Leaf treatments with a protein-based resistance inducer partially modify phyllosphere microbial communities of grapevine

Protein derivatives and carbohydrates can stimulate plant growth, increase stresstolerance, and activate plant defense mechanisms. However, these molecules can alsoact as a nutritional substrate for microbial communities living on the plant phyllosphereand possibly affect their biocontrol activity against pathogens. We investigated themechanisms of action of a protein derivative (nutrient broth, NB) against grapevinedowny mildew, specifically focusing on the effects of foliar treatments on plant defensestimulation and on the composition and biocontrol features of the phyllosphere microbialpopulations. NB reduced downy mildew symptoms and induced the expression ofdefense-related genes in greenhouse- andin vitro-grown plants, indicating the activationof grapevine resistance mechanisms. Furthermore, NB increased the number ofculturable phyllosphere bacteria and altered the composition of bacterial and fungalpopulations on leaves of greenhouse-grown plants. Although, NB-induced changeson microbial populations were affected by the structure of indigenous communitiesoriginally residing on grapevine leaves, degrees of disease reduction and defense genemodulation were consistent among the experiments. Thus, modifications in the structureof phyllosphere populations caused by NB application could partially contribute to downymildew control by competition for space or other biocontrol strategies. Particularly,changes in the abundance of phyllosphere microorganisms may provide a contributionto resistance induction, partially affecting the hormone-mediated signaling pathwaysinvolved. Modifying phyllosphere populations by increasing natural biocontrol agents withthe application of selected nutritional factors can open new opportunities in terms ofsustainable plant protection strategies.