A Comprehensive Review on the Heavy Metal Toxicity and Sequestration in Plants

Abstract

Heavy metal (HM) toxicity has become a global concern in recent years and is imposing a severe threat to the environment and human health. In the case of plants, a higher concentration of HMs, above a threshold, adversely affects cellular metabolism because of the generation of reactive oxygen species (ROS) which target the key biological molecules. Moreover, some of the HMs such as mercury and arsenic, among others, can directly alter the protein/enzyme activities by targeting their –SH group to further impede the cellular metabolism. Particularly, inhibition of photosynthesis has been reported under HM toxicity because HMs trigger the degradation of chlorophyll molecules by enhancing the chlorophyllase activity and by replacing the central Mg ion in the porphyrin ring which affects overall plant growth and yield. Consequently, plants utilize various strategies to mitigate the negative impact of HM toxicity by limiting the uptake of these HMs and their sequestration into the vacuoles with the help of various molecules including proteins such as phytochelatins, metallothionein, compatible solutes, and secondary metabolites. In this comprehensive review, we provided insights towards a wider aspect of HM toxicity, ranging from their negative impact on plant growth to the mechanisms employed by the plants to alleviate the HM toxicity and presented the molecular mechanism of HMs toxicity and sequestration in plants.

Physiological Importance of Molybdate Transporter Family 1 in Feeding the Molybdenum Cofactor Biosynthesis Pathway in Arabidopsis thaliana

“…However, as Mo is a heavy metal, it might negatively affect some cellular processes as it is known to do with other heavy metals such as cadmium or copper. These metals underlie the complex regulatory mechanisms to prevent toxic effects [ 59 ]. The fact that molybdate is nonetheless needed for Moco biosynthesis is, however, challenging for plant cells.…”

Mercury Phytotoxicity and Tolerance in Three Wild Plants during Germination and Seedling Development

“…Another investigation indicated the absence of correlation between soil and T. officinale tissue metal concentration [ 34 ], indicating the possible presence of repressive mechanisms in this species, or a high root susceptibility to this metal. The inhibitory mechanisms of seed germination and embryo development caused by Hg are practically unknown, but Hg replaces the -SH groups of proteins with S–Hg–S bridges, affecting amylase and protease activities crucial for seed germination [ 18 ]. Furthermore, it is possible that damage to the seed, physiological alterations, and the generation of reactive oxygen species (ROS) [ 35 ] impact T. officinale to a great extent during its germination.…”

“…These tests have been created in the context of contamination indicator methodologies, but their use as a preliminary tolerance test of new plants for Hg phytoremediation has not been fully exploited. Heavy metal toxicity is observed at all stages of a plant’s life cycle, but this effect is most pronounced during germination and radicle growth [ 18 , 19 ]. For this reason, it is possible that certain species that are tolerant during germination and seedling development can also be tolerant at later stages of their life cycle.…”

Quantitative Estimation of Synergistic Toxicity of Cu and Zn on Growth of Arabidopsis thaliana by Isobolographic Method

“…HM reduces the productivity of primary producers, such as plants and cultivars, by interfering with their growth and development. HMs also induce adverse physiological and morphological alterations in plants, by creating reactive oxygen species (ROS), limiting metabolic enzymes and photosynthesis, and spending energy for chelation, transport, and dislocation of absorbed HMs [1][2][3]. It is commonly understood that nonessential metals in soil, such as As, Cd, Ni, and Pb are extremely toxic to plants.…”