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.
