Date of Award

12-2015

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Botany and Plant Pathology

Advisor

Benildo G. de los Reyes

Second Committee Member

Robert Gundersen

Third Committee Member

David De Koeyer

Abstract

The late blight disease in plants is caused by the oomycete Phytophthora infestans. Of historic significance, this disease caused the ‘Great Potato Famine’ in Ireland over 150 years ago, resulting in mass death and immigration. Currently, the pathogen remains a large threat to food-security in developing regions where potato is increasingly relied upon as a staple; and global losses associated with the disease are estimated at $3-5 billion, annually. The modern day germplasm of cultivated potato generally lacks the novel genetic variation necessary to develop strong late blight resistance, and the pathogen demonstrates enhanced resistance to heavily used fungicides. Behind the exceptional infectiousness that is characteristic of the pathogen, is an extensive genetic repertoire of virulence mechanisms, which permits the rapid evolution of novel factors involved in chemical resistances and those that override host defense mechanisms.

An anticipated solution to the late blight problem is the introgression of novel genes from wild species with inherent durable qualities of resistance, against highly diverse selections of pathogen races. In particular, the identification and isolation of major resistance genes (R-genes) from wild species has conferred strong resistance when placed in the genetic background of an otherwise susceptible cultivar. These newly gained resistances are primarily attributable to an R-gene’s ability to perceive pathogen-derived molecules during the infection process, and in turn, rapidly initiate cellular defenses. Research has provided great insight to the activation of R-genes and subsequent cellular responses, but much remains unknown about the full scope of cellular responses and the structures of regulatory networks which modulate these responses.

This study utilizes a unique set of differential genotypes of potato to enhance understanding of the genetic signatures involved in R-gene mediated late blight resistance. Previously, we identified a backcross derivative of the cultivated potato (Solanum tuberosum) with introgressed late blight resistance from the donor wild species Solanum bulbocastanum. Genetic screening attributed the gained resistance in the backcross derivative Sbu 8.5, to the presence of RB, a novel R-gene native to Solanum bulbocastanum. Transcriptome libraries were constructed, utilizing RNA-Seq technology for S. bulbocastanum, Sbu 8.5 (RB+), and the susceptible introgression sibling Sbu 8.15 (RB-) for control and late blight challenged conditions. Transcriptomic responses of Sbu 8.5 (RB+) appear to be intermediate to that seen in the wild species and susceptible sibling. Quantitative real-time PCR was used to elucidate temporal expression profiles for candidate genes of interest detected through transcriptomic analyses. Expression profiles of Sbu 8.5 (RB+) mirrored that seen in its susceptible sibling, Sbu 8.15 (RB-) and its recipient parent, Solanum tuberosum cv. Ivory Crisp. Two putative transcription factors, orthologous to WRKY70 in Arabidopsis thaliana, demonstrate high responsiveness in Solanum bulbocastanum, but in dissimilar fashion in the other three lines. The observed transcriptomic and temporal gene expression signatures depict an absence of complementation between RB and the downstream responses when in a Solanum tuberosum background, possibly attributable to a lack of certain genes and/or regulatory mechanisms that are present within the wild species. The expression profile of WRKY70 indicates an aspect of Solanum bulbocastanum’s responses to late blight that was not detected in Solanum tuberosum backgrounds with or without RB.

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