Photosynthates provide an endogenous source of acetyl-CoA by activated acetyl-CoA synthetase in the stroma, from free acetate, or from the cytosolic conversion of glucose to pyruvate during glycolysis (Somerville et al., 2000; Schwender and Ohlrogge, 2002). This acetyl-CoA is preferentially transported from the cytosol to the plastid, where it is converted to the fatty acid and subsequently to TAG, which again is transported to the cy­tosol and forms the lipid bodies (Figure 8.1). The acetyl-CoA pool will be maintained through the Calvin cycle, glycolysis and pyruvate kinase (PK) mediated synthesis of py­ruvate from PEP, which occur in the chloroplast in addition to the cytosol. The first reaction of the fatty acid biosynthetic pathway towards the formation of malonyl-CoA from acetyl — CoA and CO2 is catalyzed by the enzyme Acetyl-CoA carboxylase (ACCase). (Ohlrogge and Browse, 1995). Figure 8.2 illustrates the conversion of acetyl-CoA to malonyl-CoA by utilizing ATP. During this process, seven molecules of acetyl-CoA and seven molecules of CO2 form seven molecules of malonyl-CoA. This malonyl Co-A undergoes synthesis of long carbon-chain fatty acids through repeating multistep sequences, as represented in Figures 8.2 and 8.3. A saturated acyl group produced by this set of reactions becomes the substrate for subsequent condensation with an activated malonyl group (Ohlrogge and Browse, 1995).

FIGURE 8.3 Sequential chain elongation steps and formation of precursor molecules (palmitic acid) from CO2