Date of Award

12-2009

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Botany and Plant Pathology

Advisor

Benildo G. de los Reyes

Second Committee Member

Stellos Tavantzis

Third Committee Member

Dave Lambert

Abstract

Plants are sessile organisms unable to move when environmental conditions change in an unfavorable way. Different species have evolved novel mechanisms that allow varying degrees of adaptation to cold and dehydration stress. The types of responses vary significantly within a genus with only few species able to cold-acclimate (2°C) and survive freezing temperatures. Most species are easily injured by exposure to above freezing temperature (13°C). Solanum species that are unable to acclimate are generally more susceptible to cold injury compared to species that are able to acclimate. S. lycopersicum and S. trifidum are highly sensitive to chilling while S. tuberosum (cv. Red Pontiac) is chilling tolerant but unable to acclimate. S. commersonii is able to withstand freezing temperatures (to -11°C) following acclimation at 2°C for two to three weeks. This study utilized semi-global gene expression analysis to investigate variation among Solarium species with respect to their responses to cold acclimating condition (2°C). The goal was to identify patterns of responses unique to either acclimating, freeze tolerant species (S. commersonii) or non-acclimating, cold sensitive species (S. tuberosum cv. Red Pontiac) by surveying the transcriptome during 14-day exposure to 2°C. Gene expression at 2, 4, 7, 10, and 14 days post-exposure was profiled using a 10,000-cDNA TIGR microarray. Additional analysis of selected regulatory and defense-related genes was performed on a wider spectrum of tolerance (S. lycopersicum, S. trifidum) at early time points (2, 6, 12, 24, and 48 hours post exposure to 2°C) by quantitative real-time PCR (qPCR). Microarray analysis showed that the low temperature response transcriptome is larger in S. tuberosum than S. commersonii, thus the number of genes that respond to low temperature does not seem to correlate well with cold hardiness (physiological). Ploidy level may be a contributing factor. Response in S. commersonii occurred much earlier and in a highly modulated manner. This was characterized by rapid induction that lasted for several hours to few days. In contrast, S. tuberosum exhibited a delayed response that lasted nearly towards the end of the treatment period (two weeks). Downregulation of energy consuming processes during the early stage of cold exposure was the dominant trend in S. commersonii. Based on these results, S. commersonii is able to establish a para-dormant state much earlier than S. tuberosum, and this appears to be the fundamental mechanistic difference related to the differential ability of these species to cold acclimate. Establishment of a para-dormant state in S. commersonii is also supported by the early induction of several regulatory genes with known function in the regulation of dormancy via ABA-mediated mechanisms (frigida, Srela, Sti). In summary, modulated gene expression plays a crucial role in differential ability of Solanum species to cold acclimate. S. commersonii is able to adjust its metabolic status under sub-optimal condition better than S. tuberosum and other chilling sensitive species.

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