Mechanisms of cytokinin action

Cytokinins are involved in the control of numerous and important processes associated with plant growth and development. They take part in the control of cell division, chloroplast development, bud differentiation, shoot init iation and growth or leaf senescence. In contrast to the wide knowledge of cytokinin effects, the mechanisms of cytokinin action remain largely unknow n. Therefore, it is still difficult to explain how a group of molecules can control so many different biological responses. In this review, we propose some arguments in order to solve this question. In a first part, we underl ine that cytokinins act in concert with other signals for the control of bi ological responses. Therefore, the effects observed in responses to cytokin ins would be not only related to cytokinins but would also be dependent on all other environmental and hormonal signals perceived by plant cells. In a second part, we present the different actors which could be involved in th e signalling pathways of cytokinins. It is very likely that these different elements could be implicated in different cytokinin signalling pathways. T herefore, we propose that the diversity of the cytokinin responses could be also correlated with a diversity of cytokinin signalling pathways. (C) Els evier, Paris.