Bio-control agents activate plant immune response and prime susceptible tomato against root-knot nematodes

Beneficial microorganisms are generally known to activate plant defense against biotic challenges. However, the molecular mechanisms by which activated plants react more rapidly and actively to pests remain still largely unclear. Tomato plants pre-treated with a mixture of beneficial bio-control agents (BCAs), as soil-drenches, were less sensitive to infection of the root-knot nematode (RKN) Meloidogyne incognita. To unravel the molecular mechanisms of this induced resistance against RKNs, we used qRT-PCR to monitor the expression, in tomato roots and leaves, of 6 key defense genes. Gene transcripts were detected until the 12(th) day after BCA treatment(3, 7, 8, 12 dpt) and3 and 7 days after nematode inoculation of pre-treated plants. Early after BCA treatment, the salicylic acid (SA)-dependent pathogenesis related gene (PR-gene), PR-1b, marker of the systemic acquired resistance (SAR), was systemically over-expressed. Another PR-gene, PR-5, was over-expressed at later stages of BCA-plant interaction, and only in roots. Activation of defense against RKNs was attested by the early up-regulation of 4 genes (PR-1, PR-3, PR-5, ACO) in pre-treated plants after inoculation. Conversely, the expression of the JA/ET-dependent gene JERF3 did not increase after nematode inoculation in primed plants. A catalase gene (CAT)was highly over-expressed by nematode infection, however, this over-expression was annulled at the earliest stages or limited at the later stages of infection toBCA-treated roots. Enzyme activities, such as glucanase and endochitinase, were enhanced in roots of pre-treated inoculated plants with respect to plants left not inoculated as a control. These findings indicate that BCA interaction with roots primes plants against RKNs. BCA-mediated immunity seems to rely on SA-mediated SAR and to be associated with both the activation of chitinase and glucanase enzyme activities and the inhibition of the plant antioxidant enzyme system. Immunity is triggered at the penetration and movements inside the roots of the invading nematode juveniles but probably acts at the feeding site building stage of nematode infection.