Class II chitinase accumulated in the bark tissue involves with the cold hardiness of shoot stems in highbush blueberry (Vaccinium corymbosum L.)

To understand the development of cold hardiness in highbush blueberry plants (Vaccinium corymbosum L.), we investigated seasonal changes in the protein compositions of current-year and overwintered shoots. Electrophoresis using SDS-polyacrylamide gels indicated that the amounts of a few proteins increased in autumn, in association with an enhancement of cold hardiness. Of these proteins, 65- and 60-kDa proteins were confirmed to be dehydrins by western blotting. While the levels of most of the accumulated proteins decreased in April, a 27-kDa protein maintained its level in the overwintered stem during spring. The amino acid sequence deduced from a cDNA for this protein showed significant similarities with known chitinases. The stem segments from overwintered shoots sampled in July showed higher cold hardiness against sub-zero temperature than those from current-year shoots, and the bark tissue of overwintered shoots had higher cold resistance than that of current-year shoots. The 27-kDa protein (BC27) was mostly found in the bark tissue of stems. Appreciable accumulation of this protein in floral buds was not detected. The purified BC27 protein exerted a cryoprotective effect on protoplasts subjected to a freeze–thaw treatment. These results suggest that the stems of highbush blueberry develop a system of cold hardiness different from that of the buds and apices, and that the accumulation of chitinase is involved in tolerance to low temperature in winter or unseasonably low temperature in spring.