The breeding process of YNSM
YNSM was bred using Yuetai13, a high-yield and disease-resistant variety, as male parent, and good quality variety Huanghuazhan as female parent. The F1 seeds were planted and harvest for F2 seeds. Pedigree selection was performed to select the target line, and quality and resistance identification were carried out throughout the whole breeding process. Natural identification in the blast nurseries were used for blast resistance, artificial inoculation method was adopted for bacterial blight resistance, and appearance identification and cooking and tasting screening were used for rice quality. Through 6 generations pedigree selection, comparative tests were carried out and regional tests and production tests were conducted. So far, YNSM has been certificated in Guangdong province (2011), Hainan province (2013), Jiangxi province (2017) and Hubei province (2017), China.
The milling and appearance quality of YNSM
The grain size of YNSM is between YXZ and XYXZ, the grain length is 9.28 mm, grain width is 2.3 mm, and the length/width is 3.2-3.4, which is different from the small size variety YXZ and the thin-long grain variety XYXZ (Fig. 1A-D). The grain size is mainly controlled by genetic factors rather than environmental factors. In order to reveal the regulatory factors of YNSM grain size, we used the known molecular markers to detect. The results showed that all of YNSM, YXZ and XYXZ were GS9, GW5, and GW8. For the GS3 gene, long grain variety YNSM and XYXZ is gs3, small variety YXZ is GS3. Moreover, compared to YNSM and YXZ, XYXZ is controlled by GW7 (Fig. 1E).
For the milling quality was different in different seasons. In the early season, brown rice rate, white rice rate and whole white rice rate of YNSM was similar to YXZ and XYXZ, about 70%, 60% and 55%, respectively. However, in the late season, all of the milling quality of YNSM was much better than YXZ and XYXZ (Fig. 2A, B).
Generally, the white grain rate and chalkiness rate were greatly affected by seasons, and the rice quality is better in the late season than in the early season. The transparency level of YNSM was 3, which was better than YXZ and XYXZ. The chalkiness rate and chalkiness degree in the early season was 11.84% and 1.61%, respectively. While in the late season, the chalkiness rate and chalkiness degree was 6.71% and 0.84%, which were significantly lower than YXZ, but equal or better than XYXZ (Fig. 2C).
Cooking quality characters of YNSM
The AC of YNSM in the early and late season was 17.05% and 17.24%, respectively, which were lower than in YXZ (25.05% and 26.47%), and similar to XYXZ (15.9% and 18.2%). The gel consistency (GC) of YNSM was 72.0 mm and 75.0 mm, which was in accordance with XYXZ, but significantly higher than YXZ (44 mm and 46 mm). These results showed that YNSM has low AC and long GC as the high quality varieties. The eating score of YNSM (85.6-87.1/100) was similar to XYXZ (85.7-88.1/100), significantly higher than XYZ(57.0-63.8/100).
For Wx is the main gene that controls amylose synthesis, the genotype of Wx was performed in the YNSM (Zhou, Wang et al. 2016). The results showed that the YNSM and XYXZ is Wxb, while YXZ is Wxa, which is conformed to the AC (Fig. 3A).
The RVA profile of YNSM
Both in early and late seasons, the starch paste of the detected rice varieties displayed a process “up-down-up”. In the early season, the PV of YNSM was 248.92 RVU, which was half of YXZ and similar to XYXZ (Table 1). The HV was 88.02 RVU, significantly less than YXZ (312.09 RVU), but slightly higher than XYXZ (52.38 RVU). The CPV of YNSM was 163.88 RVU, which was substantially lower than YXZ (446.25 RVU) and more than XYXZ (99.21 RVU). In the early season, the BD, CS and SB of YNSM was 160.84 RVU, -85.04 RVU and 75.8 RVU, respectively. These values were similar to XYXZ, but the BD was higher than YXZ, the BD and CS was lower than YXZ. In the late stage, HPV, CPV and SB of YNSM was similar to XYXZ, but the PV, HPV and PeT with no significant difference, which revealed CPV, SB and CS is significantly correlated to the taste quality.
Protein and fatty acid content of YNSM
Rice quality includes the nutritional quality such as storage protein, fatty acid, anthocyanin and mineral (Dyer and Mullen 2008, Wang et al. 2015). Rice grain contains a certain proportion of protein and fat, which are related to nutrition, luster and palatability. In the early season, YNSM had the highest content, nonetheless had no significant difference in the late season (Fig. 4). The protein contents of YNSM is 7.68% in early season and 7.52% in late season, which is similar to XYZ and higher than XYXZ. Interestingly, the content of fatty acids reduced greatly in the late season (Fig. 4).
Quality-yield breeding applications of YNSM
YNSM is not only excellent in rice quality, but also outstanding in comprehensive agronomic traits. It is highly resistant to blast and bacterial blight. Meanwhile, YNSM is also an excellent restorer line (R1212) of two-line and three-line hybrid rice. At present, more than 20 hybrid rice combinations have been prepared with YNSM as restorer line, and most of the rice quality is above grade 3, which has passed the national examination (Table 2, http://www.ricedata.cn/variety/). Among them, the whole polished rice rate of Guangtaiyou-YNSM, Jitianyou-YNSM, Huiliangyou-YNSM, Cliangyou-YNSM, Xinrongyou-YNSM, Zaoyou-YNSM was above 65%, the glue consistency was 61-80 mm, the AC was 14.6-19.2%, and the rice quality is up to grade 2. These results indicated that YNSM can availably improved the grain quality of hybrid rice.