Utilization of Legume-Nodule Bacterial Symbiosis in Phytoremediation of Heavy Metal-Contaminated Soils

Abstract

With the increasing industrial activity of the growing human population, the accumulation of various contaminants in soil, including heavy metals, has increased rapidly. Heavy metals as non-biodegradable elements persist in the soil environment and may pollute crop plants, further accumulating in the human body causing serious conditions. Hence, phytoremediation of land contamination as an environmental restoration technology is desirable for both human health and broad-sense ecology. Legumes (Fabaceae), which play a special role in nitrogen cycling, are dominant plants in contaminated areas. Therefore, the use of legumes and associated nitrogen-fixing rhizobia to reduce the concentrations or toxic effects of contaminants in the soil is environmentally friendly and becomes a promising strategy for phytoremediation and phytostabilization. Rhizobia, which have such plant growth-promoting (PGP) features as phosphorus solubilization, phytohormone synthesis, siderophore release, production of beneficial compounds for plants, and most of all nitrogen fixation, may promote legume growth while diminishing metal toxicity. The aim of the present review is to provide a comprehensive description of the main effects of metal contaminants in nitrogen-fixing leguminous plants and the benefits of using the legume–rhizobium symbiosis with both wild-type and genetically modified plants and bacteria to enhance an efficient recovery of contaminated lands.

Adaptation to mercury stress by nitrogen-fixing bacteria is driven by horizontal gene transfer and enhanced gene expression of the Mer operon

“…It is noteworthy that the number of nodules is greater in both host-plants when in symbiosis with AMp08 compared with AMp07, but the nodule number was even greater in the HM304 host plant with AMp08 following the stress treatment [75]. Because higher nitrogen content in the host plants will improve plant tness under stress conditions, by proxy the nodule number phenotype could be the result of the rhizobia strain tolerance and its ability to x nitrogen under Hg stress conditions [33].…”

“…In addition to heavy metal detoxi cation responses in rhizobia, it will also be important to identify regulatory changes in nitrogen xation genes (e.g. nif) that would potentially limit nitrogen export to host-plants during symbiosis if these genes are impacted by heavy metals [33]. Expression of the nif gene cluster by rhizobia controls the production of nitrogenase, the necessary enzyme for converting atmospheric nitrogen N 2 to ammonia (NH 3 ) or nitrate (NO 3 − ), that can be used as a macronutrient by the host plant.…”

Local adaptation to mercury contamination by nitrogen-fixing rhizobia is driven by horizontal gene transfer, copy number, and enhanced gene expression

“…It is noteworthy that the number of nodules is greater in both host-plants when in symbiosis with AMp08 compared with AMp07, but the nodule number was even greater in the HM304 host plant with AMp08 following the stress treatment (Sharma et al, in preparation ). Because higher nitrogen content in the host plants will improve plant fitness under stress conditions, by proxy the nodule number phenotype could be the result of the rhizobia strain tolerance and its ability to fix nitrogen under Hg stress conditions (Jach et al, 2022).…”

“…In addition to heavy metal detoxification responses in rhizobia, it will also be important to identify regulatory changes in nitrogen fixation genes (e.g. nif ) that would potentially limit nitrogen export to host-plants during symbiosis if these genes are impacted by heavy metals (Jach et al, 2022). Expression of the nif gene cluster by rhizobia controls the production of nitrogenase, the necessary enzyme for converting atmospheric nitrogen N 2 to ammonia (NH 3 ) or nitrate (NO 3- ), that can be used as a macronutrient by the host plant.…”

Role of PGPB in phytoremediation of heavy metal contaminated soils and the biochemical responses of Vicia faba-soil system

“…Phytoremediation is a method of bioremediation process using plants for treatment of polluted soils, in this context, legumes-rhizobia symbiosis become a natural choice when searching for plants that can grow on contaminated soils with poor nutritional values. Interestingly, this eco-friendly symbiosis is one of the bene cial plant-microbe interactions that constitute an elite group for novel and effective soil restoration (Jach et al 2022). Recently, scientists have been interested in nitrogen-xing microbes as a type of plant growth-promoting bacteria (PGPB) in symbiosis with legumes for phytoremediation of contaminated soils (Chiboub et Furthermore, this symbiosis could be a good biofertilizer that enhances soil fertility by enhancing P and N bioavailability (Jebara et al2020;Abdelkrim et al 2020), since these symbiotic microorganisms are quali ed by plant growth promoting (PGP) traits such as nitrogen xation, phosphorus solubilization, siderophore and phytohormone production such as indole-3-acetic acid (IAA), cytokinins.…”