Rapid repression of vacuolar invertase in mungbean hypocotyl segments and regulation by sucrose, auxin and light

Plant vacuolar invertase is implicated in the rapid regulation ofcellular energy demand by environmental changes. In etiolated mungbeanhypocotylsegments, a source-limited system, the soluble acid invertase activity andtranscript level of a vacuolar invertase gene, Vr-AI1 wasrapidly down regulated when hypocotyls were excised from the intact seedlings.Kinetic data show that the enzyme activity dropped to 50% of the initial levelwithin 3 h and the steady state level ofVr-AI1 mRNA was sharply decreased as well. Exogenoussupplyof sucrose suppressed the decrease in both enzyme activity and the transcriptlevel. Treatment with indole-3-acetic acid (IAA) and/or light also retarded thespeed of down-regulation. Studies with α-amanitin, an inhibitor of eukaryoticRNA polymerase II showed that the Vr-AI1 mRNA was morestable in the presence of IAA. Cycloheximide treatment also increased thetranscript level, raising the possibility that the Vr-AI1gene is controlled by mRNA stability. There was a discrepancy that theVr-AI1 mRNA disappeared much earlier than the enzymeactivity. In the light condition, moreover, the decay kinetics and IAAdose-response curve for the enzyme activity was markedly different from thosefor the Vr-AI1 mRNA level. Our results imply that multipledistinct mechanisms are involved in the regulation of soluble acid invertaseactivity by auxin and light