Metal and Metalloid Toxicity in Plants: An Overview on Molecular Aspects

Abstract

Worldwide, the effects of metal and metalloid toxicity are increasing, mainly due to anthropogenic causes. Soil contamination ranks among the most important factors, since it affects crop yield, and the metals/metalloids can enter the food chain and undergo biomagnification, having concomitant effects on human health and alterations to the environment. Plants have developed complex mechanisms to overcome these biotic and abiotic stresses during evolution. Metals and metalloids exert several effects on plants generated by elements such as Zn, Cu, Al, Pb, Cd, and As, among others. The main strategies involve hyperaccumulation, tolerance, exclusion, and chelation with organic molecules. Recent studies in the omics era have increased knowledge on the plant genome and transcriptome plasticity to defend against these stimuli. The aim of the present review is to summarize relevant findings on the mechanisms by which plants take up, accumulate, transport, tolerate, and respond to this metal/metalloid stress. We also address some of the potential applications of biotechnology to improve plant tolerance or increase accumulation.

Keywords: abiotic stress; metal toxicity; plant tolerance.

Figure 1

Schematic representation of the physiological and molecular processes of absorption/translocation of metals into plants. The uptake of heavy metals ((e.g., Pb, Cd, As, Zn, etc.) (colored circles)) occurs through the root cells, where the presence or high concentration of these metals triggers different signaling pathways inside the cell. The metal sensing signals initiate a defense response in plants such as the release of mitochondrial-derived OAs that form complexes with the metallic ions outside the root cell (a), or the introduction of metals and metal–OA complexes to cells through transporters (ABC-type, ZIPs, CDF, ATPase H⁺ metal, etc.) (b). In the cytosol, these metals form complexes with protein chelators (MTs and PCs) (c) that are then transported into vacuoles, also by metal transporters (ABC-type, NRAMP, CAX, and MTP), to accumulate there or to another organelle such as the Golgi (d). Heavy metals also can be translocated to the xylem by transporters (ZIP2 and ZNT1) and ultimately transported to the shoots (e), where they can also be introduced into the cell vacuoles, Golgi (MTP11), and chloroplasts (HMA) by transporters (f). Orange circles represent organic acids (OAs). MT, metallothionein, PC, phytochelatin.

Figure 2

General diagram of the Gene Ontologies (GOs) of up- and downregulated genes under different metal stress conditions in plants by transcriptomic analysis. Venn diagram showing the differentially expressed genes (DEGs) up- (green arrow) and down- (red arrow) regulated that belong to the GOs of reactive oxygen species (ROS) production and antioxidative machinery, transporter-like genes (plasma membrane or vacuoles), metabolic pathways (such as the TCA cycle, which provides OAs) and phytohormonal genes in responses to As(V), Cd, Pb, and other metals. Colored circles: heavy metals, CHO: carbohydrates, TCA: tricarboxylic acid, APX: ascorbate peroxidase, ERF: Ethylene Response Factor, AUX/IAA: Auxin/Indole-3-Acetic Acid, ABP1: Auxin binding protein 1, TIR1: Transport inhibitor response 1, SAUR: small auxin upregulated RNA, OsSAUR21: auxin-responsive SAUR gene family member of Oryza sativa.