Improvement of japonica rice resistance to sheath blight by pyramiding qSB-9TQ and qSB-7TQ

• The suitability of two QTLs was confirmed to be utilized in improving the resistance of japonica rice varieties to sheath blight disease.
• The pyramiding effect of the two QTLs was significantly larger than that of either of them, and no genetic interaction was found between the two QTLs.
• The introgression of both QTLs could achieve a 14% reduction in yield loss under serious disease condition.

Sheath blight (SB) is one of the most important rice diseases worldwide. It has been difficult to generate SB resistant rice varieties by relying on traditional breeding because resistance to rice SB is a typical quantitative trait. Two QTLs, qSB-7 and qSB-9 on chromosomes 7 and 9, respectively, with SB resistance have been detected in the indica variety Teqing (TQ). Here, we used TQ as the donor to introduce these two resistance QTLs into different commercial japonica varieties through marker-assisted selection and field screening and disease evaluation. Moreover, we have pyramided these two QTLs in the genetic background of japonica variety WLJ1. SB resistance test results showed that the backcrossed lines carrying each of the two QTLs were significantly more resistant to SB than the controls without either of the QTLs, but were significantly less resistant than the lines carrying both QTLs. In the ‘0–9’ disease rating system, qSB-7TQ and qSB-9TQ each reduced disease severity by approximately 1.0, while two QTLs pyramided reduced disease severity by 1.7. These results suggest that there is little genetic interaction between the two QTLs. The backcrossed lines carrying either a single QTL or both showed no significant differences in major agronomic and productivity traits as their recurrent parent, demonstrating the suitability of these two QTLs utilized in these japonica varieties. Moreover, we found that these two SB resistance (SBR) QTLs achieved a significant yield loss reduction (14%) from 22.3% to 7.5% in our experimental field conditions. These results constitute an important advancement in SBR research and will facilitate the utilization of qSB-7TQ and qSB-9TQ in developing japonica rice varieties with excellent SB resistance.