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Phosphorus is one of the most important macronutrients required for plant growth and development. The importance of phosphorylation modification in regulating phosphate (Pi) homeostasis in plants is emerging. We performed phosphoproteomic profiling to characterize proteins whose degree of phosphorylation is altered in response to Pi starvation in rice root. A subset of 554 proteins, including 546 down-phosphorylated and eight up-phosphorylated proteins, exhibited differential phosphorylation in response to Pi starvation. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis with the differentially phosphorylated proteins indicated that RNA processing, transport, splicing and translation and carbon metabolism played critical roles in response to Pi starvation in rice. Levels of phosphorylation of four mitogen-activated protein kinases (MAPKs), including OsMAPK6, five calcium-dependent protein kinases (CDPKs) and OsCK2β3 decreased in response to Pi starvation. The decreased phosphorylation level of OsMAPK6 was confirmed by Western blotting. Mutation of OsMAPK6 led to Pi accumulation under Pi-sufficient conditions. Motif analysis indicated that the putative MAPK, casein kinase 2 (CK2) and CDPK substrates represented about 54.4%, 21.5% and 4.7%, respectively, of the proteins exhibiting differential phosphorylation. Based on the motif analysis, 191, 151 and 46 candidate substrates for MAPK, CK2 and CDPK were identified. These results indicate that modification of phosphorylation profiles provides complementary information on Pi-starvation-induced processes, with CK2, MAPK and CDPK protein kinase families playing key roles in these processes in rice. [ABSTRACT FROM AUTHOR]

Phosphate (Pi), as the main form of phosphorus that can be absorbed by plants, is one of the most limiting macro-nutrients for plants. However, the mechanism for maintaining Pi homeostasis in rice (Oryza sativa) is still not well understood. We identified a P i-starvation- i nduced E 3 ligase (OsPIE1) in rice. Using an in vitro self-ubiquitination assay, we determined the E3 ligase activities of OsPIE1. Using GUS staining and GFP detection, we analyzed tissue expression patterns of OsPIE1 and the subcellular localization of its encoded protein. The function of OsPIE1 in Pi homeostasis was analyzed using OsPIE1 overexpressors and ospie1 mutants. OsPIE1 was localized to the nucleus, and expressed in epidermis, exodermis and sclerenchyma layers of primary root. Under Pi-sufficient condition, overexpression of OsPIE1 upregulated the expression of OsPT2, OsPT3, OsPT10 and OsPAP21b, resulting in Pi accumulation and acid phosphatases (APases) induction in roots. OsSPX2 was strongly suppressed in OsPIE1 overexpressors. Further comparative transcriptome analysis, tissue expression patterns and genetic interaction analysis indicated that the enhancing of Pi accumulation and APase activities upon overexpression of OsPIE1 was (at least in part) caused by repression of OsSPX2. These results indicate that OsPIE1 plays an important role in maintaining Pi homeostasis in rice. [ABSTRACT FROM AUTHOR]