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

TLDR

Plants adapt to limited phosphate by altering biochemical, metabolic, and developmental processes, and the MYB‑CC transcription factor family, including PHR1 and PSR1, plays a key role in this signaling. The study aimed to identify mutants that alter Pi‑starvation regulation by screening an EMS‑mutagenized Arabidopsis line with an AtIPS1::GUS reporter. An EMS‑mutagenized Arabidopsis population was generated and screened for altered AtIPS1::GUS expression under Pi starvation. The phr1 mutant showed diminished AtIPS1::GUS induction and broad defects in Pi‑starvation responses, and PHR1, a MYB‑CC transcription factor related to PSR1, localizes to the nucleus, is weakly Pi‑responsive, binds as a dimer to imperfect palindromic motifs in promoters of Pi‑responsive genes, and thus functions downstream in the Pi‑starvation signaling pathway.

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 (phosphate starvation response 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.