Potential of Lactic Acid Bacteria as Plant-Growth-Promoting Bioinoculant Candidates with In Vitro Antifungal Activity: Insights from Brassica napus as a Model System

Lactic acid bacteria (LAB) are promising microbial candidates for sustainable agriculture, but their combined plant-growth-promoting and biocontrol potential remains underexplored. In this study, 32 previously isolated LAB strains were evaluated for in vitro antifungal activity against four phytopathogens, plant-growth-promoting traits, antimicrobial biosynthetic markers, safety attributes, and in vivo effects on Brassica napus. Antagonistic activity was strong but strain-dependent, with the greatest inhibition observed against Pythium heterothallicum and several isolates showing broad-spectrum activity against Fusarium equiseti, Geotrichum citri-aurantii, and Botrytis cinerea. Functional screening revealed complementary traits, including phosphate solubilization, acid phosphatase and cellulase activities, and the production of ammonia, siderophores, and hydrogen cyanide. PCR screening detected putative markers associated with iturin and subtilosin biosynthesis in a subset of strains. No hemolytic or DNase activity was detected, and no broadly concerning antibiotic resistance profiles were observed. No strain consistently outperformed all others across inoculum densities, highlighting a nonlinear dose–response relationship and the need for strain-specific inoculum optimization. Overall, several LAB strains emerged as promising dual-function bioinoculant candidates.