Transcriptomic and functional genomics of russeting on apple

Abstract

Russeting is a common, economically important fruit skin disorder that occurs in many fruit crops, including apples (Malus x domestica, Borkh.). This skin disorder is caused by environmental factors such as high humidity, prolonged periods of surface moisture, and wounding. While previous studies have focused on the later stages of russeting, the initiation sequence of this disorder remains unclear. To address this knowledge gap, this dissertation aimed to: (1) establish an induction system for russeting in apple fruit using prolonged periods of surface moisture, (2) determine the sequence of russet formation on a histological level under prolonged periods of surface moisture, (3) investigate the behavior of candidate genes involved in periderm and cuticle-related processes on the gene expression as well as metabolic changes, (4) compare the sequence of wound- and moisture-induced russeting, (5) provide a transcriptomic resource for the initial processes during russet formation in apples, and (6) to provide preliminary data on potential candidate genes involved in russeting. Application of surface moisture using a fixed polyethylene tube to developing apple fruit was found to induce microcracks in the fruit skin during early fruit development, and an increase in water vapor permeance was observed in microcracked compared to non-microcracked fruit skins. Microcracked fruit surfaces developed russeting. Histological, gene expression, and metabolic analyses revealed a biphasic behavior during the formation of russeting in response to surface moisture. In Phase I, microcracks appeared within 2 d of moisture treatment and expanded over time, accompanied by a decrease in cuticle-related genes and cutin- and wax-specific metabolites. In Phase II, microcracking decreased after moisture removal, and periderm formation was observed starting 4 d after moisture removal, accompanied by an increase in periderm related genes and suberin specific metabolites. Russeting was observed during Phase II only after at least 6 d of moisture exposure in Phase I. Histological, gene expression, and metabolic analyses showed that the sequence of russeting initiation during Phase II induced by surface moisture was similar to that induced by skin wounding. Transcriptomic analyses revealed that Phase I was characterized by a decrease in cell cycle, cell wall, and cuticle-related genes and an increase in stress-related genes, whereas Phase II was characterized by an increase in meristematic activity, followed by an increase in abscisic acid, lignin, and suberin-related genes. Overall, the studies of this dissertation provide for the first time information on the initial processes of russeting in apple fruit skin and are a valuable resource for future research on the molecular mechanisms underlying this phenomenon.