Fungal and bacterial-derived CAZymes targeting fungal and oomycete cell walls for biological control of crop diseases.

Cell wall-degrading Carbohydrate-Active enZymes (CAZymes) have substantial potential for biological control of crop diseases by targeting the cell walls of filamentous plant pathogens (fungi and oomycetes) and could substitute for environmentally hazardous pesticides. The cell walls of fungi and oomycetes share common components but also differ in important ways, and there have been considerable recent advances in understanding dynamic structural differences across fungal species. The damage caused to cell walls by CAZymes and the potential of CAZymes in biocontrol are reviewed using studies with purified CAZymes and genetic studies in which CAZyme mutant strains have been tested for altered antagonism and biocontrol. The primary scope encompasses fungal mycoparasite CAZymes (e.g., β-1,3-glucanase, β-1,6-glucanase, α-1,3-glucanase, chitinase, cellulase, and carbohydrate-binding modules), and the secondary, more selective, scope includes bacterial and plant-derived CAZymes that offered novel insights into cell wall damage or facilitated comparisons with fungal-derived CAZymes. Mycoparasite regulation of cell wall-degrading CAZymes is complex, with few transcription factors and small-molecule inducers well understood. Plant pathogen responses to mycoparasite cell wall-degrading CAZymes involve compensatory changes mediated via cell wall integrity signaling. We discuss strategies to improve the effectiveness of CAZymes in antagonism and biocontrol, and ways to engineer a hyper-mycoparasite alongside risk assessments of adverse or unintended effects. These strategies partly depend on bridging knowledge gaps in the cell wall structures of oomycetes and fungal plant pathogens, and in how both mycoparasites and plant pathogens protect their own cell walls from the damaging effects of cell wall-degrading CAZymes.