Inoculation of plant-growth-promoting rhizobacteria and earthworms in the rhizosphere reinstates photosynthetic attributes and secondary metabolites in Brassica juncea L. under chromium toxicity

Aims The accretion of Cr in soils is pervasive and toxic to plants. Recently, plant-growth-promoting rhizobacteria and earthworms have attracted attention in relieving metal toxicity in plants. The aim of the current study was to assess the effect of earthworms (Eisenia fetida) and PGPR (Pseudomonas aeruginosa and Burkholderia gladioli) on the morphological and biochemical parameters of Brassica juncea plants under Cr stress. Methods Plants were grown with or without supplementation of PGPR and earthworms in Cr-treated soils under natural conditions. Different morphological and biochemical parameters were studied during the vegetative and reproductive phases of growth. Results Cr treatment inhibited root and shoot length, photosynthetic pigments and gaseous exchange attributes, whereas the content of osmoprotectants and phenolic compounds was high in plants raised under Cr stress. Root length was increased by 118.3% and 83.6% in 40- and 70-day-old plants, respectively, with combined treatment of EM1M2. The chlorophyll content was enhanced by 73.8% in 40-day-old plants and 55.6% in 70-day-old plants. The carotenoid content was enhanced by 122.6% and 71.7% in 40-day-old and 70-day-old plants, respectively. The level of total phenols was increased by 40% at 40 days and 26.4% in 70-day-old plants with the EM1M2 treatment. The contents of total carbohydrates, trehalose, total amino acids and proline were also increased as a result of combined treatment of PGPR and earthworms (EM1M2). Conclusions The outcomes of the current study demonstrated that PGPR and earthworms helped to ameliorate the toxic effects of Cr on plants by enhancing growth, and various metabolites in plants.