Antagonistic potential of bacterial endophytes and induction of systemic resistance against collar rot pathogen Sclerotium rolfsii in tomato

A diverse pool of 310 bacterial endophytes isolated from tomato plants growing in the Indo-Gangetic plains of India was evaluated for their antagonistic potential against three soil-borne fungal pathogens causing root rot (Rhizoctonia solani), collar rot (Sclerotium rolfsii) and, fungal wilt (Fusarium oxysporum f. sp. lycopersici). Preliminary screening indicated that most of the isolates could inhibit R. solani and F. oxysporum f. sp. lycopersici to the greater extent but failed to reduce mycelia growth of S. rolfsii by more than 50 percent. The only exceptions were 1PR7a, 2P2, and 2PR9b which were further characterized and evaluated against S. rolfsii. Molecular identification of 15 isolates based on 16s rRNA gene sequence similarity indicated the presence of three genera viz. Bacillus, Lysinibacillus, and Stenotrophomonas. In planta trial in tomato challenged with pathogen S. rolfsii in presence and absence of endophyte inoculation revealed that Bacillus sp. 2P2 showed the highest protection against S. rolfsii. These strains elicited induced systemic resistance of plant and significantly higher activity (p ≤ 0.05) of phenylalanine ammonia lyase, peroxidase, polyphenol oxidase, and ascorbate oxidase indicating the further strengthening of cell wall barrier through lipid peroxidation, cross linking of cell walls, lignifications, suberization and other cell wall strengthening processes. It was further confirmed by confocal scanning laser micrographs of upper collar region. It was evident that the inoculation of endophyte inhibited the colonization and movement of the pathogen. In addition, endophytes upregulated the expression of three pathogenesis-related genes PR1a, PR2a, and PR3, which are responsible for production of glucanases and chitinases contributing to pathogen inhibition. Further, oxidative stress alleviation was evident from decreased superoxide accumulation and enhanced dry matter content. Results of the present study indicated suppressive potential of endophyte Bacillus sp. 2P2 against S. rolfsii which could be useful in collar rot management in the nursery as well as after transplantation.