Coping with stress: morphological changes in response to environmental stimuli in a fungal plant pathogen

During their life cycles, pathogens have to adapt to many biotic and abiotic environmental stresses to maximize their overall fitness. Morphological transitions are one of the least understood of the many strategies employed by fungal plant pathogens to adapt to constantly changing environments, even though different morphotypes may play important biological roles. We characterized the responses of the wheat pathogen Zymoseptoria tritici to a series of environmental stresses in order to understand the effects of changing environments on fungal morphology and adaptation. We found that all tested stresses induced morphological changes, but different responses were found among four different strains. A transcription analysis showed that morphogenesis and virulence factors are co-regulated. We discovered that Z. tritici forms chlamydospores and demonstrated that these structures are better able to survive extreme cold, heat and drought than other cell types. We also show that blastospores (the “yeast-like” form of the pathogen typically found only in laboratory conditions) can form from germinated pycnidiospores on the surface of wheat leaves, suggesting that this morphotype can play an important role in the natural history of Z. tritici . Our findings illustrate how changing environmental conditions can affect cellular morphology and lead to the formation of new morphotypes, with each morphotype having a potential impact on both pathogen survival and disease epidemiology.