Date of Award

Spring 2007

Project Type

Dissertation

Program or Major

Genetics

Degree Name

Doctor of Philosophy

First Advisor

Thomas M Davis

Abstract

To investigate the genetic basis of verticillium wilt resistance in mint (Mentha L., Lamiaceae), wild-collected germplasm obtained from the United States Department of Agriculture was employed to develop breeding populations for wilt resistance screening and molecular genetic study, including cloning of candidate verticillium wilt resistance genes.

A collection of fourteen Mentha longifolia accessions from Europe, Asia and South Africa was analyzed for morphological traits, oil composition, and verticillium wilt resistance. In addition, a preliminary molecular diversity assessment was conducted utilizing randomly amplified polymorphic DNA (RAPD) markers. The accessions were found to be diverse regarding all observed traits and the South African accessions in particular were shown to possess unique features. Most importantly, highly wilt-resistant and highly wiltsusceptible accessions were observed.

A collection of twenty-seven resistance gene analogs (RGAs) was isolated from M. longifolia accessions using a PCR-based approach with primers targeting the conserved nucleotide binding site (NBS) domain found in most plant disease resistance genes. The mint RGAs shared predicted amino acid sequence similarity with disease resistance genes and RGAs from various other plant species, and were grouped into seven distinct families based on DNA and predicted amino acid sequence similarity. In addition to the NBS-related RGAs, a fragment of a verticillium wilt resistance gene candidate was isolated from a verticillium-resistant M. longifolia accession using a combination of PCR-based approaches that exploited known sequences of tomato Ve (Verticillium resistance) genes.

Finally, the complete coding region of the mint verticillium wilt resistance candidate gene, mVe1, was cloned and sequenced. Alleles of mVe1 were compared among four M. longifolia accessions used as crossing parents. These seven alleles were highly similar to each other (96.2-99.6% nucleotide identity) and had ∼50% predicted amino acid sequence identity to the tomato Ve genes. F1 and F2 populations were genotyped with respect to mVe1 alleles, and individuals from these populations were screened for wilt resistance. No correlation was found between any mVe allele and resistance or susceptibility to verticillium wilt in plants in the studied populations. However, this result does not discount the possibility that an mVe1 gene product plays a role in mint verticillium wilt resistance.

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