Exploration Of Novel Wheat Powdery Mildew Resistance Genes And Cloning Of PM2026

Powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is one of the most devastating foliar diseases of wheat. Many powdery mildew resistance genes have been identified in various kinds of germplasm. However, because of the co-evolution of pathogen avirulent genes and host resistance genes, discovering new genes for powdery mildew resistance is a constant task for wheat breeders. An in-depth understanding of host resistance mechanism through cloning and functional analysis of resistance genes will shed light on how to continuously control this disease.Wild emmer wheat accession TA1410 is resistant to powdery mildew. To identify and transfer powdery mildew resistance genes from TA1410 into common wheat, a resistant F3 line 380-13, derived from the cross of TA1410 x durum wheat line Zhongyin1320 was crossed with susceptible wheat cultivar Yangmail58. The homozygous resistant BC5F2 lines derived from the backcross with Yangmai158 exhibited susceptibility at seedling stage and conferred increasing resistance when the plants were closer to heading stage. In two segregating BC5F3 families investigated at heading stage, the segregation of the resistance fit a 3:1 ratio, suggesting that a single dominant gene controls the resistance. This resistance gene, designated as HSM1 (for heading stage powdery mildew resistance), was mapped to the 0.6 cM Xmag5825.1-Xgwm344 interval on chromosome 7AL and cosegregated with Xrga-C3 and Xrga-C6. A mapping position comparison with other powdery mildew resistance genes on this chromosome suggested that HSM1 belongs to the Pml resistance gene cluster.Tetraploid wheat Khapli was known to carry more than one powdery mildew resistance genes, but only Pm4a has been identified and transferred into common wheat. To find out the unknown powdery mildew resistance gene, resistance spectrum test was first applied to Khapli and Pm4a near-isogenic line CI14123 through a detached leaf assay with 14 powdery mildew pathogen isolates. Compared to Khapli showing resistant to all the isolates, Pm4a has 4 virulent isolates. A total of 95 recombinant inbred lines derived from the cross of susceptible tetraploid wheat Jiutoumai and Khapli were then evaluated by Pm4a virulent isolate. Forty-three plants showed resistance and 52 were susceptible. This resistance segregation fit the 1:1 ratio determined by a single resistance gene. Molecular marker mapping showed that this gene was located on chromosome 2AL, with 10.3 cM from Xgwm256 and 11.4 cM away from Pm4a.The recessive resistance gene pm2026 was identified from the einkorn wheat accession TA2026 and narrowed down to a 17 kb region in the previous study. Sequence comparison between TA2026 and the susceptible parent M389 in the candidate region revealed that different from the pm2026 interval, the counterpart region in M389 had 20 kb and 92 kb insertions mainly composed of dispersed repetitive DNA without any protein-encoding genes. Transient expression assay indicated neither of the two protein-encoding genes affected the penetration efficiency of wheat powdery mildew. However, the flanking gene F showed significant different expression level between resistant and susceptible parents. Haplotype analysis showed the 92 kb insertion might be responsible for the high-level expression of the downstream gene F and susceptibility.