PLANT DEFENSE RESPONSE AGAINST PLANT PATHOGENS AND THE EQUILLIBRIUM BETWEEN REACTIVE OXYGEN SPECIES AND THE ANTIOXIDANTS

The earliest cellular response of a plant on recognition of pathogen is production of reactive oxygen species such as hydrogen peroxide (H₂O₂), the superoxide anion, hydroxyl radical (OH) and singlet oxygen that inactivate enzymes and damage cellular components. Hydrogen peroxide reacts with proteins (attacking cysteine and methionine residue) and DNA, superoxide reacts with Fe-S proteins, dismutases to H₂O_2, whereas OH is extremely reactive with DNA, RNA, lipids and proteins (Dumanovic et al., 2021). The antioxidant defense system of the plant can counter reactive oxygen species (ROS). Antioxidant are component of signaling and defense mechanism in few plants, where they are precursors of compounds of much more complex nature, the modulator of plant growth and defense system against plant pathogens and predators (Barreca 2021).

ROS are produced in plant as by-product of aerobic metabolic reaction such as phototsynthesis and respiration. Plant generate ROS as signaling molecule to control infection caused by pathogen causing rapid cell damage leading to programmed cell death (Apel and Hirt 2004). Production of excessive ROS can lead to the lignin production, peroxidation of membrane lipids, glycation/oxidation/nitration of proteins, inactivation of enzymes, DNA damage and mutation, phytoalexin production and hypersensitive response (Baker and Orlandi 1995; Das and Roychoudhury 2014; Irato and Santovito 2021). The equilibrium between ROS generation and detoxification is maintained by enzymatic and non-enzymatic antioxidants. Antioxidants protect plant cell from oxidative stress. ROS associated with the response to pathogen attack are generated by NADPH oxidases or apoplastic peroxidases (Daudi et al., 2012).The chloroplast, mitochondria and peroxisomes perceives the external stimuli as pathogen invasion and triggers resistance response using ROS as signal (Camejo et al., 2016). The generation of ROS in the chloroplast is involved in hypersensitive response resulting in cell death is mediated by a mitogen activated protein kinase cascade (Liu et al., 2007).

Chloroplast and mitochondria are the power house of photosynthetic cell. The oxidation-reduction (redox) cascade of the photosynthetic and electron transport chain provides the driving force for metabolism as well as generate redox signals which may regulate gene expression and translation to enzyme (Foyer and Noctor 2003). Plastoquinone thioredoxin and reactive oxygen display signaling function. Further more the intrinsic involvement of molecular oxygen in electron transport processes as well the inherent generation of superoxide, H₂O₂ and singlet oxygen generates powerful signal. There are studies of accumulation of ascorbate and glutathione in redox signal transduction (Foyer and Noctor 2003). Ascorbate peroxidase, monodehydroascorbate reductase were present in mitochondria and peroxisomes (Jimenez et al., 1997). These ROS are short lived and are subject to cellular antioxidant mechanisms such as super oxide dismutases, peroxidases, the ascorbate/glutathione cycle and catalase (Baker and Orlandi 1995).

Enzymatic and non-enzymatic antioxidant molecules are present in microorganisms, plants and animal. Antioxidant defense system constitutes enzymatic components (Sharma et al., 2012; Irato and Santovito 2021; Vanacker et al., 1998) such as:

Superoxide Dismutase (SOD)
Catalase (CAT) and peroxidase (POX)
Glutathione peroxidase (GPX), glutathione reductase (GR), glutathione S-transferases (GST), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR)

Ascorbate peroxidase is a hydrogen peroxide-scavenging enzyme in plants that protect chloroplast and other cell constituents from damage by H₂O₂ and hydroxyl radicals produced from it (Asada 1992).

Non-enzymatic Antioxidants (Das and Roychoudhury 2014; Irato and Santovito 2021; Racchi 2013):

Ascorbic acid
Reduced Glutathione
Carotenoids
α-Tocopherols and Tocotrienols
Phenolic Compound, alkaloid, flavonoids

Vitamin E (α-tocopherol) prevents photo-oxidative deterioration of bio membranes (Fryer 1992).

Antioxidants can also be categorized in different ways such as (Nimse and Pal 2015):

Water-soluble antioxidant example vitamin C are present in the cellular fluid like cytosol or cytoplasmic matrix. They are free radical scavenger
Lipid-soluble antioxidants example vitamin E, carotenoids and lipoic acid that are predominantly located in cell membranes.
Small molecule antioxidants and large molecule antioxidants:
The small molecule antioxidants neutralize the ROS in a process known as radical scavenging. The antioxidant of this category are vitamin C, vitamin E, carotenoid and glutathione.
The large molecule antioxidant are enzymes SOD, CAT, GPX and sacrificial protein (albumin) that absorb ROS and prevent them from attacking other essential proteins.

Foyer and Noctor (2003) suggest antioxidative component have dual function of scavenging and signaling. A model for perception of increased ROS production via the antioxidant system was presented:

Under optimal condition: ROS produced by many metabolic reactions are removed by detoxification (detox-scavenging).
An oxidative stress caused by increased production of ROS or imbalance between production and accumulation of ROS i.e. decreased antioxidant activity causes increased ROS concentration. A regulatory role for glutathione is that they may influence the expression of many gene important for plant response to biotic and abiotic stress (Mullineaux and Rausch 2005).

Nonenzymatic components of Antioxidative Defense System:

Ascorbate (AsA) is present in abundance and is a low molecular weight antioxidant. This powerful antioxidant provides protection against oxidative damage caused by enhanced level of ROS (Noctor and Foyer 1998). AsA has the ability to donate electrons in several enzymatic and non-enzymatic reaction (Sharma et al., 2012). Overexpression of glutathione reductase in the chloroplast increases antioxidant capacity of the leaves improving the capacity to withstand oxidative stress (Foyer et al., 1995).

Infection with an avirulent strain causes production of reactive oxygen intermediate. The H₂O₂ produced from this oxidative burst drives cross-linking of cell wall structural proteins but also triggers programmed cell death in challenged cell as well as act as a diffusible signal for the induction in adjacent cells of genes encoding cellular protectants such as glutathione S-transferase and glutathione peroxidase (Levine et al., 1994).

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