Plant Hormones: Gibberellins and Abscisic acid
Just as the human body uses hormones to regulate numerous processes throughout the body, the same is true for the plant body. Plant hormones are chemicals that play a key role in the regulation of a wide variety of vital processes in plants, including growth, development, and reproduction. They are also responsible for the distinctive tastes, scents, and colours of plants. While there are several plant hormones, we shall be looking closely at two specific ones namely, Gibberellins and Abscisic acid.
Introduction to plant hormones
The term ‘phytohormone’ is often used to refer to a group of phytochemicals produced by plants that affect other cells in the same plant or other plants. For example, Abscisic acid (ABA) is a phytohormone that acts directly on plant cells to control the flow of water and nutrients.
Phytohormones are the plant equivalent of human hormones.
Plant hormones (also called phytohormones) have a wide range of effects on plants, including the control of plant growth, the development of plant organs, the control of plant cell division, the control of the flow of nutrients throughout the plant, and the control of the movement of molecules within cells. The plant hormones are not structurally or chemically identical to animal hormones. Instead, they are often more complex molecules that carry out their function in a variety of different ways. Phytohormones found in plants may be classified as either growth-promoting or growth-inhibiting. Growth-promoting hormones are known to stimulate the growth of the host plant through activities such as cell division, cell enlargement, flowering, fruiting, and seed formation. Some examples of these hormones are auxins, gibberellins, and cytokinins.
The first identified plant hormone was Auxin in the late nineteenth century.
On the contrary, growth-inhibiting hormones are involved in hindering plant growth such as through dormancy and abscission. These hormones play a vital role during times of biotic or abiotic stress. Abscisic acid comes under this group. Ethylene, which is a gaseous hormone, is an exception to this classification. It could belong to either of the groups (promoting or inhibiting) because of its wide range of effects. Nonetheless, it largely plays an inhibitory role.
Gibberellins are acidic growth-promoting plant hormones. There are over 100 types of gibberellins that have been discovered till date. They have been reported in a wide range of organisms, ranging from fungi to higher plants. These are all derived from the amino acid tyrosine. Gibberellic acid ‘GA 3’ was the first gibberellin to be discovered and is, as a result, the most extensively studied.
Though all of them are derived from the same precursor (i.e. gibberellic acid), each type of gibberellin has unique functions and effects on the plant.
The discovery of gibberellic acid was initiated in Japan during the early 19th century while studying the ‘bakanae’ i.e., foolish seedling disease in rice. The disease was caused by the fungus Gibberella fujikuroi. E. Kurosawa (1926) observed symptoms of the disease in rice seedlings due to secretions of the fungus. The seedlings showed symptoms such as sterility and seedling overgrowth.
Later, this active substance was identified as gibberellic acid.
Gibberellin (GA) plays a critical role in plants. They are involved in the growth and development of organs and tissues such as the embryo, the seed and the root.
Below are some of the ways in which gibberellins contribute to plant growth and development:
- Gibberellins play a role in enhancing the yield of certain crops, such as Sugarcane. They increase the length of stem in sugarcane crops thereby, increasing the yield by several tonnes per acre.
- They have the ability to increase the length of grape stalks by increasing the length of their axis.
- Gibberellins lead to improvement in the shape of fruits such as apples by causing their elongation.
- They play a major role in extending the market period of fruits by delaying senescence.
- Gibberellins cause early seed production if sprayed on juvenile conifers.
Abscisic acid (ABA) is a plant hormone that is produced by plants in response to a variety of stress conditions. Hence, it is often referred to as the ‘stress hormone’. Abscisic acid levels are usually high in seedlings and decrease throughout the growing season so that plants can focus all of their energy on growth and development.
Overall, Abscisic acid acts as a growth inhibitor. In some cases, ABA is produced in excess, which can lead to plant injury and death. Plants produce ABA to respond to and recover from stressful conditions, such as drought and to avoid being damaged by the stress of desiccation, which is caused by water loss.
Abscisic acid was first reported during the 1960s as three different types of inhibitors by three independent scientists: Inhibitor-B, abscission II and dormin. Later on, all three of them were found to be identical and named abscisic acid.
Effects of Abscisic Acid
ABA is synthesized in plants primarily as a response to stress, such as light, water and temperature restriction, and is then secreted from the plant to nearby cells to elicit a response. Some other wide-ranging effects of ABA are:-
- ABA induces seed dormancy and therefore helps seeds to withstand unfavourable conditions for growth.
- It prevents water loss by inducing the closure of stomata and limits the extent of necrosis.
- It also helps the plant to respond to drought stress by slowing down growth in order to conserve water and energy.
- It inhibits seed germination
- ABA is a plant metabolism inhibitor.
Antagonistic Effects of Gibberellins and Abscisic acid
Due to the wide range of roles played by multiple phytohormones, there is a high chance of some of these roles being antagonistic. In many ways, Abscisic acid being a growth-inhibitory hormone acts antagonistically to Gibberellins which by nature are growth-promoting hormones.
- Gibberellins delay senescence whereas abscisic acid promotes senescence in older parts of the plant.
- Abscisic acid induces seed dormancy while gibberellins break it. Dormant seeds germinate when the effects of ABA are suppressed by gibberellins.
- Gibberellins cause the seeds to germinate more rapidly while abscisic acid inhibits seed germination.
Plant hormones are considered as chemical signals that help a plant respond to its environment. Some of these signals help the plant respond to stress, such as abscisic acid while others help it grow and develop, as in the case of gibberellins.
While gibberellins and abscisic acid may work antagonistically, they share a common goal of maximising plant life span and responding to the environment accordingly. Therefore, both these hormones are extremely essential for a plant to remain healthy.
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