Plant Growth Regulators Chemical Messengers

They provide for plant growth and development Hormones Hormones are “chemical messengers” directing plant development & survival. They provide for plant growth and development

Other methods of regulation are accomplished by Chemical reactions to stimuli Sunlight on shoots Water on roots Insects (chewing) on leaves Stimulate signal transduction pathways

Hormones = Growth Regulators 6 main plant hormones 1. Auxins 2. Gibberellins 3. Cytokinins 4. Ethylene 5. Brassinosteroids 6. Abscisic acid

A good example of hormones at work: Etiolation Plant growth in the absence of sunlight Environmental “awareness” Accomplished by hormones.

Auxins Stimulate cell growth thru cell elongation Produced in apical & lateral buds & meristems Auxins move mainly from apex (top) down. Auxins

Remember zones of elongation?

To demonstrate: Cells elongate under the auxin

Auxins Greatest concentration in terminal bud. Ensure apical dominance. Lateral bud growth is inhibited by other hormones

In addition…(auxin’s roles) Flower development Root growth (Rootone) –

Indoleactic acid (IAA)(natural). examples of Auxin Indoleactic acid (IAA)(natural). 2,4-dichlorophenoxy acetic acid (2-4D)(synthetic)

Next: Gibberellins (Gibberellic Acid GA) Stimulates cell division in stem, root, & leaf growth

Gibberellic acid LOOK! Functional groups!

Brassinosteroids are also growth regulators (no need to know) Brassinosteroids are also growth regulators Cell elongation ( sound familiar?) Vascular differentiation Stress tolerance

Cytokinins regulate growth Produced in roots Transported throughout plant. Affect cell division, apical dominance

Cytokinins Esp. in root meristems Interact with auxins, stimulate bud formation.

Cytokinins inhibits leaf senescence (seasonal leaf drop).

Ethylene gas Diffuses easily throughout a plant Facilitates ripening of fruits, apoptosis (?) leading to loss of leaves & flowers.

Ethylene (gas) “the aging hormone” In fruits & leaves, breaks down chlorophyll and lets colors show through.

Plant growth Inhibitor Abscisic Acid (ABA) Widespread in plants. moves freely through plant ABA made by the leaves. Interacts with other hormones in the plant; counteracts the growth effects of auxins & gibberellins.

Abscisic Acid Predominant during plant dormancy Produced when plants are under stress

Abscisic Acid Maintains seed dormancy Responsible for seed germination occurring only under favorable conditions

Additional ways plants respond to changes in their environment: It ain’t all hormones! The role of photoreceptors and signal transduction pathways

PHYTOCHROMES: 1 isomer stops growth, the other (far red) starts it.

Photoperiodism Short day plants Long day plants

Experiments have confirmed that it is actually the length of darkness, not light, that affects flowering. Greenhouses apply this science to get flowers to be ready for certain seasons

Other environmental stimuli (phototropism) Gravitropism Thigmotropism Temperature stresses Responses to insect infestations Plants produce R gene products