The involvement of cytokinins in plant responses to environmental stress

Cytokinin (CK) levels tend to decrease under adverse environmental conditions. A general view has emerged that during stress, a reduction of CK supply from the root alters gene expression in the shoot and thereby elicits appropriate responses to ameliorate the effects of stress. However, recent studies have indicated that transcription of many stress-inducible genes can also be caused by CK application. This review attempts to highlight some of these apparently paradoxical findings and to suggest potential approaches for future research in this area. Changes in patterns of gene expression associated with responses to adverse environmental conditions are most probably the end products of hierarchical changes in regulatory controls exercised at hormonal, developmental and morphological levels. We consider possible roles for CKs in affecting overall hormonal balance following the imposition of environmental stresses, and thereby playing a role in systemic responses to stress. Particular attention is devoted to the interactions of CKs with ABA, ethylene, salicylic acid and jasmonates, all of which are known to be involved in plant stress responses. The recent emergence of MAPK-type cascades in mediating responses to several environmental modifications is discussed in relation to their possible modulation by CKs.