Nematicidal rhizobacteria with plant growth-promoting traits associated with tomato in root-knot infested polyhouses

Abstract

Background: In polyhouse cultivation of tomato (Solanum lycopersicum) in root-knot (Meloidogyne incognita) infested soils, several accessions remained free of nematode infection. It was hypothesized that such plants that were free of root-knot infection in the selected polyhouses were possibly harboring potent nematode antagonistic microbes. Therefore, attempts were made to isolate root-associated rhizobacteria from nematode infected and uninfected roots of 7 tomato accessions. Eighty morphologically distinct isolates were multiplied in nutrient broth for 48 h to test the presence of nematotoxic metabolites. Laboratory bioassays were conducted using their cell-free filtrates to observe mortality in M. incognita juveniles (J2s) at 24 h. The most potent ones were identified through molecular characterization (16S rDNA) and assayed for plant growth promotion traits, seed germination and seedling vigor and bioefficacy against M. incognita infecting polyhouse tomato. Results: Rhizobacterial densities were estimated from the root samples using 3 media. In all the media, the average numbers of colony forming units (cfu) from uninfected roots were significantly higher than the infected roots. The maximum numbers of colonies were observed in soil plate count agar. The extracellular metabolites caused juvenile mortality in the range of 60.67–100% from isolates associated with nematode uninfected roots of accessions H308, H299 and H266, as compared to isolates from nematode infected roots of accessions H195, BSS99, H178/F4 and H88, which caused mortality in the range of 52.33 to 73.67%. Four isolates (B9, B16, B26 and B31) caused 100% J2 mortality, followed by 13 isolates (B7, B8, B13, B14, B17, B18, B21, B22, B25, B28, B29, B32 and B33) that caused 90–95%, 12 isolates caused 80–85% and 51 isolates caused < 85% J2 mortality, when exposed to bacterial metabolites in cell-free filtrates for 24 h. The isolates B9, B16, B26 and B31 were identified based on 16S rRNA sequence analysis as Bacillus pumilus, B. megaterium, B. subtilis and B. cereus, respectively. These 4 bacterial isolates possessed plant growth promotion traits like production of IAA, ammonia, catalase and chitinase with the ability to solubilize zinc and phosphate. These enhanced tomato seed germination and seedling vigor, and their application in soil resulted in significant increase in root and shoot length of tomato seedlings. Conclusions: In the present investigation, the 21% bacterial colonies (17 isolates) that caused nemato toxicity in the range of 90 to 100% and another 15% (12 isolates) that caused juvenile mortality in the range of 80 to 90% were from uninfected tomato accessions, indicating their role in protecting the plants against root-knot nematode (RKN) infection. The 4 bacterial isolates characterized from uninfected plants possessed high nematicidal potential with plant growth promotion (PGP) traits that enhanced tomato seed germination and seedling vigor index. The isolates can be utilized for root-knot nematode pest management in polyhouse cultivation.