PHOSPHATE MODULATES TRANSCRIPTION OF SOYBEAN VSPB AND OTHER SUGAR-INDUCIBLE GENES

The soybean vegetative storage protein genes VspA and VspB encode vacu olar glycoprotein acid phosphatases. Transcription of the Vsp is syner gistically activated by jasmonic acid or methyl jasmonate (MeJA) and s oluble sugars. The action of these modulators is mediated by two diffe rent DNA domains in the VspB promoter. In this study, we present new d ata regarding VspB regulation by sucrose and inorganic phosphate, whic h suggest a common mechanism of transcriptional control for Vsp and ot her sugar-inducible genes. We found that the sugar-mediated activation of VspB expression was inhibited by phosphate. Deletion analysis and transient assays in tobacco protoplasts identified a 130-bp DNA domain in the VspB promoter that mediates both sucrose induction and phospha te inhibition. Transcription mediated by this DNA domain was induced b y phosphate elimination from the protoplast incubation medium, even in the absence of sucrose. The effect of sucrose and phosphate on VspB e xpression was studied in vivo in several ways. Depletion of phosphate from soybean cell cultures by the addition of mannose stimulated VspB expression, even in the absence of sucrose or MeJA. In illuminated soy bean leaves treated with MeJA, inhibition of photosynthetic electron t ransport by DCMU decreased VspB expression. In contrast, VspB expressi on in soybean leaves stimulated by phosphate depletion was not influen ced by DCMU. Moreover, sucrose-stimulated expression of the sugar-resp onsive genes lipoxygenase A and chalcone synthase of soybean and prote inase inhibitor II and class I patatin of potato was inhibited by phos phate. Like VspB, these genes were stimulated by phosphate depletion i n the absence of exogenous sucrose. We propose that sugar-responsive g enes are activated, in part, by accumulation of sugar-phosphates and c oncomitant reduction of cellular phosphate levels. These data may help explain recruitment of the Vsp, which encode acid phosphatases, as ve getative storage proteins.