Zhi Xu

Xiangwanxian 13 (XWX13), an elite fragrant indica rice, is highly susceptible to rice blast and accumulates cadmium (Cd) in grain above the food safety limit in Cd-contaminated paddies, severely constraining its commercial use. Despite these shortcomings, the variety is widely grown for its high yield and superior grain quality. To overcome these limitations, we conducted marker-assisted backcrossing (MABC) complemented by genome-wide background selection. Four major genes, namely Pi1, Pi2, OsHMA3, and OsNramp5, were precisely introduced into XWX13. Two preferable BC₃MF₅ improved lines iXWX13-1 (stacking Pi1 + Pi2 + OsHMA3) and iXWX13-2 (stacking Pi1 + Pi2 + OsNramp5) were obtained with genomic background recovery rates of 94.44% and 94.63%, evaluated by using the RICE 1K SNP array, respectively. Seedling resistance spectrum assays demonstrated more than a 97% blast resistance rate against 39 Magnaporthe oryzae isolates, and both lines showed enhanced leaf and panicle neck blast resistance in natural nurseries. Multi-site field trials revealed grain Cd concentrations of 0.009–0.077 mg kg⁻¹ in iXWX13-2, 90.98–98.87% lower than those in XWX13. Importantly, yield, major agronomic traits, and grain quality remained indistinguishable from the original variety. This study provides the first demonstration that MABC coupled with SNP array background selection can simultaneously enhance blast resistance and reduce grain Cd in XWX13 without yield or quality penalties, offering a robust strategy for pyramiding multiple desirable genes into elite cultivars.

Xiangwanxian 13 (XWX13), a fragrant indica rice variety with high yield and good grain quality, is generally welcomed by farmers in China. However, the fact that XWX13 is highly susceptible to rice blast and its high grain cadmium (Cd) accumulation in Cd polluted paddy fields has been limiting its commercial utilization. Therefore, a genomics-assisted selection approach was employed to improve its blast resistance and lower grain Cd accumulation. In this study, four major genes, namely Pi1, Pi2, OsHMA3 and OsNramp5, were precisely introduced into XWX13 with the KASP marker-assisted selection followed by evaluation of genomic background with the genome-wide RICE 1K SNP array. Two preferable improved lines iXWX13-1 (stacking Pi1+Pi2+OsHMA3) and iXWX13-2 (stacking Pi1+Pi2+OsNramp5) were obtained with a genomic background recovery rates of 94.44% and 94.63%, respectively. Resistance spectrum assay showed that the two improved lines at the seedling stage exhibited high-level resistance to 39 Magnaporthe oryzae isolates, with more than 97.44% blast resistance frequency. Also, an enhanced resistance was observed for leaf and panicle neck blast in the two improved lines than that of the original XWX13 in natural blast nurseries. For grain Cd content, a multi-site field trial showed that a trace amount of grain Cd accumulation (0.009 to 0.077 mg/kg) was detected in the iXWX13-2, 90.98%-98.87% lower than that of XWX13. Moreover, the high similarity was observed in the yield, main agronomic traits and grain quality when comparing the improved lines to the original XWX13. This work was the first successful practice to apply molecular marker assisted backcrossing with genomics-based background selection to improve the blast resistance and lower grain Cd accumulation of XWX13 without penalty of the yield, main agronomic traits and grain quality. This proposed strategy would pave the way to integration of multiple desirable genes into the elite varieties in crop genetic improvement.

The damage caused by the white-back planthopper (WBPH, Sogatella furcifera) and brown planthopper (BPH, Nilaparvata lugens), as well as southern rice black-streaked dwarf virus (SRBSDV), considerably decreases the grain yield of rice. Identification of rice germplasms with sufficient resistance to planthoppers and SRBSDV is essential to the breeding and deployment of resistant varieties and, hence, the control of the pests and disease. In this study, 318 rice accessions were evaluated for their reactions to the infestation of both BPH and WBPH at the seedling stage using the standard seed-box screening test method; insect quantification was further conducted at the end of the tillering and grain-filling stages in field trials. Accessions HN12-239 and HN12-328 were resistant to both BPH and WBPH at all tested stages. Field trials were conducted to identify resistance in the collection to SRBSDV based on the virus infection rate under artificial inoculation. Rathu Heenati (RHT) and HN12-239 were moderately resistant to SRBSDV. In addition, we found that WBPH did not penetrate stems with stylets but did do more probing bouts and xylem sap ingestion when feeding on HN12-239 than the susceptible control rice Taichung Native 1. The resistance of rice accessions HN12-239, HN12-328, and RHT to BPH, WBPH, and/or SRBSDV should be valuable to the development of resistant rice varieties.

Barley (Hordeum vulgare L.) is the most important cereal crop in the Qinghai-Tibet Plateau, and the yield has been seriously threatened by Blumeria graminis f. sp. hordei (Bgh) in recent years. To understand the virulence and genetic traits of different Bgh populations, 229 isolates of Bgh were collected from Tibet, Sichuan, Gansu and Yunnan provinces of China during 2020 and 2021, and their pathogenicity to 21 barley lines of different genotypes was assessed. A total of 132 virulent types were identified. The Bgh isolates from Yunnan showed the highest diversity in terms of virulence complexity (Rci) and genetic diversity (KWm), followed by those from Sichuan, Gansu, and Tibet, in that order. Single nucleotide polymorphism (SNP) in genes coding for alternative oxidase (AOX), protein kinase A (PKA), and protein phosphatase type 2A (PPA) were detected at seven polymorphic sites. Nine haplotypes (H1–H9) with an average haplotype diversity (Hd) and nucleotide diversity π of 0.564 and 0.00034, respectively, were observed. Of these, haplotypes H1 and H4 accounted for 88.8% of the isolates, and H4 was predominant in Tibet. Genetic diversity analysis using the STRUCTURE (K = 2) and AMOVE indicated that the inter-group variation accounted for 54.68%, and inter- and intra-population genotypic heterogeneity accounted for 23.90% and 21.42%, respectively. The results revealed the recent expansion of the Bgh population in Tibet, accompanied by an increase in virulence and a loss of genetic diversity.