Genetic and Genomic Analysis of Nonhost Resistance to Wheat Stem Rust in Brachypodium distachyon

Abstract

Wheat stem rust, caused by the fungus Puccinia graminis f.sp. tritici (Pgt), is a devastating disease that has been under control for decades. However, new races of this pathogen have emerged that overcome many important wheat stem rust resistance genes, and their spread toward important areas of wheat production threatens global wheat production. Nonhost resistance in plants, which provides durable and broad-spectrum resistance to non-adapted pathogens, may hold great potential to help in the control of wheat stem rust, but the genetic and molecular basis of nonhost resistance is poorly understood. This research project employed the model plant Brachypodium distachyon (Brachypodium), a nonhost of Pgt, for genetic analysis to map loci associated with nonhost wheat stem rust resistance. Using bulked segregant analysis, next-generation sequencing, and bioinformatics approaches, seven quantitative trait loci were found to contribute to nonhost stem rust resistance in a recombinant inbred population derived from a cross between two Brachypodium genotypes with differing levels of resistance. In a second study, analysis of a Brachypodium recombinant inbred population segregating for an induced mutation that confers susceptibility to wheat stem rust led to the identification of a one base pair deletion in a gene that may be the cause of the mutant’s susceptibility. The gene is a homolog of the Arabidopsis gene TIME FOR COFFEE (TIC), which plays a role both in circadian clock regulation and jasmonate signaling. Collectively, the findings of this research project advance our understanding of the genetic basis of nonhost resistance to wheat stem rust, and will guide future research aiming to identify genes essential to the nonhost resistance response, as well as their mechanisms of action.