Dephosphorylation of Nitrate Reductase Protein Regulates Growth of Rice and Adaptability to Low Temperature

Abstract Nitrate reductase (NR) is an important enzyme for nitrate assimilation in plants, and its activity is regulated by post-translational phosphorylation. To investigate the effect of NIA1 protein dephosphorylation on the growth of rice and its adaptability to low temperature, we analyzed phenotype, chlorophyll content, nitrogen utilization, and antioxidant capacity at low temperature in lines with a mutated NIA1 phosphorylation site ( S532D and S532A ), an OsNia1 over-expression line ( OE ), and wild-type Kitaake rice (WT). Plant height, dry matter weight, and chlorophyll content of S532D and S532A were lower than those of WT and OE under normal growth conditions but were higher than those of WT and OE at low temperature. Compared with WT and OE , the nitrite, H 2 O 2 , and MDA contents of S532D and S532A leaves were higher under normal growth conditions. The difference in leaf nitrite content between transgenic lines and WT was narrower at low temperature, especially in S532D and S532A , while H 2 O 2 and MDA contents of S532D and S532A leaves were lower than those in WT and OE leaves. The NH 4 + -N and amino acid contents of S532D and S532A leaves were higher than those of WT and OE leaves under normal or low temperature. qRT-PCR results revealed that transcription levels of OsNrt2.4 , OsNia2 , and OsNADH-GOGAT were positively correlated with those of OsNia1 , and the transcription levels of OsNrt2.4 , OsNia2 , and OsNADH-GOGAT were significantly higher in transgenic lines than in WT under both normal and low temperature. Phosphorylation of NR is a steady-state regulatory mechanism of nitrogen metabolism, and dephosphorylation of NIA1 protein improved NR activity and nitrogen utilization efficiency in rice. Excessive accumulation of nitrite under normal growth conditions inhibits the growth of rice; however, accumulation of nitrite is reduced at low temperature, enhancing the cold tolerance of rice.