An allele of the 1-aminocyclopropane-1-carboxylic acid (ACC) synthase gene (Md-ACS1), the transcript and translated product of which have been identified in ripening apples (Malus domestica), was isolated from a genomic library of the apple cultivar, Golden Delicious. The predicted coding region of this allele (ACS1-2) showed that seven nucleotide substitutions in the corresponding region of ACS1-1 resulted in just one amino acid transition. A 162-bp sequence characterized as a short interspersed repetitive element retrotransposon was inserted in the 5 -flanking region of ACS1-2 corresponding to position ؊781 in ACS1-1. The XhoI site located near the 3 end of the predicted coding region of ACS1-2 was absent from the reverse transcriptase-polymerase chain reaction product, revealing that exclusive transcription from ACS1-1 occurs during ripening of cv Golden Delicious fruit. DNA gel-blot and polymerase chain reaction analyses of genomic DNAs showed clearly that apple cultivars were either heterozygous for ACS1-1 and ACS1-2 or homozygous for each type. RNA gel-blot analysis of the ACS1-2 homozygous Fuji apple, which produces little ethylene and has a long storage life, demonstrated that the level of transcription from ACS1-2 during the ripening stage was very low.
Two MdERFs (ethylene-response factors) were isolated from ripening apple (Malusxdomestica Borkh. cv. Golden Delicious) fruit. The features of their conserved motifs indicated that MdERF1 and MdERF2 belong to group VII and group IX categories in Arabidopsis, respectively. MdERF1 was expressed predominantly in ripening fruit, although a small degree of expression was also observed in non-fruit tissues, whereas MdERF2 was expressed exclusively in ripening fruit. The increased expression in ripening fruit was repressed by treatment with 1-methylcyclopropene (1-MCP: a potent antagonist of ethylene receptors), indicating that transcription is regulated positively by the ethylene signalling system. Indeed, it was a tendency for cultivars with low ethylene production to show lower MdERFs expression than those with high ethylene production. On the basis of concomitant analyses of the expression of some genes related to ripening, the functions of MdERFs and the role of ethylene in the ripening process are discussed.
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