Identification and Characterization of the Arabidopsis PHO1 Gene Involved in Phosphate Loading to the Xylem

Abstract

The Arabidopsis mutant pho1 is deficient in the transfer of Pi from root epidermal and cortical cells to the xylem. The PHO1 gene was identified by a map-based cloning strategy. The N-terminal half of PHO1 is mainly hydrophilic, whereas the C-terminal half has six potential membrane-spanning domains. PHO1 shows no homology with any characterized solute transporter, including the family of H ؉ -Pi cotransporters identified in plants and fungi. PHO1 shows highest homology with the Rcm1 mammalian receptor for xenotropic murine leukemia retroviruses and with the Saccharomyces cerevisiae Syg1 protein involved in the mating pheromone signal transduction pathway. PHO1 is expressed predominantly in the roots and is upregulated weakly under Pi stress. Studies with PHO1 promoter-␤ -glucuronidase constructs reveal predominant expression of the PHO1 promoter in the stelar cells of the root and the lower part of the hypocotyl. There also is ␤ -glucuronidase staining of endodermal cells that are adjacent to the protoxylem vessels. The Arabidopsis genome contains 10 additional genes showing homology with PHO1 . Thus, PHO1 defines a novel class of proteins involved in ion transport in plants. INTRODUCTIONThe radial movement of ions from root epidermal and cortical cells to the xylem can be mediated by two major pathways. In the apoplastic pathway, ions move radially toward the stele through the extracellular space, whereas in the symplastic pathway, ions move intracellularly from cell to cell via plasmodesmata (Bowling, 1981;Clarkson, 1993). Although a third pathway is possible, namely, one in which ions move from cell to cell through a successive uptake and release of ions from and into the extracellular space, the high energy requirement of this pathway makes it unlikely to play a major role in ion transport to the xylem.The movement of ions and water through the apoplast of the root is blocked at the level of the endoderm by the Casparian strip, a zone in which the cell wall is impregnated with hydrophobic compounds such as suberin and lignin. Thus, passage of ions beyond the Casparian strip and toward the stele must proceed via the symplasm. Once in the cells of the stele, the release of ions into the xylem requires their efflux out of the stelar cells. Thus, radial transport of ions from the external solution to the xylem requires a minimum of two passages across the plasma membrane, once for the uptake of ions into the epidermal, cortical, or outer surface of the endodermal cells, and then again for the efflux of ions out of the stelar cells before entering the xylem vessel (Bowling, 1981;Clarkson, 1993).The uptake of anions such as Pi into a cell is an energyrequiring process. The negatively charged phosphate ion (HPO 4 Ϫ 2 or H 2 PO 4 Ϫ ) must move against an electrical gradient, the interior of the cell being negatively charged ( ‫ف‬ Ϫ 100 mV), as well as against a concentration gradient, the intracellular concentration of Pi being 1000 to 10,000 times higher than the extracellular concentration (the concentration of P...

Integration of responses within and acrossArabidopsisnatural accessions uncovers loci controlling root systems architecture

“…We focused on a subset of 11 genes that each contain highly significant SNP associations (FDR: q < 0.05) (Table S2). Among these candidate genes is PHOSPHATE 1 or PHO1 (At3g23430) shown previously to play a role in inorganic phosphate loading into the xylem and aspects of root development (20,21). Several other uncharacterized genes were also identified.…”

A Constitutive Expressed Phosphate Transporter, OsPht1;1, Modulates Phosphate Uptake and Translocation in Phosphate-Replete Rice

“…The PHO1 protein, primarily expressed in the root vascular cylinder, is known for mediating Pi efflux in loading Pi to root xylem in Arabidopsis (Hamburger et al, 2002;Stefanovic et al, 2011). In rice, it has been shown that OsPHO1;2 plays an important role in transferring Pi from roots to shoots (Secco et al, 2010).…”

Natural Variation of Root Traits: From Development to Nutrient Uptake

“…One particular allometric root phenotype that captures the proportion of LRs along the PR length was used for GWASs, and two genes underlying this trait were identified (PHOSPHATE1 [PHO1] and ROOT SYSTEMS ARCHITECTURE1 [RSA1]; Rosas et al, 2013). PHO1 was previously characterized as a phosphate transporter (Poirier et al, 1991;Hamburger et al, 2002), whereas RSA1 was a novel gene identified in the study to be involved in regulating root allometry . Interestingly, these allometric phenotypes were conditional to specific environments, and RSA1 and PHO1 mediated these responses in the context of auxin, ABA, and nitrate signaling pathways .…”