Phytohormones 2

Plant hormones control all aspects of growth and development :-

embryogenesis,

regulation of organ size,

pathogen defense,

 stress tolerance,

reproductive development.

The term 'phytohormone' was coined by Went and Thimann, in their book ”Phytohormone” in 1937.

Chemical compounds synthesized by humans which are used to regulate the growth of cultivated plants, weeds, and in vitro-grown plants and plant cells; are called plant growth regulators ( PGRs ).

Initial research into plant hormones identified five major classes: abscisic acid, auxin, cytokinins, ethylene and gibberellins.  The list was later expanded to include brassinosteroids, jasmonates, salicylic acid and strigolactones.

Brassinosteroids

Brassinolide, a major brassinosteroid Brassinosteroids are a class of polyhydroxysteroids, the only example of steroid based hormones in plants. Brassinosteroids control cell elongation and division, gravitropism, resistance to stress, and xylem differentiation. They inhibit root growth and leaf abscission.

Brassinolide was the first identified brassinosteroid and was isolated from extracts of rapeseed (Brassica napus) pollen in 1979.

Jasmonates ( JAs )

Jasmonates are lipid-based hormones that were originally isolated from jasmine oil. JAs are especially important in the plant response to attack from herbivores and necrotrophic pathogens.

The most active JA in plants is jasmonic acid. Jasmonic acid can be further metabolized into methyl-JA, which is a volatile organic compound. This unusual property means that methyl-JA can act as an airborne signal to communicate herbivore attack to other distant leaves within one plant and even as a signal to neighboring plants.  In addition to their role in defense, JAs are also believed to play roles in seed germination, the storage of protein in seeds and root growth.

Salicylic Acid ( SA )

Salicylic acid is a hormone with a related structure to phenol. It was originally isolated from an extract of white willow bark (Salix alba) and is of great interest to human medicine, as it is the precursor of the painkiller, aspirin.

In plants, SA plays a critical role in the defense against biotrophic pathogens. In a similar manner to JA, SA can also become methylated. Like methyl-JA, methyl-SA is volatile and can act as a long distance signal to neighboring plants to warn of pathogen attack. In addition to its role in defense, SA is also involved in the response of plants to abiotic stress (particularly drought, temperature, heavy metal and osmotic stress).

Strigolactones ( SLs )

Strigolactones are originally discovered through studies into the germination of the parasitic weed Striga lutea. It was found that the germination of Striga species was stimulated by the presence of a compound exuded by the roots of its host plant.   It was later shown that SLs that are exuded into the soil promote the growth of symbiotic arbuscular mycorrhizal (AM) fungi.  More recently, another role of SLs was identified in the inhibition of shoot branching.  This discovery of the role of SLs in shoot branching led to a dramatic increase in the interest in these hormones, and it has since been shown that SLs play important roles in leaf senescence, phosphate starvation response, salt tolerance and light signalling.