The significance of different diacylgycerol synthesis pathways on plant oil composition and bioengineering

Abstract

The unique properties of vegetable oils from different plants utilized for food, industrial feedstocks, and fuel is dependent on the fatty acid (FA) composition of triacylglycerol (TAG). Plants can use two main pathways to produce diacylglycerol (DAG), the immediate precursor molecule to TAG synthesis: (1) De novo DAG synthesis, and (2) conversion of the membrane lipid phosphatidylcholine (PC) to DAG. The FA esterified to PC are also the substrate for FA modification (e.g., desaturation, hydroxylation, etc.), such that the FA composition of PC-derived DAG can be substantially different than that of de novo DAG. Since DAG provides two of the three FA in TAG, the relative flux of TAG synthesis from de novo DAG or PC-derived DAG can greatly affect the final oil FA composition. Here we review how the fluxes through these two alternate pathways of DAG/TAG synthesis are determined and present evidence that suggests which pathway is utilized in different plants. Additionally, we present examples of how the endogenous DAG synthesis pathway in a transgenic host plant can produce bottlenecks for engineering of plant oil FA composition, and discuss alternative strategies to overcome these bottlenecks to produce crop plants with designer vegetable oil compositions.

Keywords: acyl editing; biotechnology; diacylglycerol; fatty acid; hydroxylase; oilseed; phosphatidylcholine; triacylglycerol.

Figure 1

De novo DAG/TAG synthesis (Kennedy pathway). Blue arrows indicate reactions involved in de novo DAG(1) synthesis. Substrate abbreviations: DAG, diacylglycerol; G3P, glycerol-3-phosphate; LPA, lyso-phosphatidic acid; PA, phosphatidic acid; PC, phosphatidylcholine; TAG, triacylglycerol. Enzymatic reactions are underlined: A, FA elongation; DGAT, acyl-CoA:DAG acyltransferase; FAS, fatty acid synthesis; GPAT, acyl-CoA:G3P acyltransferase; LPAT, acyl-CoA:LPA acyltransferase; PAP, PA phosphatase.

Figure 2

Acyl editing can provide PC-modified FAs for de novo DAG/TAG synthesis. Blue arrows indicate reactions involved in de novo DAG(1) synthesis. Red arrows indicate reactions involved in acyl editing. Substrate abbreviations: DAG, diacylglycerol; G3P, glycerol-3-phosphate; LPA, lyso-phosphatidic acid; LPC, lyso-phosphatidylcholine; mFA, PC-modified FA; PA, phosphatidic acid; PC, phosphatidylcholine; TAG, triacylglycerol. Enzymatic reactions are underlined: A, modification of FA esterified to PC; CPT, CDP-choline:DAG cholinephosphotransferase; DGAT, acyl-CoA:DAG acyltransferase; FAS, fatty acid synthesis; GPAT, acyl-CoA:G3P acyltransferase; LPAT, acyl-CoA:LPA acyltransferase; LPCAT, acyl-CoA:LPC acyltransferase; PAP, PA phosphatase; PDAT, phospholipid:DAG acyltransferase.

Figure 3

PC-derived DAG/TAG synthesis. Blue arrows indicate reactions involved in de novo DAG(1) synthesis. Purple arrows indicate reactions involved in PC-derived DAG(2) synthesis. PC-derived DAG/TAG synthesis may also utilize acyl editing as in Figure 4. Substrate abbreviations: DAG, diacylglycerol; G3P, glycerol-3-phosphate; LPA, lyso-phosphatidic acid; mFA, PC-modified FA; PA, phosphatidic acid; PC, phosphatidylcholine; TAG, triacylglycerol. Enzymatic reactions are underlined: A, modification of FA esterified to PC; B, reversible CPT or phospholipase C or phospholipase D/PAP DAG production. CPT, CDP-choline:DAG cholinephosphotransferase; DGAT, acyl-CoA:DAG acyltransferase; FAS, fatty acid synthesis; GPAT, acyl-CoA:G3P acyltransferase; LPAT, acyl-CoA:LPA acyltransferase; PAP, PA phosphatase; PDCT, PC:DAG cholinephosphotransferase.

Figure 4

Combined DAG/TAG synthesis pathways and initial labeling fluxes. Red arrows indicate possible initial fluxes of [¹⁴C]glycerol-labeled metabolites. Blue arrows indicate possible initial fluxes of [¹⁴C]acetate-labeled metabolites. Black arrows represent fluxes that will not be initially labeled with either [¹⁴C]glycerol or [¹⁴C]acetate, but will accumulate labeled metabolites over time. Substrate abbreviations: DAG, diacylglycerol; G3P, glycerol-3-phosphate; LPA, lyso-phosphatidic acid; mFA, PC-modified FA; PA, phosphatidic acid; PC, phosphatidylcholine; TAG, triacylglycerol. Enzymatic reactions are underlined: A, FA elongation; B, modification of FA esterified to PC; C, reversible CPT or phospholipase C or phospholipase D/PAP DAG production. CPT, CDP-choline:DAG cholinephosphotransferase; DGAT, acyl-CoA:DAG acyltransferase; FAS, fatty acid synthesis; GPAT, acyl-CoA:G3P acyltransferase; LPAT, acyl-CoA:LPA acyltransferase; LPCAT, acyl-CoA:LPC acyltransferase; PAP, PA phosphatase; PDAT, phospholipid:DAG acyltransferase; PDCT, PC:DAG cholinephosphotransferase.