4.3 Further breeding efforts for the genetic improvement of IR64+qEMF3 and other backgrounds
Some yield components were significantly different between genotypes. SN tended to be higher while 1000GW was lower in IR64+qEMF3 than in IR64 (Table 4, 5; Table S3). However, PN was not different between genotypes (Table 4; Table S3). IR64+qEMF3 had slightly larger flag leaf size compared to IR64 (Table 3; Table S3). Smaller 1000GW in IR64+qEMF3 was mainly due to the smaller length of the grains (Table S4). These results suggest that physical constraints due to small husk size, and not insufficient source supply, would be responsible for the smaller 1000GW observed. Ohsumi et al., (2011) demonstrated that there is a negative correlation between spikelet number per panicles and 1000GW using NILs for increased spikelet number. Whether qEMF3has pleiotropic effects on SN and 1000GW as well as the EMF trait must be further investigated. Introgression of QTL for grain size detected in genetic background of Indica group cultivars (Kato et al., 2011; Qing et al., 2018) may retrieve 1000 GW of IR64+qEMF3 to similar levels as those of IR64. Regarding other agronomic traits, minor change in days-to-heading were observed (Table 3, 5; Table S3). IR64 is a popular Indica group cultivar that is most widely grown in Asia (Brennan & Malabayabas, 2011) and its progeny continues to be grown quite widely. Our study provides breeders some fundamental information on the changes in plant phenotypes that can occur due to the introgression ofqEMF3 to IR64 genetic background. Transfer of qEMF3 to popular cultivars in the temperate, subtropical, and tropical regions by marker-assisted breeding is an ongoing project. The effects ofqEMF3 on agronomic traits and grain yield on other genetic backgrounds need to be further investigated.
CONCLUSIONS
This is the first field study to clearly demonstrate that the EMF trait can enhance rice grain yield through stabilization of PFG under stressful conditions in heat-vulnerable regions. Significant advancement of FOT due to qEMF3 was observed in wide range of environments including temperate, subtropical, and tropical regions, while the changes in agronomic traits due to the introgression of qEMF3into genetic background of IR64 were minimal. This result indicates the possibility that qEMF3 can also be effective in enhancing rice grain yield in other heat-vulnerable regions and genetic backgrounds. A breeding program to transfer qEMF3 to local cultivars grown in hot dry seasons through marker-assisted selection is an appropriate strategy that is recommended.