Search

ABSTRACTDegradation of the appearance of rice grains due to high temperatures during ripening is a serious problem being worsened by global warming. QTLs for tolerance to high temperature during ripening must be identified in order to develop new cultivars efficiently. Here, we developed recombinant inbred lines (RILs) from crosses between the high-temperature-tolerant ‘Genkitsukushi’ and the high-temperature-sensitive ‘Tsukushiroman’. The occurrence of white-back grains is significantly lower in ‘Genkitsukushi’ than in ‘Tsukushiroman’. We evaluated their high-temperature tolerance and performed quantitative trait locus (QTL) analysis. A QTL for white-back grains on chromosome (chr.) 6 and a large-effect QTL on chr. 8 were identified in the two-year experiments. The QTL on chr. 8 was close to one we previously detected using RILs derived from ‘Chikushi 52’ × ‘Tsukushiroman’. The results indicate that the QTL on chr. 6 is unique to ‘Genkitsukushi’ × ‘Tsukushiroman’, and that on chr. 8 could be identical between ‘Genkitsukushi’ and ‘Chikushi 52’. Backcrossed lines which harbor the QTL region on chr. 8 of ‘Genkitsukushi’ in the background of ‘Tsukushiroman’ verified its effects. We also analyzed the marker genotypes near the QTLs on chr. 6 and 8 in relatives of ‘Genkitsukushi’. We discuss the interaction between these QTLs and their relationship with high-temperature tolerance.

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.

Rice productivity is related to the ability of plants to adapt to heat stress. The heat-tolerant cultivars ‘Nikomaru’ and ‘Chikushi 64’ and heat-sensitive cultivar ‘Hinohikari’ were grown at 30ºC and 25ºC for 49 days after flowering. At 30ºC, only a few white immature kernels were produced in ‘Nikomaru’ and ‘Chikushi 64’, but about 22% of grains had immature kernels in ‘Hinohikari’. The high temperature(30ºC) caused no significant changes in grain dry weight, water content, and the NMR T1 value during the early ripening stage in ‘Nikomaru’ and ‘Chikushi 64’. It also did not affect grain development, especially with respect to the nucellar epidermis, in ‘Nikomaru’ and ‘Chikushi 64’, but caused clear cessation of development of the nucellar epidermis at 14 days after flowering in ‘Hinohikari’. In addition, high temperature decreased the amylose content and increased hardnessvs. adhesion ratio of cooked rice in both ‘Nikomaru’ and ‘Chikushi 64’ resulting a softer, less sticky texture, but not in ‘Hinohikari’. The maximum viscosity and breakdown values were increased, and final viscosity decreased at 30ºC in all three cultivars. These results suggested that starch in the endosperm of grains changed from a fluid state to a doughy state more slowly in ‘Nikomaru’ and ‘Chikushi 64’ than in ‘Hinohikari’, in which the water content and NMR relaxation time decreased, and transported assimilates accumulated slowly during grain development.