Identification of Changes in the Glycome Profile of Cell Walls During Blueberry Fruit Abscission

Abscission is a highly coordinated and regulated process which responds to developmental cues, and various biotic and abiotic stresses. Organ separation occurs at abscission zones (AZs) and involves the breakdown of cell walls. The spatial and temporal regulation of the dissolution of primary cell wall polysaccharides and middle lamella is not completely understood. Previous studies have indicated that the loss of pectins and other polysaccharides from the middle lamella and the primary cell wall is associated with abscission. The focus of this study was to obtain a comprehensive view of the changes within the cell wall during blueberry fruit abscission. In blueberry, fruit abscission occurs primarily at the pedicel-peduncle junction (BAZ). Abscission agents, such as Methyl Jasmonate (MJ) and Ethephon, induce abscission at the BAZ. Rabbiteye (‘Briteblue’) blueberries were given 3 treatments: Control (only adjuvant), Ethephon (1000 ppm + adjuvant) and MJ (20 mM + adjuvant). Approximately 50% of the berries abscised in response to Ethephon and MJ within 48 h. BAZs and adjacent pedicel tissue (PZ) were collected at 24 h after treatment.  These samples were used to perform plant cell wall glycome profiling using over 200 glycan-directed monoclonal antibodies. Overall, the glycome profiles of the control and treated samples from BAZ and PZ were similar in the glycan compositions. However, some differences were notable between BAZ and PZ. For example, the oxalate extractable homogalacturonan (HG) epitopes and HG backbone-1 are significantly higher in the PZ. Also, notably more hemicellulosic polysaccharides were extracted by carbonate in the PZ compared to the BAZ. Two specific differences associated with abscission agent application were: 1) hemicellulosic polysaccharides in the carbonate fraction: more Xyloglucan (XG) epitopes were released in MJ treated tissues of BAZ, suggesting that loosening of XG is a BAZ specific phenomenon brought about by MJ mediated mechanisms; and 2) the relative abundance of hemicellulosic epitopes (Xylan and XG) released in the chlorite extract of treated walls was reduced, potentially as a result of reduced lignin-hemicellulose association within the cell walls of the active abscission zone under the induction of abscission by Ethephon and MJ. Together, these data indicate that blueberry fruit abscission is mediated by specific changes in the composition of the cell walls.