PLANT HORMONESPROMOTE CELL DIVISION: CYTOKININS

WHERE IT PRODUCED ?

Cytokinin are synthesized primarily in actively growing tissues, particular in roots, embryos and fruits and transported to other organs, although there are many minor sites of production as well.

Cytokinin produced in the root reach to their target tissues by moving up the plant in the xylem sap.

HOW IT CONTROL CELL DIVISION

Cytokinin will stimulate cell division and influenced the pathway of differentiation when acting with auxin hormone.

In some experiment, a piece of parenchyma tissue from stem is cultured. This is some observation of the effect on cytokinin in cell tissue

Abcent of

cytokinin

Added cytokinin & auxin Added cytokinin

only

The cell grow very large but do not

divide

The cell is grow and divided

No effect

HOW IT CONTROL CELL DIVISION

The ratio of cytokinins to auxin controls the cell differentiate.

When the concentration of these 2 hormone are at certain level, the mass of cell continues to grow, but remains undifferentiated called a callus.

Cytokinin level increase Auxin level increase

Shoot buds develop from callus

Root form

FUNCTION OF CYTOKININ

Stimulates cell division. 

Stimulates morphogenesis

(shoot initiation/bud formation) in

tissue culture. 

Stimulates the growth of

lateral buds-release of apical

dominance. 

Stimulates leaf expansion

resulting from cell

enlargement. 

Cytokinins can slow down the aging of some plant organs. 

Slow deterioration of leaves on intact

plant. 

A list of some of the known physiological effects caused by cytokinins are listed

below

STIMULATES ABSCISION & THE REPENING OF FRUIT : ETHYLENE

WHERE IT PRODUCED?

Ethylene is the gaseous hormone that can be produced by most all part of the plant.

It produced in high concentrations during senescence, leaf abscission, and the ripening of some type of fruits.

This hormone synthesis is also stimulated by wounding & stress (drought, flooding, mechanical pressure, injury, & infection).

The triple response to mechanical stress

Triple response is occur when ethylene initiate a

seedling to perform a growth maneuver

In the triple response, stem elongation is slow,

the stem thicken & curvature causesthe stem

to start growing horizontally

It induced by mechanical stress on the stem tip

senescene

Senescence is natural process aging in plants

Ethylene play importance role e.g shedding of a leaf in autumn, withering of flower, & death of an annual after

flowering

Apoptosis = the onset of programmed cell death,During apoptosis , newly

formed enzyme breakdown chemical compound

Leaf abscission

The process of abscission occurs in autumn when a leaf falls

A change in the ratio of ethylene to auxin controls

abscission.

An aging leaf produces less & less auxin and this make the cell of the abscisssion

layer more sensitive to ethylene

Promotes bud & seed dormancy abscisic

Where it produced?

Almost plant has ability to synthesize ABA.

It presence has been detected in every major organ & living tissue and may be transported in the phloem/xylem

ABSCISIC ACID (ABA)

ABA is one of the hormone plant. Also called as abscisic II & dormin

Function: inhibit growth, promote seed dormancy & inhibit early germination,

promote stomatal closure during drought stress.

ABA inhibits stem elongation probably by its inhibitory effect on gibberellic acid

ABA also promotes abscission of leaves and fruits

EFFECT OF ABSCISIC ACID SEED DORMANCY

• One of major affect of ABA is seed dormancy. Seed dormancy has great survival value because it ensure that seed will germinate only when there are optimal conditions of light, temperature, & moisture.

• Many types of dormant seed germinate when ABA is remove or inactive.

DROUGHT TOLERANCE

• ABA is the primary internal signal- That enables plants

to withstand drought - signaling turns on the

expression of genes encoding proteins that protect cells — in seeds as well as in vegetative tissues — from damage when they become dehydrated

a) ABA in roots that elongate the root

for water absorption

b) ABA in the stem that allow the stem to elongate to get sufficient light

Example of past year question

1) Name three (3) common types of plant hormones and discuss the main functions of each hormone (APR 2009)

A. CytokininB. EthyleneC. Abscisic

• stem apex, auxillary buds promotes cells division (release

of apical dominance)• leaves inhibits senescence

Cytokinins

• Guard cells close stomata• seed coat inhibits seed

germination

Abscisic acid

• Stem inhibits cell elongation

• fruits promotes ripening

Ehtylene

2) Which plant hormones might be used to enhance stem elongation and fruit growth? (APR 2010)

A. Auxins and gibberellinsB. Ethylene and cytokininsC. Abscisic acid and phytochromeD. Phytochrome and flowering hormone