Mycorrhiza-induced resistance against the root-knot nematode Meloidogyne incognita involves priming of defense gene responses in tomato

Arbuscular mycorrhizal fungi (AMF) have great potential as biocontrol organisms against the root-knot nematode Meloidogyne incognita which causes severe gall formation in plants, but knowledge about the underlying molecular mechanisms involved in the biocontrol of nematodes is scarce. In the present study, suppression subtractive hybridization (SSH) was used to investigate plant genes that are specifically up-regulated in tomato roots (Solanum lycopersicum cv. Marmande) pre-colonized by the AMF Glomus mosseae (BEG 12) and 12 days after soil inoculation with M. incognita juveniles. Nematode infection was significantly lower in the mycorrhizal roots as compared to the non-mycorrhizal roots, and identified genes were classified mainly in the categories of defense, signal transduction and protein synthesis and modification. The higher expression of a selection of defense-related plant genes specifically in the biocontrol interaction compared to in plants that were only mycorrhizal or only nematode-infected was confirmed, which pleads for the existence of mycorrhiza-induced priming of plant defense responses. In conclusion, by focusing on up-regulated gene expression in the biocontrol interaction between mycorrhizal tomato and M. incognita, new insights were found into the molecular mechanisms underlying the mycorrhiza-induced resistance against root-knot nematodes. In particular, the involvement of the phenylpropanoid pathway and reactive oxygen species (ROS) metabolism could explain the reduced root-knot nematode infection in mycorrhizal tomato roots, processes that have also been reported to play a pivotal role in plant resistance to nematodes.