SPX1 is an important component in the phosphorus signalling network of common bean regulating root growth and phosphorus homeostasis.

Proteins containing the SPX domain are believed to play vital roles in the phosphorus (P) signalling network in plants. However, the functions of SPX proteins in legumes remain largely unknown. In this study, three SPX members, PvSPX1-PvSPX3 were cloned from common bean (Phaseolus vulgaris L.). It was found that the transcripts of all three PvSPX members were significantly enhanced in both bean leaves and roots by phosphate (Pi) starvation. Among them, the expression of nuclear localized PvSPX1 showed more sensitive and rapid responses to Pi starvation. Consistently, only overexpression of PvSPX1 resulted in increased root P concentration and modified morphology of transgenic bean hairy roots, such as inhibited root growth and an enlarged root hair zone. It was further demonstrated that PvSPX1 transcripts were up-regulated by overexpressing PvPHR1, and overexpressing PvSPX1 led to increased transcripts of 10 Pi starvation-responsive genes in transgenic bean hairy roots. Taken together, it is suggested that PvSPX1 is a positive regulator in the P signalling network of common bean, and is downstream of PvPHR1.

Characterization of two putative protein phosphatase genes and their involvement in phosphorus efficiency in Phaseolus vulgaris.

Protein dephosphorylation mediated by protein phosphatases plays a major role in signal transduction of plant responses to environmental stresses. In this study, two putative protein phosphatases, PvPS2:1 and PvPS2:2 were identified and characterized in bean (Phaseolus vulgaris). The two PvPS2 members were found to be localized to the plasma membrane and the nucleus by transient expression of PvPS2:GFP in onion epidermal cells. Transcripts of the two PvPS2 genes were significantly increased by phosphate (P(i) ) starvation in the two bean genotypes, G19833 (a P-efficient genotype) and DOR364 (a P-inefficient genotype). However, G19833 exhibited higher PvPS2:1 expression levels than DOR364 in both leaves and roots during P(i) starvation. Increased transcription of PvPS2:1 in response to P(i) starvation was further verified through histochemical analysis of PvPS2:1 promoter fusion ß-glucuronidase (GUS) in transgenic Arabidopsis plants. Analysis of PvPS2:1 overexpression lines in bean hairy roots and Arabidopsis showed that PvS2:1 was involved in root growth and P accumulation. Furthermore, expression levels of two P(i) starvation responsive genes were upregulated and the APase activities were enhanced in the overexpressing PvPS2:1 Arabidopsis lines. Taken together, our results strongly suggested that PvPS2:1 positively regulated plant responses to P(i) starvation, and could be further targeted as a candidate gene to improve crop P efficiency.