CBFs, DAM, and DELLA Genes: The coordinated regulation of cold hardiness, dormancy, and growth in apple

CBFs are cold-induced transcription factors (TFs) that regulate the expression of many cold-acclimation-related genes. To better understand and improve cold hardiness in apple, transgenic lines of ‘M26’ apple have been created that overexpress a peach CBF gene or various native apple CBF genes. Alternatively, silencing constructs have been used to generally or specifically inhibit native CBF genes. These transgenic lines have been studied in both laboratory and field studies over the past several years. Overexpression of a peach CBF gene in apple improved cold hardiness and unexpectedly also made trees enter dormancy earlier in the fall, delayed budbreak in the spring, and significantly decreased growth. These effects have been verified in 3 years of field studies. Transcriptomic studies indicate that CBF genes regulate approximately 8.5% of the total gene set identified in apple during normal cold acclimation, including a number of other transcription factors, including dormancy-associated-MADs box (DAM) genes. A bioinformatic analysis has revealed that several apple DAM genes possess one to several C-repeats (CBF protein binding motif) in their upstream promoter sequences. CBFs have also been proposed to regulate growth by inducing the accumulation of growth repressing DELLA proteins in the nucleus. Thus it appears that, in addition to cold acclimation, CBF TFs have the potential to integrate the regulation of both dormancy and growth. A bioinformatic analysis of CBF and Inducer of CBF Expression(ICE) genes in several fruit (grape, apple, peach, citrus, and blueberry) and forest tree (poplar, eucalyptus) species has also been conducted and will be discussed.