## 3 Non-Enzymatic Defensive Mechanisms
### a-Tocopherol  
\(\alpha\)-tocopherol with its three methyl substituents is considered to be one of the most reactive antioxidants among the four well-known lipoperophilic isomers oftocopherol (\(\alpha\), \(\beta\), \(\gamma\), and \(\delta\)) [26]. Regardless of present tocopherol groups, they are responsible for scavenging lipid peroxy radicals, oxygen free radicals and singlet oxygen [230; 231; 232]. Tocopherols are often localised in the green tissues of plants where photosynthetic organelles and pigments are found [152]. After tocopherol synthesis is completed in the inner envelope of chloroplasts, it has been proposed that they transfer and accumulate in all chloroplast membranes [233; 234]. Given that tocopherols are generally incorporated in the protection of lipids, and proteins and pigments are incorporated in the photosynthetic apparatus against oxidative stress, loss of them would be expected to adversely affect growth and photosynthesis in plant systems [235]. There are indications showing that tocopherols presence in chloroplasts reinforced plant tolerance to salinity, chilling and water deficit stresses [236; 237; 238; 239].  
A combination of five catalysing enzymes (i.e., 4-hydroxyphenylpyruvate dioxygenase, 2-methyl-6-phytylbenzoquinol methyl-transferase, homogentisate phytyl transferases, \(\gamma\)-tocopherol methyl-transferase, locopherol cyclase (VTE\({}_{1}\)), and two precursors (homogentisic acid and phytyl diphosphate) compounds are required for tocopherol biosynthesis [240]. Notably, the enzyme \(\gamma\)-tocopherol methyl-transferase (\(\gamma\)-TMT) has been found to catalyse the biosynthesis of \(\alpha\)-tocopherol from \(\gamma\)-tocopherol [26; 241]. The penultimate step in tocopherol biosynthesis in leaves is dependent on the availability of the VTE1 enzyme [235]. This suggests that VTE\({}_{1}\) deficiency would result in a significant reduction in tocopherol synthesis [242].  
The way tocopherols are utilised to indirectly protect the structure and function of photosystem II (PSII) is to establish a chemical reaction with O\({}_{2}\) and physically quench its excess energy, therefore protecting lipids and other components in the double-membrane of chloroplasts [243]. The prevention of membrane lipid autoxidation through \(\alpha\)-tocopherols interaction with lipid radicals (i.e., RO\({}^{\bullet}\), ROO\({}^{\bullet}\), and RO\({}^{\circ}\)) has identified them as protectors of biological membranes [241; 244]. On the other hand, the free radical trapper \(\alpha\)-tocopherol represses the chain propagation step in the lipid peroxidation (LPO) cycle [26; 152; 245]. Cellular regeneration of oxidised tocopherols (TOH\({}^{\bullet}\)) is often driven by coenzyme Q [246] or AA and GSH [247; 248]. More specifically, the benzoquinone ring (after full substitution) or the phytyl chain (after full reduction) in tocopherols may act as an efficient antioxidant for \({}^{1}\)O\({}_{2}\) neutralisation [249; 250; 251]. In addition to the inhibition of non-enzyme based LPO under stress conditions [241], \(\alpha\)-tocopherols are capable of protecting seed storage lipids, activating plant defence responses, functioning in membrane stability, or involving in seedling germination, transcript regulation and intracellular signalling [252; 253; 254].