Identification of endophytic Bacillus velezensis ZSY-1 strain and antifungal activity of its volatile compounds against Alternaria solani and Botrytis cinerea

Soil microbes can release volatile organic compounds (VOCs) that influence plant growth and pathogen resistance. Screening these microbial VOCs for their antifungal activity is a new strategy for developing biopesticides. However, the ability of endophytic bacteria to synthesize VOCs is understudied. Here, we characterized VOCs produced by Bacillus velezensis ZSY-1, and tested their antifungal activity against plant pathogenic fungi. Strain ZSY-1, isolated from Chinese catalpa, was identified as B. velezensis, based on sequences of its 16S rRNA, gyrA, and gyrB genes. Bioassay of VOCs from ZSY-1 was conducted using the two-sealed base-plates method. Volatile organic compounds from ZSY-1 exhibited significant antifungal activity against Alternaria solani, Botrytis cinerea, Valsa mali, Monilinia fructicola, Fusarium oxysporum f. sp. capsicum, and Colletotrichum lindemuthianum; the inhibition rates were 81.1%, 93.8%, 83.2%, 80.9%, 76.7%, and 70.6%, respectively. Gas chromatography–mass spectrometry of solid phase microextraction samples (SPME-GC–MS) revealed 29 unique VOCs released by strain ZSY-1. Among these compounds, 2-tridecanone, pyrazine (2,5-dimethyl), benzothiazole, and phenol (4-chloro-3-methyl) had much higher peak areas than the main compound types. However, the most common VOC types were ketones (seven unique compounds), alcohols (six), and alkanes (six). Pyrazine (2,5-dimethyl), benzothiazole, 4-chloro-3-methyl, and phenol-2,4-bis (1,1-dimethylethyl) had significant antifungal activity against A. solani and B. cinerea. In fact, we report for the first time that phenol (4-chloro-3-methyl) synthesized by B. velezensis can inhibit the growth of plant pathogenic fungi. Based on our study of antifungal activity, pyrazine (2,5-dimethyl), benzothiazole, phenol (4-chloro-3-methyl), and phenol-2,4-bis (1,1-dimethylethyl) are promising bioagents for controlling tomato fungal diseases such as early blight and grey mold.