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Abstract
The Mi gene in tomato (Solanum lycopersicum ) confers resistance against some isolates of several pests, including root-knot nematode (Meloidogyne spp.), potato aphid ( Macrosiphum euphorbiae), sweetpotato whitefly (Bemisia tabaci ), and psyllid (Bactericerca cockerelli). The molecular mode in which Mi acts to achieve reductions in aphid populations is only beginning to emerge in recent literature. We have designed a microarray study to compare transcriptional responses of tomato after challenge by an avirulent potato aphid isolate at four different time points (6h, 12h, 24h, and 48h) in order to identify genes involved in the Mi-mediated resistance cascade and temporal differences in their expression. We identified 226 genes which are differentially regulated in the susceptible response to aphid challenge and 157 genes in the resistant response, of which only 79 genes overlapped between treatments. The genes identified belong to functional categories that include responses to oxidative stress, ethylene biosynthesis, cell wall synthesis and modification, and pathogenesis-related genes. We identified one gene, a WRKY transcription factor (WRKY3) that is up-regulated in response to aphids at 6h in the resistant plants but not until 24h in the susceptible plants, indicating our ability to detect early signaling responses to aphid feeding, and that Mi-mediated resistance has a signaling component that is involved in early recognition of aphids. The early responses and higher magnitude of expression identified in response to aphid feeding in the resistant genotype indicates that the overlap with the susceptible genotype may be due to temporal and quantitative, rather than qualitative, differences in expression.
A second goal of this research was to identify aphids that differ in virulence on resistant plants carrying the Mi gene. We conducted bioassays with two aphid isolates (WU11 and WU12) to determine the affect of Mi-mediated resistance on population growth, feeding rates, survival, fecundity, juvenile development and mortality, and host-preference of the isolates. Mi-mediated resistance reduces aphid populations of both aphid isolates, but WU12 populations are reduced to a lesser extent than populations of WU11. An antixenotic component of Mi-mediated resistance deters the WU11 isolates from reaching the phloem, ultimately leading to starvation of this isolate. Feeding assays did not indicate an antixenotic component of Mi-mediated resistance for the WU12 isolates, but a host-preference assay indicated a mild effect of antixenosis that deters these isolates from settling on resistant plants when given a choice. Life table analysis revealed an antibiotic component of Mi-mediated resistance that affects WU12 juveniles by extending juvenile development times and increasing juvenile mortality on resistant plants. Additionally, plant symptom development assays indicated that Mi-mediated resistance does not confer tolerance to resistant plants.
