Differential responses to high temperature during maturation in heat-stress-tolerant cultivars of<i>Japonica</i>rice

High-temperature stress during the grain-filling stage reduces grain quality of rice, and this is a serious problem in Japan, especially in the Kyushu region. To solve this problem, various heat-tolerant cultivars have been bred, such as ‘Nikomaru’, ‘Kumasannochikara’, ‘Genkitsukushi’, ‘Sagabiyori’, and ‘Otentosodachi’. When cultivated under high temperature after flowering, these heat-tolerant cultivars had lower percentages of chalky grains than in the heat-sensitive cultivar ‘Hinohikari’. All the heat-tolerant cultivars markedly decreased the nonstructural carbohydrate content in the stem under the high temperature compared to control condition during early grain-filling stage, which is considered to be a common trait of heat tolerance. Notably, ‘Sagabiyori’, ‘Genkitsukushi’, and ‘Nikomaru’ maintained a nucellar epidermis at 17 days after flowering (DAF) under high temperature, whereas the nucellar epidermis disappeared in ‘Hinohikari’. In addition, the expression of AGPS2b, thought to be a rate-limiting enzyme in starch synthesis, in ‘Kumasannochikara’, ‘Otentosodachi’, and ‘Nikomaru’ did not decrease under high temperature, whereas ‘Hinohikari’, ‘Sagabiyori’, and ‘Genkitsukushi’ could not maintain expression of the gene at 17 DAF. Moreover, the expression of Amy3E, a starch-degradation-related gene considered to induce grain chalkiness, in ‘Kumasannochikara’ at 17 DAF was not increased by high temperature. These results suggest that the heat-stress-tolerant cultivars have various mechanisms for dealing with high-temperature stress.