Sugar Tags on Plant Enzymes Decide Between Fungal Defense and Growth

Plants face a dilemma that has puzzled immunologists for decades: the very molecule that alerts them to fungal invasion can also sabotage their own development. Chitin, a structural polymer woven into the cell walls of nearly all fungi, is a reliable danger signal for plants. When plant receptors detect chitin fragments, they launch a battery of defensive responses. Yet when chitin signalling is activated constitutively, inside the cell and without an actual pathogen present, plants stop growing. How plants manage to enjoy the protective benefits of chitin perception while avoiding its growth-suppressing costs has remained one of the unresolved paradoxes of plant immunity. A new study published in Nature Plants offers a strikingly elegant answer, and it hinges on a single sugar molecule attached to a single amino acid.

A team led by Jian-Feng Li at Sun Yat-sen University in Guangzhou, working with collaborators at South China Agricultural University, the Chinese University of Hong Kong and the University of Macau, identified two paralogous chitinases in the model plant Arabidopsis thaliana that they named MIC1 and MIC2. Both genes are induced by chitin and by other microbe-associated molecular patterns, including the bacterial flagellin-derived peptide flg22 and the elongation factor Tu-derived peptide elf18. Both proteins are secreted into the apoplast, the fluid-filled space outside the plant cell membrane where fungal contact first occurs, and both possess direct antifungal activity in vitro, breaking down chitin in the fungal cell wall itself. On the surface, the two enzymes look like redundant copies of the same tool.

The resemblance is deceptive. When the researchers generated CRISPR/Cas9 knockout mutants, the two paralogs pulled in opposite directions. Plants lacking MIC1 became markedly more resistant to the grey mould pathogen Botrytis cinerea and to the vascular wilt fungus Verticillium dahliae, while plants lacking MIC2 became more susceptible. The team also showed that the single mutants and the mic1 mic2 double mutant grow normally under standard conditions, which means the antagonistic effects of these enzymes are specific to immune outcomes rather than general developmental defects. The picture that emerged is of two enzymes with the same substrate but opposing roles in tuning the immune response.

The key to this antagonism lies in the products of chitin digestion. Chitin is a polymer of N-acetylglucosamine units, conventionally abbreviated NAG, and the length of the oligomers released by hydrolysis determines their biological meaning. Using high-performance liquid chromatography to track the fate of defined chitin oligomers, the researchers found that MIC2 cleaves long chitin chains into intermediate fragments, principally NAG4 to NAG6, which are precisely the sizes that plant chitin receptors recognize as immunogenic elicitors. These fragments activate mitogen-activated protein kinase signalling, a canonical early readout of chitin-triggered immunity, and pretreatment with NAG4 primed Arabidopsis leaves for enhanced resistance against B. cinerea. MIC2, in other words, is an alarm bell: it chews the fungal wall in a way that generates the molecular signals that rally the plant’s defenses.

MIC1 does the opposite. Rather than stopping at immunogenic intermediates, MIC1 processes chitin all the way down to NAG2 dimers and NAG1 monomers, fragments that are too short to activate receptor signalling. In seedling assays, exogenous chitin normally inhibits root growth, a visible manifestation of constitutive chitin signalling, and MIC1 is the enzyme responsible for preventing this self-inflicted damage. When the researchers tested additional Arabidopsis chitinases from other clades, they found that the MIC1-like degradation pattern, exemplified by the clade IV enzyme chitIV1, is functionally conserved, and that deleting whole clusters of MIC1-like genes reproduced the chitin-hypersensitive growth phenotype. MIC1 and its relatives therefore act as a molecular off-switch, extinguishing the growth-inhibitory signal once the immune alarm has sounded.

What makes one enzyme stop at immunogenic fragments while its near-identical paralog grinds chitin to dust? The answer, uncovered through a combination of mass spectrometry, PNGase digestion assays and site-directed mutagenesis, is N-glycosylation, the attachment of sugar chains to asparagine residues of secreted proteins. Both MIC1 and MIC2 carry N-glycans, and these modifications are not decorative: glycosylation-deficient variants of either protein failed to be secreted efficiently into the apoplast, and a glycosylation-deficient MIC2 could no longer complement the enhanced susceptibility of mic1 mic2 double mutants to B. cinerea. Glycosylation is thus required for these enzymes to reach their site of action at all.

