ELEVATION OF CYTOSOLIC CALCIUM PRECEDES ANOXIC GENE-EXPRESSION IN MAIZE SUSPENSION-CULTURED CELLS

Based on pharmacological evidence, we previously proposed that intracellular Ca2+ mediates the perception of O-2 deprivation in maize seedlings. Herein, using fluorescence imaging and photometry of Ca2+ in maize suspension-cultured cells, the proposal was further investigated. Two complementary approaches were taken: (1) real time analysis of anoxia-induced changes in cytosolic Ca2+ concentration ([Ca](i)) and (2) experimental manipulation of [Ca](i) and then assay of the resultant anoxia-specific responses. O-2 depletion caused an immediate increase in [Ca](i), and this was reversible within a few seconds of reoxygenation. The [Ca](i) elevation proceeded independent of extracellular Ca2+. The kinetics of the Ca2+ response showed that it occurred much earlier than any detectable changes in gene expression. Ruthenium red blocked the anoxic [Ca(])i elevation and also the induction of adh1 (encoding alcohol dehydrogenase) and sh1 (encoding sucrose synthase) mRNA. Ca2+, when added along with ruthenium red, prevented the effects of the antagonist on the anoxic responses. Verapamil and bepridil failed to block the [Ca](i) rise induced by anoxia and were equally ineffective on anoxic gene expression. Caffeine induced an elevation of [Ca](i) as well as ADH activity under normoxia, The data provide direct evidence for [Ca]i elevation in maize cells as a result of anoxia-induced mobilization of Ca2+ from intracellular stores. Furthermore, any manipulation that modified the [Ca](i) rise brought about a parallel change in the expression of two anoxia-inducible genes. Thus, these results corroborate our proposal that [Ca](i) is a physiological transducer of anoxia signals in plants.