植物螯合肽及其在重金属耐性中的作用

综述植物螯合肽的生物合成及其在重金属耐性中的作用．有毒重金属在土壤中的积累不仅影响作物的生长和产量形成,而且严重威胁农产品的安全性．植物对重金属的耐性和积累在种间和基因型之间存在着很大的差异,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谷胱苷肽是合成PC的前体,PC可与重金属螯合,并进一步转运至液泡贮存,使细胞质的重金属浓度降低,从而达到解毒效果．重金属诱导植物合成PC的遗传机理和生化途径有赖于分子生物学的深入研究,cD-敏感型拟南芥突变体Cad1—1(缺失GSH)和Cad2-1(缺失PC合成诱导酶)的分离及相关研究,佐证了PC在Cd—解毒中起关键作用．对PC在重金属污染土壤或水体的植物修复和农作物安全生产中的意义进行了讨论．     

植物对重金属耐性的分子生态机理

 植物适应重金属元素胁迫的机制包括阻止和控制重金属的吸收、体内螯合解毒、体内区室化分隔以及代谢平衡等。近年来,随着分子生物学技术在生态学研究中的深入应用,控制这些过程的分子生态机理逐渐被揭示出来。菌根、根系分泌物以及细胞膜是控制重金属进入植物根系细胞的主要生理单元。外生菌根能显著提高寄主植物的重金属耐性,根系分泌物通过改变根际pH、改变金属物质的氧化还原状态和形成络合物等机理减少植物对重金属的吸收。目前,控制菌根和根系分泌物重金属抗性的分子生态机理还不清楚。但细胞膜跨膜转运器已得到深入研究,相关金属离子转运器被鉴定和分离,一些控制基因如铁锌控制运转相关蛋白(ZIP)类、自然抵抗相关巨噬细胞蛋白(Nramp)类、P1B_type ATPase类基因已被发现和克隆。金属硫蛋白（MTs）、植物螯合素(PCs)、有机酸及氨基酸等是植物体内主要的螯合物质,它们 通过螯合作用固定金属离子,降低其生物毒性或改变其移动性。与MTs合成相关的MT_like基因已经被克隆,PCs合成必需的植物螯合素合酶(PCS), 即γ-Glu-Cys二肽转肽酶 (γ-ECS) 的编码基因已经被克隆,控制麦根酸合成的氨基酸尼克烟酰胺(NA)在重金属耐性中的作用和分子机理也被揭示出来。ATP结合转运器(ABC)和阳离子扩散促进器(CDF)是植物体内两种主要膜转运器,通过它们和其它跨膜方式,重金属被分隔贮藏于液泡内。控制这些蛋白转运器合成的基因也已经被克隆,在植物中的表达证实其与重金属的体内运输和平衡有关。热休克蛋白(HSP)等蛋白类物质的产生是一种重要的体内平衡机制,其分子机理有待进一步研究。重金属耐性植物在这些环节产生了相关响应基因或功能蛋白质,分子克隆和转基因技术又使它们在污染治理上得到了初步的应用。     

植物对铅的吸收、转运、累积和解毒机制研究进展

揭示植物对铅的吸收、转运、累积和解毒的分子机制,可以明晰农作物吸收铅的关键过程,阻控铅在粮食、蔬菜中的积累,降低重金属的食用风险;也可以阐明某些铅超积累植物的耐性与解毒机制,分离并克隆铅超积累的功能基因,培育高效的铅污染土壤修复植物.本文从铅进入植物的两个重要途径(叶片的吸附与吸收以及根系的吸收与转运)出发,系统总结讨论了植物对铅的吸收、转运、累积和分布的研究进展;采用胞外至胞内的空间顺序,分别从植物根系分泌物的解毒、细胞壁的固定和动态响应、细胞质膜的选择透过性作用、液泡的区隔化作用以及金属有机配体的螯合等方面论述植物铅耐性和解毒的分子机理,并在此基础上提出存在的问题和今后研究的重点.     

