Identification and Functional Characterization of Necrotrophic Effectors in Parastagonospora Nodorum

Abstract

The necrotrophic fungus Parastagonospora nodorum (teleomorph; Phaeosphaeria nodorum), is the causal agent of Septoria nodorum blotch (SNB) on common wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum L.). SNB is a serious foliar and glume disease which causes significant yield losses in major wheat growing areas and has serious impact on grain quality. P. nodorum produces necrotrophic effectors (NEs) that are recognized by and interact with dominant host sensitivity genes in an inverse gene-for-gene manner. The NE-host interaction is critical to induce necrotrophic effector-triggered susceptibility (NETS), resulting in SNB disease. To date, nine NE-host sensitivity gene interactions, following a NETS model, have been identified in the P. nodorum-wheat pathosystem. One of the NE-host sensitivity gene interactions, SnTox6-Snn6 interaction was characterized in this study. The SnTox6-Snn6 interaction was shown to be light dependent and Snn6 was located to a major disease susceptibility QTL on wheat chromosome 6A. SnTox1, another NE first identified in our lab, interacts with the corresponding wheat sensitivity gene Snn1. SnTox1 was further characterized in this study. The SnTox1 protein harbors C-terminal domains with a high degree of structural homology to plant chitin binding proteins and was subsequently shown to bind chitin, a main component of the fungal cell wall. Therefore, SnTox1 was hypothesized to compete with wheat chitinases to bind chitin, preventing fungal cell wall degradation. To investigate this hypothesis, the SnTox1 binding affinity with chitin was tested, as well as its potential function in the protection against chitinases during fungal mycelial growth. To identify additional NE regions, genome wide association study (GWAS) technology was used. A global collection of 191 P. nodorum isolates were genotyped using a restriction-site associated DNA genotyping by sequencing (RAD-GBS) protocol to identify SNP markers. Phenotypic data including fungal inoculations and culture filtrate infiltrations were collected using 191 P. nodorum isolates across several wheat lines. GWAS analyses were performed by regressing the phenotypic data and genotypic data by running multiple GWAS models.