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.