LAUSR

Plants are naturally associated with a great diversity of fungi. Plant-colonizing fungi are considered to have evolved from saprotrophic fungi via the acquisition of a capacity to colonize plant tissues and to obtain nutrients from the host. A transition from saprotrophic to plant-associated life forms provided a new niche without high competition among fungi. However, once fungi began to associate with plants, they had to contend with plant defense responses that aim to detect and eliminate anything that is “non-self”. To overcome or evade plant defense responses, parasitic fungi had to develop diverse infection strategies. Instead of a parasitic habit, some fungi had to adopt a mutualistic habit that provided benefits to hosts in exchange for benefits from the hosts. It is crucial to note that the degree of plant responses to such parasitic or mutualist fungi would vary depending on the environmental and host conditions (Hiruma et al. 2018). However, since most studies on plant–microbe interactions are done in relatively stable laboratory conditions, little is known regarding the factors influencing fungal infection strategies in a changing environment. Colletotrichum, an ascomycete genus with numerous species, is one of the most economically important groups of plant pathogens. Colletotrichum species cause anthracnose diseases in a wide range of economically important crops (Kubo and Takano 2013). Notably, various Colletotrichum species have reportedly been isolated from healthy plants after surface disinfection, suggesting that the fungi are endophytes that colonize plant tissues without disease symptoms (García et al. 2013). Therefore, species of Colletotrichum provide key fungal strain resources for the study of diverse infection strategies ranging from parasitic to potentially mutualistic strategies. Here, I summarize recent findings on the unique infection strategies of both parasitic and mutualistic Colletotrichum fungi during colonization in Arabidopsis thaliana.