1997, Stress-mduced abicisic acid transients and siimulus-response-coupling. -Physiol. Pianl. !(X): 491^99,Loss of cell turgttr and distortion of the plasma membrane occur a.s a result of dehydration and precede tbe stress-induced bulk increase in concentration of tissue abscisic acid. The latter ha-, been correlated v.iih induction of stress-related gene expression. However, several different stresses may trigger the same coupling mechanism. Thus, al least ibree signalling pathways have been proposed: abscisic acid-requiring, abscisic acid-responsive, and mechanosensors. In this paper, the role and contribution ol slress-induced ahscisic acid transients is examined in an attempi lo explain apparent abscisic acid-dependenl and -independeni stimulus-re.sponse-coupling. Early, intermediate, and late response stages are defined within the stress-induced abscisic acid transient and at leas! four signalling mechanisms are identified. These include, early and laie intracellular modulaiion of gene expression through derepression and/or negative regulation, rapid membrane-initialed calcium release and ion channel activation, and late tskm) ht}rmone-receptor induction of gene expression. An assessment of these proposed .ABA signalling mechanisms in lerms of .AB.A-dependem and -independent stimulus-response-coupling strongly suggests that rapid responses ma\ not be a prerequisite for slow responses and thai the receptor proteins involved have different stehc requiretments. i.e., they are tissue-and/or cell-specific,

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