Graduation date: 2007Solanum species differ in their degree of frost tolerance and cold acclimation capacity. Cultivated potato species of Solanum tuberosum L. are frost-sensitive, incapable of cold acclimation, and have a maximum freezing tolerance of –3°C. Solanum commersonii Dun is frost-tolerant and can survive to –5ºC pre-acclimation and –10ºC post-acclimation. Breeding attempts to improve potato freezing tolerance and cold acclimation capacity have been largely unsuccessful in S. tuberosum. Arabidopsis CBF genes encode cold-induced transcription factors that are involved in plant cold acclimation. In this study, S. tuberosum (cv. Umatilla) and S. commersonii (PI243503 clone13), were transformed with three Arabidopsis CBF genes (AtCBF1-3) driven by either the constitutive 35S or stress-inducible rd29A promoter to assess the role CBFs play in, and their effects on, potato freezing tolerance and cold acclimation capacity. Constitutive AtCBF1 and AtCBF3 overexpression increased freezing tolerance in S. tuberosum by 2°C, and in S. commersonii by 4°C, while AtCBF2 failed to increase freezing tolerance. Cold acclimation capacity was improved for S. commersonii, but was absent from S. tuberosum. During cold treatment, leaves of wildtype S. commersonii, but not S. tuberosum, showed a significant thickening due to palisade cell lengthening and enlargement of intercellular spaces. Ectopic AtCBF1 activity mimicked cold acclimation by increasing proline and total sugar content in S. commersonii in the absence of cold. An increased chlorophyll content of transgenic S. commersonii leaves coincided with an enhanced photosynthetic capacity that was maintained during cold treatment. However, constitutive expression of all three AtCBF genes caused a variety of negative phenotypic alterations, including the reduction or elimination of tuber production, limiting their agronomic potential. The stressinducible rd29A::AtCBF transgene versions had identical gains in freezing tolerance capacity while minimizing the negative phenotypic effects and allowing essentially normal tuber production levels. Ectopic AtCBF transgene expression was confirmed to induce expression of cold-regulated genes likely involved in potato frost tolerance under warm conditions. Collectively these results suggest an endogenous CBF pathway is involved in potato frost tolerance and cold acclimation. Cold-inducible overexpression of a CBF transgene may be a practical approach to improving frost tolerance while minimizing detrimental effects on tuber production in potato.