Triacylglycerols Biosynthesis in Sunflower Seeds

The amount of oil in seeds of different species varies from 1% to as much as 60% of seed dry weight. The oilseed storage reserve provides the necessary energy for the germination and establishment of the subsequent seedling. In sunflower, triacylglycerols (TAGs) represent the main component of seed oil. Sunflower storage lipid synthesis occurs in developing seeds over a period of about two weeks. The biosynthesis of triacylglycerols (TAGs) is a complex process consisting of three well-differentiated phases: (1) de novo synthesis of fatty acids in the plastid, (2) formation of the acyl-CoA pool in the cytosol, and (3) TAG assembling in the endoplasmic reticulum of developing seed tissues. Synthesized TAGs are then stored into densely packed oil bodies (oleosomes) until their utilization. TAG assembly in the ER is closely associated with membrane metabolism and involves the action of several membrane-bound acyltransferase enzymes. Until recently TAG biosynthesis was viewed as a sequential linear process where acyl-CoAs were sequentially assembled onto the sn-1, sn-2, and sn-3 positions of the glycerol backbone, through the so-called Kennedy or glycerol phosphate pathway. However, in the last decade, new activities involved in alternative pathways also operating in the TAG biosynthesis have been discovered and characterized. Therefore, multiple interconnected pathways for TAG synthesis have actually been elucidated in plants. Overall, the sunflower TAG synthesis model highlights a complex network of enzymatic reactions mediating the movement of acyl groups between PC, acyl-CoA, DAG, PA, and TAG and contributes to the final stereospecific composition of storage TAG.