types1. Short - Day Plants

2. Long - Day Plants

3. Day - Neutral Plants

Short-Day Plants Flower when days are shorter than a critical

period (long nights). Ex: Mums

Poinsettias

Long-Day Plants Flower when days are longer than a critical

period (short nights). Ex: Spinach

Iris Lettuce

Day-Neutral Plants Flower whenever they have enough energy. Ex: Roses

African Violets

Night Length Actually controls flowering response, not day

length. Proof – experiments show that if you interrupt

the dark period, you reset the “clock”.

Comment Length of night not absolute, but relative for

the response to be triggered.

Question

What detects day/night length changes? Phytochrome - plant pigment involved with

photoperiodism.

Phytochrome Forms

Pr - responds to

660nm (red light).

Pfr - responds to

730nm (far red).

Phytochrome Changes between the two forms.

Ratio or accumulation of enough Pfr triggers

the responses

In Red light: Pr

Pfr Far-red light or darkness:

Pfr Pr

Photoperiodism

Very sensitive (1 minute difference).

Sets clocks for plant responses.

Other Effects Seed Germination Stomatal Opening Leaf Drop

Lettuce Germination

Responses to Stress Stress – an environmental condition that can

have an adverse effect on a plant’s growth, reproduction and survival.

Plant Response1. Developmental changes

2. Physiological changes

Water Deficit During high Ts, guard cells may close. Young leaves may slow expansion. Leaves may roll to reduce surface area.

Oxygen Deprivation Common in roots in water-logged soils. Air tubes in roots may bring oxygen to the

cells.

Salt Stress Damages the plant through unfavorable soil

water potentials and toxic ions. Some plants can concentrate and excrete salt

through salt glands (ex. halophytes).

Heat and Cold Stress Heat - use heat-shock proteins to protect

other proteins from denaturing. Cold – lipid shifts to keep lipid bilayers

“liquid”. Cold – solute changes to lower freezing

point.

Herbivores Plants have many physical and chemical

defenses against herbivores. Physical – thorns Chemical – crystals, tannins and other toxic

compounds.

Herbivores Often trigger a plant to release chemicals to

attract predators or to warn other plants to increase their production of toxins.

Pathogens First Defense – epidermis Second Defense – chemical events to restrict

or kill the invader.

SAR

Systemic Acquired Resistance: chemicals that spread the “alarm” of an infection to other parts of the plant.

Possible Candidate: salicylic acid

Summary Know the general plant hormones and their

effects. Know tropisms. Know photoperiodism. Know general ideas about how plants respond

to stress.