Identification of transcription factors that bind to the 5'-UTR of the barley PHO2 gene.

In barley and other higher plants, phosphate homeostasis is maintained by a regulatory network involving the PHO2 (PHOSPHATE2) encoding ubiquitin-conjugating (UBC) E2 enzyme, the PHR1 (PHOSPHATE STARVATION RESPONSE 1) transcription factor (TF), IPS1 (INDUCED BYPHOSPHATESTARVATION1) RNA, and miR399. During phosphate ion (Pi) deprivation, PHR1 positively regulates MIR399 expression, after transcription and processing mature miR399 guides the Ago protein to the 5'-UTR of PHO2 transcripts. Non-coding IPS1 RNA is highly expressed during Pi starvation, and the sequestration of miR399 molecules protects PHO2 mRNA from complete degradation. Here, we reveal new cis- and trans-regulatory elements that are crucial for efficient PHO2 gene expression in barley. We found that the 5'-UTR of PHO2 contains two PHR1 binding sites (P1BSs) and one Pi-responsive PHO element. Using a yeast one-hybrid (Y1H) assay, we identified two candidate proteins that might mediate this transcriptional regulation: a barley PHR1 ortholog and a TF containing an uncharacterized MYB domain. Additional results classified this new potential TF as belonging to the APL (ALTERED PHLOEM DEVELOPMENT) protein family, and we observed its nuclear localization in barley protoplasts. Pi starvation induced the accumulation of barley APL transcripts in both the shoots and roots. Interestingly, the deletion of the P1BS motif from the first intron of the barley 5'-UTR led to a significant increase in the transcription of a downstream β-glucuronidase (GUS) reporter gene in tobacco leaves. Our work extends the current knowledge about putative cis- and trans-regulatory elements that may affect the expression of the barley PHO2 gene.