Deciphering the Integrated Regulation of Dormancy, Cold Hardiness, and Growth in Apple

The timing of cold acclimation and deacclimation, dormancy, and budbreak play an integral role in the life cycle of woody plants. The molecular events that regulate these parameters have been the subject of much study, and numerous reviews. In most studies, however, these events have been investigated independently of each other, with the focus being on either dormancy, cold acclimation, or budbreak, and the systems used have been advantageous for providing information specific to one of these events. We previously reported that ectopic expression of a peach CBF (ppCBF1) in apple increased the level of both non-acclimated and acclimated freezing tolerance in the transgenic apple (T166), relative to the untransformed control (‘M26’), but also inhibited growth, and rendered the T166 plants sensitive to short day (SD) photoperiod, and induced early bud set and leaf senescence. The observed sensitivity to SD was novel and unexpected since apples are typically not sensitive SD in terms of inducing growth cessation.  These results were confirmed in three years of field studies, including the observation of delayed budbreak. Thus, this transgenic line may serve as a model for studying the integration of the regulation of freezing tolerance, dormancy, budbreak, and growth in woody plants. The current study examined differences in the seasonal expression of genes (CBF, DAM, DELLA, and EBB) that have been reported to be associated with freezing tolerance, dormancy, growth, and budbreak, respectively, in the T166 line of transgenic apple and the untransformed ‘M26’ control.