High temperature is one of the major environmental factors influencing rice growth and productivity. Song et al. (2011) showed that 45°C high temperature resulted in suppressed root and stem growth. In addition, high temperature caused low SSR and reduced yield especially during flowering period. One of the main reasons is that heat stress can cause bad anther dehiscence, which resulted in low pollen germination of stigma and reduced pollen production and spikelet fertility (Prasad et al. 2006; Rang et al. 2011). Therefore, spikelet fertility and SSR were commonly used as indicators of HT in rice (Yeet al. 2015a; Zhao et al. 2016; Zhu et al. 2017). In this study, SSR was selected for HT evaluation and QTL mapping at rice heading stage.
We identified five detected QTLs (qSSR6-1, qSSR7-1, qSSR8-1, qSSR9-1 and qSSR11-1) associated with HT (Fig. 1, Table 4). Among them, qSSR9-1 was identified on chromosome 9 and accounted for 7.66% of the phenotypic variations. Similarly, Shanmugavadivel et al. (2017) also identified a QTL (qSTIPSS9.1) for HT of rice on chromosome 9 through using a 5K SNP array. It is worthy to note that two HT QTLs, qHt9a (RM108-RM242) and qHt9a (RM242-RM566), which shared the same location of qSSR9-1, were previously reported using a set of RILs (Chen et al. 2008), indicating that qSSR9-1 is a major QTL for the HT of rice. qSSR8-1 located on chromosome 8 and accounted for 6.11% of the phenotypic variations. Two QTLs, qDFT8 and qHT-8, were also previously identified for HT in rice and located on chromosome 8, which explained 31.10% and 51.67% of the phenotypic variation, respectively (Zhao et al. 2016; Liu et al. 2017); however, they were distinctly different from qSSR8-1 according to their mapping results, suggesting that qSSR8-1 might be a novel QTL for HT. qSSR7-1 was found to be a major QTL located on chromosome 7, which explained up to 26.35% of the phenotypic variance. According to Zhao et al. (2016), qPSLht7, which was associated with spikelet fertility under high temperature in “Sasanishiki”/“Habataki” CSSLs population across three environments, located adjacent to qSSR7-1 on chromosome 7 and explained 79.0% of the phenotypic variation. Therefore, qSSR7-1 might also be a new QTL. qSSR6-1 and qSSR11-1 explained 5.83% and 14.21% of the phenotypic variation, respectively. On the basis of data obtained from the QTL Annotation Rice Online Database [Q-TARO, http://qtaro.abr.affrc.go.jp/] and comparison with previously reported QTLs, the two minor QTLs (qSSR6-1, qSSR11-1) detected in this study might be new.
Since extremely high temperatures caused significant loss in rice production, breeding heat-tolerant rice varieties or identifying heat-tolerant rice varieties from pre-existing germplasm has become a big concern to rice breeders. Ishimaru et al. (2010) utilized the Early-Morning Flowering (EMF) trait from O. glaberrima and screened heat-tolerant rice from improved and traditional rice varieties. Compared with EMF trait screen, the progeny selection of heat resistant plants in a traditional crossing program requires high labor and economic costs. Therefore, the MAS breeding has become more and more essential for breeders to breed heat-tolerant rice varieties (Zhao et al. 2016). In recent years, many putative QTLs for HT have been identified in rice; however, the QTLs of stable effect still remain rare. Hence, we designed this research to explore and further confirm useful QTLs associated with HT of rice in heading stages. Of the five QTLs identified in this study, qSSR9-1 is consistently identified with previous studies (Chen et al. 2008; Ye et al. 2015a) and can be used for rice HT improvement by MAS, while the others are novel and need to be further confirmed. These findings would contribute to better understanding of the genetic basis of HT in rice and accelerating the process of breeding heat-tolerant rice varieties.