In conclusion, our data provided very clear evidence for the contribution of LACS4 and LACS9 to lipid trafficking between ER and plastids. Surprisingly, LACS9 which was historically always associated with the export of fatty acids from the plastid seem to be much more important for the transfer of fatty acids in the opposite direction. Moreover our analyses of the lacs4 lacs9 and the lacs1 lacs4 double mutants revealed that LACS4 is involved in very different metabolic pathways by overlapping with different other members of the LACS enzyme family. Since transcriptomic data showed for LACS4 the highest expression levels of all family members and the most widely spread distribution in different tissue it seems legitimate to propose that LACS4 is the most important activity of the LACS protein family in Arabidopsis.