A systematic review and meta-analysis on the root effects and toxic mechanisms of rare earth elements

Rare earth elements (REEs) have attracted much attention because of their unique physical and chemical properties. The root system is the plant organ most directly in contact with REEs, and it is critical to understand the mechanisms of interaction between the two. This paper investigates the effects of REEs on plant enrichment and fractionation, as well as on various developmental and toxicity indices of the root system. REEs are more likely to be deposited on the root surface under the influence of root secretion. The complexation between the two affects the uptake and fractionation of REEs and the altered pattern of root secretion. The toxicity mechanisms of REEs on plant root cells were lied in: (1) REEs generate reactive oxygen species after entering the plant, leading to oxidative stress and damage to plant cells; (2) REEs with higher charge-to-volume ratios compete for organic ligands with or displace Ca²⁺, further disrupting the normal function of plant root cells. It was shown that the sensitivity of inter-root microorganisms to REEs varied depending on the content and physicochemical properties of REEs. The paper also concluded with a meta-analysis of phytotoxicity induced by REEs, which showed that REEs affect plant physiological parameters. REEs, as a source of oxidative stress, triggered lipid peroxidation damage in plants and enhanced the activity of antioxidant enzymes, thus revealing the significant toxicity of REEs to plants. The phytotoxic effects of REEs increased with time and concentration. These results help to elucidate the ecotoxicology of rare earth-induced phytotoxicity.