Jacks of metal/metalloid chelation trade in plants—an overview

Abstract

Varied environmental compartments including soils are being contaminated by a myriad toxic metal(loid)s (hereafter termed as “metal/s”) mainly through anthropogenic activities. These metals may contaminate food chain and bring irreparable consequences in human. Plant-based approach (phytoremediation) stands second to none among bioremediation technologies meant for sustainable cleanup of soils/sites with metal-contamination. In turn, the capacity of plants to tolerate potential consequences caused by the extracted/accumulated metals decides the effectiveness and success of phytoremediation system. Chelation is among the potential mechanisms that largely govern metal-tolerance in plant cells by maintaining low concentrations of free metals in cytoplasm. Metal-chelation can be performed by compounds of both thiol origin (such as GSH, glutathione; PCs, phytochelatins; MTs, metallothioneins) and non-thiol origin (such as histidine, nicotianamine, organic acids). This paper presents an appraisal of recent reports on both thiol and non-thiol compounds in an effort to shed light on the significance of these compounds in plant-metal tolerance, as well as to provide scientific clues for the advancement of metal-phytoextraction strategies.

The role of organic acids in heavy metal tolerance in plants

“…It is essential to note that in buckwheat and in Amaranthus hypochondriacus L., which are oxalate accumulators, no increase in secretion of oxalate (or citrate) was detected under exposure to Cd, in contrast to that observed in the presence of Al (Zhu et al, 2011;Fan et al, 2016). On the other hand, when maize plants were exposed to 100 µm Cu or Cd in a hydroponic culture, significant secretion of citrate was not observed, in contrast to that observed in the presence of Al (Chaffai, Tekitek, and El Ferjani, 2006). It should be noted that the extracellular production of organic acids is considered an important mechanism in increasing heavy metal tolerance not only in higher terrestrial plants, but also in fungi (Sazanova et al, 2015) and in hydrophytes (Kurilenko and Osmolovskaya, 2015).…”

“…Along with the direct or indirect involvement of organic acids in the uptake of metals by plants, there is conclusive evidence of organic acids involvement in metals transport from roots to shoots (Lopez-Bucio, Nieto-Jacobo, Ramirez-Rodriguez, Herrera-Estrella, 2000), as well as in the detoxification of metals on the cellular level (Titov, Kaznina, and Talanova, 2014;Anjum et al, 2015). Heavy metal binding in cell cytosol involving organic ligands such as organic acids and their sequestration and immobilization in the vacuoles is considered one of the most important mechanisms of metal tolerance in plants (Lux, Martinka, Vaculik, and White, 2011;Titov, Kaznina, and Talanova, 2014).…”

“…Organic acid metabolism is of fundamental importance at the cellular level for several biochemical pathways, including energy production, formation of precursors for amino-acid biosynthesis and, at the whole plant level, in modulating adaptation to the environment (Lopez-Bucio, Nieto-Jacobo, Ramirez-Rodriguez, and Herrera-Estrella, 2000). The basis of the phenomenon of metal ion detoxification with organic acids is the ability of acids such as citrate, malate, oxalate, malonate, aconitate and tartrate to form strong bonds with heavy metal ions through metal chelatation with carboxyl groups carrying the function of donor oxygen in metal-ligands (Anjum et al, 2015).…”

Polyamines may influence phytochelatin synthesis during Cd stress in rice

“…One potential defence mechanism, chelation followed by the compartmentalisation of the metal-chelate to the vacuoles, effectively ensures low concentrations of free metals in the cytosol. Metal-chelation can be performed by compounds of thiol origin, such as glutathione (GSH) or phytochelatins (PCs), or non-thiol origin, such as polyamines (PAs) [2]. Both thiol compounds and PAs are reported to play a role in heavy metal stress tolerance.…”

Induction of phytochelatins and flavonoids in cadmium polluted Berberis thunbergii

“…Plants avoid the damaging effects of cadmium toxicity using defense strategies involving the metal ion binding to the cell walls and its immobilization, exclusion of the plasma membrane or chelation and subcellular compartmentation in the vacuoles (Anjum et al, 2015). In particular, chelation is the most widespread mechanism for the maintenance of low free metals and their detoxification (Clemens, 2001;Mejáre & Bülow, 2001).…”