ABSTRACT
Plant growth hormones, are naturally occurring small organic molecules or substances which influence physiological processes in plants at very low concentrations. Growth regulators are of two types, namely growth promoting substances and growth inhibiting substances. Auxins, gibberellins and cytokinins are growth promoting substances, whereas, dormin, ethylene and abscissic acid are growth inhibiting substances. The imbalance in growth promoting and growth inhibiting substances causes hypertrophy (Excessive increase in cell size) and atrophy (decrease in cell size). Cytokinins are necessary for cell growth and differentiation. It inhibits breakdown of proteins and aminoacids and thereby inhibit senescence and they have the capacity to direct the flow of aminoacids and other nutrients towards high cytokinin concentration. The plant hormones ethylene, jasmonic acid, and salicylic acid play a central role in the regulation of plant immune responses. Also, the phytohormone systems mediated by auxin, cytokinins, gibberellins, and abscisic acid were involved physiologically in plant susceptibility towards pathogen invasion.
