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Seven ERF cDNAs were cloned from two Japanese plum (Prunus salicina L.) cultivars, ‘Early Golden’ (EG) and ‘Shiro’ (SH). Based on the sequence characterization, these Ps-ERFs could be classified into three of the four known ERF families. Their predicted amino acid sequences exhibited similarities to ERFs from other plant species. Functional nuclear localization signal analyses of two Ps-ERF proteins (Ps-ERF1a and -1b) were carried out using confocal microscopy. Expression analyses of Ps-ERF mRNAs were studied in the two plum cultivars in order to determine the role of this gene family in fruit development and ripening. The seven Ps-ERFs displayed differential expression pattern and levels throughout the various stages of flower and fruit development. The diversity in Ps-ERFs accumulation was largely due to the differences in their responses to the levels of ethylene production. However, other plant hormones such as cytokinin and auxin, which accumulate strongly throughout the various developmental stages, also influence the Ps-ERFs expression. The effect of the plant hormones, gibberellin, cytokinin, auxin, and ethylene in regulating the different Ps-ERF transcripts was investigated. A model was proposed in which the role played by the plant hormone auxin is as important as that of ethylene in initiating and determining the date and rate of ripening in Japanese plums.

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.

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