Regulation of OsSPX1 and OsSPX3 on expression of OsSPX domain genes and Pi-starvation signaling in rice.

The rice (Oryza sativa L.) genome contains at least six genes exclusively with an SPX (SYG1/PHO81/XPR1) domain at the N-terminal, designated as OsSPX1-6. Here we report the diverse expression patterns of the OsSPX genes in different tissues and their responses to Pi-starvation. Among them, five genes, OsSPX1, 2, 3, 5 and 6 are responsive to Pi-starvation in shoots and/or in roots. The subcellular localization analysis indicates that OsSPX1 and OsSPX2 is exclusively located in nucleus, OsSPX3 in the cytoplasm, and OsSPX4 is a membrane localization protein. OsSPX1 regulates OsSPX2, 3 and 5 at the transcription level and is positively involved in the responses of the genes to Pi-starvation. Overexpression of OsSPX3 downregulates OsSPX5 in shoots under Pi-sufficiency. OsSPX3 negatively regulates the PSI (Pi-starvation induced) gene, OsIPS1 and is involved in the responses of miR399 and OsPHO2 to Pi-starvation. Our results suggest that OsSPX1 may be a regulator involved in the transcriptions of OsSPX2, 3 and 5. OsSPX3 plays a role in OsIPS1/miR399 mediated long distance regulation on OsPHO2. Our results also indicate that OsSPX3 is involved in plant tolerance to Pi-starvation stress.