Fengycin deficiency in Bacillus subtilis Pfen-1 increases antagonism and reveals a non-functional role against Sclerotinia sclerotiorum

Bacillus subtilis is a widely used agricultural probiotic for plant disease control. We previously reported the novel B. subtilis HF1, which produces abundant fengycin and surfactin lipopeptide homologues. This study investigates fengycin's functions in HF1. Surprisingly, the fengycin-deficient mutant Pfen-1 exhibited enhanced antagonism against Sclerotinia sclerotiorum. Using Pfen-1 as an ideal tool to probe fengycin function, we tested lipopeptide extracts against 20 plant pathogenic fungi. Results indicated that fengycin is the primary antifungal lipopeptide in HF1 against most fungi but not against S. sclerotiorum. We elucidated the specific, efficient biocontrol mechanism of Pfen-1 against S. sclerotiorum in rapeseed. Pfen-1 consistently showed higher activity than HF1 in some key enzymes including cellulase, proteinase, chitinase, amylase, and β-1,3-glucanase - across different culture stages, a finding corroborated by transcriptome data. Additionally, Pfen-1 significantly boosted defensive enzyme activity in rapeseed, contributing to its excellent biocontrol efficacy. This work provides new insights into fengycin's functions and offers a promising mutant strain for developing more effective agents against S. sclerotiorum in rapeseed.