BHLH32 modulates several biochemical and morphological processes that respond to P_i starvation in Arabidopsis

P_i (inorganic phosphate) limitation severely impairs plant growth and reduces crop yield. Hence plants have evolved several biochemical and morphological responses to P_i starvation that both enhance uptake and conserve use. The mechanisms involved in P_i sensing and signal transduction are not completely understood. In the present study we report that a previously uncharacterized transcription factor, BHLH32, acts as a negative regulator of a range of P_i starvation-induced processes in Arabidopsis. In bhlh32 mutant plants in P_i-sufficient conditions, expression of several P_i starvation-induced genes, formation of anthocyanins, total P_i content and root hair formation were all significantly increased compared with the wild-type. Among the genes negatively regulated by BHLH32 are those encoding PPCK (phosphoenolpyruvate carboxylase kinase), which is involved in modifying metabolism so that P_i is spared. The present study has shown that PPCK genes are rapidly induced by P_i starvation leading to increased phosphorylation of phosphoenolpyruvate carboxylase. Furthermore, several Arabidopsis proteins that regulate epidermal cell differentiation [TTG1 (TRANSPARENT TESTA GLABRA1), GL3 (GLABRA3) and EGL3 (ENHANCER OF GL3)] positively regulate PPCK gene expression in response to P_i starvation. BHLH32 can physically interact with TTG1 and GL3. We propose that BHLH32 interferes with the function of TTG1-containing complexes and thereby affects several biochemical and morphological processes that respond to P_i availability.