Bacillus mycoides PM35 in combination with titanium dioxide (TiO2)⎯nanoparticles enhanced morpho-physio-biochemical attributes in Barley (Hordeum vulgare L.) under cadmium stress

Plant growth-promoting rhizobacteria (PGPR) are naturally occurring soil bacteria and are known to induce plant growth promotion and titanium dioxide (TiO2)⎯nanoparticles (NPs) used in a range of applications that need increased whiteness, improved corrosion resistance and photocatalytic activity. Keeping in view the stress mitigation potential of TiO2⎯NPS and B. mycoides PM35, the existing research work was premeditated to inspect the beneficial role of seed priming with using different levels of TiO2⎯NPs i.e., [(0 no TiO2⎯NPs), 25 and 50 μg/ml] and soil incubation plant growth promoting rhizobacteria (B. mycoides PM35) i.e., [(0 no B. mycoides PM35), 10 and 20 μL] on biochemical, morphological and physiological characteristics of Barley (Hordeum vulgare L.) plants under different levels of Cd in the soil i.e., [(0 Cd), 50 and 100 mg kg-1]. Results from the present study showed that the increasing levels of Cd in the soil significantly (P < 0.05) decreased plant growth and biomass, photosynthetic pigments, gas exchange attributes, sugars, and nutritional contents from the roots and shoots of the plants. In contrast, increasing levels of Cd in the soil significantly (P < 0.05) increased oxidative stress indicators in term of malondialdehyde, hydrogen peroxide, and electrolyte leakage, and also increased organic acid exudation patter in the roots of H. vulgare. Although, the activities of enzymatic antioxidants and the response of their gene expressions in the roots and shoots of the plants and non-enzymatic such as phenolic, flavonoid, ascorbic acid, and anthocyanin contents were initially increased with the exposure of 50 mg kg-1 Cd, but decreased by the increasing the Cd concentration 100 mg kg-1 in the soil. The negative impact of Cd toxicity can overcome the application of PGPR (B. mycoides PM35) and TiO2⎯NPs, which ultimately increased plant growth and biomass by capturing the reactive oxygen species, and decreased oxidative stress in H. vulgare by decreasing the Cd contents in the roots and shoots of the plants. Our results also showed that the TiO2⎯NPs were more sever and showed better results when we compared with PGPR (B. mycoides PM35) under the same treatment of Cd in the soil. Research findings, therefore, suggest that the combined application of PGPR (B. mycoides PM35) and TiO2⎯NPs can ameliorate Cd toxicity in H. vulgare, resulting in improved plant growth and composition under metal stress, as depicted by balanced exudation of organic acids.