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Functional characterization of the three Oryza sativa SPX-MFS proteins in maintaining phosphate homeostasis.

Plant vacuoles serve as the primary intracellular compartments for phosphorus (P) storage. The Oryza sativa genome contains three genes that encode SPX (SYG1/PHO81/XPR1)-MFS (Major Facility Superfamily) proteins (OsSPX-MFS1-3). The physiological roles of the three transporters under varying P conditions in laboratory and field are not known. To address this knowledge gap, we generated single, double, and triple mutants for three OsSPX-MFS genes. All the mutants except Osspx-mfs2 display lower vacuolar Pi concentrations and OsSPX-MFSs overexpression plant display higher Pi accumulation, demonstrating that all OsSPX-MFSs are vacuolar Pi influx transporters. OsSPX-MFS3 plays the dominant role based on the phenotypes of single mutants in terms of growth, vacuolar and tissue Pi concentrations. OsSPX-MFS2 is the weakest and only functions as vacuole Pi sequestration in an Osspx-mfs1/3 background. The vacuolar Pi sequestration capacity was severely impaired in Osspx-mfs1/3 and Osspx-mfs1/2/3, which resulted in increased Pi allocation to aerial organs. High P in the panicle impaired panicle and fertility in Osspx-mfs1/3 and Osspx-mfs1/2/3. Osspx-mfs2 resulted more stable yield compared to the wildtype under low P in field grown plants. The results suggest that alteration of vacuolar Pi sequestration may be a novel effective strategy to improve rice tolerance to low phosphorus in cropping systems. This article is protected by copyright. All rights reserved.