Tomato Root Colonization by Exogenously Inoculated Arbuscular Mycorrhizal Fungi Induces Resistance against Root-Knot Nematodes in a Dose-Dependent Manner

Arbuscular mycorrhizal fungi (AMF) are generally recognized to induce plant growth and prime plants against soil-borne parasites, such as plant parasitic nematodes. However, the effectiveness of commercial formulates containing AMF has been questioned. Increasing amounts per plant of one commercial AMF-containing formulate, reported in the text as Myco, were used to detect the effects on growth of tomato plants and the resistance induced against root-knot nematodes (RKNs) The doses used per plant (0.5, 1.0, 2.0 g, reported as Myco1, Myco2, Myco3, respectively) were soil-drenched to growing potted plants; the effects of such treatments were analyzed both in plants not inoculated or inoculated by Meloidogyne incognita juveniles. Consistent increases in plant weight were apparent as soon as 7 days only after Myco2 treatments. Moreover, only treatments with Myco2 induced a consistent repression of the nematode infection observed in untreated plants. Conversely, treatments with Myco1 and Myco3 did not produce such an early growth improvement; some plant weight increase was observable only at 28 dpt. Accordingly, such Myco doses did not restrict the level of infestation observed in untreated plants. Control of infection was dependent on the dose of Myco provided to plants five days before nematode inoculation. About one month after all Myco treatments, several areas of roots were found to be colonized by AMF, although in Myco2-treated plants, three genes involved in the AMF colonization process (SlCCaMK, SlLYK9, and SlLYK13) were found to be over-expressed already at 7 dpt; over-expression was generally less consistent at 14 and 21 dpt. The expressions of two key genes of plant defense, the hypersensitive cell death inducer PR4b gene and the glutathione peroxidase-encoding GPX gene, were monitored in roots of Myco2-treated plants 3 and 7 days after nematode inoculation. PR4b was over-expressed and GPX was silenced in treated plants with respect to untreated plants. The repressive effect of Myco2 treatment against RKN infection was completely abolished when Myco2 suspensions were autoclaved to sterilization or treated with the potent anti-fungal agent amphotericin B, thus indicating that the biological control agents contained in the commercial formulate were living fungi.