Storage of potato (Solanum tuberosum) tubers at 4 degrees C is associated with the accumulation of several transcripts. DNA sequence analysis of cDNA clone CI21, which corresponds to one of the cold-induced transcripts, revealed high homology to transcripts of tomato (Lycopersicon esculentum) and wild potato (Solanum chacoense) induced by ripening and water stress. Two homologous, nonallelic genes, ci21A and ci21B, were isolated and sequenced. Northern blot analysis showed that CI21 transcripts were present at the highest levels in cold-stored tubers, at lower levels in stems and roots, and at the lowest levels in leaves and tubers stored at room temperature. Treatment with abscisic acid, heat, and a high concentration of salt had no marked effect on CI21 transcript levels in tubers and leaves. Drought was the only stress treatment that induced CI21 transcripts in leaves, but it did not do so in tubers. Western blot analysis detected CI21 protein only in tubers. Chimeric gene constructs between the putative ci21A promoter region and the uidA reporter gene were tested in transgenic potato plants for induction of beta-glucuronidase activity by low temperature. A 2-fold increase of beta-glucuronidase activity in response to tuber storage at 4 degrees C was observed for fragments between 380 and 2000 bp of the ci21A promoter region.