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