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

Plants have evolved a number of adaptive responses to cope with growth in c onditions 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 specif ically responsive to Pi starvation (AtIPS1:: GUS), and screened for mutants altered in Pi starvation regulation. One of the mutants, phr1 (phosphate s tarvation response 1), displayed reduced response of AtIPS1:: GUS to Pi sta rvation, and also had a broad range of Pi starvation responses impaired, in cluding the responsiveness of various other Pi starvation-induced genes and metabolic responses, such as the increase in anthocyanin accumulation. PHR 1 was positionally cloned and shown be related to the PHOSPHORUS STARVATION RESPONSE 1 (PSR1) gene from Chlamydomonas reinhardtii. A GFP:: PHR1 protei n 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 contr ast to PSR1, is only weakly responsive to Pi starvation. PHR1, PSR1, and ot her 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-C C family. Therefore, PHR1 was found to bind as a dimer to an imperfect pali ndromic sequence. PHR1-binding sequences are present in the promoter of Pi starvation-responsive structural genes, indicating that this protein acts d ownstream in the Pi starvation signaling pathway.