Botanical Institute, The University of Cologne, Zuelpicher Str. 47b, 50674 Cologne, Germany.
Department of Plant Cytology and Embryology, Jagiellonian University, Gronostajowa 9 Str., 30-387 Cracow, Poland.
J Plant Physiol. 2017 Dec;219:45-61. doi: 10.1016/j.jplph.2017.08.014. Epub 2017 Sep 7.
Among heavy metal plants (the metallophytes), facultative species can live both in soils contaminated by an excess of heavy metals and in non-affected sites. In contrast, obligate metallophytes are restricted to polluted areas. Metallophytes offer a fascinating biology, due to the fact that species have developed different strategies to cope with the adverse conditions of heavy metal soils. The literature distinguishes between hyperaccumulating, accumulating, tolerant and excluding metallophytes, but the borderline between these categories is blurred. Due to the fact that heavy metal soils are dry, nutrient limited and are not uniform but have a patchy distribution in many instances, drought-tolerant or low nutrient demanding species are often regarded as metallophytes in the literature. In only a few cases, the concentrations of heavy metals in soils are so toxic that only a few specifically adapted plants, the genuine metallophytes, can cope with these adverse soil conditions. Current molecular biological studies focus on the genetically amenable and hyperaccumulating Arabidopsis halleri and Noccaea (Thlaspi) caerulescens of the Brassicaceae. Armeria maritima ssp. halleri utilizes glands for the excretion of heavy metals and is, therefore, a heavy metal excluder. The two endemic zinc violets of Western Europe, Viola lutea ssp. calaminaria of the Aachen-Liège area and Viola lutea ssp. westfalica of the Pb-Cu-ditch of Blankenrode, Eastern Westphalia, as well as Viola tricolor ecotypes of Eastern Europe, keep their cells free of excess heavy metals by arbuscular mycorrhizal fungi which bind heavy metals. The Caryophyllaceae, Silene vulgaris f. humilis and Minuartia verna, apparently discard leaves when overloaded with heavy metals. All Central European metallophytes have close relatives that grow in areas outside of heavy metal soils, mainly in the Alps, and have, therefore, been considered as relicts of the glacial epoch in the past. However, the current literature favours the idea that hyperaccumulation of heavy metals serves plants as deterrent against attack by feeding animals (termed elemental defense hypothesis). The capability to hyperaccumulate heavy metals in A. halleri and N. caerulescens is achieved by duplications and alterations of the cis-regulatory properties of genes coding for heavy metal transporting/excreting proteins. Several metallophytes have developed ecotypes with a varying content of such heavy metal transporters as an adaption to the specific toxicity of a heavy metal site.
在重金属植物（金属植物）中，兼性物种既可以在受到重金属过度污染的土壤中生存，也可以在未受影响的地方生存。相比之下，专性金属植物则局限于污染地区。由于物种已经发展出不同的策略来应对重金属土壤的不利条件，因此金属植物提供了引人入胜的生物学特性。文献中将超积累、积累、耐受和排斥金属植物区分开来，但这些类别的界限是模糊的。由于重金属土壤通常干燥、养分有限，而且不是均匀分布的，而是在许多情况下呈斑块状分布，因此文献中常将耐旱或低养分需求的物种视为金属植物。在极少数情况下，土壤中重金属的浓度如此之高，以至于只有少数经过专门适应的植物，即真正的金属植物，才能应对这些不利的土壤条件。目前的分子生物学研究集中在遗传上可操作和超积累的拟南芥 halleri 和十字花科的 Noccaea (Thlaspi) caerulescens。滨藜海莲子亚种利用腺体将重金属排出体外，因此是重金属排斥者。西欧的两种特有锌堇菜，亚琛-列日地区的堇菜黄色亚种和东威斯特伐利亚州布伦克罗德铅铜沟的堇菜黄色亚种，以及东欧的三色堇生态型，通过与重金属结合的丛枝菌根真菌使细胞免受过量重金属的影响。石竹科的普通矢车菊和小堇菜显然会在重金属过载时丢弃叶子。所有中欧金属植物都有亲缘关系，它们生长在重金属土壤以外的地区，主要在阿尔卑斯山，因此过去被认为是冰河时代的遗迹。然而，目前的文献倾向于认为，重金属的超积累对植物来说是一种抵御食草动物攻击的防御手段（称为元素防御假说）。在 A. halleri 和 N. caerulescens 中实现重金属的超积累能力是通过编码重金属转运/排泄蛋白的顺式调控特性的重复和改变来实现的。一些金属植物已经发展出具有不同重金属转运体含量的生态型，以适应特定重金属地点的毒性。
