Sphingolipid and sterol accumulation during olive fruit abscission

Sphingolipids, found in membranes of eukaryotic cells, have been demonstrated to carry out functions in various processes in plant cells. However, the roles of these lipids in fruit abscission remain to be determined in plants. The biochemical approach has been adopted to investigate the accumulation of sphingolipids during mature-fruit abscission in olive (Olea europaea L. 'Picual'). Mature-fruit abscission resulted in quantitative and qualitative changes in sphingolipid long-chain bases (LCB) in the olive abscission zone (AZ). The total LCB increase was due essentially to the increase of t18:1(8E) LCBs, suggesting that C-4 hydroxylation and.8 desaturation with a preference for (E)-isomer formation were quantitatively the most important sphingolipids in olive AZ during abscission. However, our results also showed a specific association between the dihydroxylated LCB sphinganine (d18:0) and the mature-fruit abscission. These results indicate a clear correlation between the sphingolipid composition and mature-fruit abscission. Moreover, measurements of endogenous sterol levels in the olive AZ revealed that it accumulated sitosterol and campesterol with a concomitant decrease in cycloartenol during abscission. In addition, underlying the distinct sterol composition of AZ during abscission, genes for key biosynthetic enzymes for sterol synthesis, for obtusifoliol 14a-demethylase (CYP51) and C-24 sterol methyltransferase2 (SMT2), were upregulated during mature-fruit abscission, in parallel to the increase in sitosterol content. The differences found in AZ lipid content and the relationships established between LCB and sterol composition offer new insights about sphingolipids and sterols in abscission.