## 5 Can Bacterial Antioxidant Activity Reduce Oxidative Stress towards Cropped Plants?
Plants will use ROS as defense mechanisms to stop a pathogen from successful infection, often by releasing H\({}_{2}\)O\({}_{2}\) into the environment or using oxidative burst [40,95,96]. For the proteobacterium _Xanthomonas citri_, enhanced production of KatG allows it to deal with the challenge of oxidative burst by citrus plants during infection of leaves [97]. It also allows for protection from UV radiation and subsequent biofilm formation [97]. Interestingly, KatG also confers resistance to microbicidal H\({}_{2}\)O\({}_{2}\), and promotes virulence of _Actinobacter_ spp. towards humans [98], suggesting that this bifunctional KatG enzyme may have a specialized role in defense and virulence for bacterial species [70,97,98]. As a bifunctional catalase, KatG has been shown to interact with other substrates such as guaiacol [70], and in _M. tuberculosis_ infection in mammals, it is able to inactivate the anti-tuberculosis drug isoniazid [99]. Plants need to balance ROS production to allow for healthy growth, whilst maintaining respiration and energy production [69]. When grown under ideal conditions or moderately stressful conditions, plants reduce levels of ROS through CAT, SOD, ascorbate peroxidase, glutathione, and phytochelatin activity [51,69]. Under constant high stress from either insufficient nutrient availability or over exposure to high levels, these plants systems become insufficient and plant growth and prosperity is lost with plant death being the final outcome. One of the potential ways oxidative stress in cropped plants could be reduced is through inoculating plants with a beneficial microbe, which has the capacity to produce high levels of peroxidase/catalase enzymes that may directly reduce the oxidative stress in the plants through their interactions. Bacteria can also be highly capable of solubilizing nutrients from their environment and making them available through their production of siderophores. In nutrient scarce conditions, some rhizospheral bacteria produce siderophores that chelate Fe previously unavailable for plant uptake, and deliver them to plants enabling both plant and bacteria to thrive [100; 101]. Many bacteria are highly adept at tolerating fluctuation in trace metal availability. For example, in the model system _Nostoc punctiforme_, the highest non-toxic levels of trace elemental tolerance for Co, Cu, Mn, Ni, and Zn were determined to be 2 uM, 0.5 uM, 500 uM, 1 uM, and 18 uM, respectively [102]. However, for most bacterial cells, total intracellular trace metal levels are observed in the millimolar range (10\({}^{-3}\)). Despite intracellular levels being very high, levels of trace metals that promote active uptake and efflux channels are usually in the femtomolar range (10\({}^{-15}\)). Bacterial cells are able to combat the stress challenges presented by high concentrations of metals, such as Zn or Mn, by regulating efflux transporters embedded in the cytoplasmic membrane, hence excluding the metals from the intracellular environment and overcoming potential toxicity effects [103]. Although the bacteria increase the availability of metals to plants when they have limited access to them, they can help the plants meet their requirement to have ideal functioning of their own systems and enzymes--such as the aforementioned ROS reduction systems (CAT, SOD, ascorbate peroxidase, glutathione, and phytochemicalatin) listed above. In environments with toxic levels of trace elements, rhizospheral bacteria can protect plant roots from stress by acidifying, chelating, precipitating, complexing, or inducing redox reactions that reduce toxicity of those metals [104]. Under high environmental metal conditions, bacteria present in the soil may provide a buffering effect; where the metals will bind with high affinity to the bacteria cells (predominantly by charge with the cell walls) potentially reducing their direct availability to plants, therefore indirectly reducing oxidative stress towards cropped plants [102; 105].