P-300 - A substrate-binding domain defines the dual function of fungal chitinases in antagonism and host immune evasion

Root endophytic fungi must evade host immunity and interact with the surrounding microbiota to establish their niche. Thus, successful proliferation in the rhizosphere and host colonization are preceded and accompanied by multiple cross-kingdom interactions. While the ability to engage in these interactions is encoded in the fungal genome, the underlying molecular mechanisms remain poorly understood. We analyzed the transcriptome of the root endophyte Serendipita indica for genes involved in the antagonism against the pathogen Bipolaris sorokiniana and identified a secreted GH18-CBM5 chitinase, CHIT1, as a promising candidate. The recombinant chitinase exhibited both, antifungal and plant-protective activity against B. sorokiniana. Interestingly, removal of the non-catalytic C-terminal CBM5 domain resulted in a drastic reduction of antifungal and plant-protective activity. In addition, we identified another GH18 chitinase, CHIT2, which is expressed exclusively during plant colonization and dampens chitin-induced ROS generation, aiding immune evasion. Fusion of the CBM5 to CHIT2 activated its antifungal potential, but at the expense of immune suppression. This suggests that a CBM5 domain may act as a switch between two biological functions of fungal chitinases: niche defense against fungal competitors and suppression of plant host immunity to facilitate colonization. This highlights how gains and losses of individual domains drive the evolution of fungal effector functions.