植物对重金属的吸收和分布

植物修复是利用植物来清除污染土壤中重金属的一项技术。该技术成功与否取决于植 物从土壤中吸取金属以及向地上部运输金属的能力。植物对金属的吸收主要取决于自由态离子活度。许多螯合剂能诱导植物对重金属的吸收。金属离子在液泡中的区域化分布是植物耐 重金属的主要原因。同时,细胞内的金属硫蛋白、植物螯合肽等蛋白质以及有机酸、氨基酸等在金属贮存和解毒方面也起重要作用。本文还论述了重金属在植物体内运输的生理及分子 方面的研究进展。     

植物对重金属的吸收和分布

植物修复是利用植物来清除污染土壤中重金属的一项技术。该技术成功与否取决于植物从土壤中吸取金属以及向地上部运输金属的能力。植物对金属的吸收主要取决于自由态离子活度。许多螯合剂能诱导植物对重金属的吸收。金属离子在液泡中的区域化分布是植物耐重金属的主要原因。同时,细胞内的金属硫蛋白、植物螯合脓等蛋白质以及有机酸、氨基酸等在金属贮存和解毒方面也起重要作用。本文还论述了重金属在植物体内运输的生理及分子方面的研究进展。     

植物螯合肽的转运与功能研究进展

植物螯合肽（phytochelatins,PCs）是由植物螯合肽合酶催化谷胱甘肽合成的一类生物小分子,结构式为（γ-Glu-Cys）n-Gly（n=2-11）,在真菌和高等植物耐受重金属胁迫机制中具有重要作用。近年来,人们在Pc介导重金属脱毒害的分子机理研究上取得了重要进展,发JLSpHMT1和SpABC2是PC在裂殖酵母中介导重金属液泡区室化的主要转运蛋白,鉴定了拟南芥液泡膜PC转运蛋AtABCC1和AtABCC2。此外,PCs也可能在超积累植物细胞内对重金属脱毒害具有重要功能。     

植物对铅胁迫的耐性及其解毒机制研究进展

植物对重金属元素的耐性、积累特性是利用植物修复铅污染土壤的前提,因而需要全面理解植物对铅吸收、转运、积累和解毒的一系列生理机制.本文从植物自身对铅的适应和防御机制出发,综述了细胞壁和液泡在植物细胞钝化与铅积累中的功能;根系分泌物对铅生物有效性的影响;超氧化物歧化酶（SOD）、过氧化物酶（POD）、过氧化氢酶（CAT）、谷光苷肽还原酶（GR）、抗坏血酸过氧化物酶（APX）和植物螯合肽、谷胱甘肽在铅解毒中的作用,以及金属硫蛋白和铅特异基因表达的研究进展.并对未来该领域的研究以及铅污染环境植物修复技术的发展进行了展望.     

植物重金属转运蛋白研究进展

土壤中的有毒重金属不仅对植物有害,也可通过食物链危害人类和动物的健康.重金属转运蛋白在植物吸收、抵抗重金属的复杂机制中起着关键作用.植物重金属转运蛋白分为吸收蛋白和排出蛋白,其中,吸收蛋白转运必需重金属进入细胞,同时也会因为必需重金属的缺乏或离子之间的竞争而运载有毒重金属;排出蛋白是一类解毒蛋白,可将过量的或有毒的重金属逆向转运出细胞,或区室化于液泡中.目前,细胞内多种重金属转运蛋白基因的转录水平与重金属离子积累之间的联系已被揭示,并分离克隆出诸多相关蛋白家族成员.本文综述了近年来发现并鉴定的主要重金属转运蛋白的金属亲和性、器官表达特异性及细胞内定位等的研究进展.     

锰对植物毒害及植物耐锰机理研究进展

含锰矿渣的排放造成了严重的土壤锰污染。揭示锰毒害和植物的耐锰机制对于污染土壤治理具有重要意义。研究表明,高浓度的Mn2＋能够抑制根系Ca2＋、Fe2＋和Mg2＋等元素的吸收及活性,引起氧化性胁迫导致氧化损伤,使叶绿素和Rubisco含量下降、叶绿体超微结构破坏和光合速率降低。而锰超累积植物则具有多种解毒或耐性机制,如区域化、有机酸螯合、外排作用、抗氧化作用和离子交互作用等。根系主要通过有机酸的螯合作用促进植物对Mn^2＋的转运解毒,同时能够将过量的Mn^2＋区域化在根细胞壁中;叶片可通过酚类物质或有机酸螯合Mn^2＋,并将其区域化在叶片表皮细胞和叶肉细胞的液泡中（或通过表皮毛将Mn^2＋排出体外）。其中,金属转运蛋白在植物对Mn^2＋的吸收、转运、累积和解毒过程中发挥着重要作用。     

