Sphingolipid long chain bases as mediators of cell death in olive fruit abscission

Plant sphingolipids are lipophilic membrane components essential for different cellular functions but they also act as signaling molecules in various aspects of plant development. However, the interaction between plant sphingolipids and abscission remains largely uncharacterized. Here, the possible role of sphingolipids in regulating fruit abscission was examined in the abscission zone (AZ) of olive fruit. To this end, sphingolipid levels were manipulated through the application of exogenous sphingolipid long-chain bases (LCBs) or biosynthesis inhibitors, and their effects on fruit abscission as well as sphingolipid LCB/gene expression, hormones, reactive oxygen species (ROS) and cell death levels were examined in the AZ of olive fruit. Our data indicated that exogenous sphinganine (d18:0) induced fruit abscission, whereas the application of sphingosine (d18:1) or phytosphingosine (t18:0) or their phosphorylated derivatives did not have an effect on fruit abscission. Moreover, inhibition of LCB kinase or ceramide synthase, which increases sphingolipid LCB levels in the AZ, reduced fruit break strength. This induction of fruit abscission is associated with elevated ROS levels and cell death in the AZ enriched in salicylic acid (SA) and jasmonic acid (JA). Along the same line, programmed cell death (PCD) was particularly evident on the distal side of the AZ. These data suggest that endogenous d18:0 plays a key cellular role as signaling molecule functioning upstream of the SA/JA signaling pathway in mediating PCD spatially regulated in the AZ during fruit abscission. Overall, the findings reported here provide insight into the complex connection between PCD and plant sphingolipid LCBs, uncovering their interaction in the abscission process.