Genomics of Resistance to Mite Herbivores Associated with Drought Stress in Cereals

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Crop losses to pests, including animals like caterpillars or aphids, present a major challenge for sustainable agricultural practices. Among herbivores that feed on plant tissues, insects are by far the best characterized. However, plant-feeding mites, which are distant relatives of insects, can also be major pests in agriculture. In particular, many mite species thrive on and damage drought-stressed plants, including major cereal crops like maize and wheat. Under these situations, mites pose a significant challenge for control as they are often resistant to commonly used pesticides. Plants can defend themselves against attack and damage by herbivores. For example, for a number of plant species, certain varieties are known to be more resistant to mites than others because of their ability to produce compounds toxic to herbivores. The objective of this research is to understand the genetic and genomic nature of plant defense pathways that function to deter agriculturally important mites. The mite species to be examined are the two-spotted spider mite (Tetranychus urticae) and the Banks grass mite (Oligonychus pratensis) that are associated with significant outbreaks on drought-stressed maize or other cereal species. The research will use genomic and genetic methods to elucidate which plant defense pathways in maize (Zea mays ssp. mays) and barley (Hordeum vulgare L.) respond to mite herbivory, and how variation in these pathways impacts plant resistance. Moreover, the effect of drought (water stress) on these defense pathways will be assessed, an objective motivated by the observation that each of these pests can be economically damaging during droughts. This project has relevance, therefore, to efforts to develop more mite resistant crop plants. All data will be accessible to the public through long-term public repositories such as the Gene Expression Omnibus (GEO) and MaizeGDB. Biological resources generated during the project will be available upon request. As part of the project, the work will train undergraduate, graduate and post-graduate scholars. In addition, educational material will be developed on plant-mite interactions for distribution to the public on the Learn.Genetics website (http://learn.genetics.utah.edu/).To identify genetic pathways in plants that respond to spider mite pests, gene expression profiling using the RNA-seq method (high throughput sequencing of cDNA) will be performed on maize and barley leaves infested with both two-spotted spider mites (a generalist pest on both dicot and monocot plants) and Banks grass mites (a specialist pest on grasses). Further, to understand the effect of an agriculturally important abiotic stress on plant defense pathways, expression profiling for mite responses will be performed on both well watered and water stressed plants. Comparisons of plant transcriptional response to the two herbivores, and as a function of an abiotic stress relevant to mite outbreaks, will shed light on the specificity of plant responses to generalist and specialist herbivores, and how the environment affects the interaction in plants. A specific outcome will be the identification of gene regulatory networks underlying mite resistance. Further, susceptibility to mite damage is known to vary among individuals within plant species, including maize. To investigate this, a collection of inbred maize lines used to create the Nested Association Mapping (NAM) community genetic mapping resource will be screened for susceptibility or resistance to each mite herbivore. Gene expression studies with sensitive and resistant maize lines will then be used to assess the genomic basis of variation in plant mite resistance. In parallel, and to relate plant molecular phenotypes of mite response to plant performance upon mite infestation in an agricultural setting, field trials for resistance will be performed on plants under high water and drought stress conditions for a subset of maize inbred lines.