Cytokinin Function in Drought Stress Response and Subsequent Recovery

Plants respond to exogenous stimuli, including abiotic stress, by modulation of their behavior, which is partially mediated by plant hormones. Understanding of the role of different hormones could improve the choice of a suitable strategy for enhancement of plant stress tolerance. Cytokinins (CKs) affect multiple processes in plants, including stimulation of cell division, enhancement of sink strength and promotion of photosynthesis. As CKs counteract many processes induced by water stress which are mediated by the ‘stress hormone’ abscisic acid (e.g., stomata closure or senescence acceleration) their role in drought stress response does not seem straightforward. Exogenous CKs were reported to increase the tolerance to mild stress and to speed up the recovery (e.g., Itai et al., 1978). As reported by Zhang et al. (2000) and Huynh et al. (2005) the increased cytokinin content following over-expression of CK biosynthetic gene (IPT) driven by senescence-inducible promoter SAG12 positively correlated with elevated flooding tolerance. This together with reported role of CKs in the sink-source polarization during mild water stress in tobacco (Cowan et al., 2005) indicates that CKs play an important in plant responses to drought.

In present contribution changes in CK pool during drought stress and subsequent recovery were investigated. The stress response of tobacco plant over-expressing trans-zeatin O-glucosyltransferase (ZOG1) gene from Phaseolus lunatus (Martin et al., 2001) under either constitutive (35S) promoter (causing uniform elevation of total CKs in whole plant already before stress initiation) or senescenceinducible (SAG12) promoter (inducing CK increase related to the time and location of the stress) was compared with the corresponding wild type plants (WT).