Control

Systems

in Plants

Plant Hormones

• What is a Plant Hormone ?

• Compound produced by one

part of an organism that is

translocated to other parts

where it triggers a response in

target cells and tissues.

Functions of Plant Hormones

• Control plant growth and

development by affecting division,

elongation, and cell differentiation

• Effect depends on size of action,

stage of plant growth and hormone

concentration

• Hormonal signal is amplified by

gene expression, enzyme activity,

or membrane properties

Table 39.1 An Overview of Plant Hormones

Auxins

(IAA) indoleacetic acid:

natural auxin in plants

• Promotes elongation & secondary growth

• Apical meristem is the major site of auxin production

• Inhibits lateral growth

• Induces female floral parts & fruit

Figure 39.8 Apical dominance: with apical bud (left), apical bud removed (right)

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

Cytokinins• Move from the roots to tissues

by moving up xylem

• Stimulates protein synthesis

• Made in roots

• Functions:

–1. Cell division and differentiation

–2. Apical dominance

–3. Anti-aging hormones

• slow protein deterioration

Gibberellins

• Stimulate elongation of cells

• Inhibits root growth

• Stimulate flower part

development- bolting

• Signals seeds to break

dormancy and germinate

Figure 39.9 “Foolish seedling disease” in rice

Figure 39.11 The effect of gibberellin treatment on seedless grapes

Abscisic Acid (ABA)

• Growth inhibitor

• returns seeds to dormancy

• inhibits cell division in vascular

cambium

• causes rapid closing of stoma

during dry periods

• promotes positive geotropism

Figure 39.12 Precocious germination of mutant maize seeds

Ethylene

• Gaseous hormone

• High [auxin] induces release of

ethylene

• Causes senescence (aging)

• Fruit ripening – one bad apple

does spoil the whole bunch

• Abscission – loss of leaves on

deciduous trees

Figure 39.16 Abscission of a maple leaf

Which hormones cause the following….

• Apical dominance from apical bud

• Abscission

• Stimulates growth of axillary buds

• Root growth

• Stimulates closing of stomata

• Causes fruit ripening

• Stimulates seeds to break dormancy and germinate

• Growth inhibitor

• Cell division and differentiation

• Cell elongation

• Seedless fruit

Answers to Previous Slide:

•Auxin

•Ethylene

•Cytokinins

•Cytokinins

•Abscisic acid

•Ethylene

•Gibberellins

•Abscisic acid

•Auxins and cytokinins

•Auxins

•Auxins

Which hormone is made at each

location?

• Made in roots and transported

upwards

• Found in meristems of apical

buds and seed embryos

• Found in tissues of ripening fruit

• Leaves stems, roots and green

fruit

Answers to Previous Slide:

•Cytokinins

•Auxins

•Ethylene

•Abscisic Acid

Plant Movement• A. Tropisms:

– growth response toward or away from stimuli

• 1. Phototropism– cells on darker side of

shoot elongate faster than cells on bright side due to auxin distribution

– auxin move laterally across the tip from the bright to dark side by an unknown mechanism.

– Cells on the dark side grow

• 2. Gravitropism (geotropism)– gravity

– roots --> positive geotropism

– stems---> negative geotropism

• Statoliths– starch grains in root

cap cells, they trigger calcium redistribution which results in auxin movement in root

– auxin inhibits cell elongation

– upperside of root elongates faster than bottom

3. Thigmotropism

• growth in response to touch

• tendril contacts solid and coils

• increased production of ethylene

4. Hydrotropism

• growth toward water

• willow tree