Five major types of hormones regulate plant growth and development

Five major types of hormones regulate plant growth and development

Plant hormones exert control by affecting cell division, elongation, and differentiation

1- Auxins

2- Cytokinins

3- Gibberellins

4- Abscisic acid

5- Ethylene

Auxin

Auxin stimulates the elongation of cells in young shoots

1- Auxins are a class of chemicals that promote seedling elongation

2- The apical meristem of shoot tips is a major source of IAA synthesis

3- In moderate concentrations, auxin promotes cell elongation in stems

4- In high concentrations, auxin reduces cell elongation in stems

5- Auxins affects cell elongation in roots at lower concentrations

Auxin has a number of effects other than elongation

- Promotes growth in stem diameter

 Promotes growth of fruit-

- Can induce fruit development without pollination

Cytokinin

1 Cytokinins stimulate cell division (cytokinesis)

2- Cytokinins are produced in actively growing organs or tissues as roots, embryos and fruits.

 3- Cytokinins and auxins interact to control apical dominance:

- Auxins inhibit axillary bud growth (reducing lateral branching)

- Cytokinins counter the action of auxin by promoting axillary bud growth

- The ratio of auxins to cytokinins controls axillary bud growth

Gibberellins

Gibberellins affect stem elongation and have numerous other effects

1- They are plant hormones that promote stem and leaves elongation by increasing cell division

2- They promote fruit development and seed germination

3- Enhance the action of auxin

Ethylene

1- Ethylene triggers fruit ripening and other aging processes

2- Plants produce ethylene in response to stress as mechanical pressure, injury, infection and drought or flood

3- Ethylene promotes leaf abscission in fall by breaking down cells at the base of the petiole

4- Growers inhibit ethylene production using CO₂ to inhibit ripening in stored fruits

Abscisic acid (ABA)

Abscisic acid inhibit many plant processes:

1- ABA is a plant hormone that slow or inhibits growth

2- High concentrations of ABA promote seed dormancy

3- Ratio of ABA to gibberellins controls germination

4- ABA influences plant water relations

- Accumulation of ABA in wilted leaves promotes stomatal closure

- ABA produced in roots can signal low soil moisture conditions  and triggers plants to conserve water by closing stomata.