Differential mRNA Degradation of Two β-Tubulin Isoforms Correlates with Cytosolic Ca(2+) Changes in Glucan-Elicited Soybean Cells

Transgenic soybean (Glycine max) culture cells expressing apoaequorin, a Ca(2+) indicator, were exposed to glucan fragments derived from Phytophthora sojae or to chitin oligomers. The effects of these elicitors on cytosolic Ca(2+) concentrations and on mRNA levels of two β-tubulin isoforms, tubB1 and tubB2, were investigated. The glucan elicitors, to which the cells are known to react with a biphasic cytosolic Ca(2+) increase, induced a down-regulation of the tubB1 mRNA levels while the tubB2 mRNA level remained constant. The decrease of tubB1 mRNA level was observed after 1 hour of glucan treatment. In contrast, chitin oligomers, known to provoke a monophasic Ca(2+) increase of short duration, did not affect the tubB1 mRNA level. Pre-incubation with 10 mm 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid, an extracellular Ca(2+) chelator, blocked the cytosolic Ca(2+) increase as well as the decrease of tubB1 mRNA levels induced by glucan elicitors. Likewise, pre-incubation with 1 mm neomycin, which reduced only the second glucan-induced Ca(2+) peak, blocked the decrease of tubB1 mRNA level. Experiments with cordycepin, a transcription inhibitor, indicated that glucan fragments induced the degradation of tubB1 mRNA. In conclusion, the glucan-induced cytosolic Ca(2+) changes are correlated with a strong increase in tubB1 mRNA degradation.