The effects of cold acclimation on ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase) were examined in leaves of moderately cold-hardy 'Valencia' orange (Citrus sinensis L. Osbeck) and in two cold-hardy genotypes, 'Trifoliata' orange (Poncirus trifoliata L. Raf.) and 'FF-6-7-2' citrus hybrid (Citrus ichangensis Swing. X Fortunella margarita Lour. Swing.). Soluble protein extracted from leaves and analyzed by electrophoresis showed significant differences in composition among the three citrus genotypes. A 12 kD polypeptide with unknown function was present in large quantity in 'Valencia', but not in the other two genotypes. Probing with antibodies specific for Rubisco and PEPCase indicated that 'Trifoliata' had two polypeptides of Rubisco small subunit (13 and 14 kD) whereas 'Valencia' and 'FF-6-7-2' had only one (14 kD). Cold acclimation differentially altered the expression of the two carboxylases. In all three genotypes, cold acclimation resulted in increased amount of Rubisco large subunit, but decreased amount of the small subunit. However, the amount of PEPCase was substantially enhanced by cold acclimation in the cold-hardy genotypes ('FF-6-7-2' and 'Trifoliata'), but was largely reduced by the cold treatment in the moderately cold-hardy genotype ('Valencia'). The enhanced expression of PEPCase during cold acclimation may be important for the induction of freezing tolerance in citrus.
- cold acclimation
- protein expression