Grasses use diverse mechanisms to defend themselves against arthropod herbivores. Despite substantial progress in understanding resistance to insects, less is known about resistance to spider mites. Two spider mite herbivores, the generalist Tetranychus urticae and the grass specialist Oligonychus pratensis, can cause significant yield losses in maize, especially under drought stress. To assess the molecular-genetic basis of maize resistance to spider mite herbivory, we employed transcriptomic studies, assessed mite feeding on wildtype plants and plants with mutations in a known defense pathway, tested diverse maize lines for resistance to spider mites, and identified loci for high-level mite resistance. We found that within hours maize plants undergo similar transcriptional reprogramming in response to herbivory by both the specialist and generalist spider mite species. Among many genes induced by spider mite herbivory were BX genes that produce benzoxazinoids, which are defensive compounds known to deter insect herbivores. Strikingly, T. urticae performance on maize plants lacking benzoxazinoids was elevated as compared to wildtype plants, while no difference was observed for the grass specialist O. pratensis. Further, we screened a large panel of inbred maize lines for resistance to both mite species, and found three inbred lines that exhibit high-level resistance to T. urticae (none were resistant to O. pratensis). Quantitative trait locus (QTL) mapping in crosses between these resistant lines and a susceptible maize line identified two large-effect resistance QTL. Despite our finding that benzoxazinoids impact T. urticae, the location of the resistance QTL on chromosomes 1 and 6, along with other data, suggest that high-level resistance to T. urticae may be (at least in part) independent of maize’s benzoxazinoid defenses. Our findings will aid in efforts to develop T. urticae-resistant cereal varieties.
Coauthors: Huyen Bui – University of Utah;Robert Greenhalgh – University of Utah;Meiyuan Ji – University of Utah;Aubrey Hawks – University of Utah;Sarah Gross – University of Utah;Gunbharpur Gill – Utah State University;Ricardo Ramirez – Utah State University