Phosphoproteomic Profiling Reveals the Importance of CK2, MAPKs and CDPKs in Response to Phosphate Starvation in Rice.

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 OsCK2ß3 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.

The role of OsNLA1 in regulating arsenate uptake and tolerance in rice.

Arsenic (As) contamination in agricultural soil can cause phytotoxicity and lead to As accumulation in crops. Rice (Oryza sativa) feeds half of the world's population, but the molecular mechanism of As detoxification is not well understood in rice. In this study, the role of OsNLA1 in arsenate uptake and tolerance in rice was analyzed. OsNLA1 expression was induced in response to As(V) stress. The osnla1 mutant was more sensitive to As(V) stress than those of the wild type (WT). When exposed to As(V), mutation of OsNLA1 resulted in 30% greater As accumulation in roots and shoots of the WT. Although OsPT8 expression was induced after As(V) exposure, the amount of its protein was reduced. Unexpectedly, the osnla1 mutant showed a significant increase in punctate structures of OsPT8-GFP in response to As(V) stress, while the amount of the OsPT8-GFP protein in the osnla1 mutant was greater than in the WT. Combining OsNLA1 mutation with OsPT8 overexpression resulted in As(V) hypersensitivity, As hyperaccumulation, and higher shoot to root ratio of As in rice. These results indicated that OsNLA1 plays an important role in arsenate uptake and tolerance, mainly via regulating the amount of Pi transporters.