The role of phytohormones in plant stress: too much or too little water

Due to their sessile nature higher plants cannot avoid adverse environmental conditions. Plants encounter shifts in environmental conditions, such as flooding, drought, temperature extremes and physical stress, during their entire life cycle. In order to optimize survival and reproduction, plants must acclimatize to these environmental changes (Bradshaw 1965). The resistance of plants to stress can be subdivided into three strategies: (i) escape (a dormant phase of the life cycle survives a stressful period, e.g. in annual species dormant seeds survive harsh seasons); (ii) avoidance (e.g. waterlogginginduced oxygen deficiency of root tips can be relieved on a tissue level by internal aeration via shoot and root aerenchyma); and (iii) true tolerance at biochemical level (e.g. a set of anaerobic stress proteins is synthesized in response to anoxic conditions to enable anaerobic ATP generation) (Jones & Jones 1989). Phytohormones, especially abscisic acid (ABA) and ethylene, play a predominant role in the conversion of stressful environmental signals into changes in plant gene expression. For example, ABA is involved in the desiccation tolerance of established plants and seeds. Ethylene, in concert with auxin, gibberellins and ABA, plays an important role in enhanced shoot elongation in response to submergence, as observed in many semi-aquatic plant species (Voesenek el al. 1992). Both ABA and ethylene are involved in the change of plant gene expression upon wounding, resulting in the production of a set of proteins involved in both wound healing and prevention of subsequent pathogen attacks. Evidence accumulates for the existence of at least two different wound stimulus transduction pathways, one acting via ABA and one via ethylene (Sanchez-Serrano et al. 1991).