Dualities in plant tolerance to pollutants and their uptake and translocation to the upper plant parts

There is a duality in plant tolerance to pollutants and its response to the pollutants’ stress.On the one hand some plants, (hyper)tolerant to heavy metals, are able to hyperaccumulate these metals in shoots, which could be beneficial for phytoremediation purposes to clean-up soil and water. On the other hand tolerant food crops, exposed to heavy metals in their growth medium, may be dangerous as carriers of toxic metals in the food chain leading to food toxicity. There is an additional duality in plant tolerance to heavy metals and that is in food crops that are tolerant and/or hyperaccumulators, which could be used on one hand for phytoremediation, under controlled conditions and on the other hand for food fortification with essential metals.Similarly, plants are also exposed to a large number of xenobiotic organic pollutants. Because they generally cannot avoid these compounds, plants take up, translocate, metabolize and detoxify many of them. There is a large variability in tolerance (defence) mechanisms against organic pollutants among plant species. This includes production of reductants but also scavenger molecules like ascorbate and glutathione and expression of the P-450 defence system, and superfamilies of the enzymes glutathione- and glucosyl-transferases. Again, with view to organic pollutants, plant detoxification mechanisms might well protect the plant itself, but produce compounds with some deleterious potential for other organisms.In this review we discuss these dualities on the basis of examples of agricultural and ‘wild’ species exposed to metal contaminants (mainly Cd) and organic pollutants. Differences in uptake and translocation of various pollutants and their consequences will be considered. We will separately outline the effects of the organic and non-organic pollutants on the internal metabolism and the detoxification mechanisms and try to indicate the differences between both types of pollutants. Finally the consequences and solutions of these dualities in plant tolerance to pollutants will be discussed.     

Mechanisms of metal resistance in plants: aluminum and heavy metals

Plants have evolved sophisticated mechanisms to deal with toxic levels of metals in the soil. In this paper, an overview of recent progress with regards to understanding fundamental molecular and physiological mechanisms underlying plant resistance to both aluminum (Al) and heavy metals is presented. The discussion of plant Al resistance will focus on recent advances in our understanding of a mechanism based on Al exclusion from the root apex, which is facilitated by Al-activated exudation of organic acid anions. The consideration of heavy metal resistance will focus on research into a metal hyperaccumulating plant species, the Zn/Cd hyperaccumulator, Thlaspi caerulescens, as an example for plant heavy metal research. Based on the specific cases considered in this paper, it appears that quite different strategies are used for Al and heavy metal resistance. For Al, our current understanding of a resistance mechanism based on excluding soil-borne Al from the root apex is presented. For heavy metals, a totally different strategy based on extreme tolerance and metal hyperaccumulation is described for a hyperaccumulator plant species that has evolved on naturally metalliferous soils. The reason these two strategies are the focus of this paper is that, currently, they are the best understood mechanisms of metal resistance in terrestrial plants. However, it is likely that other mechanisms of Al and/or heavy metal resistance are also operating in certain plant species, and there may be common features shared for dealing with Al and heavy resistance. Future research may uncover a number of novel metal resistance mechanisms in plants. Certainly the complex genetics of Al resistance in some crop plant species, such as rice and maize, suggests that a number of presently unidentified mechanisms are part of an overall strategy of metal resistance in crop plants.     

Biochemical changes and adaptive strategies of plants under heavy metal stress

Heavy metal contamination of soil, aqueous waste stream and ground water causes major environmental and human health problems. Heavy metals are major environmental pollutants when they are present in high concentration in soil and show potential toxic effects on growth and development in plants. Due to unabated, indiscriminate and uncontrolled discharge of hazardous chemicals including heavy metals into the environment, plant continuously have to face various environmental constraints. In plants, seed germination is the first exchange interface with the surrounding medium and has been considered as highly sensitive to environmental changes. One of the crucial events during seed germination entails mobilization of seed reserves which is indispensable for the growth of embryonic axis. But, metabolic alterations by heavy metal exposure are known to depress the mobilization and utilization of reserve food by affecting the activity of hydrolytic enzymes. Some plants possess a range of potential mechanisms that may be involved in the detoxification of heavy metals by which they manage to survive under metal stress. High tolerance to heavy metal toxicity could rely either on reduced uptake or increase planned internal sequestration which is manifested by an interaction between a genotype and its environment. Such mechanism involves the binding of heavy metals to cell wall, immobilization, exclusion of the plasma membrane, efflux of these toxic metal ions, reduction of heavy metal transport, compartmentalization and metal chelation by tonoplast located transporters and expression of more general stress response mechanisms such as stress proteins. It is important to understand the toxicity response of plant to heavy metals so that we can utilize appropriate plant species in the rehabilitation of contaminated areas. Therefore, in the present review attempts have been made to evaluate the effects of increasing level of heavy metal in soils on the key behavior of hydrolytic and nitrogen assimilation enzymes. Additionally, it also provides a broad overview of the strategies adopted by plants against heavy metal stress.     