植物螯合肽及其功能

植物螯合肽（phytochelatin,PC）是一类富含Cys、由PC合酶以GSH为底物催化合成的小分子多肽,能通过Cys的-SH络合重金属.研究PC的合成机理及其重金属解毒机制、研究PC合酶和PC合酶基因的表达模式及其功能对于运用植物修复技术治理重金属污染的土壤和水体具有重要意义.     

Molecular mechanisms of heavy metal hyperaccumulation and phytoremediation

A relatively small group of hyperaccumulator plants is capable of sequestering heavy metals in their shoot tissues at high concentrations. In recent years, major scientific progress has been made in understanding the physiological mechanisms of metal uptake and transport in these plants. However, relatively little is known about the molecular bases of hyperaccumulation. In this paper, current progresses on understanding cellular/molecular mechanisms of metal tolerance/hyperaccumulation by plants are reviewed. The major processes involved in hyperaccumulation of trace metals from the soil to the shoots by hyperaccumulators include: (a) bioactivation of metals in the rhizosphere through root–microbe interaction; (b) enhanced uptake by metal transporters in the plasma membranes; (c) detoxification of metals by distributing to the apoplasts like binding to cell walls and chelation of metals in the cytoplasm with various ligands, such as phytochelatins, metallothioneins, metal-binding proteins; (d) sequestration of metals into the vacuole by tonoplast-located transporters. The growing application of molecular-genetic technologies led to the well understanding of mechanisms of heavy metal tolerance/accumulation in plants, and subsequently many transgenic plants with increased resistance and uptake of heavy metals were developed for the purpose of phytoremediation. Once the rate-limiting steps for uptake, translocation, and detoxification of metals in hyperaccumulating plants are identified, more informed construction of transgenic plants would result in improved applicability of the phytoremediation technology.     

Cellular mechanisms for heavy metal detoxification and tolerance.

Heavy metals such as Cu and Zn are essential for normal plant growth, although elevated concentrations of both essential and non-essential metals can result in growth inhibition and toxicity symptoms. Plants possess a range of potential cellular mechanisms that may be involved in the detoxification of heavy metals and thus tolerance to metal stress. These include roles for the following: for mycorrhiza and for binding to cell wall and extracellular exudates; for reduced uptake or efflux pumping of metals at the plasma membrane; for chelation of metals in the cytosol by peptides such as phytochelatins; for the repair of stress-damaged proteins; and for the compartmentation of metals in the vacuole by tonoplast-located transporters. This review provides a broad overview of the evidence for an involvement of each mechanism in heavy metal detoxification and tolerance.     

植物超积累重金属的生理机制研究进展

土壤重金属污染已经成为一个全球性问题。重金属超积累植物在修复土壤重金属污染中具有重要的应用前景。重金属超积累植物通常具备三个基本特征,即：根系具有从土壤中吸收重金属的强大能力、能从根到地上部分高效转运重金属、在叶片中能解毒和隔离大量重金属。本文总结了重金属超积累植物吸收、转运、隔离和解毒重金属的生理机制研究进展,以期为进一步阐明植物超积累重金属的机制及其在植物修复中的应用提供参考。     

根系分泌物在重金属污染土壤生态修复中的作用机制研究进展

我国土壤重金属污染问题日益突出。作为一种绿色、安全的生物修复技术,植物修复技术备受关注。根系分泌物作为植物-土壤-微生物三者物质交换与信息传递的重要载体,是植物响应外界胁迫的重要生理生态指征,在植物修复过程中发挥关键作用。研究表明,根系分泌物能够有效调控根际微环境,提升植物抗逆能力,影响重金属在根际微域中的环境行为。传统的根际生态研究更多聚焦在养分调控等方面,而关于根系分泌物在污染场地生态修复中的作用机制研究相对较少。本文综述了根系分泌物对土壤理化性质、土壤微生物、植物生理代谢过程以及重金属在土壤-植物系统中迁移转化等方面的影响,系统总结了植物根系分泌物响应重金属胁迫的生理生态机制,并对未来土壤重金属污染生态修复研究方向进行了展望。     

