Chromium (VI) Toxicity and Active Tolerance Mechanisms of Wheat Plant Treated with Plant Growth-Promoting Actinobacteria and Olive Solid Waste

The present study aimed to assess the potential of plant growth-promoting Actinobacteria and olive solid waste (OSW) in ameliorating some biochemical and molecular parameters of wheat (Triticum aestivum) plants under the toxicity of high chromium levels in the soil. With this aim, a pot experiment was conducted, where the wheat plants were treated with a consortium of four Actinobacterium sp. (Bf treatment) and/or OSW (4% w/w) under two levels of nonstress and chromium stress [400 mg Cr(VI) per kg of soil] to estimate the photosynthetic traits, antioxidant protection machine, and detoxification activity. Both Bf and OSW treatments improved the levels of chlorophyll a (+47–98%), carotenoid (+324–566%), stomatal conductance (+17–18%), chlorophyll fluorescence (+12–28%), and photorespiratory metabolism (including +44–72% in glycolate oxidase activity, +6–72% in hydroxypyruvate reductase activity, and +5–44% in a glycine to serine ratio) in leaves of stressed plants as compared to those in the stressed control, which resulted in higher photosynthesis capacity (+18–40%) in chromium-stressed plants. These results were associated with an enhancement in the content of antioxidant metabolites (+10–117%), of direct reactive oxygen species-detoxifying enzymes (+49–94%), and of enzymatic (+40–261%) and nonenzymatic (+17–175%) components of the ascorbate–glutathione cycle in Bf- and OSW-treated plants under stress. Moreover, increments in the content of phytochelatins (+38–74%) and metallothioneins (+29–41%), as markers of detoxification activity, were recorded in the plants treated with Bf and OSW under chromium toxicity. In conclusion, this study revealed that the application of beneficial Actinobacteria and OSW as biofertilization/supplementation could represent a worthwhile consequence in improving dry matter production and enhancing plant tolerance and adaptability to chromium toxicity.