Functional characterization of the three Oryza sativa SPX-MFS proteins
in maintaining phosphate homeostasis
Abstract
Plant vacuoles serve as the primary intracellular compartments for
phosphorus (P) storage and play a central role to maintain P
homeostasis. The Oryza sativa (rice) genome contains three genes
that encode SPX (SYG1/PHO81/XPR1)-MFS (Major Facility Superfamily)
proteins (OsSPX-MFS1, 2, 3). OsSPX-MFS1 and OsSPX-MFS3 were shown
previously to have vacuolar phosphate (Pi) transporter activities, but
the physiological role of the three transporters under varying P
conditions and under field grown conditions for a crop plant is not
known. To address this knowledge gap, we generated single, double, and
triple mutants (7 mutants with at least two lines of each) for the three
rice Os SPX-MFS genes. All the mutants except osspx-mfs2
display lower vacuolar Pi concentrations and all Os SPX-MFSs
overexpression plant lines display higher Pi accumulation, demonstrating
that all three OsSPX-MFSs are vacuolar Pi influx transporters.
OsSPX-MFS3 plays the dominant role based on the phenotypes of three
single mutants in terms of growth, vacuolar and tissue Pi
concentrations. OsSPX-MFS2 is the weakest and only functions as vacuole
Pi sequestration under osspx-mfs1/3 background. The vacuolar Pi
sequestration was severely impaired in osspx-mfs1/ 3 and
osspx-mfs1/2/ 3, which led to Pi toxicity and subsequently
increased Pi allocation to aerial organs. High Pi in the panicle result
in necrotic symptoms on husks and impaired panicle and grain development
in osspx-mfs1/ 3 and osspx-mfs1/2/ 3 mutant
lines. The mutation in the weak vacuolar Pi transporter OsSPX-MFS2
resulted more stable yield compared to the wildtype under low P field
conditions. The results suggest that alteration of vacuolar Pi
sequestration may be a novel effective strategy to improve rice (crop)
tolerance to low phosphorus field conditions and maintain yield.