## 2 Enzymatic Defensive Mechanisms
### Catalase  
As a Fe-containing homotetrameric enzyme, catalase (CAT, EC 1.11.1.6) is responsible for the catalysation of H\({}_{2}\)O\({}_{2}\) overproduced during light respiration or photorespiration in peroxisomes, H\({}_{2}\)O\({}_{2}\) produced during \(\beta\)-oxidation of fatty acids in glycosylomes, or H\({}_{2}\)O\({}_{2}\) produced by SOD [4; 75]. In all these cases, CAT mediates the dismutation of oxygenated water (H\({}_{2}\)O\({}_{2}\)) into water and oxygen in an energy-efficient way [26]. Peroxisomes are the core sites of CAT activity in response to H\({}_{2}\)O\({}_{2}\) production as a result of oxidative stress, purine catabolism, photorespiration, and not least the \(\beta\)-oxidation of fatty acids [10; 76]. Nevertheless, a more recent report highlights ther presence in the cytosol, chloroplast, mitochondria, and other subcellular compartments [77]. Despite having a high specificity for H\({}_{2}\)O\({}_{2}\), CAT showed a low affinity for organic peroxides [77]. One of the unique characteristics of this enzyme is its high turnover rate and low demand for reducing equivalents [26]. This enzyme is encoded by different CAT genes in several plant species [11]. For example, these include seven CAT genes reported in cotton (_Gossyipium hirsutum_ L.) [78], four in cucumber (_Cucunis sativus_ L.) [79], and in rice (_Oryza sativa_ L.) [80], three in (_Arabidopsis thaliana_ L.) [PERSON]. [81], pumpkin (_Cucurbita Linn._) [82], maize (_Zea mays_ L.) [83], Tex-Mex tobacco (_Nicotiana plumbaginfilia_ [PERSON]) [84], two in common barley (_Hordeum vulgare_ L.) [85], and in tomato (_Lycopersicon esculentum_ Mill.) [86], and one in sweet potato (_Ipomoea batatas_ L.) Poir. [87], and in castor bean (_Ricinus communis_ [PERSON]) [88], among others (Table 1). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Plant Species** & **Common Name** & **Genomic DNA or Protein** & **Subcellular Localisation and Site of Detection in Plant** & **References** \\ \hline \multirow{4}{*}{_Brassica napus_} & & & Peroxisomes (CAT\({}_{2}\)–CAT\({}_{3}\), CAT\({}_{6}\)–CAT\({}_{8}\), & \\  & & & CAT\({}_{11}\)–CAT\({}_{14}\)), cytoskeleton (CAT\({}_{1}\)), & \\  & & & cytoplasm (CAT\({}_{4}\)), mitochondon (CAT\({}_{9}\)), & [89] \\  & & & and chloroplast (CAT\({}_{5}\), CAT\({}_{10}\)) in root, & \\  & & & leaf, stem, and silique samples & \\ \hline \multirow{2}{*}{_Heliantlus anmus_} & \multirow{2}{*}{Sunflower} & CAT\({}_{1}\)–CAT\({}_{8}\) & Peroxisomes in cotyledons and roots & [90; 91; 92] \\ \cline{2-2}  & & & & \\ \hline _Gossyipium_ & \multirow{2}{*}{Cotton} & CAT\({}_{1}\)–CAT\({}_{7}\) & Peroxisomes in leaves & [78; 93] \\ \cline{2-2}  & & & & \\ \hline _Pisum sativum_ & \multirow{2}{*}{Pea} & CAT\({}_{1}\)–CAT\({}_{5}\) & Peroxisomes in leaves and the whole fruit & [94; 95] \\ \hline \multirow{2}{*}{_Cucunis sativus_} & \multirow{2}{*}{Cucumber} & CAT\({}_{1}\)–CAT\({}_{4}\) & Peroxisomes in roots, stem, leaves, & \\  & & & flowers, and fruits & \\ \hline \multirow{4}{*}{_Oryza sativa_ L.} & \multirow{2}{*}{Rice} & \multirow{2}{*}{OsCATA–OsCATD} & Cytosol (OsCATA–OsCATC), & \\  & & & peroxisomes (OsCATB, OsCATC), and & \\ \cline{1-1}  & & & plasma membrane (OsCATD) in & [80; 96; 97] \\ \cline{1-1} \  
In Arabidopsis, the first two (CAT\({}_{1}\) and CAT\({}_{2}\)) genes are predominantly present in glycosylomes, peroxisomes and cytosol, and the latter (CAT\({}_{3}\)) is present in mitochondria and cytosol, with no presence reported in chloroplasts [4; 77]. Recent reports indicate that these genes are primarily expressed in seeds (CAT\({}_{1}\), CAT\({}_{2}\), CAT\({}_{3}\)), pollems (CAT\({}_{1}\)), roots (CAT\({}_{2}\)), photosynthetic tissues (CAT\({}_{2}\)), and vascular tissues (CAT\({}_{3}\)) [26; 121; 122; 123; 124]. The involvement of CATs in the growth, development, or senescence of organs such as roots [122], shoots, flower parts (ovule, pollen), seeds [121; 123], and leaves [77; 125] has been explicitly documented.