RESUMEN
Suppression subtractive hybridization and differential display polymerase chain reactions were used to identify genes that were differentially expressed in cold-hardened and dehardened leaves of winter rye (Secale cereale L.). The transcripts of nine genes declined during dehardening at 22 degrees C of cold-hardened 4 degrees C-grown leaves, indicating some role in cold-acclimation. Among the genes that were strongly expressed in cold-hardened leaves were five genes of photosynthetic metabolism, the gene of the antioxidative enzyme peptide methionine sulfoxide reductase (PMSR) and three genes of RNA and protein metabolism. Four genes were identified that were more strongly expressed during dehardening of cold-hardened leaves at 22 degrees C. A full-length cDNA for a presumed cytosolic PMSR (EC 1.8.4.6) of rye leaves was identified. After heterologous expression in Escherichia coli, an antiserum against the ScPMSR was produced. The content of the ScPMSR protein, visualized by immunoblotting, was much higher in cold-hardened than in non-hardened leaves and declined during dehardening. In non-hardened leaves the mRNA of ScPMSR increased only slowly during exposures to 4 degrees C in light and was not affected by exposure to 4 degrees C in darkness. However, the ScPMSR mRNA was also induced by prolonged exposure (48 h) to high light at 22 degrees C, or by treatment with 2 muM paraquat. Consequently, the induction of cytosolic ScPMSR is a late response to prolonged photooxidative stress conditions, as expected during growth at low temperature in light. In cold-hardened leaves, PMSR may protect proteins from photodamage and thus prevent their degradation and the need for repair.