Date of Award

12-2012

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Plant Science

Advisor

Benildo G. de los Reyes

Second Committee Member

Andrei Alyokhin

Third Committee Member

Stellos Tavantzis

Abstract

Plants are constantly exposed to various environmental adversities and being sessile mean they cannot readily avoid drastic changes brought about by both physical and biological factors. Major biotic factors affecting the fitness of plants include insect herbivores and disease-causing microbes. Strategies used to protect crops against insect herbivores and pathogens are vastly dependent on chemicals, either synthetic or organic. These short-term remedies in turn have more long-term effects on the environment, besides being highly cost-ineffective to farmers. Crop breeders are therefore always interested in built-in protection against pests and pathogens (host plant resistance mechanisms). Cultural practices from time immemorial have always been selective breeding depending on the desired product of the crop. Consequently, many important defense-related genes have been left behind in the gene pool during the process of crop domestication and breeding. Wild species, however, retain such desirable traits enabling them to hardily withstand much harsher conditions in comparison to cultivars, which are grown under managed and ideal conditions. There is a need to bring back the traits that have been left behind in the gene pool into the cultivated germplasm for a more sustainable pest and disease control strategies. The use of novel genes or alleles from exotic germplasm in breeding for host-plant resistance mechanisms into modem cultivars is a major focus of crop breeding programs. However, with the use of conventional plant breeding methodologies alone, it often takes between 7-10 years to establish a resistant variety. Usually, by the time a new breeding line is developed, the predominant biotypes/genotypes of the pests or pathogens might have already changed due to the tremendous selection pressure applied on them by indiscriminate use of pesticides and fungicides, rendering the new breeding line less effective against a newer genotype of pest/pathogen. More effective methodologies for introgressing novel genes into the breeding pipeline are essential to ensure fast and continuous deployment of new resistance mechanisms into cultivars.

The goal of this study was to contribute to the understanding of the molecular processes involved in the expression of resistance against potato-peach aphids (Myzus persicae) in the wild relatives of cultivated potato (Solarium tuberosum) and its introgression to modem cultivars. The study specifically focused on the wild species Solanum bulbocastanum, which is known to be a rich source of novel resistance genes. The study revolved around the use of the S. bulbocastanum-derived backcross introgression population to understand the genetic basis of resistance against M. persicae. The study also detected co-introgression of aphid resistance mechanisms and resistance against two aggressive genotypes of Phytophthora infestans (US-8 , US-14), the causal organism for recent disease outbreak of potato crops in the state of Maine. This study has also identified and validated dual and single resistance against both M. persicae and P. infestans in specific introgression lines. Derivatives from a cross (BC4 /BC5) between a doubly susceptible S. tuberosum cultivar Ivory Crisp as recurrent parent and doubly resistant donor parent S. bulbocastanum Acc. 243510 showed different patterns of resistance introgression.

This study also investigated the component genes involved in the expression of the introgressed insect and disease resistance loci by transcript profiling using the technique of suppressive subtractive hybridization. The general trends established from the transcriptome data revealed that many genes with either direct or indirect roles in plant responses to biotic and abiotic stresses are overrepresented among the upregulated transcripts. Although none of the known R-genes previously associated with the broadspectrum resistance of S. bulbocastanum to late blight could be implicated in the dual resistance of introgression lines, a possible role of a gene-for-gene-type resistance mechanism was postulated based on the occurrence of transcriptome signature for hypersensitive response mechanism, oxidative burst and programmed cell death. Overall, the results of this study should catalyze further studies to pinpoint regulatory genes that might be responsible for the acquired resistance to aphid and late blight o f the S. tuberosum x S. bulbocastanum-derived introgression lines.

Download

Share