Characterizing the Fitness Cost to Wheat (Triticum Aestivum L.) Genotypes with Multiple Leaf Rust Resistance Genes

Leaf rust, caused by Puccinia recondita Roberge ex Desmaz. f. sp. tritici., has evolved as a race-specific pathogen of wheat (Triticum aestvium L.). It causes significant economic losses annually. Wheat has co-evolved with resistance genes, such as Lr10 and Lr21, which result in a hypersensitive reaction to a specific race. Another gene, Lr34 reacts to the pathogen by expressing slow-rusting. Pyramiding leaf rust resistance genes is one possible strategy to enhance the level of resistance and/or extend the longevity of resistance. However, expression of resistance may exact an excessive energy cost, and hypersensitivity and cell death results in the loss of photosynthetic tissue. Thus, the objective of this study was to identify if the cost of pyramiding Lr10, Lr21, and Lr34 counteracts the benefits of pyramiding them. ‘Thatcher’ and nearisogenic lines (NILs) of Thatcher with one and two different Lr genes were compared along with Thatcher F1 hybrids that combined two different Lr genes in a heterozygous state. Known molecular markers to the genes were utilized to try and achieve and confirm the pyramids. A split-plot design was employed, with main plots represented by treatments - inoculation with or without a host non-specific crown rust pathogen (Puccinia coronata Corda var. avenae W.P. Fraser Ledingham) - and subplots represented by genotypes - an oat genotype, Thatcher, Thatcher NILs and F1s. Grain yield, above-ground biomass, and seed shape, size, and weight were quantified to determine the potential cost of resistance to the plants. Crown rust inoculations did not definitively invoke gene expression or hypersensitive reactions with an absence of pustule formation in the wheat genotypes. However, genotypes expressing two resistance genes generally exhibited some reduction in reproduction in comparison to genotypes without resistance genes, or expressing a single gene. Breeders should consider the potential negative consequences to plant fitness before pyramiding resistance genes that result in hypersensitive reactions. Molecular markers, specifically for Lr10 and Lr21 were difficult to use for verifying pyramids compared to Lr34. Geneticists and breeders should be cognizant of the fact that very similar Lr genes are not easily or readily verified in pyramids, even using molecular markers