Synergism of arbuscular mycorrhizal fungi and bacteria in bioremediation and restoration of metal-stressed environments

Metal pollution poses significant ecological and economic concerns for many countries, resulting from anthropogenic activities such as intensive farming, mining, and other industrial sectors. Many of these metals can be toxic, affecting not only plant and animal nutrition but also human health. Phytoremediation of metal-polluted soils is now regarded as one of the most promising nature-based solutions for removing metals from contaminated environments. It can be enhanced by plant inoculation with beneficial microorganisms, such as arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR). For about two decades, the combined use of PGPR and AMF has attracted interest. This review summarizes the studies carried out on this subject, highlighting the complementary mechanisms of these two types of microbes and their synergistic effects, which improve the plant's mineral nutrition and tolerance to heavy metals, as well as better metal neutralization through stabilization in the plant's aerial and root organs and in the soil. Among these mechanisms, AMF intervene by mobilizing essential minerals due to their external mycelium, which explores a large volume of soil. AMF also contribute to reducing soil erosion through the soil-binding capacity of their extraradical mycelium and glomalin production, which enhances soil aggregation and stability. These symbionts contribute efficiently to metal toxicity alleviation in plants. PGPR can improve plant growth through various mechanisms, including hormone production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, nitrogen fixation, and the secretion of different chelating substances. Metals can be neutralized by a variety of processes, including binding, biosorption, transformation, and immobilization. Mycorrhiza helper bacteria are associated with AMF and can stimulate their mycelial growth, spore production, and spore germination, thus increasing mycorrhizal colonization. The selection of bacteria and AMF for phytoremediation purposes should be based on these different complementary properties. Furthermore, genomic and transcriptomic studies may be utilized to identify the most active genes in terms of their positive effects on the plant and phytoremediation mechanisms. This approach enables a more rigorous selection of strains. Field experiments with co-inoculation of AMF and bacteria are rare at present and need to be developed in different edaphic and climatic conditions.