Carbon reserve and remobilization in leaf sheaths during the grain-filling stage in response to leaf early senescence

To elucidate the contribution of non-structural carbohydrates (NSCs) stored in leaf sheaths to developing grains and the corresponding molecular regulatory mechanisms of rice plant when source leaves suffered the early aging during the grain-filling stage, the carbohydrate changes and transcriptional expression of several gene families related to carbohydrate metabolism were comprehensively investigated in leaf sheaths of two rice cultivars: early senescence leaf (esl) mutant and its wild type. Results showed that the translocation rate of NSCs in leaf sheaths and the contribution of NSCs to grain development were significantly enhanced in source leaves with early senescence at the grain-filling stage. The decrease of starch in leaf sheaths of esl mutant rice between the heading and harvest stages was lower than that in the wild type, whereas the translocation of sucrose and fructose in leaf sheaths of esl mutant rice was significantly higher than those in the wild type. The expression of SUT4 in leaf sheaths of esl mutant rice at the early grain-filling stage was probably responsible for the rapid sucrose loading into the phloem of leaf sheaths. The remarkably low expression of CIN1, SuSy1, and SuSy2 in leaf sheaths of esl mutant rice reflected the low sucrose metabolism. Meanwhile, the transcriptional levels of genes for starch metabolism-related enzymes in leaf sheaths of esl mutant rice were significantly lower than those of the wild type. Therefore, starch metabolism in leaf sheaths of esl mutant rice was depressed when source leaves underwent the early senescence during the grain-filling stage. The possible relationships among NSC translocation, carbohydrate metabolism, and transcription regulation in leaf sheaths of esl mutant rice were also discussed.