Identification of key molecular and cellular events of table olive (Olea europaea L.'Manzanillo') fruit abscission zone formation during maturation and upon ethephon treatment
Table olive (Olea europaea L. 'Manzanillo') is a non-climacteric fruit that requires high concentration of ethylene-releasing compounds to reduce fruit removal force at harvest, posing challenges for mechanical harvest efficiency. To elucidate the mechanisms of fruit abscission zone (FAZ) development, we examined physiological, transcriptomic, and cellular changes during natural maturation and after ethephon treatment. Analysis of ethylene and carbon dioxide emissions revealed that prior to color transition, the fruit releases a low amount of ethylene, accompanied by reduction of fruit removal force. After 1500 ppm ethephon dipping, a significant reduction of fruit removal force was observed in treatment group compared to the control after one week. To identify the molecular players involved during both natural maturation and ethephon-induced abscission, transcriptome analyses were conducted on the FAZs of immature, horticulturally mature, and ethephon-treated fruits, categorized by growing degree days and fruit removal force. A subset subtracting strategy was used to prevent mesocarp responses dominating the expression profile and help to identify FAZ-specific responses. Transcriptome analysis indicated FAZ-specific genes associated with phenylpropanoid metabolism, beta-1,3-glucanases, and pectate lyase showed increased expression in both horticulturally mature and ethephon-treated samples compared to immature samples. Microscopy-based analyses of FAZ indicate lignification, pH increase, diminishing of plasmodesmata callose, reduction of low-methylesterified homogalacturonan and non-fucosylated xyloglucan are key events of cell wall remodeling of FAZ development. Our study elucidates the mechanisms underlying olive fruit abscission zone development and identifies key events as markers for breeding high-abscission-at-maturity cultivars and management practices for improving mechanical harvest efficiency.