Exploring the utility of Brachypodium distachyon as a model pathosystem for the wheat pathogen Zymoseptoria tritici

Zymoseptoria tritici, the causative organism of Septoria tritici blotch disease is a prevalent biotic stressor of wheat production, exerting substantial economic constraints on farmers, requiring intensive chemical control to protect yields. A hemibiotrophic pathogen with a long asymptomless phase of up to 11 days post inoculation (dpi) before a rapid switch to necrotrophy; a deficit exists in our understanding of the events occurring within the host during the two phases of infection. Brachypodium distachyon has demonstrated its potential as a model species for the investigation of fungal disease resistance in cereal and grass species. The aim of this study was to assess the physical interaction between Z. tritici (strain IPO323) and B. distachyon and examine its potential as a model pathosystem for Z. tritici.

Septoria tritici blotch symptoms developed on the wheat cultivar Riband from 12 dpi with pycnidial formation abundant by 20 dpi. Symptoms on B. distachyon ecotype Bd21-1 were visible from 1 dpi: characteristic pale, water soaked lesions which developed into blotch-like lesions by 4 dpi. These lesions then became necrotic with chlorotic regions expanding up to 7 dpi. Sporulation on B. distachyon tissues was not observed and no evidence of fungal penetration could be obtained, indicating that Z. tritici was unable to complete its life cycle within B. distachyon ecotypes. However, observation of host responses to the Z. tritici strain IPO323 in five B. distachyon ecotypes revealed a variation in resistance responses, ranging from immunity to a chlorotic/necrotic phenotype.

The observed interactions suggest that B. distachyon is an incompatible host for Z. tritici infection, with STB symptom development on B. distachyon comparable to that observed during the early infection stages on the natural host, wheat. However first visible symptoms occurred more rapidly on B. distachyon; from 1 dpi in comparison to 12 dpi in wheat. Consequently, we propose that the interaction between B. distachyon and Z. tritici as observed in this study could serve as a suitable model pathosystem with which to investigate mechanisms underpinning an incompatible host response to Z. tritici.