An allele of the 1-aminocyclopropane-1-carboxylate synthase gene (Md-ACS1)accounts for the low level of ethylene production in climacteric fruits ofsome 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 inse rted retroposon-like sequence. Apple species can be classified into three g roups that are heterozygous or homozygous for the ACS1-1 and ACS1-2 alleles . We measured the internal ethylene concentration (IEC) in climacteric frui t of 35 apple cultivars with respect to genotype. Eleven ACS1-2 homozygous cultivars exhibited much lower IECs than cultivars homozygous or heterozygo us for ACS1-1. Furthermore, F-1 ACS1-2 homozygous progeny derived from cros ses between heterozygous cultivars had fruit with a very low IEC. These res ults are in accord with previous data indicating the absence of transcripti on 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 fo r ethylene production during ripening.