But one glycan does far more than ensure secretion. Mass spectrometry mapped three N-glycosylation sites on MIC1 and two on MIC2, and the decisive difference came down to a single residue: asparagine 137 in MIC1. When the researchers swapped variable segments between the two proteins to create chimeric enzymes, the exchange that transferred the MIC1 region containing Asn137 into MIC2 converted MIC2 into a MIC1-like enzyme, digesting NAG7 into more dimers and monomers and fewer immunogenic pentamers and tetramers. Conversely, the corresponding chimeric version of MIC1 lost its characteristic band shift and behaved more like MIC2. In complementation tests, the chimeric MIC2 carrying the MIC1 glycosylation region partially suppressed chitin-induced growth inhibition, whereas wild-type MIC2 did not, and it slightly increased susceptibility to B. cinerea instead of restoring resistance. Removing the Asn137 glycan from MIC1 converted it into a MIC2-like enzyme, demonstrating that this one sugar attachment dictates the chitinolytic specificity of the entire protein.

Structural modelling with AlphaFold showed that MIC1 and MIC2 are highly similar in sequence and fold, with their catalytic glutamates aligned and their glycosylation sites positioned on surface loops, including the Loop III region implicated in substrate handling in GH19 family chitinases. The Asn137 glycan apparently reshapes the local architecture of the catalytic cleft in a way that changes how many NAG units the enzyme removes per cleavage event. This is a remarkable example of post-translational modification acting as a functional diversification code: two genes born of duplication, nearly interchangeable in sequence, are sorted into opposing immune roles by a single covalently attached oligosaccharide. The authors describe this as an N-glycosylation code governing the functional specialization of paralogous chitinases.

The implications extend well beyond Arabidopsis. A bioinformatic survey revealed that MIC1-like chitinases are widespread across the plant kingdom, and the Asn137-equivalent residue is highly conserved among MIC1 orthologs from different Arabidopsis ecotypes. The team turned to rice, the world’s most important staple crop, and generated CRISPR knockout mutants of the rice ortholog OsMIC1. The osmic1 mutants showed enhanced resistance to rice blast and to bacterial blight, the latter an intriguing bonus suggesting the mechanism is not strictly antifungal, while their grain length, grain width and thousand-grain weight remained indistinguishable from wild-type plants. Like their Arabidopsis counterparts, however, osmic1 seedlings did exhibit chitin-induced root growth inhibition, confirming that the dampening function of OsMIC1 is conserved.

For breeders, this is the rare kind of result that points directly at a molecular target. Fungal pathogens destroy a substantial fraction of global crop yields each year, and conventional resistance breeding often trades yield for defense. The MIC1/MIC2 system suggests a third path: tune the enzymatic processing of a universal pathogen-derived signal so that immunity is amplified without the growth penalty that constitutive immune signalling imposes. Editing or selecting variants of MIC1-like chitinases, or manipulating the glycosylation machinery that controls them, could in principle produce crops that detect and respond to fungal attack more vigorously while growing as if no pathogen were present. A single sugar on a single asparagine, it turns out, may hold the balance between feast and famine in the plant kingdom.

Subject of Research: N-glycosylation-dependent functional divergence of paralogous plant chitinases balancing immunity and growth

Article Title: N-glycosylation code diversifies paralogous chitinases to balance plant immunity and growth

Article References: Li, C., Lin, X.-X., Tian, L., Sun, W., Liu, J., Liu, Q., Du, Y., Jiang, X., Liang, Z., Deng, Y. Z., Au, S. W. N., Wan, J.-B., & Li, J.-F. (2026). N-glycosylation code diversifies paralogous chitinases to balance plant immunity and growth. Nature Plants. https://doi.org/10.1038/s41477-026-02420-7

Image Credits: AI Generated

DOI: 10.1038/s41477-026-02420-7

Keywords: chitinases, N-glycosylation, plant immunity, chitin signalling, Arabidopsis thaliana, MIC1, MIC2, growth-defense trade-off, fungal resistance, rice breeding, apoplast, CRISPR