The role of CBF orthologs in the cold acclimation of blooming sour cherry (Prunus cerasus) and strawberry (Fragaria x ananassa)

Many temperate fruit crops suffer yield reductions due to freeze damage. Several early blooming rosaceous crops, such as strawberry (Fragaria × ananassa), and many of the cultivated Prunus species, are particularly prone to floral freeze damage. The pistil and young fruitlet, is often the most susceptible to freezing injury. Many cultural techniques are employed to mitigate freeze damage to flowers, but these techniques are often costly and ineffective. Fruit cultivars with improved floral freezing tolerance would have a major effect on solving this problem.; Cold acclimation is the process by which a plant exposed to low temperature increases its freezing tolerance. Studies on cold acclimation of Arabidopsis have identified a family of cold-inducible transcription factors, CBF1, 2, and 3, that appear to be key regulators of cold acclimation and many cold regulated (COR) genes in a diverse number of plant families. Several COR genes appear to not be expressed in the pistils of Arabidopsis following exposure to low temperature despite expression in many vegetative and other reproductive tissues. Therefore, it was hypothesized that the CBF-COR gene system will be conserved in the Rosaceae and that the expression of these genes will be compromised in pistils of rosaceous plants.; Following 16 days of 4°C treatment to whole plants, the freezing tolerance of strawberry receptacles was significantly enhanced, from −3.8 to −5.3°C for ‘Honeoye’, with similar increases observed for many genotypes. In contrast, sour cherry pistils showed no significant increase in freezing tolerance for the 3 cultivars examined.; Orthologs of CBF1 were cloned from strawberry and sour cherry with degenerate PCR primers. The putative orthologs FaCBF and PcCBF have 48% amino acid identity to CBF1 and mRNA levels were up-regulated in leaves of both crops following exposure to 4°C from 15 minutes to 24 hours. mRNA expression of FaCBF and PcCBF in pistils of strawberry and sour cherry was not detected following 4°C exposure at any time point tested.; Two transgenic lines were regenerated that expressed the transgene at low levels in both leaves and receptacles with pistils. Receptacles of the transgenic lines showed no significant change in freezing tolerance when compared to wild type plants. However, the temperature at which 50% electrolyte leakage occurred in detached leaf-discs from the two transgenic lines was −8.2°C and −10.3°C, respectively. These freezing tolerance values were significantly greater than the value for the wild-type ‘Honeoye’ leaf discs of −6.4°C.