CBF1 Orthologs in Sour Cherry and Strawberry and the Heterologous Expression of CBF1 in Strawberry

Orthologs of CBF1 , a cold-induced transcription factor important in the cold acclimation response in Arabidopsis thaliana were cloned from strawberry ( Fragaria × ananassa Duchesne) and sour cherry ( Prunus cerasus L.) with degenerate PCR primers. The putative orthologs [ Fragaria × ananassa CBF1 ( FaCBF1 ) and Prunus cerasus CBF1 ( PcCBF1 )] 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. However, mRNA of FaCBF1 and PcCBF1 was not detected in pistils of strawberry and sour cherry following 4 °C exposure. Agrobacterium -mediated transformation of a CaMV35S-CBF1 construct was conducted on Fragaria × ananassa `Honeoye' crown discs. Two transgenic lines were regenerated that expressed the transgene at low levels in both leaves and receptacles. Receptacles of the transgenic lines showed no significant change in freezing tolerance when compared to wild type plants, although 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.