Abstract
ABSTRACTLate pear cultivars such as Passe‐Crassane (PC) require a long chilling treatment before they are capable of ripening. Early cultivars such as Old‐Home (OH) have no cold prerequisite. The regulation of 1‐aminocyclopropane‐1‐carboxylic acid synthase (ACS) genes was studied in OH, PC and in OH × PC hybrids in order to determine the role of this gene family in the cold requirement. Of the seven Pc‐ACS cDNAs isolated, four (Pc‐ACS1a/b and Pc‐ACS2a/b) showed differential expression associated with the cold requirement. Pc‐ACS1a transcripts accumulated throughout the cold treatment and, with Pc‐ACS2a, during ripening of cold‐dependent cultivars. Pc‐ACS1b and Pc‐ACS2b were detected only during ripening of cold‐independent genotypes. Furthermore, Pc‐ACS2a transcript accumulation was negatively regulated by ethylene, whereas Pc‐ACS2b was positively regulated by the hormone. Pc‐ACS3, 4 and 5 transcript accumulation was similar in all genotypes. Genetic analyses of OH, PC, and 22 OH × PC progenies demonstrated that late, cold‐dependent cultivars were homozygous for Pc‐ACS1a and 2a whereas early, cold‐independent cultivars were heterozygous for Pc‐ACS1(a/b) and homozygous for Pc‐ACS2b. A model is presented in which differences in Pc‐ACS alleles and gene expression between cold‐ and non‐cold‐requiring pears are critical in determining the ripening behaviour of the cultivars.
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