Pyrenophora tritici-repentis : investigation of factors that contribute to pathogenicity

Graduation date: 2013Pyrenophora tritici-repentis (Ptr) is the necrotrophic fungus responsible for tan spot of wheat (Triticum aestivum). Ptr causes disease on susceptible wheat cultivars through the production and secretion of host-selective toxins (HSTs). HSTs are compounds that are only known to be produced by fungi and considered to be primary determinants of pathogenicity. Infiltration of these toxins into sensitive wheat elicits the same symptoms as the pathogen, which simplifies investigations of host- pathogen interactions due to exclusion of the pathogen. These characteristics make HSTs ideal molecules to dissect molecular plant-microbe interactions. Known HSTs of Ptr include Ptr ToxA (ToxA), Ptr ToxB (ToxB) and Ptr ToxC (ToxC). ToxA is the most characterized toxin of Ptr, as well as the first proteinaceous HST identified. The proposed mode-of-action for ToxA includes internalization into sensitive wheat\ud mesophyll cells, localization to the chloroplast, photosystem perturbations and elicitation of high amounts of reactive oxygen species (ROS), all of which lead to necrosis. However, it is still unknown how ToxA is transported to the chloroplast. To identify additional interacting components involved in ToxA symptom development, genes were silenced in tobacco plants (Nicotiana benthamiana) using the tobacco rattle virus (TRV) virus-induced gene-silencing (VIGS) system. Four genes were identified that potentially could play a role in ToxA-induced cell death: a 40S ribosomal subunit, peroxisomal glycolate oxidase (GOX), a thiamine biosynthetic enzyme (Thi1), and the R-gene mediator, Sgt1. Ptr exhibits a complex race structure determined by the HST(s) produced and the symptom(s) elicited on sensitive wheat cultivars. Currently, there are eight characterized races and other HSTs and races have been proposed. Isolate SO3 was discovered in southern Oregon and elicits ToxA-like symptoms on a wheat differential set, yet lacks the ToxA gene. The transcriptome of SO3 was sequenced, assembled, and aligned to a ToxA-producing isolate, Pt-1C-BFP, which will aid in the identification of the protein(s) that may be responsible for these ToxA-like symptoms. SO3 contains a set of 497 sequences that were not found in the ToxA-producing isolate Pt-1C-BFP (BFP). These sequences should be further investigated to identify those that encode small secreted proteins (SSPs) and could potentially serve as HSTs and pathogenicity factors of SO3