Memon Abdul R, Schröder Peter
TUBITAK, Marmara Research Center, Institute for Genetic Engineering and Biotechnology, P.O. Box 21, 41470, Gebze, Kocaeli, Turkey.
Environ Sci Pollut Res Int. 2009 Mar;16(2):162-75. doi: 10.1007/s11356-008-0079-z. Epub 2008 Dec 6.
BACKGROUND, AIM, AND SCOPE: Trace elements (heavy metals and metalloids) are important environmental pollutants, and many of them are toxic even at very low concentrations. Pollution of the biosphere with trace elements has accelerated dramatically since the Industrial Revolution. Primary sources are the burning of fossil fuels, mining and smelting of metalliferous ores, municipal wastes, agrochemicals, and sewage. In addition, natural mineral deposits containing particularly large quantities of heavy metals are found in many regions. These areas often support characteristic plant species thriving in metal-enriched environments. Whereas many species avoid the uptake of heavy metals from these soils, some of them can accumulate significantly high concentrations of toxic metals, to levels which by far exceed the soil levels. The natural phenomenon of heavy metal tolerance has enhanced the interest of plant ecologists, plant physiologists, and plant biologists to investigate the physiology and genetics of metal tolerance in specialized hyperaccumulator plants such as Arabidopsis halleri and Thlaspi caerulescens. In this review, we describe recent advances in understanding the genetic and molecular basis of metal tolerance in plants with special reference to transcriptomics of heavy metal accumulator plants and the identification of functional genes implied in tolerance and detoxification.
Plants are susceptible to heavy metal toxicity and respond to avoid detrimental effects in a variety of different ways. The toxic dose depends on the type of ion, ion concentration, plant species, and stage of plant growth. Tolerance to metals is based on multiple mechanisms such as cell wall binding, active transport of ions into the vacuole, and formation of complexes with organic acids or peptides. One of the most important mechanisms for metal detoxification in plants appears to be chelation of metals by low-molecular-weight proteins such as metallothioneins and peptide ligands, the phytochelatins. For example, glutathione (GSH), a precursor of phytochelatin synthesis, plays a key role not only in metal detoxification but also in protecting plant cells from other environmental stresses including intrinsic oxidative stress reactions. In the last decade, tremendous developments in molecular biology and success of genomics have highly encouraged studies in molecular genetics, mainly transcriptomics, to identify functional genes implied in metal tolerance in plants, largely belonging to the metal homeostasis network.
Analyzing the genetics of metal accumulation in these accumulator plants has been greatly enhanced through the wealth of tools and the resources developed for the study of the model plant Arabidopsis thaliana such as transcript profiling platforms, protein and metabolite profiling, tools depending on RNA interference (RNAi), and collections of insertion line mutants. To understand the genetics of metal accumulation and adaptation, the vast arsenal of resources developed in A. thaliana could be extended to one of its closest relatives that display the highest level of adaptation to high metal environments such as A. halleri and T. caerulescens.
This review paper deals with the mechanisms of heavy metal accumulation and tolerance in plants. Detailed information has been provided for metal transporters, metal chelation, and oxidative stress in metal-tolerant plants. Advances in phytoremediation technologies and the importance of metal accumulator plants and strategies for exploring these immense and valuable genetic and biological resources for phytoremediation are discussed.
A number of species within the Brassicaceae family have been identified as metal accumulators. To understand fully the genetics of metal accumulation, the vast genetic resources developed in A. thaliana must be extended to other metal accumulator species that display traits absent in this model species. A. thaliana microarray chips could be used to identify differentially expressed genes in metal accumulator plants in Brassicaceae. The integration of resources obtained from model and wild species of the Brassicaceae family will be of utmost importance, bringing most of the diverse fields of plant biology together such as functional genomics, population genetics, phylogenetics, and ecology. Further development of phytoremediation requires an integrated multidisciplinary research effort that combines plant biology, genetic engineering, soil chemistry, soil microbiology, as well as agricultural and environmental engineering.
背景、目的与范围：微量元素（重金属和类金属）是重要的环境污染物，其中许多即使在极低浓度下也具有毒性。自工业革命以来，生物圈中微量元素的污染急剧加速。主要来源包括化石燃料燃烧、金属矿石的开采与冶炼、城市垃圾、农用化学品以及污水。此外，许多地区还发现了富含大量重金属的天然矿床。这些地区通常生长着适应在富含金属环境中茁壮成长的特色植物物种。尽管许多物种避免从这些土壤中吸收重金属，但其中一些物种能够大量积累高浓度的有毒金属，其积累水平远远超过土壤中的含量。重金属耐受性这一自然现象激发了植物生态学家、植物生理学家和植物生物学家对研究拟南芥和天蓝遏蓝菜等特殊超积累植物中金属耐受性的生理和遗传学的兴趣。在本综述中，我们特别参考重金属积累植物的转录组学以及耐受性和解毒相关功能基因的鉴定，描述了植物中金属耐受性遗传和分子基础的最新进展。
植物易受重金属毒性影响，并通过多种不同方式做出反应以避免有害影响。毒性剂量取决于离子类型、离子浓度、植物物种以及植物生长阶段。对金属的耐受性基于多种机制，如细胞壁结合、离子主动转运至液泡以及与有机酸或肽形成复合物。植物中金属解毒的最重要机制之一似乎是通过低分子量蛋白质（如金属硫蛋白和肽配体植物螯合肽）对金属进行螯合。例如，植物螯合肽合成的前体谷胱甘肽不仅在金属解毒中起关键作用，还在保护植物细胞免受包括内在氧化应激反应在内的其他环境胁迫方面发挥作用。在过去十年中，分子生物学的巨大发展和基因组学的成功极大地推动了分子遗传学研究，主要是转录组学，以鉴定植物中与金属耐受性相关的功能基因，这些基因大多属于金属稳态网络。
通过为模式植物拟南芥研究开发的大量工具和资源，如转录谱分析平台、蛋白质和代谢物谱分析、基于RNA干扰（RNAi）的工具以及插入系突变体库，对这些积累植物中金属积累遗传学的分析得到了极大加强。为了理解金属积累和适应的遗传学，拟南芥中开发的大量资源可以扩展到与其亲缘关系最近且对高金属环境具有最高适应水平的物种之一，如拟南芥和天蓝遏蓝菜。
本综述论文探讨了植物中重金属积累和耐受性的机制。已提供了关于耐金属植物中金属转运蛋白、金属螯合和氧化应激的详细信息。讨论了植物修复技术的进展以及金属积累植物的重要性，以及探索这些巨大而有价值的遗传和生物资源用于植物修复的策略。
十字花科中的许多物种已被鉴定为金属积累植物。为了全面理解金属积累的遗传学，拟南芥中开发的大量遗传资源必须扩展到其他具有该模式物种所不具备特征的金属积累物种。拟南芥微阵列芯片可用于鉴定十字花科金属积累植物中差异表达的基因。整合从十字花科模式物种和野生物种获得的资源将至关重要，可以将植物生物学的大多数不同领域，如功能基因组学、群体遗传学、系统发育学和生态学结合在一起。植物修复的进一步发展需要综合多学科的研究努力，将植物生物学、基因工程、土壤化学、土壤微生物学以及农业和环境工程结合起来。
