Synthesis of C20-38 Fatty Acids in Plant Tissues

Abstract

Very-long-chain fatty acids (VLCFA) are involved in a number of important plant physiological functions. Disorders in the expression of genes involved in the synthesis of VLCFA lead to a number of phenotypic consequences, ranging from growth retardation to the death of embryos. The elongation of VLCFA in the endoplasmic reticulum (ER) is carried out by multiple elongase complexes with different substrate specificities and adapted to the synthesis of a number of products required for a number of metabolic pathways. The information about the enzymes involved in the synthesis of VLCFA with more than 26 atoms of Carbon is rather poor. Recently, genes encoding enzymes involved in the synthesis of both regular-length fatty acids and VLCFA have been discovered and investigated. Polyunsaturated VLCFA in plants are formed mainly by 20:1 elongation into new monounsaturated acids, which are then imported into chloroplasts, where they are further desaturated. The formation of saturated VLCFA and their further transformation into a number of aliphatic compounds included in cuticular waxes and suberin require the coordinated activity of a large number of different enzymes.

Keywords: cuticular waxes; desaturation of fatty acids; elongase systems; polar lipids; storage lipids; suberin; very-long-chain fatty acids.

Figure 1

The distribution of very-long-chain fatty acids (VLCFA) in plant cells and tissues. C_20–28 VLCFA are the components of polar lipids (PL) of cell membranes. C_20–38 VLCFA are found in developing seeds as a part of reserve triacylglycerols and waxes and are also included in cuticular waxes, suberin and sporopollenin, which form a surface barrier on leaves, stems, roots and pollen shells, respectively.

Figure 2

Metabolic pathways of VLCFA synthesis in plant tissues. The presented scheme shows the main pathways for the biosynthesis of saturated and unsaturated VLCFA, starting with their precursors—18:0 and 18:1 fatty acids (FA). It can be seen that, often, the final product is obtained not in one way but in several. Thus, unsaturated VLCFA can be obtained either by desaturation or by elongation of the previous unsaturated products. It is assumed that the processes of desaturation and elongation of VLCFA can alternate. Different final products can be synthesized from the same intermediate compounds. Vertical arrow—C₂-elongation and horizontal arrow-desaturation.

Figure 3

Enzymes of the VLCFA elongase complex. The elongation of VLCFA in the endoplasmic reticulum (ER) is carried out by an elongase complex that consists of four enzymes: β-ketoacyl-CoA-synthase (KCS), β-ketoacyl-CoA-reductase (KCR), β-hydroxyacyl-CoA-dehydratase (HCD) and enoyl-CoA-reductase (ECR), which catalyze four consecutive reactions: a condensation, a reduction, a dehydration and a final reduction, respectively. All these reactions give an acyl-CoA that is two carbon atoms longer than the initial acyl-CoA. ELONGATION DEFECTIVE-likes (ELO-likes) and FATTY ACID ELONGATION 1 (FAE1) are two families of condensing enzymes.

Figure 4

Genes encoding the elongation and desaturation enzymes involved in the synthesis of VLCFA and their derivatives. The ACC1 gene encodes cytosolic acetyl-CoA carboxylase, which synthesizes malonyl-CoA, used for the formation of regular-chain FA in chloroplasts and for the elongation of VLCFA in the endoplasmic reticulum (ER). Elongation of the VLCFA occurs with the help of the sequential operation of four enzymes of the elongase complex. Two gene families: FAE1 and ELO-likes encode β-ketoacyl-CoA synthase (KCS). The KCR1, GL8A and GL8B genes encode β-ketoacyl-CoA reductase (KCR); PAS2 encodes β-hydroxyacyl-CoA dehydratase (HCD) and CER10–enoyl-CoA reductase (ECR). For the synthesis of VLCFA of different lengths, which are included in the membrane, surface and storage lipids, some specific genes are needed. The Fad2, Fad3, Fad6, Fad7, Fad8, ADS2, ADS3 and ADS4 genes encode the desaturases localized in the ER and chloroplasts. The LACS gene family encodes numerous long-chain acyl-CoA synthases that are involved in several metabolic pathways, starting from FA activation, including the biosynthesis of PL, triacylglycerols (TAG), jasmonate and β-oxidation of VLCFA in peroxisomes.

Figure 5

The regulation of the synthesis of VLCFA in plant cells. The diagram shows four transcription factors (MYB30, MYB94, MYB96 and PUCHI) that positively regulate VLCFA synthesis, and two (ERF13 and DEWAX) regulate it negatively. Pathogen-induced MYB30, ABA-dependent MYB94, MYB96 and auxin-dependent PUCHI activate the expression of genes for VLCFA biosynthesis, which leads to the accumulation of VLCFA in the cell. ERF13 and darkness-induced DEWAX suppress the expression of elongase complex genes and reduce the synthesis of VLCFA in the cell. Auxin-dependent degradation of the ERF13 results in the induction of KCS8, KCS16 and KCS18 gene expression and the accumulation of VLCFA in the cell. Arrows indicate a positive regulation. Lines ending with a bar indicate a negative regulation.