Plums are climacteric fruits: their ripening is associated with a burst of ethylene production and respiration rate. Stone fruits, including plum, have a distinct pattern of growth and development, described as a double sigmoid pattern. In order to understand the developmental control of ethylene perception in plum, four ethylene perception and signal transduction components (EPSTCs) were characterized, including two ETR1-like proteins (Ps-ETR1 and Ps-ERS1), a CTR1-like protein, and an ethylene-responsive element-binding factor (ERF). Their regulation was studied throughout fruit development and ripening in early and late cultivars. Analysis of transcript levels revealed that only Ps-ERF1 and Ps-ERS1 accumulated immediately after fertilization. Increases in Ps-ETR1 and Ps-CTR1 transcript levels could not be detected before S3 of fruit development. Marked differences associated with the ripening behaviour of early ('Early Golden') and late ('Shiro') Japanese plum cultivars were observed. The early cultivar showed ripening patterns typical of climacteric fruits accompanied by sharp increases of the four transcript levels in an ethylene-dependent manner. However, the late cultivar exhibited a suppressed-climacteric pattern, with a slight increase in ethylene production related to ripening. The accumulation of the Ps-ETR1 (and not Ps-CTR1) mRNA in the late cultivar was ethylene independent. Ps-ERS1 mRNA was expressed at low, constant levels, while, Ps-ERF1 remained undetectable. The differences between the two plum cultivars in the date and rate of ripening in relation to the differences in the accumulation patterns of the four mRNAs are discussed.

The regulation of ACC synthase (ACS) genes was studied in early (‘Early Golden’) and late (‘Shiro’) Japanese plum cultivars (Prunus salicina L.) in order to determine the role of this gene family in fruit ripening. Of the four Ps-ACS cDNAs isolated, two (Ps-ACS1 and -3) showed differential expression between the two cultivars. Ps-ACS1 accumulated during fruit ripening of ‘Early Golden’ (‘EG’) and ‘Shiro’ (‘SH’) in ethylene-dependent and -independent manners, respectively. Ps-ACS3a transcripts accumulated throughout fruit development and during ‘EG’ fruit ripening. Ps-ACS3b was detected only during ripening of ‘SH’ fruit. Furthermore, Ps-ACS3a transcript accumulation was negatively regulated by ethylene, whereas Ps-ACS3b was positively induced by the hormone. In both cultivars, the expression of Ps-ACS4 and -5 is under positive and negative feedback control by ethylene, respectively. Genetic analyses of ‘EG’ and ‘SH’ cultivars demonstrated that ‘EG’ is homozygous for Ps-ACS3a whereas ‘SH’ is heterozygous for Ps-ACS3 (a/b). The role of ethylene-overproducer 1-like in delaying fruit ripening by interacting with Ps-ACS proteins was also studied. The effect of the plant hormones, auxin, gibberellin, and cytokinin, in regulating ethylene production by promoting the induction of the different Ps-ACS mRNAs in plum was investigated. A model is presented in which differences in Ps-ACS alleles and gene expression between early and late plums are critical in determining the ripening behaviour of the cultivars.