GH25 lysozyme mediates tripartite interkingdom interactions and microbial competition on the plant leaf surface

Microbial communities inhabiting plants have emerged as crucial factors in regulating plant health and defense against disease-causing pathogens. The basidiomycete yeastMoesziomyces bullatusex.AlbugoonArabidopsis(MbA)releases Glycoside Hydrolase 25 (GH25) protein which regulates the leaf microbiome by antagonizing an oomyceteA. laibachiibiotrophic pathogen MbA. Application of bothMbAand GH25 protein rescued fresh shoot weight ofA. thalianauponA. laibachiiinfection, showing its potential in plant protection. Tripartite interaction assays did not reveal antagonistic activity of GH25 towards other plant pathogenic oomycetes or fungi besidesA. laibachii. We identified a core set of bacteria are closely associated withA. laibachiiand established that GH25 inhibits members of this core group. AmongA. laibachii-associated bacteria that were inhibited by GH25,Curtobacterium sp. could override the inhibition ofA. laibachiibyMbA. We describe a tripartite antagonistic interaction in which bacterium and oomycete protect each other from growth inhibition byMbA. Curtobacterium sp., in turn, exhibits specific inhibition ofA. laibachii-associated bacteria that are not targeted byMbAbut themselves antagonizeA. laibachii. Our study reveals an inter-kingdom interaction network in which a GH25 lysozyme shapes the antagonistic relationship between yeast, a pathogenic oomycete and an oomycete-associated bacterium.