Managing heavy metal toxicity stress in plants: Biological and biotechnological tools

The maintenance of ion homeostasis in plant cells is a fundamental physiological requirement for sustainable plant growth, development and production. Plants exposed to high concentrations of heavy metals must respond in order to avoid the deleterious effects of heavy metal toxicity at the structural, physiological and molecular levels. Plant strategies for coping with heavy metal toxicity are genotype-specific and, at least to some extent, modulated by environmental conditions. There is considerable interest in the mechanisms underpinning plant metal tolerance, a complex process that enables plants to survive metal ion stress and adapt to maintain growth and development without exhibiting symptoms of toxicity. This review briefly summarizes some recent cell biological, molecular and proteomic findings concerning the responses of plant roots to heavy metal ions in the rhizosphere, metal ion-induced reactions at the cell wall-plasma membrane interface, and various aspects of heavy metal ion uptake and transport in plants via membrane transporters. The molecular and genetic approaches that are discussed are analyzed in the context of their potential practical applications in biotechnological approaches for engineering increased heavy metal tolerance in crops and other useful plants.     

解磷微生物修复土壤重金属污染研究进展

土壤重金属污染问题日益严重,具有普遍性、隐蔽性、表聚性、不可逆性等特点,已经成为环境污染治理中的热点、难点问题。解磷微生物能够依靠自身的代谢产物或通过与其他生物的协同作用,将土壤中的难溶性磷转化为可供植物吸收利用的磷,具有多重植物促生长功能和重金属解毒能力,可在重金属毒害水平下,促进植物生长、提高植物抗病能力、克服重金属对植物生长的不利影响,从而增强重金属修复植物的生存竞争力。从解磷微生物的研究现状入手,介绍了解磷微生物对土壤重金属污染的修复能力,综述了解磷微生物对土壤重金属污染修复的作用机制,分析了目前解磷微生物在重金属修复过程中存在的问题,并提出了今后研究的方向,为重金属污染土壤的修复提供了新思路。     

Physiological Aspects of Cadmium and Lead Toxic Effects on Higher Plants

Using the examples of cadmium and lead, the review considers the various toxic effects exerted by these heavy metals. Putative specific and nonspecific mechanisms of the toxic effects of the heavy metals and plant responses are discussed together with the issue of Cd and Pb accumulation in various plant organelles, cells, tissues, and organs. The basic mechanisms providing for plant resistance to excess Cd and Pb are elucidated. These data are used to schematically outline the changes in plant metabolism produced by these heavy metals.     

高等植物重金属耐性与超积累特性及其分子机理研究

由于重金属污染日益严重, 重金属在土壤_植物系统中的行为引起了人们的高度重视。高等植物对重金 属的耐性与积累性, 已经成为污染生态学研究的热点。近年来, 由于分子生态学等学科的发展, 有关植物对重金属的解毒和耐性机理、重金属离子富集机制的研究取得了较大进展。高等植物对重金属的耐性和积累在种间和基因型之间存在很大差异。根系是重金 属等土壤污染物进入植物的门户。根系分泌物改变重金属的生物有效性和毒性, 并在植物吸收重金属的过程中发挥重要作用。土壤中的大部分重金属离子都是通过金属转运蛋白进入根细胞, 并在植物体内进一步转运至液泡贮存。在重金属胁迫条件下植物螯合肽 (PC) 的合成是植物对胁迫的一种适应性反应。耐性基因型合成较多的PC, 谷胱甘肽 (GSH) 是合成PC的前体, 重金属与PC螯合并转移至液泡中贮存, 从而达到解毒效果。金属硫蛋白 (MTs) 与PC一样, 可以与重金属离子螯合, 从而降低重金属离子的毒性。该文从分子水平上论述了根系分泌物、金属转运蛋白、MTs、PC、GSH在重金属耐性及超积累性中的作用, 评述了近 10年来这方面的研究进展, 并在此基础上提出存在的问题和今后研究的重点。     

