## 5 Can Bacterial Antioxidant Activity Reduce Oxidative Stress towards Cropped Plants?
Bacteria isolated from heavy metal polluted environments have been shown to reduce toxic impacts of metals and even enhance plant growth. The Cu-resistant bacterium _Providencia vermicola_ has been shown to protect lentils from oxidative damage when exposed to high Cu soil levels--both by reducing Cu uptake by the plants and production of indole acetic acid and siderophores [106]. Moreover, _P. vermicola_ was able to increase root and shoot length, dry weight, and leaf size area of the lentils [106] compared to a negative control. In another study, two Zn-resistant _Rhizobium leguminosarum_ strains were able to enhance growth of _Trifolium repens_ (white clover) in high Zn conditions [107]. 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]. The PGPB isolate _Chrysiobacterium_ spp. C138 (previously isolated from an \(O\). _sativa_ rhizosphere) has been shown to enhance Fe uptake in Fe-starved _Solanum lycopersicum_ L. (tomato) [108]. Similarly, inoculation of maize seeds with siderophore-producing _Pseudomonas_ spp. strains lead to increases in root and shoot growth, and hence larger dry weights [109]. Chemical chelators such as ethylenediaminetetraacetic acid (EDTAA) are currently used to deliver trace elements to cropped plants with varying success [110]. Prolonged use is known to lead to ecotoxicological effects, as EDTA also increases the bioavailability of dangerous heavy metals in soils [111]. Ethylenediaminetetraacetic acid has also been shown to cause leaf necrotic lesions in _Brassica rapa_ (Chinese cabbage), and to reduce root symbiosis with arbuscular mycorrhiza in _Trifolium pratense_ (red clover) [111]. Bacterial metabolites have already been utilized for applications, including in cosmetics, as semiconductor components; and in dairy, food, and textile industries [91]. However, their use in an agricultural context as a stress reducing agent for cropped plants is relatively unexplored. Understanding the modes of action for microbial oxidative stress reduction towards cropped plants and modulating trace element availability is essential. In other instances, microbes previously identified to be plant growth promoting can induce oxidative stress towards plants in different growth systems. For instance, the _P. fluorescens_ isolate DUS1-27 was previously shown to enhance _B. napus_ L. (canola) growth in soil based systems; however, when grown in a hydroponic system with the plants, an oxidative stress response is observed [112]. This highlights the importance of understanding the modes of action for microbial antioxidant mediated stress reduction in plants, as changes in environment can negatively alter microbe associated molecular patterns. Importantly,this highlights that expanding their use requires: (1) Understanding and characterization of how these bacteria reduce oxidative stress towards plants, and (2) confirmation as to which of these are relevant under field-based conditions. Overall, abiotic stressors including soil temperature, moisture, Ph, and trace metal availability, all play a role in inducing oxidative stress towards irrigated cropped plants in the form of ROS. Ultimately, fluctuating trace metal availability can increase ROS production leading to increased levels of free radicals, such as hydroxyl radicals being formed. ROS imbalance in cropped plants not only reduces growth, but also reduces crop yields. The ability of beneficial microbes to enhance plant growth is well characterized and it is also clear that many of them have a high tolerance for trace metal fluctuations in the environment. Therefore, their characteristics and modes of action to reduce abiotic stress in cropped systems is worthy of further exploration to determine their potential to increase overall soil health and cropping sustainability, as well as to reduce input costs and increase yields.