Changes in Hormonal Profiles and Corresponding Gene Expressions During the Initiation and Development of Juice Sac Primordia in Citrus Ovaries and Fruitlets

The edible part of citrus fruit is composed of juice vesicles/sacs which develop from the endocarp, the two to three inner cell layers of the white spongy peel termed albedo. Juice sac primordia usually appear 1 week after anthesis. Hormones, especially auxin and gibberellin, play a role in pericarp development during the ovary-to-fruit transition, but their effect on juice vesicle induction has not been studied. Here, hormone profiling in the pericarp and changes in the expression of their corresponding genes in the endocarp and pericarp were compared between two citrus cultivars: Calabria citron, in which juice sacs develop normally, and Yemenite citron, in which juice sac formation does not initiate. Most of the identified hormones, abscisic acid, gibberellin A4, indole-3-acetic acid, isopentenyladenine, jasmonic acid and zeatin riboside, were at higher levels in Yemenite than in Calabria. Overall, changes in abscisic acid levels in the pericarp were very well correlated with changes in the expression of abscisic acid-related genes in the endocarp. However, the application of various hormones, including abscisic acid, to Calabria flowers failed to arrest juice sac initiation. The possible involvement of abscisic acid and other hormones in the process of juice vesicle initiation and pericarp growth is discussed.