Proteomic analysis of the response of the plant growth-promoting bacterium Pseudomonas putida UW4 to nickel stress

Background  

Plant growth-promoting bacteria can alleviate the inhibitory effects of various heavy metals on plant growth, via decreasing levels of stress-induced ethylene. However, little has been done to detect any mechanisms specific for heavy metal resistance of this kind of bacteria. Here, we investigate the response of the wild-type plant growth-promoting bacterium Pseudomonas putida UW4 to nickel stress using proteomic approaches. The mutant strain P. putida UW4/AcdS^-, lacking a functional 1-aminocyclopropane-1-carboxylic acid deaminase gene, was also assessed for its response to nickel stress.     

Halophytes--an emerging trend in phytoremediation

Halophytic plants are of special interest because these plants are naturally present in environments characterized by an excess of toxic ions, mainly sodium and chloride. Several studies have revealed that these plants may also tolerate other stresses including heavy metals based on the findings that tolerance to salt and to heavy metals may, at least partly, rely on common physiological mechanisms. In addition, it has been shown that salt-tolerant plants may also be able to accumulate metals. Therefore, halophytes have been suggested to be naturally better adapted to cope with environmental stresses, including heavy metals compared to salt-sensitive crop plants commonly chosen for phytoextraction purposes. Thus, potentially halophytes are ideal candidates for phytoextraction orphytostabilization of heavy metal polluted soils and moreover of heavy metal polluted soils affected by salinity. Some halophytes use excretion processes in order to remove the excess of salt ions from their sensitive tissues and in some cases these glandular structures are not always specific to Na+ and Cl- and other toxic elements such as cadmium, zinc, lead, or copper are accumulated and excreted by salt glands or trichomes on the surface of the leaves--a novel phytoremediation process called "phytoexcretion". Finally, the use of halophytes has also been proposed for soil desalination through salt accumulation in the plant tissue or dissolution of soil calcite in the rhizosphere to provide Ca2+ that can be exchanged with Na+ at cation exchange sites.     

植物修复重金属污染及内生细菌效应

土壤和水体的重金属污染已严重危害人类生存环境与健康。由于受重金属污染的环境分布广泛,迫切需要开发经济的清除环境重金属的技术。植物修复是通过绿色植物降解或移除环境污染物,有望成为重金属污染环境的原位修复技术。植物内生菌是指定殖于健康植物的各种组织和器官内部的细菌,被感染的宿主植物不表现出外在病症,耐重金属的内生菌在多种超富集植物中存在。在植物修复过程中,野生型内生菌或基因工程内生菌的抗性系统能降低重金属植物毒性,促进其迁移金属。耐重金属内生菌还可以通过固氮、溶解矿物元素及产生类植物激素、铁载体和ACC脱氨酶等产物促进植物的生长。主要综述目前植物-内生菌相互作用及其潜在的促进植物修复重金属污染的研究进展。     

Heavy metals toxicity in plants: An overview on the role of glutathione and phytochelatins in heavy metal stress tolerance of plants

Plants experience oxidative stress upon exposure to heavy metals that leads to cellular damage. In addition, plants accumulate metal ions that disturb cellular ionic homeostasis. To minimize the detrimental effects of heavy metal exposure and their accumulation, plants have evolved detoxification mechanisms. Such mechanisms are mainly based on chelation and subcellular compartmentalization. Chelation of heavy metals is a ubiquitous detoxification strategy described in wide variety of plants. A principal class of heavy metal chelator known in plants is phytochelatins (PCs), a family of Cys-rich peptides. PCs are synthesized non-translationally from reduced glutathione (GSH) in a transpeptidation reaction catalyzed by the enzyme phytochelatin synthase (PCS). Therefore, availability of glutathione is very essential for PCs synthesis in plants at least during their exposure to heavy metals. Here, I reviewed on effect of heavy metals exposure to plants and role of GSH and PCs in heavy metal stress tolerance. Further, genetic manipulations of GSH and PCs levels that help plants to ameliorate toxic effects of heavy metals have been presented.     

