Identification of Lipopeptide Iturin A Produced by Bacillus amyloliquefaciens NCPSJ7 and Its Antifungal Activities against Fusarium oxysporum f. sp. niveum

Bacillus amyloliquefaciens NCPSJ7 showed potential fungicidal activities for the effective control of fungal infection. From the PCR test, the key genes (srfAA, sfp, fenD, bmyB, ituD, and ituC) were detected in B. amyloliquefaciens NCPSJ7. These genes were closely related to the lipopeptides (LPs) synthesis. Next, three LPs families were identified with liquid chromatography-mass spectrometry (LC/MS), including iturin A, fengycin A, and surfactin. After purification with C18, the main active antifungal compound was proven to be C14-iturin A by ESI-HRMS, which has significant activities against fungi. These results proved that C14-iturin A played an important role in inhibiting the growth of fungi for B. amyloliquefaciens NCPSJ7. Furthermore, the isolated LP could inhibit mycelial growth and conidia germination at 30 μg/mL. SEM allowed us to observe that mycelial morphology and conidia germination were also affected. The mycelial ultrastructure TEM observations showed that the external electron-dense outer layer cell wall, which mainly consisted of glycoproteins, was affected. Furthermore, swollen mitochondria, enriched glycogen, and increased vacuoles were also found. LP also affected the intact wall and membranes, leading to their increased permeability, which was proved by propidium iodide (PI) staining and conductivity measurements. Meanwhile, the ergosterol, which has an affinity for iturin A, also increased. These results indicated that LP caused fungal dysfunction and membrane permeability increase, leading to fungal inhibition. Identifying and studying LPs is important in exploring the fungicidal activities of B. amyloliquefaciens, which promotes the use of B. amyloliquefaciens NCPSJ7 as a potential candidate for biocontrol.