Commercial formulates of beneficial microbes have been used to enrich the rhizosphere microbiome of tomato plants grown in pots located in a glasshouse. These plants have been subjected to attacks by soil-borne parasites, such as root-knot nematodes (RKNs), and herbivores, such as the miner insect Tuta absoluta. The development of both parasites and the symptoms of their parasitism were restricted in these plants with respect to plants left untreated. A mixture, named in the text as Myco, containing plant growth promoting rhizobacteria (PGPR), opportunistic biocontrol fungi (BCF), and arbuscular mycorrhizal fungi (AMF) was more effective in limiting pest damages than a formulate containing the sole AMF (Ozor). Therefore, Myco-treated plants inoculated with RKNs were taken as a model to further study which biocontrol agent (BCA) was specifically responsible of plant immunization and the molecular pathways exploited to achieve such an immunization. The PGPR contained in Myco were not able to reduce nematode infection, rather, they worsened symptoms on plants, compared with those observed on untreated plants. When plants were treated with suspensions of Bacillus subtilis at concentrations similar to those provided with Myco and then inoculated with RKNs, infection was actually more severe than that on untreated plants. Therefore, it was argued that both BCF and AMFwere the microorganisms that, colonizing roots, synergistically stimulate plant immune system against RKNs. Beneficial fungi lowered the activities of the defense supporting enzymes endochitinases and β-1,3-glucanase to be able to colonize the roots. However, as early as three days after nematode inoculation, these enzyme activities, and the expression of the encoding pathogenesis related genes (PR-2, PR-3) were found to be enhanced in roots with respect to not inoculated plants, thus indicating that plants had been primed against RKNs. Addition of paclobutrazol, which reduced salicylic acid (SA) levels in cells, and of diphenyliodonium chloride, which inhibits superoxides generation, completely abolished the repressive effect of Myco on nematode infection and presumably plant immune reaction. Inhibitors of copper enzymes and of the alternative cyanide-resistant respiration did not significantly alter resistance induction by Myco. When Myco-treated plants were subjected to a moderate water stress and inoculated with nematodes, they retained numbers of developed individuals in the roots similar to those present in regularly watered plants, in contrast to what occurred in roots of untreated stressed plants that hosted very few individuals because of poor nutrient availability.

Two wild-type field populations of root-knot nematodes (Mi-Vfield, Mj-TunC2field), and two isolates selected for virulence in laboratory on resistant tomato cultivars (SM2V, SM11C2), were used to induce a resistance reaction in tomato to the soil-borne parasites. Epigenetic and metabolic mechanisms of resistance were detected and compared with those occurring in partially or fully successful infections. The activated epigenetic mechanisms in plant resistance, as opposed to those activated in infected plants, were detected by analysing the methylated status of total DNA, by ELISA methods, and the expression level of key genes involved in the methylation pathway, by qRT-PCR. DNA hypo-methylation and down-regulation of two methyl-transferase genes (CMT2, DRM5), characterized the only true resistant reaction obtained by inoculating the Mi-1.2-carrying resistant tomato cv Rossol with the avirulent field population Mi-Vfield. On the contrary, in the roots into which nematodes were allowed to develop and reproduce, total DNA was generally found to be hyper-methylated and methyl-transferase genes up-loaded. DNA hypo-methylation was considered to be the upstream mechanism that triggers the general gene over-expression observed in plant resistance. Gene silencing induced by nematodes may be obtained through DNA hyper-methylation and methyl-transferase gene activation. Plant resistance is also characterized by an inhibition of the anti-oxidant enzyme system and activation of the defence enzyme chitinase, as opposed to the activation of such a system and inhibition of the defence enzyme glucanase in roots infested by nematodes.