Systemic Resistance in Tomato Induced by Biocontrol Bacteria Against the Root-Knot Nematode, Meloidogyne javanica is Independent of Salicylic Acid Production

Salicylic acid (SA)-mediated induction of systemic resistance by Pseudomonas aeruginosa strain 7NSK2 and P. fluorescens strain CHA0 against soil-borne fungi and viruses have been reported. The role of SA biosynthesis in the enhancement of defence mechanism against plant-parasitic nematodes by these bacterial strains in tomato is not known. To better understand the importance of SA in rhizobacteria-mediated suppression of root-knot nematodes, biocontrol potential of SA-negative or SA-overproducing mutants against Meloidogyne javanica was evaluated with their respective wild type counter parts. Culture supernatant of 7NSK2, CHA0 and their respective mutants caused significant mortality of M. javanica juveniles in vitro. SA deletion in 7NSK2 and SA overproduction in CHA0 did not influence bacterial efficacy to cause nematode deaths. Similarly, culture supernatants resulting from King’s B liquid medium amended with FeCl3 did not influence nematicidal activity of the bacterial strains. Strain CHA0 induced juvenile deaths more than 7NSK2 did. In pot experiments, the bacterial strains applied in unsterilized sandy loam soil markedly reduced final nematode population densities in roots and subsequent root-knot infection in tomato seedlings. SA-negative or overproducing derivatives prevented tomato roots in kinetics similar to those with their respective wild types. When soil iron concentration was lowered by the addition of ethylenediamine di(o-hydroxyphenylacetic acid), nematode biocontrol by the bacterial strains (both wild type and mutants) remained unaltered. To understand the mechanism involved in rhizobacteria-mediated suppression of root-knot nematode in tomato, bacterial performance was assessed in a split root trial in which one-half of the root system was treated with bacterium while the other inoculated with nematode. Compared with the controls, application of the bacterial cell suspension to one-half of the root system lowered the populations of root-knot nematode in non-bacterized nematode-treated sections indicating enhanced defence in the non-bacterized half. With respect to nematode infection, mutants induced systemic resistance to a similar extent as that caused by the wild types in both wild type tomato and NahG tomato plants. It is concluded that fluorescent pseudomonads induce systemic resistance against root-knot nematode via a signal transduction pathway, which is independent of SA accumulation in roots