Pleomorphism in Zymoseptoria tritici: adaptation and response to environmental stimuli

Adaptation and invasion of host tissue are enabled by switching between different cellular morphologies in some pathogenic fungi. This switch may be triggered by different environmental stimuli. Although the mechanisms involved in morphological changes are well known for dimorphic human pathogens, only few studies have investigated these phenomena in plant pathogens. Generally, fungal plant pathogens grow as filamentous over and inside the plant tissue to cause disease. However, the ability of some fungal species to switch between one or more morphologies, the biological function of the different cellular morphologies, and the genetic basis controlling it, are understudied. Here we investigated the responsiveness of the devastating wheat pathogen Zymoseptoria tritici (Zt) to environmental signals controlling growth form transition. Furthermore, we used a RNA-Seq approach to identify candidate genes involved in the dimorphism. All the seven tested stimuli affected the cellular morphology in the four tested Swiss Zt isolates. We focus here on the effects of carbon deprivation and high temperature stress (at 27°C). Carbon starvation induces a fast response with blastospores rapidly switching to hyphal growth within a few hours after the application of the stress. After 96 hours, ~60% of the cells presented hyphal growth. High temperature also promoted a rapid morphological response with hyphal induction after four hours of incubation. Strikingly, we also observed the formation of structures similar to chlamydospores and pseudohyphae under this condition. These structures were never before described in Zt, but are reported in other pathogenic fungi. Specially the chlamydospores are reported as long-term survival structures helping to escape from harsh environments. The viability, resistance, and pathogenicity of chlamydospore-like cells will be investigated further. We provide evidences of the sensible regulatory mechanisms by which Zt isolates detect changes in the environment and respond to these changes by promoting a diversity of cellular morphologies, according to the sensed stimuli.