Understanding molecular mechanisms for improving phytoremediation of heavy metal-contaminated soils

Heavy metal pollution of soil is a significant environmental problem with a negative potential impact on human health and agriculture. Rhizosphere, as an important interface of soil and plants, plays a significant role in phytoremediation of contaminated soil by heavy metals, in which, microbial populations are known to affect heavy metal mobility and availability to the plant through release of chelating agents, acidification, phosphate solubilization and redox changes, and therefore, have potential to enhance phytoremediation processes. Phytoremediation strategies with appropriate heavy metal-adapted rhizobacteria or mycorrhizas have received more and more attention. In addition, some plants possess a range of potential mechanisms that may be involved in the detoxification of heavy metals, and they manage to survive under metal stresses. High tolerance to heavy metal toxicity could rely either on reduced uptake or increased plant internal sequestration, which is manifested by an interaction between a genotype and its environment.A coordinated network of molecular processes provides plants with multiple metal-detoxifying mechanisms and repair capabilities. The growing application of molecular genetic technologies has led to an increased understanding of mechanisms of heavy metal tolerance/accumulation in plants and, subsequently, many transgenic plants with increased heavy metal resistance, as well as increased uptake of heavy metals, have been developed for the purpose of phytoremediation. This article reviews advantages, possible mechanisms, current status and future direction of phytoremediation for heavy-metal–contaminated soils.     

Roles of root and shoot tissues in transport and accumulation of cadmium, lead, nickel, and strontium

The review considers the roles of root and shoot tissues in transport and accumulation of heavy metals in plants of two contrast groups, i.e., excluders and hyperaccumulators. The regularities in distribution of cadmium, lead, nickel, and strontium are summarized. Effects of other cations, calcium in particular, on accumulation and distribution of heavy metals are analyzed. Specific patterns of metal distribution in hyperaccumulator plants are discussed together with morphological and functional features underlying the ability of plants to accumulate heavy metals in the aboveground organs. Based on the data available, the root and shoot tissues are classified according to their roles in transport and distribution of the metals examined.     

施硅对抑制植物吸收重金属镉的效应研究进展

施用化学改良剂是控制土壤重金属污染的有效手段。研究施硅对抑制植物吸收重金属镉的影响及其作用机制,对促进利用硅肥作为改良剂治理重金属污染土壤技术的发展有重要意义。近年来,以硅肥作改良剂对重金属污染的土壤进行治理的研究大量涌现。本文从施硅对抑制植物吸收镉及镉在植物体内的分布、迁移的影响;从植物细胞膜透性、抗氧化物酶系和抗氧化剂等新陈代谢或生理过程及硅-金属复合物的结构组成等方面对植物抗镉胁迫的生理生化效应及其抑制植物吸收重金属镉的机制进行综述,并对今后有待进一步研究的问题提出了建议。     

Toxicity of Cadmium and nickel in the context of applied activated carbon biochar for improvement in soil fertility

Toxicity induced by heavy metals deteriorates soil fertility status. It also adversely affects the growth and yield of crops. These heavy metals become part of the food chain when crops are cultivated in areas where heavy metals are beyond threshold limits. Cadmium (Cd) and nickel (Ni) are considered the most notorious ones among different heavy metals. The high water solubility of Cd made it a potential toxin for plants and their consumers. Accumulation of Ni in plants, leaves, and fruits also deteriorates their quality and causes cancer in humans when such a Ni-contaminated diet is used regularly. Both Cd and Ni also compete with essential nutrients of plants, making the fertility status of soil poor. To overcome this problem, the use of activated carbon biochar can play a milestone role. In the recent past application of activated carbon biochar is gaining more and more attention. Biochar sorb the Cd and Ni and releases essential micronutrients that are part of its structure. Many micropores and high cation exchange capacity make it the most acceptable organic amendment to improve soil fertility and immobilize Cd and Ni. In addition to improving water and nutrients, soil better microbial proliferation enhances the soil rhizosphere ecosystem and nutrient cycling. This review has covered Cd and Ni harmful effects on crop yield and their immobilization by activated carbon biochar. The focus was made to elaborate on the positive effects of biochar on crop yield and soil health.     

How Plants Cope with Cadmium: Staking All on Metabolism and Gene Expression

Environmental pollullon is one of the major problems for human health. Toxic heavy metals are normally present as soil constituents or can also be spread out in the environment by human activity and agricultural techniques. Soil contamination by heavy metals as cadmium, highlights two main aspects: on one side they interfere with the life cycle of plants and therefore reduce crop yields, and on the other hand, once adsorbed and accumulated into the plant tissues, they enter the food chain poisoning animals and humans. Considering this point of view, understanding the mechanism by which plants handle heavy metal exposure, In particular cadmium stress, is a primary goal of plant-blotechnology research or plant breeders whose aim is to create plants that are able to recover high amounts of heavy metals, which can be used for phytoremediation, or identify crop varieties that do not accumulate toxic metal in grains or fruits. In this review we focus on the main symptoms of cadmium toxicity both on root apparatus and shoots. We elucidate the mechanisms that plants activate to prevent absorption or to detoxify toxic metal ions, such as synthesis of phytochelatins, metallothioneins and enzymes involved in stress response. Finally we consider new plant-biotechnology applications that can be applied for phytoremediation.     

