Characterization of the phosphate starvation-induced glycerol-3-phosphate permease gene family in Arabidopsis

Phosphate (Pi) deficiency is one of the leading causes of loss in crop productivity. Plants respond to Pi deficiency by increasing Pi acquisition and remobilization involving organic and inorganic Pi transporters. Here, we report the functional characterization of a putative organic Pi transporter, Glycerol-3-phosphate permease (G3Pp) family, comprising five members (AtG3Pp1 to -5) in Arabidopsis (Arabidopsis thaliana). AtG3Pp1 and AtG3Pp2 showed 24-and 3-fold induction, respectively, in the roots of Pi-deprived seedlings, whereas Pi deficiency-mediated induction of AtG3Pp3 and -4 was evident in both roots and shoots. Furthermore, promoter-β-glucuronidase (GUS) fusion transgenics were generated for AtG3Pp2 to -5 for elucidation of their in planta role in Pi homeostasis. During Pi starvation, there was a strong expression of the reporter gene driven by AtG3Pp4 promoter in the roots, shoots, anthers, and siliques, whereas GUS expression was specific either to the roots (AtG3Pp3) or to stamens and siliques (AtG3Pp5) in other promoter-GUS fusion transgenics. Quantification of reporter gene activities further substantiated differential responses of AtG3Pp family members to Pi deprivation. A distinct pattern of reporter gene expression exhibited by AtG3Pp3 and AtG3Pp5 during early stages of germination also substantiated their potential roles during seedling ontogeny. Furthermore, an AtG3Pp4 knockdown mutant exhibited accentuated total lateral root lengths under +phosphorus and -phosphorus conditions compared with the wild type. Several Pi starvation-induced genes involved in root development and/or Pi homeostasis were up-regulated in the mutant. A 9-fold induction of AtG3Pp3 in the mutant provided some evidence for a lack of functional redundancy in the gene family. These results thus reflect differential roles of members of the G3Pp family in the maintenance of Pi homeostasis.