Resistance to Plant Parasites in Tomato Is Induced by Soil Enrichment with Specific Bacterial and Fungal Rhizosphere Microbiome

Commercial formulations of beneficial microbes have been used to enrich the rhizospheremicrobiome of tomato plants grown in pots located in a glasshouse. These plants have been subjectedto attacks by soil-borne parasites, such as root-knot nematodes (RKNs), and herbivores, such as theminer insect Tuta absoluta. The development of both parasites and the symptoms of their parasitismwere restricted in these plants with respect to plants left untreated. A mixture, named in the textas Myco, containing plant growth-promoting rhizobacteria (PGPR), opportunistic biocontrol fungi(BCF), and arbuscular mycorrhizal fungi (AMF) was more effective in limiting pest damage than aformulation containing the sole AMF (Ozor). Therefore, Myco-treated plants inoculated with RKNswere taken as a model for further studies. The PGPR contained in Myco were not able to reducenematode infection; rather, they worsened symptoms in plants compared with those observed inuntreated plants. Therefore, it was argued that both BCF and AMF were the microorganisms thatcolonized roots and stimulated the plant immune system against RKNs. Beneficial fungi colonizedthe roots by lowering the activities of the defense supporting enzymes endochitinases and -1,3-glucanase. However, as early as three days after nematode inoculation, these enzyme activities andthe expression of the encoding pathogenesis-related genes (PR-2, PR-3) were found to be enhanced inroots with respect to non-inoculated plants, thus indicating that plants had been primed against RKNs.The addition of paclobutrazol, which reduces salicylic acid (SA) levels in cells, and diphenyliodoniumchloride, which inhibits superoxide generation, completely abolished the repressive effect of Myco onnematode infection. Inhibitors of copper enzymes and the alternative cyanide-resistant respirationdid not significantly alter resistance induction by Myco. When Myco-treated plants were subjectedto moderate water stress and inoculated with nematodes, they retained numbers of developedindividuals in the roots similar to those present in regularly watered plants, in contrast to whatoccurred in roots of untreated stressed plants that hosted very few individuals because of poornutrient availability.