Date of Graduation

12-2014

Document Type

Thesis

Degree Name

Master of Science in Plant Pathology (MS)

Degree Level

Graduate

Department

Plant Pathology

Advisor/Mentor

Kenneth Korth

Committee Member

Craig Rothrock

Second Committee Member

Fiona Goggin

Third Committee Member

Robert Robbins

Keywords

Aphid, Medicago Truncatula, Nematode, Triterpene Saponins

Abstract

Plants have developed numerous ways to protect themselves from microbes and insects, including producing secondary metabolites that negatively impact an invading pathogen or pest. Saponins are specialized metabolites found in many plant species and may play a role in protecting the plant. Though saponins are found in many plant species, triterpene saponins are found primarily in dicotyledons, including legumes. Medicago truncatula is a model system for studying legume biology and some accessions accumulate high concentrations of saponins. Accessions of M. truncatula known to have differing levels of saponins in both foliar and root tissues were tested for their suitability as hosts for the necrotrophic fungus, Phoma medicaginis, the pea aphid, Acyrthosiphon pisum, and the root-lesion nematode, Pratylenchus penetrans. Fungal disease measured by whole-plant ratings or via localized development were highest on ESP105, an accession with very low foliar saponins, and A17, which is derived from a commercial cultivar compared to accessions containing high concentrations of saponins. A trend of accessions with high saponin levels in foliar tissue being more resistant to P. medicaginis infection was found. Only one accession, GRC43, limited population growth of aphids compared to other lines. No significant differences were observed in nematode populations among the accessions tested. An isolate of P. medicaginis expressing green fluorescent protein was used to visualize fungal infection and growth on the same accessions of M. truncatula. Spread of GFP-expressing hyphae was reduced on lines PRT178 and GRC43, in strong agreement with disease ratings. Finally, plant responses to pests were assessed at a sub-cellular level, and accumulation of several transcripts encoding enzymes of the saponin biosynthetic pathway were affected post inoculation with P. medicaginis, pea aphid and P. penetrans. This research validates the importance of using accessions of M. truncatula to study saponin effects on pests and suggests potential promise of saponins as biological controls for various pests.

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