Date of Graduation
12-2015
Document Type
Thesis
Degree Name
Master of Science in Plant Pathology (MS)
Degree Level
Graduate
Department
Plant Pathology
Advisor/Mentor
Kenneth L. Korth
Committee Member
Craig S. Rothrock
Second Committee Member
Andy Pereira
Keywords
Biological sciences, Medicago truncatula, Plant-insect interaction, Saponin, Spodoptera exigua, Transcriptome
Abstract
Triterpene saponins are a class of defensive plant-derived compounds, which have a
variety of functions including antimicrobial, insecticidal, and nematicidal activities. This
research assessed the performance of the beet armyworm, Spodoptera exigua, following ad
libero feeding on Medicago truncatula accessions differing in specialized metabolite and
saponin profiles. Insect performance did not directly relate to the foliar saponin levels in the
accessions, which was measured using HPLC mass spectrometry. Accession ESP105 had the lowest foliar saponins overall, yet had similar levels of soyasaponins compared to the other accessions. Conversely, accessions A17, PRT178, and GRC43 had high foliar saponins with particularly high levels of medicagenic and zanhic acids compared to accession ESP105.Saponin extract was topically applied to insect artificial diet at a gradient of concentrations. Contradictory to previous reports, no inhibition of growth of insects on diet containing saponin extract or no statistical differences in key fitness indicators were detected.
Wounding by insects greatly induced genes involved in saponin production. Biosynthesis
of saponins begins in the mevalonate pathway and the upstream biosynthetic reactions are well understood. Saponins are made up of an aglycone “sapogenin” that is modified with variable compositions and numbers of sugar moieties at variable positions on the sapogenin backbone.Sapogenins are synthesized by cytochrome P450 enzymes and glycosylation is carried out by glycosyltransferases. To date, few of these cytochrome P450’s and fewer glycosyltransferases have been functionally identified. RNA was sequenced to evaluate gene transcript abundance in response to herbivory between accessions A17 and ESP105 to illuminate potential saponin biosynthetic genes. Twenty-eight oxidation-reduction genes induced by insect feeding and several glycosyltransferases and transcription factors that showed a similar expression pattern were identified. Quantitative-PCR was used to confirm the sequencing results and to examine expression of candidate genes in the remaining accessions. Elucidation of the saponin biosynthetic pathway could facilitate incorporation of saponin compounds as pest deterrents in important economic crops. By combining bioassays, metabolite profiling and gene expression data, the effect of saponins on beet armyworm, and the genetic regulation of saponin biosynthesis induced by insect wounding in a model forage legume was determined.
Citation
Rogers, A. M. (2015). A Comparative Study of Medicago truncatula Transcriptomes as They Relate to Saponin Defenses Against Insect Pests. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/1332