Phosphate Modulates Transcription of Soybean VspB and Other Sugar-Inducible Genes.
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The soybean vegetative storage protein genes VspA and VspB encode vacuolar glycoprotein acid phosphatases. Transcription of the Vsp is synergistically activated by jasmonic acid or methyl jasmonate (MeJA) and soluble sugars. The action of these modulators is mediated by two different DNA domains in the VspB promoter. In this study, we present new data regarding VspB regulation by sucrose and inorganic phosphate, which suggest a common mechanism of transcriptional control for Vsp and other 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 phosphate inhibition. Transcription mediated by this DNA domain was induced by phosphate elimination from the protoplast incubation medium, even in the absence of sucrose. The effect of sucrose and phosphate on VspB expression 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 soybean leaves treated with MeJA, inhibition of photosynthetic electron transport by DCMU decreased VspB expression. In contrast, VspB expression in soybean leaves stimulated by phosphate depletion was not influenced by DCMU. Moreover, sucrose-stimulated expression of the sugar-responsive genes lipoxygenase A and chalcone synthase of soybean and proteinase inhibitor II and class I patatin of potato was inhibited by phosphate. Like VspB, these genes were stimulated by phosphate depletion in the absence of exogenous sucrose. We propose that sugar-responsive genes are activated, in part, by accumulation of sugar-phosphates and concomitant reduction of cellular phosphate levels. These data may help explain recruitment of the Vsp, which encode acid phosphatases, as vegetative storage proteins.