Background

Ascorbate peroxidase (APX) is an antioxidant enzyme, involved in the removal of toxic components such as reactive oxygen species (ROS) produced during physiologic and metabolic activities of the cell. Cells need to detoxify ROS efficiently as a consequence of abiotic stresses. A complex enzymatic antioxidative system could be developed by cells, which controls the production of ROS and ultimately protects the plant against oxidative damage (Ashraf et al., 2015). This metal-induced plant anti-oxidative defense mechanism is differential and largely based on the types of heavy metals and plant species. Elevated anti-oxidative enzyme activities attenuate abiotic oxidative stress and play a pivotal role in plants adapting against various environmental stresses (Singh et al., 2008). Furthermore, a decline in APX activity suggests that higher Cr toxicity might be inhibiting its ROS-eliminating role in plants. Decreased APX activity with increased metal toxicity was previously reported in Indian mustard (Mobin and Khan, 2007), oilseed rape (B. Ali, et al., 2014), and wheat (S. Ali, et al., 2015). Overall, increasing Cr toxicity triggers antioxidant production (mainly responsible for ROS quenching), representing plants’ potential to withstand Cr-polluted soils up to a certain level, which is further dependent on plant species and environmental conditions. Anjum et al. (2017) reported that at lower heavy metals concentrations the activity of antioxidant enzymes increased, whereas at higher concentrations the activity did not further increase. Samantaray et al. (1999) used peroxidase and catalase activities as enzyme markers for Cr-tolerant mung bean cultivars. The observed increase in antioxidant enzyme activity might have been in direct response to the generation of superoxide radicals by Cr-induced blockage of the electron transport chain in the mitochondria. Higher increases observed due to Cr(VI) indicated that its addition probably generated more singlet oxygen than Cr(III). The decrease in enzyme activity, as the concentration of external Cr increases, might be due to the inhibitory effect of Cr ions on the enzyme system itself.

In the present study, a comprehensive account of past developments and current trends using glycine betaine (GB) and arbuscular mycorrhizal fungi (AMF) in the research on Cr toxicity and its amelioration in sorghum plants has been attempted. Moreover, another line of plant defense strategy against heavy metals toxicity, which involves the utilization of APX and the instigation of AMF symbiotic system, as well as their possible collaboration with one another or with the plant antioxidant system, has been examined and discussed (Kumar, 2021). The study focuses on the underlying functions and detoxification capabilities of GB and AMF that are typically used by plants to enhance tolerance to Cr toxicity.