Role and Regulation of Cytokinins in Plant Response to Drought Stress

Abstract

Cytokinins (CKs) are key phytohormones that not only regulate plant growth and development but also mediate plant tolerance to drought stress. Recent advances in genome-wide association studies coupled with in planta characterization have opened new avenues to investigate the drought-responsive expression of CK metabolic and signaling genes, as well as their functions in plant adaptation to drought. Under water deficit, CK signaling has evolved as an inter-cellular communication network which is essential to crosstalk with other types of phytohormones and their regulating pathways in mediating plant stress response. In this review, we revise the current understanding of CK involvement in drought stress tolerance. Particularly, a genetic framework for CK signaling and CK crosstalk with abscisic acid (ABA) in the precise monitoring of drought responses is proposed. In addition, the potential of endogenous CK alteration in crops towards developing drought-tolerant crops is also discussed.

Keywords: CK metabolic genes; CK signaling genes; abscisic acid; cytokinin; drought tolerance.

Figure 1

A timeline of key progresses in cytokinin (CK) research [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45]. HP: Histidine phosphotransfer; IPT: isopentenyl transferase.

Figure 2

Proposed model of the regulatory functions of cytokinin (CK) signaling pathway in plant drought adaptation. Information for this pathway was suggested from previous studies [116,117]. Numbers represent regulation processes that were found in Arabidopsis (1-9) and apple (10-15). Dashed lines indicate proposed processes that have not been experimentally validated. Arrowheads represent activation, and perpendicular bars indicate inhibition. CK signaling in drought stress response induced by abscisic acid (ABA) can be divided into two unique mechanisms at the cellular level. One is the indirect regulation by ABA-responsive transcription factors [MYC (myeloblastosis) and DREB (dehydration-responsive element binding)] via the CK metabolic gene family members IPTs and CKXs, to initiate the reduction of endogenous CK levels in plant. In the other mechanism, the inhibition of CK action can be induced by ABA-responsive component SnRK2, which directly phosphorylates type-A RR5 (ARR5), a negative regulator of CK signaling. As a result, expression of stress-responsive genes is regulated to confer modifications in physiological and biochemical activities in mediating plant responses to drought conditions.