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 of cellular energy demand by environmental changes. In etiolated mungbean hypocotyl segments, a source-limited system, the soluble acid invertase activity and transcript level of a vacuolar invertase gene, Vr-AI1 was rapidly down regulated when hypocotyls were excised from the intact seedlings. Kinetic data show that the enzyme activity dropped to 50% of the initial level within 3 h and the steady state level of Vr-AI1 mRNA was sharply decreased as well. Exogenous supply of sucrose suppressed the decrease in both enzyme activity and the transcript level. Treatment with indole-3-acetic acid (IAA) and/or light also retarded the speed of down-regulation. Studies with alpha-amanitin, an inhibitor of eukaryotic RNA polymerase II showed that the Vr-AI1 mRNA was more stable in the presence of IAA. Cycloheximide treatment also increased the transcript level, raising the possibility that the Vr-AI1 gene is controlled by mRNA stability. There was a discrepancy that the Vr-AI1 mRNA disappeared much earlier than the enzyme activity. In the light condition, moreover, the decay kinetics and IAA dose-response curve for the enzyme activity was markedly different from those for the Vr-AI1 mRNA level. Our results imply that multiple distinct mechanisms are involved in the regulation of soluble acid invertase activity by auxin and light.