Role of cytokinins in stress resistance of plants

The facts of both positive and negative influences of cytokinins on stress resistance of plants are known today. Without pretending to a final choice between these points of view, we have made an attempt to analyze the details of the experiments that gave rise to conclusions about the nature of the effect of cytokinins on the resistance to stress-causing influences with a focus on their intensity and duration. The review deals with the data concerning the influence of different adverse factors on the content of endogenous cytokinins and transduction of cytokinin signals, examines the influence on plant resistance of treatment with exogenous hormone, and the effects of genetic modifications causing changes in cytokinin content and signaling. Resistance is considered not only as a mean of plant survival under severe stress but also as an instrument of maintaining growth rate in plants exposed to moderate stress. Literature data and our own results make it possible to conclude that cytokinins play an important role in formation of plant resistance to adverse influences; however, the effect of these hormones depends on stress intensity. Under moderate stress, cytokinins ensure maintenance of plant growth, whereas a drop in cytokinins hampers growth under a strong influence of adverse factors, which is a prerequisite for mobilization of limited resources characteristic of severe stress and ensures preservation of plant viability.