Figure 39.0 A grass seedling growing toward a candle’s light

Chapter 39:Plant Hormones

Review for test

Test tomorrow all multiple choice

HormonesChemical signalsProduced in one part of plant and effect

other partsTrigger responses in target cells and

tissuesSmall amounts produce substantial

changes

General plant hormones

AuxinsGibberellinsCytokininsAbscisic acidEthylene

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 matrix3. Acidity in wall increases and breaks bonds

in the cellulose fibers4. Cell wall weakens and water enters5. Cell elongates

Auxin & Auxin & CytokininsCytokinins

Auxin: Control of Apical Dominance

Effect on Stems:Auxin produced by terminal bud inhibits

growth of axillary buds. Plant grows tallerCytokinins produced by roots stimulate

axillary bud growthLower buds grow faster than those close to

terminalIf 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 branchingHigh concentrations of auxin INHIBIT root

growthCytokinins inhibit root branching

Apical Dominance

Function of cytokinins

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

GibberellinGibberellin

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

infecteduninfected

Effects of Gibberellins

Make in roots and young leavesGeneral 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 acidAbscisic 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 plantsProduced in response to stress.

EthyleneEthylene

H H \ / C = C / \ H H

Functions of ethylene

GaseousCauses fruit to ripening.Senescence and abscission.Interference with auxin transport.Initiation of stem elongation and bud

development.

Tropism

TropismsPhototropism:

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 movementGrowth 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 angleAuxin accumulates

(moves) to the shaded side

PhototropismHigh auxin

concentration promotes cell growth on shaded side

Stem grows towards light source

Positive phototropism

GravitropismShoot:

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