Date of Graduation

12-2015

Document Type

Thesis

Degree Name

Master of Science in Plant Pathology (MS)

Degree Level

Graduate

Department

Plant Pathology

Advisor

Kenneth L. Korth

Committee Member

Craig S. Rothrock

Second Committee Member

Andy Pereira

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.

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