Biocontrol of Fusarium wilt disease by endophytic Bacillus xiamenensis BS10 isolated from the root nodules of Brassica nigra (mustard plant) and analysis of defence molecular biomarkers in the greenhouse and field trial

Abstract

Endophytic bacteria possess positive effects on their host plants, including, secondary metabolite induction, disease-resistance improvement, and growth promotion. Bacillus spp. are considered to be an eco-friendly and bio-safe alternative to traditional chemical fungicides. In our study, a collection of 12 endophytic bacteria were isolated from Brassica nigra root nodules and analyzed for plant growth-promoting activities. The endophytic strain BS10 significantly solubilise phosphate (19 ± 1 mm halo zone), siderophore (17 ± 1 mm halo zone), HCN (18 ± 1 halo zone), and indole acetic acid (205.1 ± 2.8 μg/mL) significantly (p < 0.05) than other isolates. The antifungal activity of isolated endophytes was analyzed using a dual-culture screening method. The antifungal activity against Fusarium oxysporum revealed that the strain BS10 exhibited maximum activity against F. oxysporum and the inhibition rate ranged from 20 ± 1 to 70.2 ± 3.5 % after 4 days of treatment and increased after 8 days (36.9 ± 3.5–75.8 ± 3.2 %) among other endophytes. The cell-free extract decreased fungal growth and the inhibition rate varied between 12 % and 47 % (p < 0.05). The results revealed that the fungal infection of the mustard plants with wilt disease causing F. oxysporum significantly reduced the growth of the mustard plant, and root, shoot length, and dry weight compared with the control (p < 0.05). The infected plants treated with endophyte showed an improvement in the growth parameters (p < 0.05). The endophytes-treated plants showed fewer symptoms of rot disease in mustard plants. The chlorophyll content (2.27-fold), soluble leaf protein (1.22-fold), proline (26 ± 2.8 nM/g fresh weight) were improved in the plant leaves treated with endophyte. The biosynthesis of ascorbate peroxidase, phenylalanine ammonia-lyase, and catalase was improved in the endophyte-treated seeds compared with control seeds (p > 0.05). The isolated B. xiamenensis is an important resource to biocontrol Fusarium wilt diseases in the mustard plant.