Plant growth promoting rhizobacteria modulates the antioxidant defense and the expression of stress responsive genes providing Pb accumulation and tolerance of grass pea

Abstract To ensure the success of phytoremediation, it is important to consider the appropriate combination of plants and microorganisms. This study was conducted to get a better insight into the underlying molecular and biochemical mechanism of grass pea (Lathyrus sativus L.) induced by plant growth promoting rhizobacteria (PGPR), when exposed for 3, 6, 9 and 14 days to 1 mM Pb in a hydroponic system. The significant positive effect of bacterial inoculation was reproduced in various parameters. Results indicated that inoculation of PGPR significantly increased the concentration of Pb by 20%, 66%, 43% and 36%, in roots and by 46%, 55%, 37%, 46% in shoots, respectively after 3, 6, 9 and 14 days of metal exposure as compared to the uninoculated plants. The metal accumulation in grass pea plants triggered a significant elevation in the synthesis of non-protein thiols (NPT), particularly in inoculated plant leaves where it was about 3 and 2-fold higher than the uninoculated set at 6 and 9 d. Nevertheless, Pb treatment significantly increased oxidative stress and membrane damage in leaves with the highest hydrogen peroxide (H2O2) production and tissue malondialdehyde (MDA) concentration recorded in uninoculated plants. Further, the inoculation alleviated the oxidative stress, improved plant tolerance and modulated the activities of antioxidant enzymes (SOD, CAT, APX, GR, DHAR and MDHAR). Similary, the expression patterns of LsPCS, LsGCN, LsCNGC, LsGR and LsGST through qRT-PCR demonstrated that bacterial inoculation significantly induced gene expression levels in leaves 6 d after Pb treatment, indicating that PGPR act as regulators of stress responsive genes. The findings suggest the key role of PGPR (R. leguminosarum (M5) + Pseudomonas fluorescens (K23) + Luteibacter sp. + Variovorax sp.) in enhancing Pb accumulation, reducing metal toxicity, strengthening of the antioxidant system and conferring higher Pb tolerance to grass pea plants. Hence, the association Lathyrus sativus- PGPR is an effective tool to achieve the goal of remediation of Pb contaminated sites.