Methyl jasmonate regulates panicle morphogenesis by mediating the negative effects of high temperature stress on carbon and nitrogen allocation and utilization

High-temperature stress (HTS) poses a serious threat to panicle development in rice. Plant hormones, including jasmonic acid (JA), play an important role in plant organ development. While the roles of IAA, cytokinin and gibberellin in heat stress have been studied, the research on JA in rice panicle morphogenesis under HTS during the early panicle differentiation period is still limited. In this study, we showed that HTS (day/night: 38℃ / 29℃) during the panicle differentiation period significantly reduced the number of differentiated spikelets and the number of grains per panicle. The correlation results suggest that this is related to antioxidant enzymes, carbon and nitrogen metabolism, and endogenous hormones, especially to carbon and nitrogen metabolism pathways. HTS limited the rate of carbon and nitrogen accumulation and redistribution in the plant, and the plant preferentially allocated more carbon and nitrogen to spikelets development under control conditions. The expression of JA synthesis and signaling genes was down-regulated under HTS, leading to a decrease in endogenous JA and MeJA content. Exogenous MeJA treatment optimized carbon and nitrogen metabolism by significantly enhancing the activities of nitrate reductase, glutamine synthetase, glutamate synthase, sucrose synthase, and sucrose phosphate synthase. Ultimately, the accumulation and allocation of carbon and nitrogen in panicle were improved, which greatly alleviated the high-temperature-induced decrease in differentiation spikelets and grain number. Overall, our results provide insight into the physiological effects of HTS during spikelets and panicle development; but, also suggest that HTS could be relieved with a supplemental application of MeJA.