Regulation of Ascorbate-Glutathione Pathway in Mitigating Oxidative Damage in Plants under Abiotic Stress

Abstract

Reactive oxygen species (ROS) generation is a usual phenomenon in a plant both under a normal and stressed condition. However, under unfavorable or adverse conditions, ROS production exceeds the capacity of the antioxidant defense system. Both non-enzymatic and enzymatic components of the antioxidant defense system either detoxify or scavenge ROS and mitigate their deleterious effects. The Ascorbate-Glutathione (AsA-GSH) pathway, also known as Asada-Halliwell pathway comprises of AsA, GSH, and four enzymes viz. ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase, play a vital role in detoxifying ROS. Apart from ROS detoxification, they also interact with other defense systems in plants and protect the plants from various abiotic stress-induced damages. Several plant studies revealed that the upregulation or overexpression of AsA-GSH pathway enzymes and the enhancement of the AsA and GSH levels conferred plants better tolerance to abiotic stresses by reducing the ROS. In this review, we summarize the recent progress of the research on AsA-GSH pathway in terms of oxidative stress tolerance in plants. We also focus on the defense mechanisms as well as molecular interactions.

Keywords: antioxidant defense; free radicals; glyoxalase system; hydrogen peroxide; plant abiotic stress; reactive oxygen species; redox biology; stress signaling.

Figure 1

Ascorbate-Glutathione (AsA-GSH) (Ascorbate-Glutathione) pathway [ascorbate, AsA; ascorbate peroxidase, APX; monodehydroascorbate, MDHA; monodehydroascorbate reductase, MDHAR; dehydroascorbate, DHA; dehydroascorbate reductase, DHAR; glutathione, GSH; oxidized glutathione, GSSG; glutathione reductase, GR; Nicotinamide adenine dinucleotide phosphate (reduced form), NAD(P)H; Nicotinamide adenine dinucleotide phosphate (oxidized form), NAD(P)⁺].

Figure 2

Ascorbate biosynthesis and metabolism is a complex set of reactions, some involving unidentified enzymes; some of the products are reactive and potentially damaging carbonyl compounds, (A) biosynthetic pathway; and, (B) regeneration and degradation pathways in plants. The metabolites in the violate box represent the name of each biosynthetic pathway. The elaborated name of enzymes are as follows (HK: Hexokinase; PGI: glucose-6-phosphate isomerase; PMI: mannose-6-phosphate isomeras; PMM: phosphomannomutase; TC1 or GMP: GDP-d-mannose pyrophosphorylase/mannose-1-phosphate guanylyltransferase; VTC2 or GGP: GDP-d-mannose 3′,5′-epimerase, GME: GDP-l-galactose phosphorylase;VTC4 or GPP:l-galactose-1-phosphate phosphatase; GalDH: l-galactose dehydrogenase; l-GalLDH: l-galactono-1,4-lactone dehydrogenase; ?: nucleotide pyrophosphatase or sugar-1-phosphate guanyltransferase; ??: sugar phosphatase; l-GulDH: l-gulose dehydrogenase; l-GulL: l-gulonolactonase; l-GulLOX: l-gulono-1,4-lactone oxidase; PPGT: polygalacturonate 4-alpha-galacturonosyltransferase; d-GalPUT: d-galacturonate-1-phosphate uridyltransferase; d-GalUR: d-galacturonate reductase; AL: aldonolactonase; PGM: phosphoglucomutase; UDPGluPP: UDP-glucose-pyrophosphorylase; UDP-GluDH: UDP-glucose dehydrogenase; UDP-GluPUT: glucuronate-1-phosphate uridylyltransferase; d-GluPP: d-glucurono-1-phosphate phosphatase; MIOX: myo-inositol oxygenase;d-GluR: d-glucuronate reductase; MDHAR: monodehydroascorbate reductase; DHAR: dehydroascorbate reductase;l-IDH: l-Idodonate dehydrogenase).

Figure 3

Glutathione biosynthesis, metabolism, and degradation in plants. (A) Biosynthesis the first step occurred in plastid: Glu and Cys form γ-glutamylcysteine (γ-EC) catalyzed by γ-EC synthetase (γ-ECS). The second step occurred in the cytosol or in plastid: γ-EC and Gly bond together to form GSH catalyzed by GSH-S (glutathione synthase). Further, GSH participates in ROS scavenging and is converted into GSH/glutathione disulfide (GSSG) by the enzyme glutathione peroxidase (GPX), glutathione S-transferase (GST), and DHAR. Further GSSG can be recycled to GSH by the activity of glutathione reductase (GR). (B) In the degradation pathway, GSH and S-conjugated compound (GS-X) can be degraded to γ-EC and γ-EC-X by phytochelatin synthase (PCS). While, carboxypeptidase (Cpep) and γ-glutamyl transpeptidase (GGT) both could degrade GS-X to form γ-Glu-aa (aa, amino acid) and γ-EC-X, respectively. Similarly, GSSG is degraded by GGT and Cpep to form γ-Glu-aa and γ-EC, respectively. Further, the produced γ-Glu-aa is converted to 5-oxoproline (5-OP) by γ-glutamyl cyclotransferase (GGC). Besides, GSH is also converted to 5-OP. Although it is thought that this reaction is catalyzed by GGC, still it is unclear. 5-OP is converted to Glu in the next step by the action of 5-oxoprolinase (OPase).

Figure 4

The function of Ascorbate peroxidase (APX) for the abolition of excess reactive oxygen species (ROS) generation in various cellular compartments. Additional details are in the text.

Figure 5

The antioxidant of MDHAR in regenerating AsA to support the removal of reactive oxygen species (ROS) (lower left) contrasts the pro-oxidant role of MDHAR creating 2,4,6-trinitrotoluene (TNT) toxicity.

Figure 6

The mechanistic scheme, the ping-pong mechanism for the enzymatic reduction of dehydroascorbate (DHA).

Figure 7

Mechanistic scheme for the enzymatic reduction of glutathione/oxidized glutathione (GSSG) in a plant cell.

Figure 8

Abiotic stress-induced oxidative stress through the generation of ROS. Additional details are in the text.

Figure 9

AsA-GSH pathway of the antioxidant defense system and its relation with the glyoxalase system. Additional details are in the text.