Heavy Metal Stress and Some Mechanisms of Plant Defense Response

Abstract

Unprecedented bioaccumulation and biomagnification of heavy metals (HMs) in the environment have become a dilemma for all living organisms including plants. HMs at toxic levels have the capability to interact with several vital cellular biomolecules such as nuclear proteins and DNA, leading to excessive augmentation of reactive oxygen species (ROS). This would inflict serious morphological, metabolic, and physiological anomalies in plants ranging from chlorosis of shoot to lipid peroxidation and protein degradation. In response, plants are equipped with a repertoire of mechanisms to counteract heavy metal (HM) toxicity. The key elements of these are chelating metals by forming phytochelatins (PCs) or metallothioneins (MTs) metal complex at the intra- and intercellular level, which is followed by the removal of HM ions from sensitive sites or vacuolar sequestration of ligand-metal complex. Nonenzymatically synthesized compounds such as proline (Pro) are able to strengthen metal-detoxification capacity of intracellular antioxidant enzymes. Another important additive component of plant defense system is symbiotic association with arbuscular mycorrhizal (AM) fungi. AM can effectively immobilize HMs and reduce their uptake by host plants via binding metal ions to hyphal cell wall and excreting several extracellular biomolecules. Additionally, AM fungi can enhance activities of antioxidant defense machinery of plants.

Fate and Impacts of Priority Pollutant Metals in Coal Fly Ash–Soil–Switchgrass Plant Mesocosms

“…In this study, soils amended with 7.5% and 15% CFA were moderately contaminated, and the degrees of contamination were decreased by day 90. Also on day 90, the CFAamended soils containing AMF or GSH had a very low degree of contamination, which may be caused by assimilation of the metals into the cell walls of AMF hyphae (Emamverdian et al, 2015;Firmin et al, 2015), whereas exogenous GSH may allow plant assimilation of the metals (Cai et al, 2010;Wei et al, 2010;Cao et al, 2013). According to Rahman et al (2012) and Likuku et al (2013), the PLI proposed by Tomlinson et al (1980) allows the detection of pollution and the necessary actions that need to be taken.…”

“…Plants have developed strategies to avoid toxic metal accumulation in roots and subsequent translocation into the shoots (Hossain et al, 2012;Emamverdian et al, 2015). Plants association with AMF (Firmin et al, 2015) or utilization of GSH (Hossain et al, 2012) may be also be an important strategy.…”

“…Plants association with AMF (Firmin et al, 2015) or utilization of GSH (Hossain et al, 2012) may be also be an important strategy. AMF can effectively immobilize HMs and reduce their uptake by host plants during HM contamination and pollution by binding metal ions to hyphal cell walls and excreting several extracellular biomolecules (Emamverdian et al, 2015;Firmin et al, 2015). GSH serves a similar purpose by enhancing the plant's oxidative defense system (Hossain et al, 2012).…”

“…Soil fortification with exogenous glutathione (GSH) or inoculation with arbuscular mycorrhizal fungi (AMF) has been suggested as ways to protect plants against the effects of HMs (Hossain et al, 2012;Emamverdian et al, 2015;Firmin et al, 2015). Huang et al (2005) have shown that AMF (Glomus mosseae) decreased the availability of excessive Zn, Cu, and Pb for maize (Zea mays L.) grown on soil contaminated with HMs.…”

Ameliorative effect of Yeast Extract, IAA and Green-synthesized Nano Zinc Oxide on the Growth of Cu-stressed Vicia faba Seedlings

“…Among heavy metals, copper is a redoxactive micronutrient essential for normal growth and development of plants, acts as a structural element in regulatory proteins and participates in photosynthetic electron transport, mitochondrial respiration, oxidative stress responses, cell wall metabolism and hormone signaling (Yruela, 2005 andEmamverdian et al, 2015). At higher concentrations than that required for optimal growth, Cu is proved to be phytotoxic at morphological, physiological, biochemical and molecular levels.…”