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Low, non-freezing temperatures and/or short daylength (SD) regulates cold acclimation and dormancy in fruit trees. Regarding cold acclimation, C-repeat binding factor (CBF/DREB) transcriptional activator genes have the well-documented ability to induce the expression of a suite of genes associated with increased cold tolerance. We isolated a full-length cDNA of a peach CBF gene, designated PpCBF1 (GenBank Accession HM992943), and constitutively expressed it using an enhanced 35S promoter in apple. Unexpectedly, constitutive overexpression of the PpCBF1 in apple resulted in strong sensitivity to short daylength. Growth cessation and leaf senescence were induced in transgenic lines exposed to SD and optimal growth temperatures of 25°C over a 4-week period. Following 1-4 weeks of SD and 25°C trees were returned to LD and 25°C in the greenhouse. Control (untransformed) plants continued to grow while transgenic lines receiving two or more weeks of SD remained dormant and began to drop leaves. Constitutive overexpression of the PpCBF1 in apple resulted in a 4-6°C increase in freezing tolerance in both the non-acclimated and acclimated states, respectively, compared with untransformed M.26 trees. This is the first instance that constitutive overexpression of a CBF gene has resulted in SD-induction of dormancy and to our knowledge the first time apple has been shown to strongly respond to short daylength as a result of the insertion of a transgene.

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