Secondary abscission zones: understanding the molecular mechanisms triggering stylar abscission in citrus

The separation events of abscission occur in a group of functionally specialized cells known as the abscission zone (AZ). This tissue differentiates simultaneously with the development of lateral organs formed from the shoot apical meristem (SAM) in a limited number of morphological locations on the plant body (primary AZs) or after the development of lateral organs in a position that is not predetermined by the morphology of the plant (secondary AZs). In addition, differentiation and activation of secondary AZs are almost concurrent processes whereas primary AZs remain inactive until they acquire the competence to respond to abscission signals. Research on abscission has been mainly focused on the analysis of primary AZs while secondary AZs have drawn limited attention. In citrus pistils, the unit formed by the stigma and the style usually abscises several weeks after anthesis. Differentiation of the style AZ takes place in the basal portion of the style near the ovary. However, style AZ differentiation and activation does not occur in certain cultivars of citrus that retain the style throughout fruit development and maturation. In order to study style AZ differentiation and activation we performed a comparative anatomical and molecular analysis in pistils of both 'Navelate' orange and 'Bergamot of Calabria' showing regular stylar abscission and persisting styles, respectively. A better understanding of the molecular mechanisms underlying stylar abscission may be the starting point to envisage strategies to prevent fertilization and ensure the production of seedless citrus fruits without the need of avoiding pollination.