Specific Changes in the Expression of Cell Wall Carbohydrate Metabolism Related Genes during Fruit Abscission in Rabbiteye Blueberry

Organ separation through abscission occurs at abscission zones (AZs) and involves the breakdown of cell walls. During abscission, the AZ cells respond to the various signals, and activate cell wall loosening enzymes. The spatial and temporal regulation of the dissolution of primary cell wall polysaccharides and middle lamella is not completely understood. The focus of this study was to obtain a comprehensive understanding of changes in the expression of cell wall metabolism related genes during abscission in rabbiteye blueberry. In blueberry, fruit abscission occurs primarily at the pedicel-peduncle junction (PPJ). Next generation sequencing (454) was performed on a pool of potential AZs for identification of AZ related genes. Twenty eight cell wall carbohydrate metabolism related genes were selected using Blast2GO analysis to further study the changes in their expression in response to abscission agent applications. Control (only adjuvant), Ethephon (1000 ppm + adjuvant) and Methyl Jasmonate (MJ; 20mM + adjuvant) treatments were performed on rabbiteye blueberry. Approximately 60% of the berries abscised at the PPJ in response to Ethephon and MJ within 48 h. PPJ and fruit-pedicel junction (FPJ) tissue were collected at 0, 24 and 48 h after treatment. In the PPJ, expression of POLYGALACTURONASE and POLYGALCTURONASE ISOZYME increased by 4- to 5-fold, while PECTATE LYASE, BETA-GLUCANASE, and BETA-GALACTOSIDASE expression increased by more than 8- to 10-fold at 48 h after treatment with MJ and ethephon. Expression of PECTIN METHYLESTERASE and INVERTASE PECTIN METHYLESTERASE INHIBITOR was down-regulated by 2- to 7-fold as a result of MJ and ethephon treatments. Also, the expression of CELLULOSE SYNTHASE decreased as a result of the abscission agent applications. Overall, cell wall carbohydrate metabolism related genes appear to play an instrumental role in abscission, resulting in detachment of the berry. Together, these data suggest that blueberry fruit abscission is mediated by specific changes in the composition of the cell walls at the PPJ.