Antioxidants of Non-Enzymatic Nature: Their Function in Higher Plant Cells and the Ways of Boosting Their Biosynthesis

Abstract

Plants are exposed to a variety of abiotic and biotic stresses leading to increased formation of reactive oxygen species (ROS) in plant cells. ROS are capable of oxidizing proteins, pigments, lipids, nucleic acids, and other cell molecules, disrupting their functional activity. During the process of evolution, numerous antioxidant systems were formed in plants, including antioxidant enzymes and low molecular weight non-enzymatic antioxidants. Antioxidant systems perform neutralization of ROS and therefore prevent oxidative damage of cell components. In the present review, we focus on the biosynthesis of non-enzymatic antioxidants in higher plants cells such as ascorbic acid (vitamin C), glutathione, flavonoids, isoprenoids, carotenoids, tocopherol (vitamin E), ubiquinone, and plastoquinone. Their functioning and their reactivity with respect to individual ROS will be described. This review is also devoted to the modern genetic engineering methods, which are widely used to change the quantitative and qualitative content of the non-enzymatic antioxidants in cultivated plants. These methods allow various plant lines with given properties to be obtained in a rather short time. The most successful approaches for plant transgenesis and plant genome editing for the enhancement of biosynthesis and the content of these antioxidants are discussed.

Keywords: CRISPR/Cas9; antioxidants; ascorbic acid; carotenoids; flavonoids; glutathione; higher plants; isoprenoids; plastoquinone; reactive oxygen species; tocopherol; transgenesis; ubiquinone.

Figure 1

The chemical structures of the main precursors in isoprenoid synthesis. (A) Isopentenyl diphosphate; (B) L-tyrosine; (C) 4-hydroxybenzoic acid.

Figure 2

Schematic overview of biosynthetic pathways of isoprenoid antioxidants. Metabolites: 4-HBA, 4-hydroxybenzoic acid; 4-HPPA, 4-hydroxyphenylpyruvic acid; DAHP, 3-deoxy-D-arabino-heptulosonate 7-phosphate; DMAPP, dimethylallylpyrophosphate; DMPBQ, dimethyl-phytyl-benzoquinone; GGPP, geranylgeranyl diphosphate; HGA, homogentisate acid; IPP, isopentenyl diphosphate; MEP, 2C-methyl-D-erythritol-4-phosphate; MPBQ, methyl-phytyl-benzoquinone; MSBQ, methyl-solanesyl-benzoquinone; MVA, mevalonic acid; Phytyl-DP, phytyl diphosphate; PPDHB, polyprenyl-dihydroxybenzoate; PPHB, polyprenyl-hydroxybenzoate; PPPP, polyprenyl pyrophosphate; Solanesyl-DP, solanesyl diphosphate. Enzymes (colored): 4CL8, peroxisomal 4-coumarate CoA ligase; βLCY1, β-carotene cyclase; β-OHase, β-carotene hydroxylase; εLCY, ε- carotene cyclase; εOHase, ε-carotene hydroxylase; CoQ1 (SPS3), solanesyl diphosphate synthase; Coq3, Coq5, S-adenosyl-l-methionine (SAM)-dependent methyltransferases; Coq2 (PPT1), 4-hydroxybenzoate polyprenyl diphosphate transferase; Coq6, CoqF, flavin-dependent monooxygenases; CRTISO, carotenoid isomerase; FPPS, farnesyl diphosphate synthase; GGPPS, geranylgeranyl diphosphate synthase; GGPPS, geranylgeranyl diphosphate synthase; HPPD, 4-hydroxyphenylpyruvate dioxygenase; HPPR, 4-hydroxyphenylpyruvatereductase; HPT (VTE2), homogentisate phytyl transferase; HST, homogentisate solanesyl diphosphate transferase; IDI, isopentenyl diphosphate isomerase; IPPI, isopentenyl diphosphate isomerase; NXS, neoxanthin synthase; PDS, Phytoene desaturase; PSY, Phytoene synthase; SPS, solanesyl diphosphate synthases; TAT, tyrosine aminotransferase; VDE, violaxanthin de-epoxidase; VTE1, tocopherol cyclase; VTE3, MPBQ/MSBQ methyl transferase; VTE4, γ-tocopherol methyltransferase; VTE5, phytol kinase; VTE6, phytyl-phosphate kinase; ZDS, ζ-carotene desaturase; ZE, zeaxanthin epoxidase. The enzymes of the cytoplasmic stages of isoprenoid synthesis are marked in purple, the stages and enzymes of the chloroplast isoprenoid synthesis are marked in turquoise, the stages of tocopherol synthesis are yellow-brown, the stages of plastoquinone synthesis are green, and the stages of carotenoid synthesis are red. The stages and enzymes of ubiquinone synthesis are marked in brown.

Figure 3

Basic flavonoid skeleton. Benzene rings A and B and heterocycle C are shown in a flavonoid structure.

Figure 4

The pathway of flavonoid biosynthesis in plants. Each colored frame represents a different class of flavonoids. The enzyme names are abbreviated as follows: ACCase, acetyl-CoA carboxylase; ANS, anthocyanidin synthase; ANR, anthocyanidin reductase; C4H, cinnamic acid 4-hydroxylase; CHI, chalcone isomerase; CH4′GT, chalcone 4′-O-glucosyltransferase; 4CL, 4-coumarate CoA ligase; CHS, chalcone synthase; CHR, chalcone reductase; CLL-7, cinnamate–CoA ligase; DFR, dihydroflavonol 4-reductase; FGT (AGT), flavonoid glycosyltransferases; FNS, flavone synthase; FNR, flavanone 4-reductase; F3H, flavanone 3-hydroxylase; F3′H, flavanone 3′-hydroxylase; FLS, flavonol synthase; IFS, isoflavone synthase; LAR, leucoanthocyanidin reductase; PAL, phenylalanine ammonia lyase; STS, stilbene synthase.

Figure 5

The pathways of L-ascorbic acid biosynthesis in plants. The enzymes catalyzing the reactions of Smirnoff–Wheeler, L-gulose, D-Galacturonate, and Myo-inositol pathways are red, purple, blue, and green, respectively.

Figure 6

The pathways of glutathione biosynthesis in plants. The enzyme names are abbreviated as follows: γ-ECS, γ-glutamylcysteine synthetase; GSHS, glutathione synthetase; GR, glutathione reductase; GPX, glutathione peroxidase. The cytoplasmic stages are marked in purple; the plastid stages are marked in green; the mitochondrial stages are marked in brown.