Misfits of wheat stem rust resistance-­‐ Unusual solutions to a consistent problem

Abstract

Rust fungi include some of the most economically damaging pathogens of wheat. They are notorious for their ability to quickly spread in susceptible host populations and greatly reduce grain yield potential and quality when managed improperly. Puccinia graminis f. sp. tritici (Pgt), the causal agent of wheat stem rust, can cause yield losses exceeding 50%. Stem rust is controlled in the U.S.A. using several methods including the introduction of genetic resistance, selection for earlier maturing varieties, removal of the alternate host Berberis vulgaris, and the application of fungicides. Subsequently, epidemics of stem rust causing greater than 10% yield losses have not been observed in the U.S.A. since the mid 1950’s. Together, removal of B. vulgaris from wheat growing regions and the introduction of genetic resistance have accounted for much of the control of stem rust. Genetic resistance remains the dominant method of controlling stem rust in regions where removing B. vulgaris is not applicable. In more recent years, races of Pgt have been identified that overcome most widely deployed resistance genes. In 1999 race TTKSK was identified in Uganda that overcame stem rust resistance gene Sr31. Following deployment of Sr24 in Kenya, further selection for virulence resulted in the identification of race TTKST, then TTTSK (Sr36 virulence), and more recently TTKTT and TTKTK (SrTmp virulence). Major resistance genes have continually proven to not provide a durable form of resistance to wheat stem rust. Some resistance genes however have proven the test of time and remain effective to date. These resistance genes include Sr2, Lr34, Lr46, and Lr67. Each gene functions in an additive, minor-effect, and in some cases recessive manner, atypical of standard major genes, and provides and/or enhances resistance to multiple diseases including stem rust, leaf rust, stripe rust, and powdery mildew. Additionally Lr34 and Lr67 do not have the NB-LRR protein domains consistent with major genes. Durable genetic resistance to stem rust may require sources of resistance that deviate from standard mechanisms. This dissertation describes such sources of resistance. SrTm4 is a major gene identified in Triticum monococcum that functions in a recessive manner, is broadly effective, and elicits a mesothetic (intermediate-effect) infection type. The adult plant resistance observed in ‘Morocho Blanco’ was found to have two underlying QTL, Qsr.cdl.2BS.2 and Qsr.cdl.6AS.1. These two loci comprise much of the adult plant resistance in ‘Morocho Blanco’ and exhibit interactions with environment or pathogen race. The Sr12 mutants created in this dissertation were made to characterize the disease reducing capabilities of Sr12: a recessive, race specific major gene that co-locates with adult plant resistance to Sr12 virulent races. Lastly, this dissertation also describes the identification of putative susceptibility genes for rust pathogens in barley, maize, soybean, and Brachypodium distachyon. The putative susceptibility gene in B. distachyon was tested with a T-DNA insertion mutant and exhibits enhanced rust resistance, however, may be linked to changes in overall plant growth and development. Each source of rust resistance defies standard systems of characterization and includes some traits that are less desirable along with their resistance capabilities, for example: unstable expression due to environmental interactions, race specificity, or recessive gene action. However, the benefit of these sources of stem rust resistance may compensate for their less desirable traits.