An allele of the 1-aminocyclopropane-1-carboxylate synthase gene (Md-ACS1) accounts for the low level of ethylene production in climacteric fruits of some apple cultivars

An allele of the apple ripening-specific 1-aminocyclopropane-1-carboxylate (ACC) synthase gene (Md-ACS1-1) has a 5'-flanking region possessing an inserted retroposon-like sequence. Apple species can be classified into three groups that are heterozygous or homozygous for the ACS1-1 and ACS1-2 alleles. We measured the internal ethylene concentration (IEC) in climacteric fruit of 35 apple cultivars with respect to genotype. Eleven ACS1-2 homozygous cultivars exhibited much lower IECs than cultivars homozygous or heterozygous for ACS1-1. Furthermore, F-1 ACS1-2 homozygous progeny derived from crosses between heterozygous cultivars had fruit with a very low IEC. These results are in accord with previous data indicating the absence of transcription from ACS1-2 in a heterozygous cultivar. Since the low level of ACS1 mRNA in climacteric fruit was observed in several ACS1-2 homozygous cultivars, we conclude that the low level of ethylene production in some cultivars is caused by the mutated allele of ACS1, which is the main gene responsible for ethylene production during ripening.