Characterization of Mi-1-mediated defense responses in tomato to root-knot nematodes and potato aphids

Mi-1 is a unique resistance (R) gene in tomato (Solanum lycopersicum) that confers resistance to animals from distinct taxa, root-knot nematodes (Meloidogyne sp.) and potato aphids (Macrosiphum euphorbiae). Gene expression and functional genomics approaches were used to identify the components of Mi-1-mediated defense to nematodes and aphids. In this process, genes that are important for a basal defense pathway were also identified and characterized. Gene expression studies showed that both root-knot nematodes and aphids up-regulate genes from the JA signaling pathways during both compatible and incompatible interactions, suggesting a role for this pathway in basal as well as Mi-1-mediated defense in tomato. Using a tomato mutant that is defective in JA perception, jasmonic acid insensitive 1 (jai1 ), it was found that though nematode and aphid feeding regulated JA-dependent responses in tomato, this response is not required for Mi-1-mediated defense. A role for JA in basal defense to aphid was identified in a choice assay, while JA did not play a role in aphid survival and fecundity in a no-choice assay. In addition, an intact JA signaling pathway is required for tomato susceptibility to the nematode, as the jai1 mutants were less susceptible than the wild-type plants. This requirement of JA signaling pathway for susceptibility to nematodes was not associated with JA biosynthesis and SA accumulation. Microarray analyses of tomato roots 24 hours after root-knot nematodes infection identified two WRKY transcription factors orthologous to the Arabidopsis thaliana AtWRKY72, to be up-regulated. Functional characterization in tomato by virus-induced gene silencing implicated a role for these WRKYs in both Mi-1-mediated and basal defenses to root-knot nematodes and aphids as well as in basal defense to Pseudomonas syringae pv. tomato. In addition, functional characterization of the Arabidopsis AtWRKY72 showed that this gene is also involved in a basal defense to root-knot nematodes and the oomycetes, Hyaloperonospora parasitica arabidopsis strain Noco2, but not to R-gene-mediated defenses, indicating a conserved basal defense signaling pathway between tomato and Arabidopsis. Gene expression profiling in Arabidopsis identified a group of genes that are regulated by WRKY72 independent of previously characterized defense pathways.