Phosphate sensing and signalling in Arabidopsis thaliana
Phosphate (Pi) deficiency is a global problem for food production. Plants have evolved complex mechanisms to adapt to low Pi. We focused on the initial aspects of adaptation to low Pi - perception and immediate-early responses to changes in external Pi. To examine whether a labile repressor controls expression of the high affinity Pi transporter, Pht1;1, we performed electrophoretic mobility shift assays (EMSA) but observed only weak protein-DNA binding activity using extracts from Arabidopsis suspension cultures or seedlings. The regulatory role of different regions in Pht1;1 promoter was dissected by promoter deletion analysis, using uidA as a reporter. We identified two domains important for regulation: sequences between -1898 bp and - 932 bp are important for induction of Pht1;1 in low Pi; the intron in the 5’UTR impacts Pht1;1 expression in the young part of both primary and lateral root apices. A complementary approach to identify repressors of Pi starvation responses was pursued: We identified ZAT18, a putative transcription factor, as a candidate repressor. ZAT18 contains an EAR motif, a repressor domain in plants; the expression of ZAT18 responds to Pi starvation. Using transgenic lines with promoter::ZAT18-VENUS constructs, we studied its expression, localization and abundance in different levels of Pi availability: ZAT18 is mainly expressed in the nucleus of Arabidopsis root hair cells. Its accumulation was induced by 4 day Pi starvation. We also performed a microarray analysis to examine global gene expression levels during Pi starvation and rapid recovery. Our data indicated that 258 genes were induced and 188 genes were suppressed during Pi starvation. For most of these genes, responses were reversed after 4 hour Pi recovery. Further study of these genes will help to define targets of the early Pi starvation-signalling pathway.