Response: Commentary: Toward a more physiologically and evolutionarily relevant definition of metal hyperaccumulation in plants

Weed Species from Tea Gardens as a Source of Novel Aluminum Hyperaccumulators

Increased availability of toxic Al³⁺ is the main constraint limiting plant growth on acid soils. Plants adapted to acid soils, however, tolerate toxic Al³⁺, and some can accumulate Al in their aerial parts to a significant degree. Studies on Al-tolerant and Al-accumulating species have mainly focused on the vegetation of acid soils distributed as two global belts in the northern and southern hemispheres, while acid soils formed outside these regions have been largely neglected. The acid soils (pH 3.4-4.2) of the tea plantations in the south Caspian region of Northern Iran were surveyed over three seasons at two main locations. Aluminum and other mineral elements (including nutrients) were measured in 499 plant specimens representing 86 species from 43 families. Al accumulation exceeding the criterion for accumulator species (>1000 µg g^-1 DW) was found in 36 species belonging to 23 families of herbaceous annual or perennial angiosperms, in addition to three bryophyte species. Besides Al, Fe accumulation (1026-5155 µg g^-1 DW) was also observed in the accumulator species that exceeded the critical toxicity concentration, whereas no such accumulation was observed for Mn. The majority of analyzed accumulator plants (64%) were cosmopolitan or pluriregional species, with a considerable rate of Euro-Siberian elements (37%). Our findings, which may contribute to phylogenetic studies of Al accumulators, also suggest suitable accumulator and excluder species for the rehabilitation of acid-eroded soils and introduce new model species for investigating Al accumulation and exclusion mechanisms.

Defense Mechanisms of Two Pioneer Submerged Plants during Their Optimal Performance Period in the Bioaccumulation of Lead: A Comparative Study

Ceratophyllum demersum L. and Hydrilla verticillata (L.f.) Royle, two pioneer, submerged plants, effectively remove heavy metals from contaminated water. The present work evaluates the bioaccumulation and defense mechanisms of these plants in the accumulation of lead from contaminated water during their optimal performance period. C. demersum and H. verticillata were investigated after 14 days of exposure to various lead concentrations (5–80 μM). The lead accumulation in both C. demersum and H. verticillata increased with an increasing lead concentration, reaching maximum values of 2462.7 and 1792 mg kg⁻¹ dw, respectively, at 80 μM. The biomass and protein content decreased significantly in C. demersum when exposed to lead. The biomass of H. verticillata exposed to lead had no significant difference from that of the controls, and the protein content increased for the 5–10 μM exposure groups. The malondialdehyde (MDA) content and superoxide dismutase (SOD), peroxidase (POD), and polyphenol oxidase (PPO) activities were much higher in C. demersum, suggesting considerable damage from lipid peroxidation and sensitivity to lead stress. Enzyme inhibition and inactivation were also observed in C. demersum at high lead concentrations (40–80 μM). The excellent growth status, low damage from lipid peroxidation, and high activity of catalase (CAT) and phenylalanine ammonia-lyase (PAL) observed in H. verticillata illustrate its better tolerance under the same lead stress.

Divergent biology of facultative heavy metal plants

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.