The Brachypodium-Puccinia graminis System: Solving a Puzzle to Uncover the Underlying Mechanisms of Non-Host Resistance and Plant Immunity

Brachypodium distachyon is regarded as non-host to the causal agent of stem rust in wheat and barley, P. graminis f. sp. tritici (Pgt), and a near-host to the pathogens of forage grasses, P. graminis f. sp. lolii (Pgl) and P. graminis f. sp. phlei-pratensis (Pgp). Given the devastating effect of stem rust in grain production, it is important to advance our understanding of the molecular and genetic determinants that control resistance to Pgt. Acquisition of such knowledge can help identify novel and sustainable approaches to control Pgt epidemics. Brachypodium shows resistance to Pgt in a genotype-dependent manner, specifically the inbreed line Bd1-1 displays pre-haustorial resistance to Pgt and post-haustorial resistance to Pgl and Pgp. Such contrasting infection phenotypes prompted us to conduct a RNA-Seq analysis to pinpoint mechanisms that mediate pathogen recognition and defense to these rust fungi. Gene expression profiles were determined to dissect the plant responses associated to two distinctive early stages of infection, appressorium formation and fungal penetration. Here, we discuss some of the molecular and cellular activities that occurred during these interactions in the context of plant immunity and present a follow-up reverse genetics approach to evaluate the role of candidate genes in the defense against Pgt.