Control and Coordination

PLANT HORMONES

Plant hormones are chemical messengers which plays a vital role in controlling the entire life process and other activities in the plant cell-like seed germination, ripening of the fruit, growth, flowering, etc.

There are four major types of plant hormones (or phytohormones) which are involved in the control and coordination in plants. These are:

• Auxins,
• Gibberellins,
• Cytokinins, and
• Abscisic acid (ABA)
• Ethylene

Production of plant growth hormone is affected by drought, nutrients, temperature, light, and stress are the environmental factors.

Out of these, auxins, gibberellins and cytokinins are the plant hormones which promote growth of plants, while abscisic acid is a plant hormone which inhibits (or prevents) the growth.

Auxin

• Auxin hormone controls a plant’s response to light and gravity.
• Auxin is made by cells at the tip of stems and roots.
• Auxin moves away from light, and towards gravity.
• Auxin was the first phytohormones to be recognized.
• It is also called IAA- indole-3-acetic acid.
• It is essential for the growth and development of a plant.
• Auxins have the following vital roles:
• Auxin helps in cell enlargement by increasing the plasticity of the cell wall, promotes the growth of the plant’s tissue, is responsible for the phototropism and geotropism, helps in promoting root initiation and stem cutting in plants.
• Auxin also inhibits the growth of lateral bud by stimulating the production of ethylene, controls the aging and senescence of plants., helps in promoting the growth of many fruits without fertilization and also helps in delaying the shedding of leaves.

Gibberellins:

• Gibberellins are the most abundantly found hormone in plants.
• They help in breaking the dormancy in seeds and buds and also promote growth in fruits.
• There are more than 70 gibberellins isolated.
• They are GA₁, GA₂, GA₃and so on.
• The GA₃Gibberellic acid is the most widely studied plant growth regulators.
• Gibberellins help in controlling the elongation of the stem in young shoots, seeds germination, enzyme production and food production, which is required for the new cells to grow.
• They also help in promoting flowering, cell division, and growth of seeds after germination.
• Gibberellins help in increasing the number of fruits, the size of a fruit and also delay the ripening of fruits in citrus plants and are also involved in seed germination.

Cytokinins:

• Cytokinins promote cell division (cytokinesis) hence the name.
• Zeatin is a naturally occurring plant growth hormone, which is isolated from corn plant.
• Cytokininsare present in all cell division sites including root tips, leaves, fruits, and seeds.
• Cytokinins help in stomatal opening, delaying the senescence of leaves, promoting the expansion of cotyledons.
• Cytokinins are also involved in the enlargement of stem cells and promoting chloroplast maturation.

Abscisic acid:

• It is a chemical messenger produced by other parts of a plant to indicate whether the plant is experiencing water stress, this signaling is mainly produced by drought roots and leaves.
• Abscisic acid helps in promoting bud dormancy and senescence of leaves, in the closing of stomata during Transpiration.
• Abscisic acid inhibits detachment of plant parts, plays an important role in seed development, maturation and dormancy and helps in slowing the plant growth and protects undeveloped bud during the cold season.
• It promotes the wilting and falling of leaves

Ethylene:

• Ethylene is an organic compound and gaseous substance.
• It is produced from an amino acid.
• It is present in the large amount and is mainly produced by plants that are under stress.
• Ethylene helps in stimulating fruit ripening and determining the sex of a flower.
• It is involved in the production of female flowers in a male plant.
• Ethylene promotes Apo-geotropism in roots, helps in the root initiation of plants and increases the speed of senescence.