Interaction Between Ethylene and Abscisic Acid in the Regulation of Citrus Fruit Maturation

Abstract

Fruit ripening is a complex process involving a number of physiological and biochemical changes which are thought to be under hormonal and environmental control. Although the ripening of Citrus fruits is not associated with a transient increase in the biosynthesis of ethylene, recent physiological and molecular evidence has indicated that this plant hormone may be involved in the control of fruit maturation [1, 2]. Alonso et al [2] isolated a number of ethylene-induced cDNAs in the peel of Navel oranges and showed that some of the mRNAs corresponding to these cDNAs were up regulated during fruit maturation and suggested that ethylene is implicated in the regulation of some aspects of Citrus fruit maturation. Since ethylene production remains very low during the process, changes in the sensitivity of the tissue to the basal ethylene levels may operate. Furthermore, Abscisic Acid (ABA) has also been shown to be involved in the maturation of Citrus fruits but its role is far from understood. Goldschmidt et al. [3] demonstrated an increase in ABA during natural and ethylene-induced senescence of Citrus flavedo. Richardson & Cowan [4], comparing changes in ABA in different Citrus varieties, indicated that full colour was achieved when ABA in the skin started to decline. We have recently described the characterization of a spontaneous bud mutation of Navelate orange, named ‘Pinalate,’ which is affected in colour development and is ABA-deficient [5]. This mutant provides an interesting experimental system to study the involvement of ABA in the hormonal mechanisms governing fruit growth and maturation. In this work we report a comparative analysis of the maturation process in ‘Navelate’ oranges (Citrus sinensis L., referred to as wild type, WT) and ‘Pinalate’. The role of ethylene and its potential interaction with ABA in the regulation of Citrus fruit coloration are discussed.