N-glycosylation code diversifies paralogous chitinases to balance plant immunity and growth.

Balancing immunity and growth is a fundamental challenge for plants. Surprisingly, fungal cell-wall chitin elicits robust immune responses without causing growth inhibition, yet constitutive intracellular chitin signalling severely impairs growth. How plants resolve this paradox remains unclear. Here we identify two chitin-inducible, paralogous apoplastic chitinases in Arabidopsis thaliana, MIC1 and MIC2, both possessing direct antifungal activity through chitin hydrolysis. MIC2 generates immunogenic chitin oligomers that activate chitin-triggered immunity, whereas MIC1 ultimately degrades chitin into non-immunogenic dimers and monomers, thereby dampening growth-inhibitory signalling. This functional divergence is dictated by Asn137 glycosylation in MIC1. Removing this N-glycan converts MIC1 into a MIC2-like enzyme. MIC1-like chitinases are widespread in plants, and Oryza sativa Osmic1 mutants exhibit enhanced fungal resistance without growth defects. Our work defines an N-glycosylation code that governs the functional specialization of paralogous chitinases and provides a conserved molecular target for breeding crops with improved fungal resistance and uncompromised growth.