Bacillus subtilis MBI600 Promotes Growth of Tomato Plants and Induces Systemic Resistance Contributing to the Control of Soilborne Pathogens

Bacillus subtilis MBI600 (Bs MBI600) is a recently commercialized plant-growth-promoting rhizobacterium (PGPR). In this study, we investigated the effects of Bs MBI600 on the growth of tomato and its biocontrol efficacy against three main soilborne tomato pathogens (Rhizoctonia solani, Pythium ultimum, and Fusarium oxysporum f.sp. radicis-lycopersici-Forl). Furthermore, the root colonization ability of the Bs MBI600 strain on tomato roots was analyzed in vivo with a yellow fluorescence protein (yfp)-labeled strain, revealing strong colonization ability, which was affected by the root growth substrate. The application of Bs MBI600 on tomato plants resulted in significant increases in shoot and root lengths. Transcriptional activation of two auxin-related genes (SiPin6 and SiLax4) was observed. Single applications of Bs MBI600 on inoculated tomato plants with pathogens revealed satisfactory control efficacy compared to chemical treatment. Transcriptomic analysis of defense-related genes used as markers of the salicylic acid (SA) signaling pathway (PR-1A and GLUA) or jasmonic acid/ethylene (JA/ET) signaling pathway (CHI3, LOXD, and PAL) showed increased transcription patterns in tomato plants treated with Bs MBI600 or Forl. These results indicate the biochemical and molecular mechanisms that are activated after the application of Bs MBI600 on tomato plants and suggest that induction of systemic resistance (ISR) occurred.