How Plants Cope with Cadmium: Staking All on Metabolism and Gene Expression

Environmental pollullon is one of the major problems for human health. Toxic heavy metals are normally present as soil constituents or can also be spread out in the environment by human activity and agricultural techniques. Soil contamination by heavy metals as cadmium, highlights two main aspects: on one side they interfere with the life cycle of plants and therefore reduce crop yields, and on the other hand, once adsorbed and accumulated into the plant tissues, they enter the food chain poisoning animals and humans. Considering this point of view, understanding the mechanism by which plants handle heavy metal exposure, In particular cadmium stress, is a primary goal of plant-blotechnology research or plant breeders whose aim is to create plants that are able to recover high amounts of heavy metals, which can be used for phytoremediation, or identify crop varieties that do not accumulate toxic metal in grains or fruits. In this review we focus on the main symptoms of cadmium toxicity both on root apparatus and shoots. We elucidate the mechanisms that plants activate to prevent absorption or to detoxify toxic metal ions, such as synthesis of phytochelatins, metallothioneins and enzymes involved in stress response. Finally we consider new plant-biotechnology applications that can be applied for phytoremediation.     

Molecular Mechanisms and Genetic Basis of Heavy Metal Tolerance/Hyperaccumulation in Plants

Phytoremediation has gained increased attention as a cost-effective method for the remediation of heavy metal-contaminated sites. Because some plants possess a range of potential mechanisms that may be involved in the detoxification of heavy metals, they manage to survive under metal stresses. High tolerance to heavy metal toxicity could rely either on reduced uptake or increased plant internal sequestration,which is manifested by an interaction between a genotype and its environment. The growing application of molecular genetic technologies has led to increased understanding of mechanisms of heavy metal tolerance/accumulation in plants and, subsequently, many transgenic plants with increased heavy metal resistance,as well as increased uptake of heavy metals, have been developed for the purpose of phytoremediation. In the present review, our major objective is to concisely evaluate the progress made so far in understanding the molecular/cellular mechanisms and genetic basis that control the uptake and detoxification of metals by plants.     

Effect of nickel at high concentration on proliferation of quiescent center cells and initiation of lateral root primordia in wheat seedlings

DNA synthesis and cell divisions in the quiescent center as well as initiation of lateral root primordia were investigated in the course of incubation of the roots of 3-day-old wheat (Triticum aestivum L.) seedlings on the medium with 0.1 mM NiSO₄ for 72 h. It was found that the earliest effect of nickel on proliferation of the quiescent center cells was associated with an increase in the mitotic index 6 h after the beginning of its action. This effect was assumed to depend on an increase in mitosis time. Twelve hours after the beginning of the effect of nickel, mitotic index became somewhat lower, and in 18 h it sharply decreased. Some dividing cells were observed among the initial cells of certain tissues and near the quiescent center even in 72 h. The portion of DNA synthesizing cell sharply decreased in 12 h, and in 48 h such cells were lacking. The main mechanism governing the termination of cell proliferation in the quiescent center as well as in the meristem and calyptrogen of the cap is the inhibition of cell transition to DNA synthesis. The cells that had time to start DNA synthesis or already finished it and were in other phases of the cycle continued a slow progression through the cycle and completed it. Sister cells, produced as a result of divisions, left the mitotic cycle in the phase G₁ and transited to dormancy. Nickel did not inhibit initiation and development of lateral root primordia. Resumption of DNA synthesis and cell divisions occurred not only in the pericycle and endodermis participating in the initiation of lateral root primordia but also in the cortex cells in the vicinity of developing primordia. In 18 h after the beginning of the experiment when the rate of the root growth considerably decreased, the region, where primordia were initiated, was located closer to the root tip. Subsequently, when elongation of the cells was inhibited, this region moved closer to the tip until structural disturbances occurred in the nuclei of the endodermal cells located near the root tip and elongated under the effect of nickel. The results concerning the effect of nickel and other heavy metals on root cell proliferation obtained by other researchers and the role of pericycle organization in the translocation and accumulation of nickel in the tissues are discussed.