During the adaptation of plants to low temperature, changes in gene expression can be induced in a variety of tissues. Low-temperature-regulated gene expression was studied in cold-stored potato (Solanum tuberosum L.) tubers by two-dimensional electrophoresis of in vitro translation products. As a response to cold treatment, the relative amount of mRNA encoding at least 26 polypeptides changed. By differential screening of a cDNA library, 16 clones corresponding to cold-inducible transcripts were isolated. They were classified into four non-cross-hybridizing groups. RNA hybridizations using representative clones from each group revealed different temporal accumulation patterns for the cold-inducible transcripts. mRNAs homologous to the cDNA clones were first detectable after 1 to 3 d of cold treatment, and the highest level of expression was reached after 3 to 7 d. Transcripts corresponding to cDNA clones CI13 and CI19 were transiently expressed, whereas the steady-state level remained high for cDNA clones CI7 and CI21 during the cold storage period of 4 weeks. The DNA sequences of two cDNA clones, CI7 and CI19, have been determined. The polypeptide predicted from the DNA sequence of CI19 is sequence related to small heat-shock proteins from other plant species. The deduced protein sequence of CI7 exhibits strong homology to the dehydrin/RAB group of dehydration stress- and abscisic acid-inducible polypeptides and to cold-induced proteins from Arabidopsis and spinach.

Cold storage of potato tubers at 4 degrees C is associated with the accumulation of several cold-induced transcripts. By using a previously characterized cDNA (CI7) as probe, we isolated and sequenced the corresponding ci7 gene. The putative promoter of ci7 contains sequence elements that have been shown to mediate expression of stress-responsive genes of Arabidopsis thaliana. CI7 transcripts were differentially induced in response to cold, drought, high salt or exogenous ABA treatment in potato tubers and leaves. Whereas accumulation of CI7 transcript during cold storage occurred within days, induction of CI7 transcript in response to drought, ABA and salt occurred rapidly within few hours. In tubers, accumulation of CI7 protein in response to abiotic stresses and ABA was small when compared to transcript levels. In leaves, the CI7 protein was undetectable after all treatments tested. 3 kb of the 5'-flanking ci7 promoter region were fused to the GUS reporter gene and introduced into S. tuberosum plants. The analysis of tubers of independent transgenic lines did not reveal significant induction of enzymatic GUS activity in response to low temperature. When RNA gel blotting was used to analyze the level of induction of the GUS gene driven by the ci7 promoter, the heterologous GUS fusion was, however, strongly responsive to low temperature. Nuclear run-on transcription studies of the ci7 gene, in comparison with RNA gel blot analyses of the transgenic plants, indicated that most of the temperature-regulated expression of the ci7 gene in tubers may be accounted for by post-transcriptional control mechanisms.