With the increasing greenhouse effect, high temperature has become the most unfavorable environmental factor for the rice grain filling process, affecting rice yield and quality mainly through changing the composition and structure of starch in rice grains. Research has focused on the rational management of water and fertilizer, and spraying of exogenous chemicals, which have become important measures to alleviate high temperature stress of rice. As a multifunctional molecule, melatonin has the potential to improve plant stress resistance by enhancing the scavenging efficiency of reactive oxygen species (ROS), thus protecting plants from the adverse effects of abiotic stress. The present study used a typical japonica rice variety Nipponbare (NPB) as the experimental material, which was treated with high temperature and melatonin during grain-filling stages. The effects of exogenous melatonin on the rice growth and quality traits, as well as starch synthesis, in response to high temperature were analyzed systematically. Exogenous melatonin significantly increased the rice leaf photosynthetic and heat-resistance properties. Melatonin could alleviate the effects of high temperature on the key physicochemical properties related to rice quality. Furthermore, milled rice from NPB plants treated with melatonin had better endosperm appearance under high temperature. Further study found that exogenous melatonin could stabilize the chain length distribution of starch in NPB (especially amylopectin), which implied that melatonin could be used in rice cultivation to alleviate the effect of high temperature on quality, optimization of amylopectin synthesis can also improve rice quality. The results of the present study provide a new idea and research direction to alleviate high temperature stress of rice in the context of global warming.

Amylose content controlled by Wx determines rice grain quality, which is easily affected by high temperature. Wx^a and Wx^b are the two typical Wx alleles in rice, however, their effects on quality formation in response to high temperature under the backgrounds of indica rice and japonica rice have not been systematically compared. In this study, the near-isogenic lines (NILs) of Wx^a and Wx^b with japonica rice 2661 and indica rice 3611 backgrounds were treated by high temperature during the grain-filling stages. High temperature accelerated the grain ripening process, decreased the thousand-kernel weight, and increased the chalkiness degree of all rice samples. However, these traits of Wx NILs with 3611 background were more susceptible to high temperature than those with 2661 background. Furthermore, high-temperature treatment decreased the amylose contents (AC) and starch viscosities but increased the gelatinization temperature of all the Wx NILs. The 3611-Wx^a was atypical Wx^a-type rice, whose AC was more sensitive to high temperature. The AC result was consistent with quantitative analysis of GBSSI by Western blot. In addition, the effects of Wx genotype and genetic background on rice physicochemical quality (such as the gel consistencies, starch crystallinity, and the morphological structure of starch grains) in response to high temperature were systematically analyzed. These results have important guiding significance for rice-quality improvement under high-temperature climate.