Chapter 33

Control Systems in Plants

• What Are the Health Benefits of Soy?– Soy protein• Is one of the few plant proteins that contains all the

essential amino acids

– Phytoestrogens, a class of plant hormones• Are found in soy

CH3

OH

HO HO

O OH

OHO

Estrogen (Estradiol) Phytoestrogen (Genistein)

Chemical structures of a humanestrogen and a plant phytoestrogen

PLANT HORMONES• Experiments on how plants turn toward light led

to the discovery of a plant hormone– Plants exhibit phototropism• The growth of shoots in response to light

Figure 33.1A

– Microscopic observations of plants• Indicate that a cellular mechanism underlies

phototropism

Shaded side of shoot

Illuminated side of shoot

Light

Figure 33.1B

• Showing That Light Is Detected by the Shoot Tip– Charles Darwin (late 1800s) showed that the tip of a

grass seedling detects light• And transmits a signal down to the growing region of

a shoot

Light

Control Tipremoved

Tip covered byopaque cap

Tip coveredby trans-parent cap

Base coveredby opaqueshield

Tip separatedby gelatinblock

Tip separatedby mica

Darwin and Darwin (1880) Boysen-Jensen (1913)

Figure 33.1C

• Isolating the Chemical Signal- (Frits Went 1926)– The hormone Auxin• Was determined to affect phototropism• Promotes faster cell elongation on the shaded site of

the shoot

Agar

Shoot tip placed on agar block.Chemical (later called auxin)diffuses from shoot tipinto agar.

Other controls:Blocks with nochemical haveno effect.

Offset blocks withchemical stimulatecurved growth.Control

Block withchemicalstimulatesgrowth.

No light

Figure 33.1D

Five major types of hormones regulate plant growth and development

– Even in small amounts, plant hormones• Trigger signal transduction pathways• Regulate plant growth and development

Auxin

• Auxin stimulates the elongation of cells in young shoots

– Plants produce auxin (IAA)• In the apical meristems at the tips of shoots

– At different concentrations, Auxin

• Stimulates or inhibits the elongation of shoots and roots

Figure 33.3A, B

Roots

Stems

0

0.9 g/L

10–8 10–6 10–4 10–2 1 102

Increasing auxin concentration (g/L)

Elon

gatio

nIn

hibi

tion

Pro

moti

on

Hypothesis

– Auxin may act by weakening cell walls• Allowing them to stretch when cells take up water

Plasmamembrane

Cellwall H+

1

2H+

3H2O

Vacuole

Cellelongation

Cellulose loosens; cell can elongate

Cellulosemolecule

Cross-linkingmolecule

Enzyme

Cellulosemolecule

Cell wall

Cytoplasm

H+ pump(protein)

Figure 33.3C

Auxin promotes growth in stem diameter

• By stimulating the development of vascular tissues and cell division in vascular cambium

Cytokinins

• Cytokinins stimulate cell division

• Are produced by growing roots, embryos, and fruits• Promote cell division

Cytokinins from roots may balance the effects of auxin from apical meristems• Causing lower buds to develop into branches

Figure 33.4

Terminal bud

No terminal bud

Gibberellins

• Gibberellins affect stem elongation and have numerous other effects

Figure 33.5A

Gibberellins• Stimulate the development of fruit• Function in embryos in some of the early events of seed

germination• When sprayed at a certain time can produce seedless

fruits

Figure 33.5B

Abscisic Acid

• Abscisic acid inhibits many plant processes

– Abscisic acid (ABA)• Inhibits the germination of seeds

– The ratio of ABA to gibberellins• Often determines whether a seed will remain

dormant or germinate

Seeds of many plants remain dormant• Until their ABA is inactivated or washed away

Figure 33.6

– ABA also acts as a “stress hormone”• Causing stomata to close when a plant is dehydrated

Ethylene

• Ethylene triggers fruit ripening and other aging processes

– As fruit cells age• They give off ethylene, which triggers a variety of aging

processes

• Fruit Ripening– Ethylene• Triggers fruit ripening

1

2

3

Figure 33.7A

• The Falling of Leaves

– A changing ratio of auxin to ethylene• Is triggered by shorter

days• Probably causes autumn

color changes and the loss of leaves from deciduous trees

Leafstalk

Stem(twig)

Abscissionlayer

Protectivelayer

Stem Leaf stalk

LM 2

0

Figure 33.7B

CONNECTION• Plant hormones have many agricultural uses– Farmers use auxin• To delay or promote fruit drop

Figure 33.8

– Auxins and Gibberellins• Are used to produce seedless fruits

– A synthetic Auxin called 2,4-D• Is used to kill weeds• Has safety questions associated with its use