Role of Endogenous Hormonal System in the Realization of the Antistress Action of Plant Growth Regulators on Plants

The endogenous hormonal system plays a leading role in the regulation of growth and development of plants. This regulatory system responds sensitively to even slight changes in the plant environment, which is manifested in reorganization of the hormonal status. Detrimental factors lead to an imbalance in the content of different groups of phytohormones, as a rule reflected in the accumulation of abscisic acid (ABA) and in a parallel decline in the content of metabolism-stimulating hormones indole-3-acetic acid (IAA) and cytokinins. In total, these changes in hormone content in plants are related to inducing an effective stress response of plants accompanied however with inhibition of growth processes and plant productivity. This makes actual application of natural growth regulators characterized by growth-stimulating and anti-stress activity to increase both plant resistance and productivity as well. The present mini review mainly concentrates on the shifts in endogenous phytohormones (ABA, IAA and cytokinins) level caused by application of hormones such as, brassinosteroids, cytokinins and salicylic acid (SA). The main attention is focused on the role of these changes in endogenous hormones in the realization of growth-stimulating and protective function of applied plant growth regulators (PGRs). It is necessary to underline that simultaneous analysis of different groups of phytohomones allowed us to reveal a complex pattern of changes in plant hormonal system in response to treatments with exogenous growth regulators and to evaluate their contribution to the control of resistance to stress factors. _____________________________________________________________________________________________________________