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. Since CKs are likely to elicit their effects at the level of gene expression and stress-induced alterations in gene expression are usually rapid and repeatable, molecular analysis of CK-regulated stress-inducible genes may contribute to enhanced understanding of CK-mediated signal transduction in plants. In this regard, examination of stress responses of transgenic plants with altered levels of endogenous CKs as well as mutants altered in CK metabolism or signalling may be informative. Emerging evidence that CKs may impact on methylation reactions is discussed in relation to the apparent importance of methyl group transfers in the biochemical acclimation to environmental stress.