Overview of biosynthesis, accumulation, and deposition of selected fatty acids in plants.

Panel A: In plants, synthesis of neutral lipids requires the collaboration of plastids and the endoplasmic reticulum while their accumulation occurs in ER derived organelles called lipid droplets, found in all organs. Blue arrows indicate flux toward neutral lipid accumulation in organelles and selected organs. Panel B: The synthesis and elongation of FAs, bound to ACP occur in the chloroplast till 18:0 (1, fatty acyl synthase) and its desaturation into 18:1^Δ9c by Δ9 stearoyl-ACP desaturase (SAD) (2). In a similar way, 16:0 desaturation by Δ9 palmitoyl-ACP desaturase (PAD) (3) produces palmitoleic acid (16:1^Δ9), which can be further elongated to produce cis vaccenic acid 18:1^Δ11c, (1). The synthesis of Acyl-ACP can be interrupted Acyl-ACP thioesterases (4) releasing free FA with chain lengths under 18C. Panel C: Free FAs are transported through the membrane, activated into the form of CoA esters by long-chain acyl-CoA synthetases (LACS) (5) and transferred to the endoplasmic reticulum (ER) by acyl-CoA binding proteins (6), where they are further processed. Oleic acid elongation and the other FA modifications take place mostly in the ER and involve different enzymes (elongases, hydroxylases, desaturases) which use acyl bound to ACP, glycerolipids (GL), PC and acyl-CoA as substrates. Oleic acid bound to PC or GL is transformed into linoleic acid (7, Δ12 desaturase). FAD3 catalyzes the desaturation of linoleic (18:2^Δ9c,12c) acid into linolenic (18:3^Δ9c,12c,15c) (8). FADX, a fatty acid conjugase desaturase is responsible for the synthesis of eleostearic acid (18:3^Δ9c,11t,13t) (9). Vernolic acid (Δ12,13–O–18:1^Δ9c) is obtained upon the action of cytochrome P450 on 5 PC-bound linolenic acid or of delta 12 epoxygenase (Δ12 desaturase like) (10). Hydroxylation of the Δ12 position of PC bound oleic acid (fatty acid hydroxylase) by homologs of fatty acid desaturase 2 (FAD2) (11) produces ricinoleic acid (Δ12–OH–18:1^Δ9c). Ricinoleoyl-CoA, deriving from Ricinoleoyl-PC, is transformed into nebraskanic (7,18–(OH)2–24:1^Δ­15) acid upon the action of hydroxylases (12). Elongation of oleoyl-CoA by acyl-CoA elongase (13) produces eicosenoic (20:1^Δ11c) and erucic acid (22:1^Δ13c). Panel D: Esterification of FA to glycerol for accumulation into lipid droplets. Dihydroxyacetone phosphate (DHAP) originates from glycolysis and is converted to glycerol-3-phosphate (G3P) by glycerol-3-phosphate dehydrogenase (G3PDH) (14). G3P is acylated at sn1 positions by a glycerol phosphate acyltransferase (GPAT) (15) which produces lysophosphatidic acid (LPA). Acylation at sn2 position by a 1-acyl G3P acyltransferase (16) produces phosphatidic acid (PA). The dephosphorylation of PA by phosphatidic acid phosphatases (PAP) (17) produces 1,2-diacyl-sn-glycerol (DAG). Another acyl group is esterified at the sn-3 position of DAG either by acyl-CoA:1,2-diacyl-sn-glycerol acyltransferase (DGAT, acetyl-CoA dependent pathway) (18), or by the phospholipid:1,2-diacyl-sn-glycerol acyltransferase (PDAT) (19) which transfers acyl from PC to DAG. For the sake of clarity, we have focused our attention on the FAs which are exemplified in this review. For more detailed and comprehensive descriptions of FA metabolism in model plants and crops, please refer to the following very recent articles (Baud, 2018; Cahoon and Li-Beisson, 2020; Miklaszewska et al., 2021).