Background and Objective
The rapid evolution of plant pathogens has challenged the effectiveness of resistance genes as biological assets. Wheat yellow rust (Puccinia striiformis f. sp. tritici, YR) is a prominent example of this challenge. As one of the limiting factors for producing satisfactory yields, wheat yellow rust can significantly reduce yields under favorable conditions; therefore, developing strategies to confer resistance against it is particularly important. Wheat disease, once confined to colder regions, now occupies a wide geographical range, causing significant damage to wheat production. Wheat resistance to biotic and abiotic stresses, to some extent, is controlled by genetic components. This offers an opportunity to develop adaptable, yellow rust-resistant wheat varieties with higher yields. Gene pyramiding is an effective and inexpensive method for producing resistant plants by selecting genotypes that carry different combinations of genes involved in disease resistance. Research to combat this disease emphasizes the identification and development of resistance genes. Yr15 and Yr5 genes are among the known genes that have shown substantial resistance to various yellow rust disease races. Resistance genes such as Yr15, derived from closely related wild sources and retaining their effectiveness longer globally, are particularly important in developing resistant varieties. Pyramiding these two genes can confer effective resistance, with studies indicating that the resistance conferred by the combination of Yr5 and Yr15 has not been broken. This emphasizes the importance of conserving and utilizing these genes in wheat breeding programs. Various studies have recommended pyramiding the Yr5+Yr15 genes to overcome resistance breakdown. Marker-assisted breeding (MAB) for pyramiding genes is being carried out worldwide.
Materials and Methods
In this research, cultivation was conducted at the Shahid Bahonar University of Kerman's research farm from 2017 to 2025. In this study, with the ultimate aim of pyramiding Yr5 and Yr15 into the early-heading NILs of Kalheydari, two yellow rust resistance genes were transferred at the seedling stage in separate backcrossing programs using molecular markers from standard donor parents for Yr15 and Yr5 (Yr15/6*Avocet ‘S’ and Yr5/6*Avocet ‘S’) into the early-heading NILs of Kalheydari. To identify rust resistance genes in segregating populations, leaf samples were collected, and DNA was extracted. Polymerase chain reaction was performed using specific primers in a gradient thermal cycler to amplify resistance or susceptibility alleles. All molecular experiments were repeated twice throughout the project to ensure reproducibility. The resulting products were separated on agarose gel using 0.5x TBE buffer, stained with DNA Green Viewer safe stain, and visualized under ultraviolet light using a gel documentation system.
Results
After five generations of backcrossing and identification of yellow rust–resistant genotypes in the BC5F1 generation using molecular markers, genotypes carrying Yr5 and Yr15 were crossed to pyramid the genes and enhance resistance efficiency. In the next generation, through selfing and marker-assisted selection, resistant and stable isogenic lines to wheat yellow rust were obtained. Marker-assisted backcrossing has emerged as a powerful tool in plant breeding programs. Studies have demonstrated that employing Marker-assisted backcrossing in backcross breeding is more efficient than phenotypic selection. This approach accelerates the breeding process by approximately two years compared to traditional marker-free backcrossing methods.
Discussion
Yellow rust disease has consistently posed a threat to wheat production. While numerous race-specific resistance genes have been identified, the resistance conferred by these genes is unstable due to the rapid evolution of the pathogen. Transferring and pyramiding yellow rust resistance genes into wheat cultivars not only prevents yield losses during epidemic years and establishes durable resistance but also has positive environmental impacts. Utilizing a combination of classical breeding and marker-assisted backcrossing can accelerate transferring of yellow rust resistance genes into superior bread wheat cultivars and lines. Developing early-heading isogenic lines of the Kalheydari variety can help to prevent disease epidemics and economic losses during both epidemic and drought years. It can also play a crucial role in developing resistant cultivars and reducing the costs of disease control.