Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 10, 2011
Publication Date: January 28, 2011
Citation: Wisniewski, M.E., Norelli, J.L., Bassett, C.L., Artlip, T.S., Macarisin, D. 2011. Ecotopic expression of a novel peach (Prunus persica) CBF transcription factor in apple (Malus x domestica) results in short-day induced dormancy and increased cold hardiness. Planta. 233:921-983. Interpretive Summary: In order to survive freezing and winter temperatures, the ability to cold acclimate and undergo a period of dormancy is essential to temperate tree fruit crops and woody plants in general. Despite the recognized role of temperature on the onset and release of dormancy and on cold acclimation and deacclimation, the impact that climate change, including elevated levels of atmospheric CO2,will have on the phenology (dormancy, cold acclimation and deacclimation, flowering, etc.), of fruit trees is unknown. Apple trees may be especially vulnerable to erratic temperatures, because in contrast to other fruit trees, the onset of dormancy and cold acclimation are both largely dictated by low temperature exposure rather than short days. Therefore, a better understanding of the regulation of cold acclimation and deacclimation in fruit trees is needed. We isolated CBF (C-repeat binding factor) transcription factor genes from both peach and apple trees, and have studied their expression patterns and function. The present study examined the effect of overexpressing a peach CBF1 transcription factor in apple. Results indicated that overexpressing a peach CBF1 gene in apple significantly improved cold hardiness in the transgenic apple trees in both non-acclimated and acclimated conditions. Suprisingly, overexpression of the peach CBF1 gene in apple also resulted in the transgenic trees undergoing short-day induced dormancy. This was surprising since apple trees are generally insensitive to short photoperiod and rely on low temperature as a signal to begin to go dormant and to cold acclimate. Relying on daylength rather than low temperature alone may be an important trait to confer to apple in order to adapt them to climate change. The basis for the ability of the peach CBF1 gene to confer short-day induced dormancy in apple is being explored. Additionally, an APHIS field permit has been obtained so that the transgenic apple trees can be studied under field conditions. The field studies will determine if there are any adverse effects of overexpressing peach CBF1 in apple on growth and other aspects of apple phenology.
Technical Abstract: Regulation of growth and dormancy in temperate perennials such as fruit trees is essential, as is the ability to acclimate to freezing temperatures. Depending on the species, the onset of either dormancy and or cold acclimation can be triggered to varying degrees by either or both low, non-freezing temperatures and short photoperiod. C-repeat binding factor (CBF) transcriptional activator genes have the ability to induce the expression of a suite of genes (cold regulon) associated with increased cold tolerance. The role of CBF genes in initiating the process of cold acclimation in numerous plant species has been well documented. To better understand how various phenological parameters (timing of acclimation and deacclimation, flowering, fruit development, etc.) of fruit trees may be impacted by climate change, we have begun to isolate and characterize the expression and function of peach and apple CBF genes. In the present study, we isolated a full length cDNA of a peach CBF gene, designated PpCBF1, and constitutively expressed it using a dual 35S promoter in apple. Overexpression of the PpCBF1 in apple significantly increased the level of cold hardiness in both non-acclimated and acclimated plants demonstrating the functional role of PpCBF1 in cold acclimation. Transgenic T166 trees exhibited a 4 to 6 deg C increase in freezing tolerance, in the non-acclimated and acclimated state, respectively, compared to untransformed M26 trees. The highest level of freezing tolerance was associated with the highest level of PpCBF1 expression. Transgenic apple lines also exhibited increased levels of anthocyanins in leaf vein tissues, had slightly tougher leaves as measured by increased dry weights, and delayed growth during propagation. Unexpectedly, transgenic lines exhibited a strong sensitivity to short photoperiod (SD) that was not present in the untransformed apple lines. Low temperatures (LT) of 5 deg C, with or without SD (8/16 h) induced growth cessation and a basipetal pattern of leaf senescence in both control and transformed apple lines. However, increasing degrees of growth cessation and leaf senescence were induced in transgenic lines exposed to SD and high temperatures (HT) of 25 deg C over a 4-week period. Upon return to greenhouse conditions (HT and LD), control (untransformed) plants continued to grow while transgenic lines remained dormant and began to drop leaves. This is the first instance that growth cessation and leaf senescence has been shown to be regulated by a CBF gene and to our knowledge, the first time apple has been shown to have the ability to strongly respond to short photoperiod.