Control Systems in Plants

Etioloation and De-

etiolation

Plant Hormones

• What is a Plant hormone?

• Compound produced by one part of an organism that is translocated to other parts where it triggers a response in target cells and tissues.

Experiments on PhototropismDiscovery of Hormones

• 1. Darwin and Darwin– Removed the tip of the coleoptile

of a grass seedling, and it failed to grow toward light.

• 2. Boysen-Jensen– Put block of gelatin on coleoptile

tip to allow chemical diffusion– Auxin purified later by Thimann

• 3. Went– Modified Boysen-Jensen experiments– Extracted the chemical messenger responsible

Functions of Plant Hormones

• Control plant growth and development by affecting division, elongation, and cell differentiation

• Effect depends on size of action, stage of plant growth and hormone concentration

• Hormonal signal is amplified by gene expression, enzyme activity, or membrane properties

Five Classes of Plant Hormones

• 1. Auxins

• 2. Cytokinins

• 3. Gibberellins

• 4. Abscissic acid

• 5. Ethylene

Which hormones cause the following….• Apical dominance from apical bud• Abscission• Stimulates growth of axillary buds• Root growth• Stimulates closing of stomata• Causes fruit ripening• Stimulates seeds to break dormancy and

germinate• Growth inhibitor• Cell division and differentiation• Cell elongation• Seedless fruit

Which hormone is made at each location?

• Made in roots and transported upwards

• Found in meristems of apical buds and seed embryos

• Found in tissues of ripening fruit

• Leaves stems, roots and green fruit

Which hormone caused

the following?

Opposing hormones

• Which two hormones act in opposition to one another regarding apical dominance, cell division and differentiation?

• Which two hormones work in opposition regarding seed dormancy?

Phototropism

Acid Growth

Hypothesis

Plant Movement• A. Tropisms:

– growth response toward or away from stimuli

• 1. Phototropism– cells on darker side of shoot

elongate faster than cells on bright side due to auxin distribution

– auxin move laterally across the tip from the bright to dark side by an unknown mechanism.

– Cells on the dark side grow

• 2. Gravitropism (geotropism)– gravity– roots --> positive

geotropism– stems---> negative

geotropism

• Statoliths– starch grains in root cap cells,

they trigger calcium redistribution which results in auxin movement in root

– auxin inhibits cell elongation– upperside of root elongates faster

than bottom

• 3. Thigmotropism

• growth in response to touch

• tendrils contacts solid and coils

• increased production of ethylene

• 4. Hydrotropism

• growth toward water

• willow tree

Circadian Rhythms and the Biological Clock

• Circadian Rhythm- a physiological cycle with a frequency of about 24 hours that persists even when an organism is sheltered from environmental cues.

• Photoperiodism– a physiological response to DAY length– seasonal events

• Photoperiods Control of Flowering– the amount of night length controls

flowering

• 1. Short day plants– late summer, fall and winter

• 2. Long day plants– late spring and summer

• 3. Day- Neutral plants– unaffected by photoperiods

Critical Night Length• Night (dark) actually causes flowering

not light– Leaves detect the photoperiod while buds

produce flowers

• Florigen-– scientists believe this unidentified hormone

is produced in the leaves and moves to buds.

Phytochrome• Protein containing chromophore (light-

absorbing component) responsible for a plant’s response to photoperiod– Pr - red absorbing– Pfr- Far red absorbing

• Plants synthesize Pr in dark– if phytochrome illuminated then Pr Pfr since both

types of light are seen

• Pfr triggers many plant responses to light• In darkness Pfr goes back to Pr

Response to Stress

• Water deficit

• Oxygen deprivation

• Salt Stress

• Heat Stress

• Cold Stress

• Herbivores…

Defense Against Pathogens

• Gene for Gene relation between plant and pathogen

• Coevolution between plant and pathogen

Short Day Plant

Short Day Plant

• Spinach Long Day Plant

Spinach Long Day Plant

Which wave-

length of light

causes photo-

tropism?

AuxinsIAA indoleactetic acid: natural

auxin• Promotes elongation & secondary

growth

• Apical meristem is the major site of auxin production

• Inhibits lateral growth

• Induces female floral parts & fruit

Cytokinins

• Move from the roots to tissues by moving up xylem

• Works with auxin to promote differentiation• Stimulates protein synthesis• Made in roots• Function:

– 1. Cell division and differentiation– 2. Apical dominance– 3. Anti-aging hormones

• slow protein deterioration

Observe Apical Dominance

Gibberellins• Stimulate elongation of cells

• Inhibits root growth

• Stimulate flower part development- bolting(large internode)

• Works with auxin for fruit development

• Signals seeds to break dormancy and germinate

Abscisic Acid (ABA)

• Growth inhibitor

• returns seeds to dormancy

• inhibits cell division in vascular cambium

• causes rapid closing of stoma during dry periods

• promotes positive geotropism

Ethylene

• Gaseous hormone

• high [auxin] induce release of ethylene

• fruit ripening (positive feedback)

• Apoptosis:– Senescence (aging)– Abscission– leaves falling