Systemic Resistance to Strawberry Anthracnose Induced by Paenibacillus polymyxa TP3 Requires the Fusaricidin Synthetase Gene fusA

Anthracnose, caused by Colletotrichum spp., is an important fungal disease prevalent in strawberry nurseries and fruit production. Paenibacillus polymyxa, a plant growth-promoting rhizobacterium, effectively inhibits various fungal pathogens and their associated diseases. In this study, a fusaricidin synthetase gene (fusA)-disrupted mutant generated from a strawberry strain TP3 of P. polymyxa was used to demonstrate the requirement of fusA for disease suppression directed by this beneficial bacterium. In contrast to the wild-type strain TP3, this fusA-disrupted mutant was unable to produce fusaricidins but increased biofilm biomass; however, it reduced the inhibition of fungal growth and decreased the suppression of anthracnose symptom development in strawberry. Nevertheless, this fusA-disrupted mutant showed enhanced colonization on strawberry leaves and roots compared with the wild-type strain TP3. A callose deposition assay indicated that P. polymyxa TP3-directed strawberry anthracnose suppression by root-drenching required fusaricidins to enhance plant immunity. This research validates the role of fusaricidins in P. polymyxa TP3-mediated induction of systemic resistance and facilitates the application of P. polymyxa as a health enhancer for sustainable crop production, especially for strawberry crops.