Chromium (Cr) toxicity leads to the enhanced production of reactive oxygen species (ROS), which are extremely toxic to the plant and must be minimized to protect the plant from oxidative stress. The potential of plant-growth-promoting rhizobacteria (PGPR) and earthworms in plant growth and development has been extensively studied. The present study was aimed at investigating the effect of two PGPR (Pseudomonas aeruginosa and Burkholderia gladioli) along with earthworms (Eisenia fetida) on the antioxidant defense system in Brassica juncea seedlings under Cr stress. The Cr toxicity reduced the fresh and dry weights of seedlings, enhanced the levels of superoxide anion (O₂•^–), hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and electrolyte leakage (EL), which lead to membrane as well as the nuclear damage and reduced cellular viability in B. juncea seedlings. The activities of the antioxidant enzymes, viz., superoxide dismutase (SOD), guaiacol peroxidase (POD), ascorbate peroxidase (APOX), glutathione peroxidase (GPOX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) were increased; however, a reduction was observed in the activity of catalase (CAT) in the seedlings under Cr stress. Inoculation of the PGPR and the addition of earthworms enhanced the activities of all other antioxidant enzymes except GPOX, in which a reduction of the activity was observed. For total lipid- and water-soluble antioxidants and the non-enzymatic antioxidants, viz., ascorbic acid and glutathione, an enhance accumulation was observed upon the inoculation with PGPR and earthworms. The supplementation of PGPR with earthworms (combined treatment) reduced both the reactive oxygen species (ROS) and the MDA content by modulating the defense system of the plant. The histochemical studies also corroborated that the combined application of PGPR and earthworms reduced O₂•^–, H₂O₂, lipid peroxidation, and membrane and nuclear damage and improved cell viability. The expression of key antioxidant enzyme genes, viz., SOD, CAT, POD, APOX, GR, DHAR, and GST showed the upregulation of these genes at post-transcriptional level upon the combined treatment of the PGPR and earthworms, thereby corresponding to the improved plant biomass. However, a reduced expression of RBOH1 gene was noticed in seedlings supplemented under the effect of PGPR and earthworms grown under Cr stress. The results provided sufficient evidence regarding the role of PGPR and earthworms in the amelioration of Cr-induced oxidative stress in B. juncea.

Grain legumes are a rich source of dietary proteins and hence are essential for achieving food and nutritional security, worldwide. Grain legumes are negatively affected by salt stress, which results in decreased growth, yield, productivity, and other financial losses. Salt stress causes severe phytotoxicities, like specific ion toxicities, hormonal and nutritional imbalances, disruptions of stomatal conductance, and alterations in leaf water potential and rate of photosynthesis. To enhance agricultural productivity under salt stress conditions, several ecofriendly methods have been tried recently. Out of these, the use of biostimulants gained particular attention due to their ease of availability and high-stress ameliorative potential. It includes the use of non-microbial biostimulants like humic acid and seaweed extracts (SWE), as well as microbial biostimulants like mycorrhiza, plant growth-promoting and nitrogen-fixing bacteria, which promote plant growth and development. Biostimulants successfully alleviate salt stress-induced toxicities in legumes, when introduced either alone or in combination with other macro- or microorganisms in the soil. In response to salinity, plants raise their cellular levels of osmolytes (proline and glycine betaine) and polyamines, as well as phytohormones. The application of biostimulants in grain legumes alleviates the salt stress-induced toxicity by improving the seed germination, seedling growth, rate of photosynthesis, nitrogen fixation, absorption and uptake of minerals, grain development, yield, ROS homeostasis and osmoregulation. Many of the reviews have lacked the assessment of the effect of these ecofriendly biostimulants on mitigation of salt stress in grain legumes. The development of integrated, eco-friendly and sustainable use of various biostimulants and their mode of action in managing salt stress in legume production is discussed in the present review.