Effects of heavy metals' toxicity on plants and enhancement of plant defense mechanisms of Si-mediation “Review
Abstract
Today’s [e.g., “heavy metals (HMs)”] caused by anthropogenic activities have negative impacts on our environment and food productions. HMs can be classified as either essential or nonessential. A trace of essential HMs, such as Cu, Mo, and Zn, can be necessary for plant metabolism, but excess of them can harm the plant growth and development. Nonessential HMs, however, are toxic for plant metabolism and have damaging effects on enzyme activity, photosynthetic properties, cell membrane, permeability and eventually plant growth. Plants with avoidance and tolerance against stress could manage extreme HMstress in soils so that with special mechanisms, such as specific translation and metal accumulation, can elevate abiotic and biotic stress in plants. Moreover, in cells with mechanisms such as [e.g., “Metallothionein (MTs)”] (metal binding proteins) or [e.g., “Phytochelatin (PCs)”] storage and crystallization could reduce the HMstress in the cell wall, plasma membrane, cytosol, tonoplast and vacuoles. Meanwhile, the role of Si-mediation in detoxification of HMs is so bold. Si-mediation with increasing antioxidant, reducing lipid peroxidation, and increasing efficiency of photosynthetic properties elevates the HMs and other biotic and abiotic stresses in plants.
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Introduction
HMs are known as biotic stress and hazardous chemical that could affect human health by influencing the food chain and aquifers. They are, also, known as one of the reasons to inhibition of plant growth [1]. There are two types of metals in the soil: essential and non-essential. Essential HMs plays an important role in many enzyme activities as cofactor and in other protein structures which plants need them for growing and development [2,3]. However, HMs concentration is an important factor in the growth of plants so that the excess of HMs can lead to a reduction in plant growth. Heavy metals with binding to sulfhydryl group could lead to ions' substitution on protein structure [4]. In the other hand, enhancement of HMs can initiate the oxidative stress by generating ROS form oxidative stress, which in turn may disrupt the balance between pro-oxidant and antioxidant homeostasis. Additionally, observation obtained by oxidative attacking to DNA in cultured cells, and animals indicated that metal has this ability to interact with nuclear proteins and DNA [5]. Plants use a number of defense mechanisms for detoxification of toxic when encounter with abiotic stress caused by high concentration of HMs. This can help to recover and ameliorating in cells. As the first step, plants start with some avoidance and hemostasis mechanisms to prevent the onset of stress in extracellular, including binding them to micronize, cell wall, and extracellular exudates or with control efflux pumping of metals in plasma. This includes membrane and mechanisms of storage and detoxification in the vacuole and protoplasm. Among these mechanisms, the most important ones are transferring and sequestration to the vacuole, chelation mechanisms and reducing the damage heat shock proteins with renovating their [4,6]. It has been found in recent decades that Si could play an important role as one useful element in plant resistance which copes with abiotic and biotic stress and improves the plant growth [7]. In plants, silicon amplifies water-use efficiency [8], enhances cell wall rigidity [9], increases antioxidant enzyme activities, and reduces lipid peroxidation [10]. The aim of authoring this paper is to first recognize the impact of HMs on plants, and then investigate the toxicity of some non-essential HM on plants and study the plant defense mechanisms. Eventually, it is aimed to assess the Silicon as a reduction and amelioration of biotic and abiotic stresses.
Conclusion
HMs are one of the most important abiotic stresses that inhibit the growth and development in living organisms, lead to an early senescence of them [122], and menace safe food product for human over the world [123]. In plants, the excess amount of HMs leads to some symptoms such as necrosis, chlorosis, alterations of plants’ phenotype, and genotype, causes an oxidative stress, and subsequently reactivates oxygen species (ROS) so that they stimulate the plant defense mechanism such as increasing antioxidant enzymes and non-enzyme activities [124]. Hence, that is counted as the most important defense mechanism of plants against stress for cell protects [125].
Plants, in response to HMs, follow three different strategies: 1-metal excluders: it covers a majority of mechanisms, including a large group of plants in which plants prevent the HMs stress by limiting the translocation of HMs into plant aerial parts; 2-metal indicator: plant is one indicator of soil HMthat accumulates in biomass and other parts of plant; 3-metal accumulation: plants are counted as an accumulator in soil so HMs transudate and accumulate in plant tissue [126]. In cells, using strategies such as metal binding to cell wall and chelation, transforming HMs to low levels, and eventually accumulation and crystallization of them, plants could detoxify the HMs stress [127].
Si-mediation can help plant biomass production and plant growth [128]. One of the avoidance mechanisms of Si-mediation in the root is to reduce the uptake of HMs (Cd) with increasing the root oxalate exudation by enhancing the number of root tips [129].But, the major role of Si, when encounters with abiotic stress, is the elevation of the plant resistant by increasing the antioxidant enzyme activity [130].Si-mediation in cells decreases the toxic concentrations caused by HMs, symplast, apoplast, and incensement Si-absorbed in cell walls, and limits the root to shoot HMs translocations [131]. Generally, excess of HMs in agricultural soil caused by anthropogenic activities has made serious problems on the way to boost the agricultural products and improve their quality. In recent centuries, numerous studies carried out by researchers to alleviate and ameliorate the HMs toxicity in plants, which have revealed new ways for research communities to understand and solve this problem. Si-mediation is one of the beneficial elements in stress conditions that helps to increase the efficiency of antioxidant enzyme activity. In this study, we tried to identify some non-essential elements and express some plant mechanisms caused by Si-mediation in coping with abiotic and biotic stress.