Performance Story: Characterization of multiple rust resistance genes to design an optimal deployment strategy
Rust diseases, including leaf, stripe and stem rusts, are the most widely occurring diseases of wheat worldwide. Pyramiding multiple resistant genes has been proposed as the most effective way to control these diseases in wheat. However, to practise this approach, one must know the most effective gene combinations from the pool of more than 200 genes that bring resistance to the three rust diseases. Deployment the most effective gene pyramids will ensure the maintenance of durable rust resistance within Canadian wheat cultivars, thus avoid production losses from rust epidemics. Moreover, the optimization will minimize the yield penalty of resistance genes, as well as improve breeding efficiency in the development of new cultivars. In this project, we aimed to identify an optimal gene pyramid for multiple rust resistance by investigating rust resistance from a well-known International Maize and Wheat Improvement Center (CIMMYT) line, Parula, which has durable resistance to multiple rust types.
We created a population of Thatcher x Parula, which contains many adult plant resistance genes (APRs) that provide durable resistance that remains effective for long periods of time when compared to the short lifespan of many seedling resistance (R) genes that are effective at all stages of the lifecycle. We found that Parula contains four stable leaf rust adult plant resistance genes (APRs) that provide an almost an immune response to leaf rust (Lr34/Yr18/Sr57, Lr46/Yr29/Sr58, Lr_7BS (Lr72), and Lr_7BL (Lr68)). The two leaf rust QTL from 7B confer strong leaf rust resistance, but have not been deployed into the Canadian wheat program. Introgression of these genes will diversify the leaf rust resistance gene pool.
The other two of these important leaf rust resistance genes (Lr34/Yr18/Sr57, Lr46/Yr29/Sr58), along with Lr27/Yr30/Sr2, are also responsible for the largest portion of stripe rust resistance. Stem rust resistance from Lr27/Yr30/Sr2 conferred the largest resistance effect of all of the resistance genes, followed by Sr_5A, Lr34/Yr18/Sr57 and Sr12.
Findings from this project inform breeders that a foundational multi-APR cassette of Lr34/Yr18/Sr57, Lr46/Yr29/Sr58 and Lr27/Yr30/Sr2 can provide an almost immune resistance to current leaf, stripe and stem rust races from Western Canada, especially if combined with 2-3 R genes. Lr34/Yr18/Sr57 has already been an important target for breeders and our results confirm that this target needs to be maintained. Lr46/Yr29/Sr58 still needs to be further characterized, but our results indicate that it is another high value APR gene for Canadian wheat, and recent findings from our other related projects indicate it may already be deployed in Canadian wheat, which will help expedite this process. We found that Sr2/Yr30/Lr27 is a very valuable gene for rust resistance in Canada, but because of its linkage with other important genes, including resistance to Stagonospora nodorum blotch (QSng.sfr-3BS) and Fusarium head blight (FHB1), it has not been used for rust resistance in Canada. Our results show that breaking down the linkage drag and fully utilizing this gene will be important to protect Canadian wheat from rust diseases.
The relatively very small number genes of this resistant cassette improve the breeding efficiency, further aided by potential quality marker assisted selection for these genes. Because of their effectiveness across multiple rust types, this also is a solution for the emerging stripe rust issue across Western Canada. Breeders can input this gene cassette to optimize their genetic and genomic strategy and achieve desirable and durable resistance to all 3 rusts in Canadian wheat.