Plant Hormones(Plant-Growth Regulators)

• Plant-growth regulators (plants hormones) are used to improve yields
• Increase or decrease seeds germination cause early or late maturation of plants improve quality to accelerate the ripening of fruits and grains

The five main groups of plant hormones

1. Auxins

2. Cytokines

3. Ethylene

4. Abscise acid (ABA)

5. Gibberellins (GA)

1.Auxin

• Auxins promote stem elongation, inhibit the growth of lateral buds (maintains apical dominance).
•  They are produced in the stem, buds, and root tips.
•  Example: Indole Acetic Acid (IAA).
•  Auxin is a plant hormone produced in the stem tip that promotes cell elongation.
• Auxin moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant. This produces a curving of the plant stem tip toward the light, a plant movement known as phototropism.
•  Auxin also plays a role in maintaining apical dominance.
•  Auxin maintains this dormancy. As long as sufficient auxin is produced by the apical meristem, the lateral buds remain dormant.
•  If the apex of the shoot is removed (by an animal or a scientist), the auxin is no longer produced.
•  This will cause the lateral buds to break their dormancy and begin to grow.
•  In effect, the plant becomes bushier. When a gardener trims a hedge, they are applying apical dominance.

2 .Cytokines :

• Cytokines promote cell division.
• Cytokines are produced in roots and are transported throughout the plant.
• They have a variety of effects depending on the target organ.
• In addition to stimulating cell division, cytokines influence the direction of organ development (organogenesis).
• For example, the relative amounts of cytokines and auxin determine whether roots or shoots will develop.
• Cytokines also modify apical dominance in stems and promote lateral bud growth.
• They also stimulate seed germination.

3 . Ethylene :

• This is a gaseous hormone that promotes the ripening of fruit.
• During the later stages of fruit development, ethylene gas fills the intercellular air spaces within the fruit and stimulates its ripening by enzymatic breakdown of cell walls.
• In Banana, gaseous hormones ripening by enzymatic breakdown of cell walls.
• Ethylene is also involved in stimulating 

  the production of flowers.
• Ethylene is involved in the production of female flowers in a male plant.
• In addition, ethylene inhibits the elongation of roots, stems, and leaves.

4 . Abscise acid ( ABA)

• Abscise Acid (ABA) is a growth inhibitor.
• It also promotes leaf aging when plants go dormant for the winter.
• Abscise acid promotes seed dormancy and inhibits early germination.
• Dormancy in these seeds is broken by an increase in gibberellins or by other mechanisms that respond to cause such as temperature or light.

5. Gibberellins (Gibberellic Acid) :

• Gibberellins are a group of plant hormones that, like, auxin, promote plant growth.
• The more than 60 types of gibberellins are abbreviated GA1, GA2, etc.
• They are synthesized in young leaves, roots, and shoots and transported to other parts of the plant.
• Gibberellins produced in the roots and transported to shoot tips interact with auxins to stimulate shoot growth.
• Gibberellins are also involved in the promotion of fruit development and of seed germination.
• They may be sprayed commercially on certain crops such as grapes to increase the overall size of the fruit.
• Gibberellins (GA3) is not widely used on wine grapes because the application of GA3 to seeded grapes may reduce shoot and cluster counts.
• Plant growth is driven by an increase in two factors: the number of cells, and their size.
• The plant hormone gibberellins control how root cells elongate as the root grows.
• Gibberellins also regulate the number of cells in the root.