Chapter 33 Notes - Control in Plants

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings

PowerPoint Lectures forBiology: Concepts and Connections, Fifth Edition – Campbell, Reece, Taylor, and Simon

Lectures by Chris Romero

Chapter 33Chapter 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


• Soy products contain isoflavones

– A type of phytoestrogen that may provide human health benefits

Soybeans


PLANT HORMONES

33.1 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 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

• 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


33.2 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


• The major types of plant hormones

Table 33.2


33.3 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)

Elo

ngat

ion

Inhi

bitio

n

Pro

mot

ion


• 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


33.4 Cytokinins stimulated cell division

• Cytokinins

– 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


33.5 Gibberellins affect stem elongation and have numerous other effects

• Gibberellins

– Stimulate the elongation of stems

Figure 33.5A


• Gibberellins

– Stimulate the development of fruit

– Function in embryos in some of the early events of seed germination

Figure 33.5B


33.6 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


33.7 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

33.8 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

Chapter 33 Notes - Control in Plants

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