In this article we will discuss about:- 1. Meaning of Plant Growth Regulators (PGRs) 2. Characteristics of Plant Growth Regulators 3. Functions 4. Discovery.
Meaning of Plant Growth Regulators (PGRs):
Plants produce some specific chemical substances, which are capable of moving from one organ to the other, and provide physiological control on growth. These substances, which are active in very small amounts are called plant hormones or growth regulators.
Characteristics of Plant Growth Regulators:
The plant growth regulators (PGR) are small, simple molecules of diverse chemical composition. They could be indole compounds (indole-3-acetic acid IAA); adenine derivatives (N6 – furfurylamino purine, Kinetin); derivatives of carotenoids and fatty acids (abscisic acid, ABA); terpenes (gibberellic acid, GA3) or gases (ethylene, C2H4).
Plant growth regulators are variously described as plant growth substances, plant hormones or phytohormones.
The plant growth regulators (PGR) can be broadly divided into two groups based on their functions in a living plant body.
One group of PGRs are involved in growth promoting activities, such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation. These are also called plant growth promoters, e.g., Auxins, gibberellins and cytokinins.
The PGRs of other group play an important role in plant responses to wounding and stress of biotic and abiotic origin. They also are involved in various growth inhibiting activities such as promotion of dormancy and abscission. The PGR abscisic acid (ABA) belongs to this group.
The gaseous PGR, ethylene (C2H4), could fit either of the groups, but it is largely inhibits growth activities.
Functions of Plant Growth Regulators:
Each of these PGRs belonging to any group bring about a variety of growth and developmental responses, and not all responses are similar and simultaneous at any given time, e.g., a given concentration of auxin which promotes shoot growth may inhibit root growth.
Secondly, similar responses are affected by different PGRs, e.g., both auxins and gibberellins can-result in cell enlargement, while cell division could be promoted by auxins, gibberellins and cytokinins.
The PGRs are active at very low concentrations usually in 10−6 M range. However, site of production and sites of action (target cells) of PGRs could be same or different. Also they are synthesised at several sites and in different tissues within a plant body and not in any specialised gland or tissue, as in animals.
Moreover, each of the major classes of PGR bring about a variety of growth and differentiation responses.
Discovery of Plant Growth Regulators:
The discovery of each of the five major groups of PGRs have been accidental:
1. All this started with the observation of Charles Darwin and his son Francis Darwin when they observed that the coleoptiles of canary grass (Phalaris canarensis) responded to unilateral illumination by growing towards the light source (phototropism). After a series of experiments, it was concluded that the tip of coleoptile was the site of transmittable influence that caused the bending of the entire coleoptile.
Thereafter, several experiments in different laboratories, culminated in the discovery of first PGR, the auxin, by F.W. Went (1928) in coleoptile tips of Avena saliva seedlings.
2. The bakane (foolish seedling) disease of rice seedlings which resulted in weak, elongated stems that produced little or no grains. Japanese plant pathologist indentified the causal organism, Gibberella fujikoroi, a fungus.
In 1926 Kurosawa reported the appearance of symptoms of disease in uninfected rice seedlings. When they were treated with sterile filtrates of the fungus, the active substance was later identified as gibberelic acid.
3. F. Skoog (USA) and his co-workers were studying the nutritional requirements of tissue cultures derived from the internodal segments of tobacco stems. They observed that the callus proliferated only, it in addition to auxins nutrient medium was supplemented with one of the following extracts of vascular tissues or yeast extract or coconut milk or DNA.
Skoog and Miller, soon identified and crystallized the cytokinesis-promoting active substance that they termed kinetin.
4. As the research in plant growth regulators was progressing, it became apparent that certain substances of plant origin interfered with auxin responses. Advent of paper chromatography technique as an analytical tool helped to identify the PGR, abscisic acid in 1967.
5. Businessmen involved in shipping and storing fruits have been aware that ripe and rotten fruits could accelerate the ripening of other fruits stored nearby. Cousins (1910) confirmed the release of a volatile substance from ripened oranges that hastened the ripening of unripened bananas stored nearby. R. Gane (1934) provided undisputable evidence that this volatile substance was ethylene (C2H4), a gaseous PGR using gas chromatography technique.