The root-knot nematode (RKN), Meloidogyne incognita, poses significant challenges for many important crops. Identified distinctive bacterial proteases have emerged as potential nematicides. In this study, the nprE from Bacillus subtilis 168 encoded the neutral metalloprotease, effectively enhancing it through a site-directed mutation. The enzyme-specific activity of pro118, secreted by the modified strain BS118, reached 215.4 U mg^-1. It was twice that of pro168 produced by the wild-type strain BS168, which recorded an enzyme-specific activity of 101.6 U mg^-1. The protease, pro118, caused the highest mortality in M. incognita juveniles, J₂S (91.3% to 84.5%) after 48 h of in vitro treatment. Furthermore, when simultaneously applying pro118 with the nematode infection, it notably decreased the number of J₂S, root galls, and egg masses/root systems by 19.5%, 18.8%, and 26.2%, respectively, compared with its application one week after the nematode infection. This consistent trend also manifested in the plant shoot length and fresh weight enrichment, which showed an increase of 9.7% and 14.7% in the first treatment versus the second. In conclusion, the study asserts that applying neutral metalloprotease as a bioagent to biocontrol M. incognita is a promising approach for mitigating the impact of this agricultural pest.

Keywords: Site-directed mutation, Bacillus subtilis 168, neutral metalloprotease, Meloidogyne incognita, nematicidal agents, growth parameters, eggplant

Key findings: The site-directed mutation, which involves replacing the native promoter with another constitutive one, is a promising approach for constructing strains with higher protease modifications. This method’s subsequent employment as a bioagent against Meloidogyne incognita infection led to significant plant growth and health improvements.