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.

Arsenic Toxicity-Induced Physiological and Metabolic Changes in the Shoots of Pteris cretica and Spinacia oleracea

“…Once inside the plant cell, most As(V) is reduced to As(III), as only the trivalent form can undergo detoxification [ 8 ]. The cellular detoxification of As(III) in non-As-hyperaccumulators involves As complexation with phytochelatins respectively nonprotein thiols and storage of these complexes in vacuoles [ 26 , 27 ], and can be characterized as adaptive resistance. Compared to non-As-hyperaccumulators, As-hyperaccumulators had less of a tendency for complex formation [ 13 , 26 , 27 ].…”

Tissue Distribution and Biochemical Changes in Response to Copper Accumulation in Erica australis L.

“…A similar pattern was observed in Avicennia marina , where root structure alteration was not observed as a general symptom of Cu toxicity but darkened roots observed in plants cultivated at 200 µM Cu might indicate necrosis [ 2 ]. Root anatomical and physiological alterations play an important role in metal transport and plant growth [ 6 , 25 ]. The high Cu concentration in roots was responsible for the inhibition in plants growth and biomass production as a consequence of Cu toxicity.…”

“…Peroxidase and catalase activities were used as stress markers since these enzymes may scavenge the high ROS production induced by high free metal cellular contents [ 13 ]. Once the metal is absorbed and accumulated in the cytosol, it may cause oxidative stress through the production of reactive oxygen species [ 6 ]. The free metal may be chelated with amino acids and then removed by compartmentation [ 2 , 27 ], avoiding further cell damages.…”

“…The critical toxicity level for most crops is above 20–30 mg kg −1 leaf dry weight [ 3 ], while in Cu-tolerant metallophytes, leaves may contain up to 1000 µg g −1 leaf dry weight [ 4 , 5 ]. Leaf chlorosis and stunted growth are the more frequent copper toxicity symptoms observed mostly as the result of inhibition of nutrient uptake or direct interference with plant metabolism [ 2 , 6 ].…”

“…Meanwhile, increased activity of ROS scavenging systems is induced to cope with free radicals, which might generate protein damage by oxidative stress [ 13 ]. Organic compounds such as amino acids, amides and carboxylates, of known metal-complexing properties are synthesized in response to metallic stresses [ 6 ]. However, their roles in metal tolerance still need to be proved in many species.…”