Multifunctional efficacy of the nodule endophyte Pseudomonas fragi in stimulating tomato immune response against Meloidogyne incognita

Root-knot nematodes severely affect the growth and productivity of agricultural plants in the world. Conversely, endophytic bacteria can be used to prevent plant diseases and increase crop yield. The nodule endophytic bacterium Pseudomonas fragi (Eichholz) Gruber has been reported to promote crop growth, but its role in nematode control is unclear. In this study, the biocontrol effect of P. fragi Sneb1990 against Meloidogyne incognita (Kofoid & White) Chitwood was assessed in tomato ( Solanum lycopersicum L.). In vitro, the fermentation broth of P. fragi Sneb1990 provoked 76.8% juvenile mortality after 48 h and 71.9% egg mortality after 7 d. P. fragi Sneb1990 promoted plant growth and alleviated M. incognita disease symptoms in pot and field experiments. The split-root assay revealed that P. fragi Sneb1990 stimulated local and systemic immunity to suppress the penetration and development of M. incognita and that it enhanced the expression of PR1, PR2, PR3, PR5, and PDF1.2, which participate in the jasmonic acid and salicylic acid pathways. Furthermore, the flagellin polypeptide flg22 of P. fragi Sneb1990 triggered tomato immunity against M. incognita, stimulated reactive oxygen species production, and upregulated the expression of salicylic acid resistance-related genes. This is the first report of the suppression of M. incognita infection by the nodule endophyte P. fragi. In addition, the flagellin functional domain of P. fragi Sneb1990 triggered immune responses against M. incognita in tomato. Overall, our study highlights the potential of P. fragi Sneb1990 as a biocontrol agent against M. incognita.