Figure 39.0 A grass seedling growing toward a candle’s light Chapter 39: Plant Hormones

Review for test Test tomorrow all multiple choice

Hormones Chemical signals Produced in one part of plant and effect other parts Trigger responses in target cells and tissues Small amounts produce substantial changes

General plant hormones Auxins Gibberellins Cytokinins Abscisic acid Ethylene

Table 39.1 An Overview of Plant Hormones Study This!!

Hormonal Action Hormone (red) binds to receptor (receptor) which activate a 2nd messenger (blue). This triggers 1.Proton pumps 2.Production of golgi vesicles 3.Transcription and translation

H+

Figure 39.7 Cell elongation in response to auxin: the acid growth hypothesis

Acid Growth Hypothesis 1. Auxin, produced by apical mersitems in buds, trigger H+ pumps 2. H+ pumps move ions into cell wall matrix 3. Acidity in wall increases and breaks bonds in the cellulose fibers 4. Cell wall weakens and water enters 5. Cell elongates

Auxin & Cytokinins

Auxin: Control of Apical Dominance Effect on Stems: Auxin produced by terminal bud inhibits growth of axillary buds. Plant grows taller Cytokinins produced by roots stimulate axillary bud growth Lower buds grow faster than those close to terminal If terminal bud is cut, (i.e. auxin is removed) and axillary buds grow and plant gets fuller

Auxin: Control of Apical Dominance Effect on Roots: Auxin stimulates root branching High concentrations of auxin INHIBIT root growth Cytokinins inhibit root branching

Apical Dominance

Function of cytokinins Promotes cell division. Morphogenesis. Lateral bud development. Delay of senescence. Made in root, travel in xylem

Gibberellin

Discovered in association with Foolish disease of rice (Gibberella fujikuroi) uninfected infected

Effects of Gibberellins Make in roots and young leaves General cell elongation in leaves and stems (not roots). Found in embryo of seeds and leads to the breaking of dormancy. Promotion of flowering. Commercially used for spraying on Thompson’s seedless grapes

Figure 39.11 The effect of gibberellin treatment on seedless grapes

Abscisic acid

Functions of abscisic acid General growth inhibitor. Prevents seeds from germinating (if ABA is removed from seeds, germinations begins!) Causes closing of stomata “Winterization”effect on plants Produced in response to stress.

Ethylene H H \ / C = C / \

Functions of ethylene Gaseous Causes fruit to ripening. Senescence and abscission. Interference with auxin transport. Initiation of stem elongation and bud development.

Tropism

Tropisms Phototropism: Gravitropism: shoots or stems bends towards light enhancing photosynthesis Gravitropism: Plants uses statoliths to tell which way is “up” or “down” Roots display positive gravitropism (grows down towards the earth) and the shoot displays negative gravitropism (grows up out of the earth)

Growth movement

Phototropism Light straight above plants Auxin amounts equal on both sides Cell elongation equal on both sides Stems grow straight up!

Phototropism Light at an angle Auxin accumulates (moves) to the shaded side

Phototropism High auxin concentration promotes cell growth on shaded side Stem grows towards light source Positive phototropism

Gravitropism Shoot: Root: Grows away from pull of gravity Negative gravitropism Root: Grows toward pull of gravity Positive gravitropism

If plant falls over… In roots: Statoliths (starch grains) settle to the bottom of the cells Triggers the movement of Ca2+ Causes lateral movement of auxin

Roots and stems respond… …the effects of auxin on roots and shoots are opposing!!!! IN ROOTS: 1. Low auxin, cells elongate 2. High auxin, inhibitory response, no elongation Roots curve down IN SHOOTS 3. Low auxin inhibits elongation 4. High auxin, stimulates cell elongation Shoots curve up