Up-regulation of two cDNA clones encoding metallothionein-like proteins in apple fruit during cool storage

We are investigating the molecular basis of low temperature responses in apples, by identifying and characterising fruit genes which show altered expression in response to cool-storage, Two independent cold-regulated clones (pAMTI and pAMT2) were isolated from a cDNA library derived from cool-stored apple ( Malus domestics Borkh. cv. Granny Smith) fruit. These clones share only 27% amino acid identity with each other, but both show high similarity to plant metallothionein (MT)-like proteins. The polypeptide encoded by pAMTI shares similarity with type 2 MT-like sequences, while that encoded by pAMT2 is similar to others which share a different distribution of cysteine residues. We suggest, these form a 'type 3' group of MT-like clones. Genomic Southern analysis confirmed that there is a family of MT-like genes in apple. There are differing patterns of pAMTI and pAMT2 expression during apple fruit development, amt 1 RNA was abundant in flowers and during the early stages of development, and decreased as the fruit approached maturity, while amt 2 RNA was barely detectable in flowers and young fruit and accumulated with fruit development. In ripe fruit. amt 1. expression was up-regulated, while amt 2 expression was down-regulated. In leaves, both genes showed increased expression with leaf age. In Granny Smith, Cox's Orange Pippin and Braeburn apple cultivars. both genes were up-regulated in cool-stored fruit. In Granny Smith contical tissue, amt RNA levels were elevated within the first 45 min at both 0.5°C and 4°C, but not at 12.5°C. The different patterns of amt 1 and amt 2 expression during fruit development and in different tissues suggest that the respective genes have distinct controlling elements and may be functionally different. The in vivo roles of the encoded polypeptides, particularly in relation to chilling tolerance or acclimation, are as yet unknown.