Plants' molecular behavior to heavy metals: from criticality to toxicity

Abstract

The contamination of soil and water with high levels of heavy metals (HMs) has emerged as a significant obstacle to agricultural productivity and overall crop quality. Certain HMs, although serving as essential micronutrients, are required in smaller quantities for plant growth. However, when present in higher concentrations, they become very toxic. Several studies have shown that to balance out the harmful effects of HMs, complex systems are needed at the molecular, physiological, biochemical, cellular, tissue, and whole plant levels. This could lead to more crops being grown. Our review focused on HMs' resources, occurrences, and agricultural implications. This review will also look at how plants react to HMs and how they affect seed performance as well as the benefits that HMs provide for plants. Furthermore, the review examines HMs' transport genes in plants and their molecular, biochemical, and metabolic responses to HMs. We have also examined the obstacles and potential for HMs in plants and their management strategies.

Keywords: agricultural productivity; cross-tolerance; genotoxicity; hormesis; molecular responses; transport genes.

Figure 1

Various origins of heavy metals. Coal combustion, mining, refining, soil erosion, and volcanic eruptions are all sources of antimony. Sources of arsenic include smelting, mining, atmospheric deposition, pesticides, and geological sedimentation. Volcanic dust and coal and hydrocarbon combustion are sources of beryllium as well. Sources of cadmium include plastic, fertilizer, pesticides, refining, and welding. Sources of chromium include textiles, dyeing, electroplating, paint manufacturing, steel fabrication, and tanning. Copper is obtained through mining, refining, painting, plating, and printing. Coal combustion, electroplating, battery manufacturing, mining, paint, and pigments are all lead sources. Batteries, coal combustion, geothermal activities, mining, paint and paper industries, volcanic eruptions, and geological weathering are all sources of mercury. Sources of nickel include porcelain enameling, electroplating, non-ferrous metals, and pigments. Sources of selenium include coal combustion and mining. Sources of silver include the production of batteries, mining, photographic processing, and smelting. Production of cement, combustion of fossil fuels, metal smelting, and hydrocarbon refining are all sources of thallium. Brass manufacturing, mining, hydrocarbon refining, and plumbing are all sources of zinc. This figure was made using BioRender.

Figure 2

Toxic and inefficient impact of heavy metals on plants. This figure was made using BioRender.

Figure 3

Sequestration of heavy metals in plant cells within the vacuoles. The uptake of heavy metal (HMs) ions is facilitated by a variety of transporters, such as the cation diffusion facilitator (CDF) family, the heavy-metal-transporting ATPase (HM ATPase), the copper transporter (CTR), the zinc-regulated, iron-regulated transporter-like proteins (ZIP), and the natural resistance-associated macrophage protein (NRAMP). For instance, HMs, such as Cd²⁺, enter the cytosol and initiate the production of phytochelatins (PCs) after being transported by members of the ZIP family. PCs are produced through a transpeptidation reaction from reduced glutathione (GSH) in a non-translational manner. The primary function of PCs is to bind cytosolic HMs, which results in the formation of the HM–PC complex. In the case of Cd²⁺ ions, these bind with low-molecular-weight (LMW) complex and form the LMW Cd-complex. This complex is subsequently transported into the vacuole by a tonoplast-localized ATP-binding-cassette (ABC) transporter. The LMW Cd-complex is collected and converted into a high-molecular-weight (HMW) complex within the vacuole. This complex contains supplementary Cd²⁺ ions. The tonoplast-localized cation/proton exchanger (CAX) transporters facilitate the direct interaction between these HMW complexes and protons, thereby enabling them to access the vacuole. Metal tolerance proteins (MTPs) and NRAMPs are transporters that are present in the tonoplast. They are accountable for the migration of metal ions to facilitate compartmentalization or remobilization. Organic acids, amino acids, and metallothioneins (MTs) are among the chelators that contribute to the regulation of metal levels in the cytosol to a safe and low level. ROS, reactive oxygen species. This figure was made using BioRender.