## 2 Enzymatic Defensive Mechanisms
### Superoxide Dismutase  
This omnipresent metalloenzyme, which can be found in almost all aerobic organisms including plants, in all intracellular organelles and apoplastic spaces [49], is actively present at the forefront of defence against oxidative damage from ROS [4; 50]. The primary function of superoxide dismutase (SOD, EC 1.15.1.1) is to dismutase superoxide radicals (O\({}^{\bullet-}\)2) into H\({}_{2}\)O\({}_{2}\) and O\({}_{2}\), such that it eventually wards off the formation of OH\({}^{\bullet}\) by the metal-catalysed Haber-Weiss reaction [26; 49; 51]. In other words, the catalysing activity of this enzyme regulates the amount of O\({}_{2}\) and H\({}_{2}\)O\({}_{2}\), which are the Haber-Weiss reaction substrates, and reduces the risk of producing highly active OH\({}^{-}\) radicals. Based on the metal ion it binds, three main isozymes/isoforms (SODs) of this enzyme have been introduced thus far in the _Arabidopsis thaliana_ genome: Cu/Zn-SOD gene localised to the chloroplast thylakoids (CSD\({}_{2}\)), peroxisomes (CSD\({}_{3}\)), and the cytosol (CSD\({}_{1}\)), Fe-SOD gene localised to the chloroplast stroma (FSD\({}_{1}\)), cytosol (FSD\({}_{1}\)), thylakoids (FSD\({}_{2}\), FSD\({}_{3}\)), and the nucleus (FSD\({}_{1}\)), and Mn-SOD (MSD\({}_{1}\)) gene localised to the mitochondrial matrix [11; 46; 49; 52; 53; 54]. These isoforms have been identified as the main ROS scavengers under drought stress conditions [55]. Additionally, nutrient deficiency (K, P, Mg, Ca, S or N) increased SOD functioning and introduced new isoforms in maize plants [56]. The existence of coherent coordination between peroxidase (an H\({}_{2}\)O\({}_{2}\) scavenger) and enhancement of the activity of chloroplastidial Fe-SOD usually leads to plant protection when CO\({}_{2}\) assimilation is intensely reduced; or otherwise results in elevated cytotoxicity in the Haber-Weiss reaction [57; 58]. Such coordination during exposure to oxidative stress has already been observed in leaves of tea [59], tobacco [60], onion [61] and cotton [62] plants. In addition to the patented purification and characterisation techniques (e.g., anion exchange chromatography and ammonium sulfate precipitation) [63], another possible approach to increasing SOD levels in leaves of stress-tolerant plant species would be exposing them to environmental stresses such as low temperature, water deficit, or salinity [64; 65].  
Regardless of SOD types, they are initially encrypted in the nucleus and then transferred to various organs [46; 66]. In chloroplast, the SOD enzyme is attached/bonded to the thylakoid membrane as a solution in the stroma [49]. The former one dismantles the superoxide radical immediately at the site of production, and the stroma-soluble form converts the superoxide radicals released in the stroma into H\({}_{2}\)O\({}_{2}\)[4]. Most of the research conducted on SOD was centred on how environmental stresses or genotypes might affect the enzymatic antioxidants alone [57; 67] or combined [68; 69; 70] with other plant items. In a study by [PERSON] et al. [71], it was found that abiotic stress conditions can upregulate the formation of SODs in potato plants. Heavy metals such as zinc oxide nanoparticles (ZnO-NP) escalate NADP-oxidase activity and thus increase the production of superoxide radicals [58]. Apart from their beneficial antioxidative function in plants suffering from salt [72], oxidative [52], or photooxidative [73] stresses, there are indications showing that SODs have an effect on root development [53], germination [46], flowering [74], and not least, chloroplast development [52].