Quantitative phosphoproteomics reveals SnRK1-dependent phosphosites in rice sheaths during grain filling

Insufficient leaf photosynthesis promotes sheath NSC transport to the panicle during grain filling. SnRK1 regulates assimilate distribution between plant tissues and organs. However, the mechanism by which SnRK1 regulates sheath NSC transport to the panicle and its response to limited leaf assimilation remain unclear. Here, we performed leaf cutting (LC) at anthesis to simulate insufficient leaf assimilation and set no treatment as a CK. In response to LC, SnRK1 activity increased rapidly, and NSC transport was advanced. Sheath NSCs were not transported in a snrk1a mutant with deficient SnRK1 activity. These results indicated that SnRK1 activity is important for sheath NSC transport. The high correlation of T6P and sucrose and the inhibition of SnRK1 by T6P in sheaths in vitro implied that T6P slightly inhibits SnRK1 activity in rice sheaths in response to sucrose availability. The increase in SnRK1 activity was accompanied by an increase in OsSnRK1a expression and a low sucrose content. Low sucrose signaling increases SnRK1 transcription. Therefore, we speculated that OsSnRK1a expression positively regulates SnRK1 activity in response to low sucrose availability in rice sheaths. Through phosphoproteomics and further PRM verification, 20 phosphosites that depend on SnRK1 and are correlated with NSC transport were obtained. Based on the function of these sites and proteins, we found that SnRK1 regulates starch degradation, sucrose metabolism, phloem transport, sugar transport across tonoplast, and glycolysis via phosphorylation to promote sheath NSC transport. Overall, our results revealed the importance, function and regulatory mechanism of SnRK1 in sheath NSC transport.