*2.2.3 Lipid (triacylglycerol) synthesis*

Lipids (triglycerides) are formed by combining a glycerol with three fatty acids via ester linkages. Glycerol and free fatty acids do not combine to form lipids, and glycerol-3-phosphate (G-3-P) and fatty acids bind to coenzyme A (fatty acyl-CoA) or acylbearing protein (ACP). The precursor of all fatty acids in seeds is acetyl-CoA, which is

#### **Figure 3.** *Production of fatty acids from glucose in plants [40].*

#### *Seed Filling DOI: http://dx.doi.org/10.5772/intechopen.106843*

derived from sucrose in plants. After sucrose is transported into the developing seed, it is converted to hexose phosphate (Glc-6-P) and triose phosphate (Triose-P) in the plastids. The dihydroxyacetone phosphate is then reduced to yield glycerol-3-phosphate (G-3-P) in the cytosol. G-3-P is then esterified with three fatty acids in the endoplasmic reticulum to form triglycerides. Accumulation of G-3-P is an important limiting factor for the formation of new triglycerides. Glc-6-P is usually transferred to the plastids, but in some species it may also be converted to other intermediates in the cytosol or mitochondria. The first product for fatty acid synthesis is acetyl-CoA, which provides the 2C-acyl groups for the fatty acid chain. The first coupling step in fatty acid biosynthesis is the carboxylation of acetyl-CoA carboxylase (ACCase) to form malonyl-CoA. It is then converted to malonyl-ACP by addition of ACP with malonyl transacylase. Several enzymes are involved in a FAS complex that adds 2C to the extended chain and increases the length of the fatty acid at each cycle. Fatty acids are released from the ACP complex by acyl-ACP trioesterase (FAT) (**Figure 4**) [1, 40–44].
