Distribution of MdACS3 null alleles in apple (Malus × domestica Borkh.) and its relevance to the fruit ripening characters.

Expression of MdACS3a, one of the ripening-related ACC synthase genes, plays a pivotal role in initiating the burst of ethylene production by MdACS1 in apple fruit. Although previous studies have demonstrated the presence of MdACS3a-null alleles through deficiency of transcription activity or loss of enzyme activity due to amino acid substitution, which may affect the storage properties of certain fruit cultivars, an overall picture of these null alleles in cultivars is still lacking. The present study investigated the distribution of null allelic genes in 103 cultivars and 172 breeding selections by using a simple sequence repeat (SSR) marker linked to them. The results indicated that both allelic genes were widely distributed throughout the examined cultivars and selections, some occurring as the null genotype, either homozygously or heterozygously, with each null allele. The implications of MdACS3a distribution results and the influence of its null allelotypes in fruit characters are discussed.