Cadmium hyperaccumulation leads to an increase of glutathione rather than phytochelatins in the cadmium hyperaccumulator Sedum alfredii

Sedum alfredii has been reported to be a cadmium (Cd) hyperaccumulator. Phytochelatins (PCs) and other thiol (SH)-containing compounds have been proposed to play an important role in the detoxification and tolerance of some heavy metals, but it is not clear whether PCs are responsible for Cd hyperaccumulation and tolerance in S. alfredii. In this study, two geographically isolated populations of S. alfredii were studied: one population grew on an old Pb/Zn mine site, while the other on a non-mine site. The mine population of this species exhibited a stronger heavy metal tolerance than in the other population. Root-to-shoot transport of Cd was higher in population located at the mine site than at the non-mine site. Considerable amounts of Cd were accumulated in leaves and stems of mine plants, while most Cd was distributed in roots of non-mine plants. Non-protein SH in plant tissues of two populations were further investigated by a HPLC pre-column derivatization system. Upon exposure to Cd, no PCs were detected in all tissues of mine population, while an appreciable amount of glutathione (GSH) was observed in the descending order of stem>root>leaf. The concentrations of GSH consistently increased with the increase of exogenous Cd concentrations and time. On the contrary, Cd exposure strongly induced the production of PCs (mainly PC(2) and PC(3)) and GSH in plant tissues of non-mine population, and the concentrations of GSH showed an initial drop over the duration of 7-d exposure. The present results provided strong evidence that PCs are not involved in Cd transport, hyperaccumulation and tolerance in mine population of S. alfredii.     

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.     

A new pathway for heavy metal detoxification in animals. Phytochelatin synthase is required for cadmium tolerance in Caenorhabditis elegans

Increasing emissions of heavy metals such as cadmium, mercury, and arsenic into the environment pose an acute problem for all organisms. Considerations of the biochemical basis of heavy metal detoxification in animals have focused exclusively on two classes of peptides, the thiol tripeptide, glutathione (GSH, gamma-Glu-Cys-Gly), and a diverse family of cysteine-rich low molecular weight proteins, the metallothioneins. Plants and some fungi, however, not only deploy GSH and metallothioneins for metal detoxification but also synthesize another class of heavy metal binding peptides termed phytochelatins (PCs) from GSH. Here we show that PC-mediated heavy metal detoxification is not restricted to plants and some fungi but extends to animals by demonstrating that the ce-pcs-1 gene of the nematode worm Caenorhabditis elegans encodes a functional PC synthase whose activity is critical for heavy metal tolerance in the intact organism.     

Heavy metal detoxification mechanisms in halophytes: an overview

Heavy metals are among the major pollutants from anthropogenic inputs that reach mangrove ecosystem by urban and agricultural runoff, industrial effluents, boating, mining and other processes. To minimize the detrimental effects of heavy metal exposure and their accumulations, plants in general have evolved biological detoxification mechanisms, which include avoidance or exclusion, excretion and accumulation. To protect the cellular components from oxidative damage by heavy metal contamination, biological systems have developed enzymatic and non-enzymatic antioxidant mechanisms. Another detoxification mechanisms produced in plants are osmoprotectants, which are the compatible solutes which maintain a favourable water potential gradient and protect cellular structures from toxic ions. Besides these mechanisms, another heavy metal detoxification system in plants involves the chelation of metals by metal binding molecules like metallothioneins (MTs) and phytochelatins (PCs). To limit the heavy metal toxicity from mangrove ecosystem, it was found that phytoremediation is a most useful technology where in plants are used to remove pollutants from the environment and it is considered as a comparatively new, low-cost and highly promising technology for the remediation of heavy metal. Rhizofiltration, phytovolatilization, phytoextraction and phytostabilization are the important phytoremediation techniques. Among these phytoextraction and phytostabilization are found highly important in the case of mangroves and are promising means of phytoremediation.