Microbial chitinases in the plant holobiont: ecological roles and structural determinants.

Complex intra- and inter-kingdom interactions shape microbiota stability and influence the success of microbial invasion into the plant holobiont. Microbial chitinases, enzymes central to fungal physiology and nutrient acquisition, can also function as secreted effector-like proteins that modulate community dynamics by restricting access to occupied ecological niches, such as plant roots, or by contributing to the destabilization of resident consortia. Beyond these antimicrobial functions, microbial chitinases can facilitate plant colonization by interfering with chitin-triggered immunity, suggesting that chitinase diversification has accompanied microbial adaptation to distinct host-associated niches. In this review, we examine microbial chitinase functions in ecological contexts and discuss how structural variation can tune chitinolytic activity, substrate engagement, and biological function. We argue that the ecological roles of microbial chitinases cannot be inferred from catalytic activity alone, but emerge from the interplay between enzyme architecture, substrate accessibility, synergistic enzyme activities, regulated spatiotemporal expression, and the microbial or host context in which chitin is encountered.