Arabidopsis Transcription Factors PHL2 And PHL1 Regulate The Transcriptional Responses To Phosphate Starvation

Phosphorus is one of the essential macronutrients for plant growth and development. In natural and the agricultural system, however, plants always face phosphate(Pi) deficiency. When confronted with Pi starvation, plants activate an array of adaptive responses to sustain their growth. These responses, in a large extent, are controlled at the transcriptional level. Arabidopsis PHR1(PHOSPHATE RESPONSE1)and its close homolog PHL1(PHR1-like 1) belong to a 15-member family of MYB-CC transcription factors and are regarded as the key components of the central regulatory system controlling plant transcriptional responses to Pi starvation. The knockout of PHR1 and PHL1, however, causes only a partial loss of the transcription of Pi starvation-induced genes, suggesting the existence of other key components in this regulatory system. In this work, we used the transcription of a Pi starvation-induced acid phosphatase, AtPAP10, to study the molecular mechanism underlying plant transcriptional responses to Pi starvation. We showed that PHL2(PHR1-like 2), and perhaps also PHL3(PHR1-like 3), two other members of the MYB-CC family, acts redundantly with PHR1 to regulate plant transcriptional response to Pi starvation.We first identified a DNA sequence on the AtPAP10 promoter that is critical for the transcription of At PAP10, named P sequence. Using P sequence as a bait, we performed a yeast one-hybrid screen against a cDNA library made from the Pi-starved Arabidopsis seedlings. We then demonstrated that PHL2 and PHL3, two nuclear proteins,specifically bind to this DNA sequence and activate the transcription of AtPAP10.Moreover, the transcription of AtPAP10 is reduced in phl2 mutant.In silico analysis indicated that there are three P1BS-like elements(PHR1 biding site) existed in AtPAP10 promoter via in silico analysis. We also identified three binding sites in P sequence through electrophoretic mobility shift assay. Among them, two contained one copy of the P1BS-like element, respectively. No annotated cis-element in the public database, however, was found in the other binding site. Thus, we identified anovel protein binding site within the P sequence; the exact binding sequence, however,remained to be further defined. Moreover, PHR1, PHL2 and PHL3 bind to similar DNA sequence in EMSA analysis.Unlike PHR1 and PHL1, the transcription and protein accumulation of PHL2 and PHL3 are upregulated by Pi starvation. RNA-seq analyses indicated that the transcription of most Pi starvation-induced genes is impaired in the phl2 mutant,indicating that PHL2 is also a key component of the central regulatory system. Finally,we showed that PHL2 acts redundantly with PHR1 to regulate plant transcriptional response to Pi starvation.