The Cysteine Rich Necrotrophic Effector SnTox1 Produced by Stagonospora nodorum Triggers Susceptibility of Wheat Lines Harboring Snn1

Abstract

The wheat pathogen Stagonospora nodorum produces multiple necrotrophic effectors (also called host-selective toxins) that promote disease by interacting with corresponding host sensitivity gene products. SnTox1 was the first necrotrophic effector identified in S. nodorum, and was shown to induce necrosis on wheat lines carrying Snn1. Here, we report the molecular cloning and validation of SnTox1 as well as the preliminary characterization of the mechanism underlying the SnTox1-Snn1 interaction which leads to susceptibility. SnTox1 was identified using bioinformatics tools and verified by heterologous expression in Pichia pastoris. SnTox1 encodes a 117 amino acid protein with the first 17 amino acids predicted as a signal peptide, and strikingly, the mature protein contains 16 cysteine residues, a common feature for some avirulence effectors. The transformation of SnTox1 into an avirulent S. nodorum isolate was sufficient to make the strain pathogenic. Additionally, the deletion of SnTox1 in virulent isolates rendered the SnTox1 mutated strains avirulent on the Snn1 differential wheat line. SnTox1 was present in 85% of a global collection of S. nodorum isolates. We identified a total of 11 protein isoforms and found evidence for strong diversifying selection operating on SnTox1. The SnTox1-Snn1 interaction results in an oxidative burst, DNA laddering, and pathogenesis related (PR) gene expression, all hallmarks of a defense response. In the absence of light, the development of SnTox1-induced necrosis and disease symptoms were completely blocked. By comparing the infection processes of a GFP-tagged avirulent isolate and the same isolate transformed with SnTox1, we conclude that SnTox1 may play a critical role during fungal penetration. This research further demonstrates that necrotrophic fungal pathogens utilize small effector proteins to exploit plant resistance pathways for their colonization, which provides important insights into the molecular basis of the wheat-S. nodorum interaction, an emerging model for necrotrophic pathosystems.

Proximal Sensing of Plant-Pathogen Interactions in Spring Barley with Three Fluorescence Techniques

“…Moreover, minor changes in the green fluorescence intensity of the resistant varieties suggest the accumulation of pathogen or resistance specific compounds, such as lignin, and/or the production of waxes, affecting the fluorescence emission [24–28]. To this point our findings confirm and support previous studies which focussed on the process of fungal influencing the autofluorescence of leaves [29], even if we did not record the fluorescence of single cells. A completely different trend was shown in rust inoculated leaves which exhibited a significantly lower green fluorescence as their respective control leaves.…”

Evaluation and Association Mapping of Resistance to Tan Spot and Stagonospora Nodorum Blotch in Adapted Winter Wheat Germplasm

“…The four NE genes ToxA, ToxB, SnTox1, and SnTox3 have been cloned from the two fungal pathogens and heterologously expressed in the Pichia pastoris yeast strain x33 (Abeysekara et al 2010;Liu et al 2009Liu et al , 2012. The yeast strains expressing individual NE genes were grown in yeast potato dextrose broth (Liu et al 2012) for 24 to 48 h at 30°C with agitated shaking. The culture was then spun down to collect the supernatant for infiltration.…”

Roles and delivery mechanisms of fungal effectors during infection development: common threads and new directions

“…Necrotrophic pathogens turn ETI to their advantage; as per their lifestyle, they require dead cells as food sources and an ETI-like reaction that results in dead host cells provides them with the proper infection court. The wheat pathogen, Stagonospora nodorum, requires the small, secreted necrotrophic effector SnTox1 for full virulence [34]. This gene interacts with the wheat resistance gene product Snn1 and leads to all the hallmarks of a hypersensitive response, including cell death, DNA laddering and defense gene expression (Figure 1).…”