PLANT DEFENSE RESPONSE AGAINST PLANT PATHOGENS AND ANTIOXIDANT DEFENSE SYSTEM

To prevent invasion by pathogen plant metabolism must be flexible and active. The life time of reactive oxygen species (ROS) within cell is determined by the antioxidative system which provides protection against oxidative damage. Antioxidant defense system in plants detoxifies the ROS generated during plant infected with the pathogen. The antioxidative system is comprised of several enzymes and compounds of low molecular weight (Noctor and Foyer 1998). If an antioxidant is not present in sufficient quantity to neutralize ROS in plants which is very toxic and reactive,  oxidation of biomolecule such as lipid peroxidation, protein damage, oxidation of  DNA and RNA, enzyme inhibition and activation of apoptosis will occur (Gill and Tuteja 2010; Dumonovic et al., 2021). The induced defense is facilitated via defensive enzymes such as peroxidase (POD), catalase (CAT), superoxide dismutase (SOD) and ascorbate peroxidase (APX). SOD catalyses the dismutation of superoxide anion radical (O2) to hydrogen peroxide (H2O2), CAT dismutases H2O2 to oxygen and water and APX reduces H2O2 to water by utilizing ascorbate (ASC) as the specific electron donor. Antioxidant in plant are water soluble ascorbate, glutathione and phenol and liposoluble tocopherols, tocotrienols and carotenoids (Dumanovic et al., 2021).

The increased activities of  POD, APX, CAT and SOD enzymes was observed in bean leaves infected with bean yellow mosaic virus  indicating ROS scavenging systems can have an important role in managing ROS generated in response to pathogen (Radwan et al., 2010).  Suppression of POD, APX and CAT activities and induction of SOD reflect the role of salicylic acid (SA) in defense responses against the virus infection ((Radwan et al., 2010). The stimulated antioxidative processes contribute to the suppression of necrotic symptom development in leaves with systemic acquired resistance (Fodor et al., 1997). Both plant and fungi have ROS scavenging activities that detoxify ROS directly through the action of enzyme SOD, CAT, glutathione peroxidase (GPX) and peroxiredoxins (Prx) or indirectly via ascorbic acid (vitamin C), tocopherols (vitamin E) and vitamin B6 (Breitenbach et al., 2015;  Hasanuzzaman et al., 2020). Vitamin B6 is an enzymatic cofactor and potent antioxidant and is thus of importance for cellular well-being (Mooney et al., 2009). 

Plant cell consume large quantity of ascorbate. Ascorbate peroxidase eliminate H2O2. Ascorbate is a substrate for cell wall peroxidases and may play a role in the regulation of cell wall lignification during the hypersensitive response (HR), through its capacity to inhibit the oxidation of phenolic compounds by peroxidases (Mehlhorn et al., 1996; Vanacker et al., 1998). The increased apoplastic antioxidant defences indicate establishment of biotrophy in a susceptible host (Vanacker et al., 1998).

Erysiphae graminis f. sp. hordei infects barley plant causing powdery mildew disease. E. graminis f. sp. hordei infection leads to significant changes in the antioxidant metabolism of barley.  El-Zahaby et al.(1995) concluded that in compatible plant-pathogen interaction the lipid peroxidation is insignificant and antioxidant reactions are induced that inhibit tissue necrotization.

The necrotrophic fungus Botrytis cinerea infection resulted in H2O2 and SOD that occurred during plant-pathogen interaction which is crucial for resistance. Nowogorska and Patykowski (2015) suggested Phaseolus vulgaris cv Korona plant was resistant to Pseudomonas syringae pv. phaseolicola (Psp). After inoculation with this pathogen, H2O2 and SOD level changed.  Indicating induction of enzymatic response after Psp, delayed growth of this necrotrophic pathogen.  Increased peroxidase with ferulic acid (FPOD) and peroxidase with syringaldazine (SPOD) activities were observed both after Psp  and Botrytis cinerea infection, which exhibited their role in strengthening plant cell wall during different kind of infection (Nowogorska and Patykowski 2015). The aim was how plants cope with multiple infection with pathogens having different strategy.  

Rice infected with Rhizoctonia solani induces enzymatic scavenger activities of peroxidase, ascorbate peroxidase, catalase and superoxide dismutase (Paranidharan et al., 2003). R. solani is classified into fourteen anastomosis group (AGs) based on hyphal fusion (Garcia et al., 2006). Samsatly et al.(2018) identified vitamin B6 biosynthetic machinery in R. solani AG3 as an antioxidant exhibiting high ability to quench ROS similar to CAT and glutathione s-transferase.   Non-enzymatic antioxidant carbohydrate mannitol is secreted by few fungus that may function as carbohydrate storage compound and as a scavenger of ROS (Jennings et al., 2002; Voegele et al., 2005; Bleau and Spoel 2021). Since Vicia faba plant is unable to synthesize or utilize mannitol thus it becomes a strategy for the biotrophic fungal pathogen (Voegele et al., 2005).

ROS produced in plant as a defense response to pathogen infection, include modulation of cell death and defense –related gene expression. The cell death as HR enhances resistance against biotrophic pathogen but favours infection by necrotrophs. The chloroplastic ROS generated facilitate infection by the necrotrophic fungal pathogen Botrytis cinerea. The modulation of chloroplastic ROS level by the expression of cyanobacterial antioxidant flavodoxin can provide protection against necrotrophic fungal pathogen (Rossi et al., 2017).

Trichoderma reduced H2O2 which might protect membrane lipid from peroxidation. Trichoderma citrinoviride protect plant from disease by functioning as antagonist or by triggering the antioxidant defense system in plants (Sekmen Cetinel et al., 2021).

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