A conserved MYB transcription factor involved in phosphate starvation signaling both in vascular plants and in unicellular algae

Abstract

Plants have evolved a number of adaptive responses to cope with growth in conditions of limited phosphate (Pi) supply involving biochemical, metabolic, and developmental changes. We prepared an EMS-mutagenized M 2 population of an Arabidopsis thaliana transgenic line harboring a reporter gene specifically responsive to Pi starvation ( AtIPS1∷GUS ), and screened for mutants altered in Pi starvation regulation. One of the mutants, phr1 ( ph osphate starvation r esponse 1 ), displayed reduced response of AtIPS1∷GUS to Pi starvation, and also had a broad range of Pi starvation responses impaired, including the responsiveness of various other Pi starvation-induced genes and metabolic responses, such as the increase in anthocyanin accumulation. PHR1 was positionally cloned and shown be related to the PHOSPHORUS STARVATION RESPONSE 1 ( PSR1 ) gene from Chlamydomonas reinhardtii . A GFP∷PHR1 protein fusion was localized in the nucleus independently of Pi status, as is the case for PSR1. PHR1 is expressed in Pi sufficient conditions and, in contrast to PSR1 , is only weakly responsive to Pi starvation. PHR1, PSR1, and other members of the protein family share a MYB domain and a predicted coiled–coil (CC) domain, defining a subtype within the MYB superfamily, the MYB–CC family. Therefore, PHR1 was found to bind as a dimer to an imperfect palindromic sequence. PHR1-binding sequences are present in the promoter of Pi starvation-responsive structural genes, indicating that this protein acts downstream in the Pi starvation signaling pathway.