Salicylic acid promotes endosperm development and heat-tolerance of waxy maize (Zea mays L. var. ceratina Kulesh) under heat stress

•SA prolonged grain filling period, promoted the starch synthesis, and increased yield.•SA alleviated oxidative damage of endosperm, enhanced proliferation of endosperm cells.•SA stabilized plant hormone level and signal transduction in heat-tolerant variety.•SA triggered jasmonic acid signaling pathway in heat-tolerant variety.•SA initiated flavonoid signaling pathway in heat-sensitive variety. High temperature (HT) during the grain-filling stage has become an important factor restricting endosperm development and yield formation in maize. Saicylic acid (SA) is an efficient plant-protective hormone, but its specific function and molecular mechanism regulating the heat tolerance of maize grains have not been reported. In this study, two waxy maize varieties cultivated in pots were used as research materials, and exogenous SA and HT were applied at the initial grain-filling stage. Compared with HT, the application of SA prolonged grain-filling duration and increased grain numbers per panicle, thereby improving the grain weight and yield. SA enhanced the ploidy level and promoted the proliferation of endosperm cells under HT. SA promoted the biosynthesis and accumulation of sucrose and starch in endosperm under HT by regulating their metabolism. SA stabilized the balance of endogenous hormones in heat-tolerant variety, and activated the plant hormone signal transduction pathway. SA triggered alpha-linolenic acid metabolism and jasmonic acid signaling pathways of heat-tolerant variety. Furthermore, SA adjusted the phenylpropanoid and flavonoid biosynthesis pathways of heat-sensitive variety. In conclusion, the physiological, biochemical, transcription profile, and metabolite level changes induced by SA treatment form the basis for the enhancement of heat tolerance of maize endosperm.