What are plant hormones?
Plant hormones, also known as phytohormones are chemicals that regulate plant growth. The term, Phytohormone was coined by Kenneth Vivian Thimann, in 1948. In plants, these are produced by cells in one area of the plant, such as leaves, stems or roots and then transported to a different area of the plant in order for them to have a response.
How do plant hormones affect the plant?
Plant hormones shape the plant, affecting seed growth, time of flowering, the sex of flowers, senescence of leaves, and fruits. They affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant longevity, and even plant death. Hormones are vital to plant growth, and, lacking them, plants would be mostly a mass of undifferentiated cells. So they are also known as growth factors or growth hormones.
How do plants transport hormones within the body?
Plants lack glands that produce and secrete hormones. Instead, each cell is capable of producing hormones. Hormones are transported within the plant using localised movement and cytoplasmic streaming within cells and slow diffusion. Phloem and Xylem are vascular tissues that also help in the transportation of hormones from one part of the plant to another.
Types of plant hormones
Plant hormones can be classified into five major categories, some of which are made up of many different chemicals that can vary in structure from one plant to the next. The classifications is based on chemical structural similarities and their effects on plant physiology. Each class has positive as well as inhibitory function and work in tandem with each other, interplaying to affect growth regulation.
1. Abscisic acid
- Abscisic acid, also called ABA and/or Dormin, is one of the most important plant growth regulators. It inhibits growth/germination of seeds. It is produced mainly in conditions that are unfavourable to plant growth. It induces seed and bud dormancy in winters.
- As the temperatures change, the levels of Abscisic acid reduce/dissipate in plants to enable flowering in plants and trees and for germination of new seeds to occur. It also prevents seeds from germinating when still within a fruit.
- In plants, during water stress, Abscisic acid, helps to close the stomata, to conserving water.
- Auxins were the first class of growth regulators discovered. They affect cell elongation by altering cell wall plasticity. They are responsible for stimulating the development of xylem (water transporting tissues throughout the plant) and inhibit growth of buds lower down the stems, ensuring the plant grows with the leading tip(apical dominance). They promote root growth and branching. In high doses, it however inhibits root growth and expansion.
- Cytokinins or CKs are a group of chemicals that influence cell division and shoot formation. They also delay senescence of tissues and are responsible for ensuring proper transport of the hormone auxin to where it is required in the plant’s body.
- Cytokinins and auxins together have a synergistic effect with auxins and the ratios of these two groups of plant hormones affect major growth period in a plant’s lifetime. Cytokinins, in conjunction with ethylene promote abscission (fall or drop) of leaves and fruit.
- Ethylene is also known as the ripening hormone. It is a gas that forms through the breakdown of methionine, which is in all cells. Its production increases when the seeds are mature, ensuring the fruit is released only when the seeds are ripe for germination.
- Have you noticed how when you keep a ripe fruit with unripe ones, the unripe ones also ripen rapidly. This is because of ethylene.
- Ethylene also affects cell shape and cell elongation. When, a growing shoot faces an obstacle, etheylene production increases and the stem becomes thicker, exerting more pressure to overcome the obstacle.
- Gibberellins were first discovered when the fungus, Gibberella fujikuroi, induced abnormal growth in rice plants.
- Gibberellins or GAs include a large range of chemicals that are produced by plants and fungus. They are important for seed germination, initiating mobilisation of nutrients stored within the seed. Absorption of water by the seed causes the production of Gibberellins.