Cataloging proteins putatively secreted during the biotrophy-necrotrophy transition of the anthracnose pathogen Colletotrichum truncatum.

Hemibiotrophic phytopathogenic fungi cause devastating diseases in agronomically important crops. These fungal pathogens exploit a stealth bi-phasic infection strategy to colonize host plants. Their morphological and nutritional transition from biotrophy (characterized by voluminous intracellular primary hyphae) to necrotrophy (characterized by thin secondary hyphae) known as the biotrophy-necrotrophy switch (hemibiotrophy) is critical in symptom and disease development. To establish successful hemibiotrophic parasitism, pathogens likely secrete suites of proteins at the switch that constitute the biotrophy-necrotrophy switch secretome. To catalogue such proteins, a directional cDNA library was constructed from mRNA isolated from infected Lens culinaris leaflet tissues displaying the switch of Colletotrichum truncatum, and 5000 expressed sequence tags (ESTs) were generated. Four potential groups (hydrolytic enzymes, cell envelope-associated proteins [CEAPs], candidate effectors and proteins with diverse functions) were identified from pathogen-derived ESTs. Expression profiling of transcripts encoding CEAPs and candidate effectors in an infection time-course revealed that the majority of these transcripts were expressed or induced during the necrotrophic phase and repressed during the biotrophic phase of in planta colonization, indicating the massive accumulation of proteins at the switch. Taken together, our data suggest that the hemibiotrophic mode of fungal proliferation entails complex interactions of a pathogen with its host wherein the pathogen requires live host cells prior to switching to the necrotrophic phase. The microbial proteins employed during pathogenesis are likely to have defined roles at specific stages of pathogenesis.