Identification of a grapefruit cDNA belonging to a unique class of citrus dehydrins and characterization of its expression patterns under temperature stress conditions

Citrus fruits are sensitive to low temperatures and this often results in the development of chilling injuries during postharvest storage. In order to gain more insight into the molecular mechanisms involved in the acquisition of fruit chilling tolerance, we initiated a grapefruit (Citrus paradisi, cv. Marsh Seedless) flavedo cDNA sequencing project and used it to identify a cDNA similar to other Poncirus trifoliata and Citrus unshiu dehydrin genes reported to be responsive to low temperatures. The grapefruit dehydrin cDNA, designated cor15, encodes a predicted polypeptide of 15.1 kDa, that is almost completely identical with other reported citrus dehydrin proteins, except that it contains two large amino acid repeats, whereas P. trifoliata COR11 has only one such repeat and P. trifoliata COR19 and C. unshiu COR19 have three repeats. Together, the various grapefruit, P. trifoliata and C. unshiu dehydrins form a closely related and unique dehydrin gene family that differs from most other plant dehydrins in having an unusual K-segment similar to that of gymnosperms and in having a serine cluster (S-segment) at an unusual position at the carboxy-terminus. The grapefruit cor15 gene is consistently expressed in the fruit peel tissue at harvest, but its message levels dramatically decrease during storage at 2 degrees C. However, a pre-storage hot water treatment, which enhances fruit chilling tolerance, elicited retention of the constant level of cor15 gene expression during cold storage and eliminated its decline. The hot water treatment had no inductive effect on cor15 gene expression when the fruit were held at non-chilling temperatures. The effects of other stresses, such as exposure to ethylene, UV irradiation and wounding, on cor15 gene expression, were temporary and persisted for 1-2 days after the treatments.