## 5 Can Bacterial Antioxidant Activity Reduce Oxidative Stress towards Cropped Plants? Plant growth promoting bacteria (PGPB) are microbes that form part of the plant holobiont [81]. They are found in the rhizosphere (the region of soil directly surrounding the roots), and are able to enhance plant growth through increased nutrient availability, production of plant growth hormones, and by providing protection from pathogens [81; 82; 83]. Well characterized bacterial plant growth promotion activities include: Nitrogen (N) fixation [84], 1-Aminocyclicpropane-1-carboxylate (ACC) deaminase activity (reduction of ethylene induced stress signaling) [85], increased phosphate solubilization [86], indole-3-acetic acid production (IAA--cell elongation and cell growth factor) [87], and pathogen suppression [81]. Bacteria have their own antioxidant enzymes for dealing with oxidative stress. Bacterial catalases, like those of plants, are also classified as monofunctional (HPII) or bifunctional (HPI) catalases,as well as a third group of Mn-catalases [70]. The common catalase enzymes in bacteria are KatA, KatB, KatC, KatE, KatG, and KatM [70,88]. Bifunctional catalases (HPI) are generally activated when initially challenged by oxidative stress, whereas monofunctional catalases (HPII) are utilized as the cells grow into stationary phase [70]. Some soil bacteria, including _Pseudomonas fluorescens_, have high antioxidant and CAT activity, potentially being able to remove excess H\({}_{2}\)O\({}_{2}\) from the rhizosphere, produced during photosynthesis by plants or from abiotic stressors [89,90]. Mn-Catalases lack a heme prosthetic group and instead contain a dimanganese cluster [70]. These catalases are known to be important in reducing stress associated with abiotic stress factors, including Fe deficiency, microaerophilic environments, temperature fluctuation, high saline environments, desiccant conditions, and exposure to chemicals such as cyanide [91,92]. Cyanobacteria from the genus _Anabaena_ (known rice biofertilizers) have been shown to tolerate oxidative stress induced by high levels of salt and desiccation via their KatB Mn-CAT enzyme [92]. Bacterial CAT enzymes serve to reduce ROS accumulation associated with stressors rather than mediating, as both stressor reducers and intracellular signaling molecules (as seen in plants) [70]. The root nodule bacterium _Sinorhizobium meliloti_ utilizes KatA, KatB, and KatC to cope with the production of H\({}_{2}\)O\({}_{2}\) generated by bacterial N-fixation [89]. In _S. meliloti_, KatA activity primarily increases in response to exogenous H\({}_{2}\)O\({}_{2}\) exposure, whilst KatC activity increases in response to abiotic heat and salinity stress [89]. The KatB enzyme is constitutively expressed throughout the whole growth period of the bacteria as an initial defense against oxidative stress [89]. In addition to reducing oxidative stress, _S. meliloti_ utilizes catalase and other antioxidants to form symbioses with legumes, including _Medicago sativa_ (alfalfa) [93], as part of the symbiosis process requiring the breakdown of plant produced H\({}_{2}\)O\({}_{2}\) for nodule organogenesis [93]. Bacterial inoculants have also been shown to increase the antioxidant activity of plants when exposed to drought stress. Leaves of _Lactuca sativa_ L. (Lettuce) were inoculated with _Pseudomonas mendocina_ Palleroni, and either the mycorrhizal fungi _Glomus intraradices_ or _Glomus mosseae_, or alone. The highest amount of CAT activity was observed in the _P. mendocina_-inoculated plants grown under severe stress conditions. Under moderate drought conditions, total peroxidase and CAT activity increased with bacterial and fungal inoculation, whilst SOD activity decreased [94], suggesting that these beneficial microbes could be used to alleviate oxidative stress in the lettuce. While many soil microbes have the potential to enhance plant growth using antioxidants, some plant pathogens exploit their antioxidant systems during infection.