Genomics and physiological evidence of heavy metal tolerance in plants: Effectiveness of legume-PGPB symbioses in phytoremediation of heavy metal-contaminated soils and defence response mechanisms

Abstract

Heavy metals (HMs) contamination of soils poses a serious threat to plants and microorganisms in the environment and attacks human health via the food chain. Phytoremediation is an environmentally friendly approach that consists in the use of plants and microorganisms to alleviate HMs contamination. Currently, there is growing interest in the usefulness of legumes inoculated with HMs-resistant plant growth-promoting bacteria (PGPB) symbioses in phytoremediation. PGPB, including rhizobia and endophytes, stimulate legume plant growth through plant growth-promoting (PGP) traits, such as nitrogen fixation, phosphorus solubilization, phytohormone synthesis and siderophore release. Legumes-PGPB symbioses induce mobility of metals and nutrients by different manners, such as acidification, precipitation, bioaccumulation and chelation, and the formed complexes are transported into cell vacuoles via protein transporters. PGPB can enhance plant uptake of HMs in the phytoextraction process or reduce HMs accumulation in the aerial part of plants in the phytostabilization process. Elevated HMs in soils cause generation of reactive oxygen species (ROS) in plant organs, which can cause oxidative stress, and inoculated legumes respond by stimulating several antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POX), catalase (CAT) and ascorbate peroxidise (APX). Furthermore, analysis of quantitative gene expressions involved in HMs tolerance, such as F-box, phytochelatin synthase (PCS) and metallothioneins (MTs), showed modulation of responses under HMs stress and co-inoculation with PGPB. Results suggest that certain antioxidant enzymes and genes can be involved in HMs tolerance mechanisms of legumes co-inoculated with HMs-resistant PGPB.