Effectiveness of Metal–Metal and Metal–Organic Compound Combinations Against Plutella xylostella: Implications for Plant Elemental Defense

Copper, lead and zinc interactions during phytoextraction using Acer platanoides L.-a pot trial

Of the many environmental factors that modulate the phytoextraction of elements, little has been learnt about the role of metal interactions. The study aimed to show how different concentrations of Cu, Pb and Zn in the cultivation medium influenced the biomass, plant development and phytoextraction abilities of Acer platanoides L. seedlings. Additionally, the impact on the content and distribution of Ca, K, Mg and Na in plant parts was studied with an analysis of phenols. Plants treated with a mixture of two metals were characterised by lower biomass of leaves and higher major elements content jointly than those grown in the salt of one element. Leaves of A. platanoides cultivated in Pb5 + Zn1, Pb1 + Zn1 and Pb1 + Zn5 experimental systems were characterised by specific browning of their edges. The obtained results suggest higher toxicity to leaves of Pb and Zn present simultaneously in Knop solution than Cu and Pb or Cu and Zn, irrespective of the mutual ratio of the concentrations of these elements. Antagonism of Cu and Zn concerning Pb was clearly shown in whole plant biomass when one of these elements was in higher concentration (5 mmol L-1) in solution. In the lowest concentrations (1 mmol L-1), there was a synergism between Cu and Zn in plant roots. Plants exposed to Zn5, Cu1 + Pb5, Pb5 + Zn1 and Cu1 + Zn1 were characterised by higher total phenolic content than the rest plants. Both the presence and the concentration of other elements in the soil are significant factors that modulate element uptake, total phenolic content, and plant development.

Soil contamination with permissible levels of lead negatively affects the community of plant-associated insects: A case of study with kale

This study investigated whether lead (Pb), at concentrations allowed for soil, affects the community of insects that live in the aerial part of plants. We evaluated the effect of Pb concentrations on accumulated species richness, composition, and abundance of different functional groups of insects. Kale plants were grown in soil experimentally contaminated with four concentrations of lead nitrate: 0 (control), 144 (T1), 360 (T2), and 600 (T3) mg/kg of soil. The experiment was conducted in an open greenhouse for the natural colonization of insects. Insects were collected twice using trap bags attached to the plant leaf and by direct removal. The concentration of Pb in the stem and leaf samples increased with the increased soil contamination, even showing values above the limit allowed by the legislation for this plant species. Control plants showed a higher richness of accumulated insect species. In addition, the treatments had an effect on the community composition, in which Diaeretiella rapae (primary parasitoid) was found as an indicator of the control + T1 treatments and the top species Pachyneuron sp. (parasitoid of predators) was associated with the control. The abundance of chewing and sucking herbivores, their respective parasitoids, predators, and parasitoids of predators were negatively affected. Hyperparasitoid abundance was not affected, but their accumulated species richness was. This study was innovative in demonstrating that soil contamination by different concentrations of a heavy metal (Pb) can negatively affect the community of plant-associated insects, even at concentrations allowed for soil, reflecting possible damage to the ecosystem.

Are the soils and vegetation of a forest close to tailings ponds affected by metals and arsenic?

This study aimed to determine the transfer of metals (Cd, Pb and Zn) and As to a Mediterranean forest close to five tailings ponds in Cartagena-La Union mining district (SE Spain). In addition, the effect of the rhizosphere of two native plant species, Olea europaea (OE) and Pistacia lentiscus (PL), on soil properties and chemical speciation of metal(oid)s was evaluated. Results showed there was no influence of the rhizosphere in the total concentration of metal(loid)s in soil, decreasing as Pb > Zn > As > Cd. Chemical partitioning revealed that only Cd and As can be considered hazardous, with a high percentage of these elements in the soil-labile fractions (20-40%). The accumulation in vegetal tissues was only high for Pb in PL roots, which makes it a suitable species for phytostabilization. Additionally, translocation factors showed transfer of Pb and Zn in OE, and Zn in PL to aerial parts, although no toxicity evidences for plants or animals were found. Finally, soil properties affected metal(loid)s accumulation in plants. The OE species was related to soil-labile metal(loid) fractions and pH, total N, organic carbon and silt content. The PL species were associated with immobilized metal(loid) fractions, sand content, electrical conductivity and total concentrations of As, Cd and Pb. Hence, mining activity has affected native adjacent soils, with accumulation of metals in plant species, although translocation was low, likely due to physiological strategies of the studied species to protect themselves against hazardous elements, and to the high soil pH, which limits metals' mobility.

Metal bioaccumulation alleviates the negative effects of herbivory on plant growth

Metalliferous soils can selectively shape plant species' physiology towards tolerance of high metal concentrations that are usually toxic to organisms. Some adapted plant species tolerate and accumulate metal in their tissues. These metals can serve as an elemental defence but can also decrease growth. Our investigation explored the capacity of natural metal accumulation in a tropical tree species, Eremanthus erythropappus (Asteraceae) and the effects of such bioaccumulation on plant responses to herbivory. Seedlings of E. erythropappus were grown in a glasshouse on soils that represented a metal concentration gradient (Al, Cu, Fe, Mn and Zn), and then the exposed plants were fed to the herbivores in a natural habitat. The effect of herbivory on plant growth was significantly mediated by foliar metal ion concentrations. The results suggest that herbivory effects on these plants change from negative to positive depending on soil metal concentration. Hence, these results provide quantitative evidence for a previously unsuspected interaction between herbivory and metal bioaccumulation on plant growth.

Testing the elemental defense hypothesis with a woody plant species: Cadmium accumulation protects Populus yunnanensis from leaf herbivory and pathogen infection

Elemental defense hypothesis states that metals accumulated in plant tissues may serve as defense against herbivores and pathogens. However, evidences collected so far are inconsistent and studies using woody plants as model species are still lacking. In this study we used a local woody plant species, Populus yunnanensis, to investigate whether cadmium (Cd) accumulation in leaves can protect plants from leaf herbivory and pathogen infection. Plants grown with or without Cd supplementation in the soil were subjected to herbivory by a specialist (Botyodes diniasalis) and a generalist (Spodoptera exigua), or to pathogen infection by a leaf pathogenic fungus (Pestalotiopsis microspora). Two additional tests with artificial media amended with a series of Cd concentrations were conducted for S. exigua and P. microspora to investigate the toxicity of Cd independently of other organic defenses present in P. yunnanensis leaves. The results showed that both herbivores strongly preferred control leaves over leaves containing high Cd. Feeding on leaves from Cd-treated plants significantly reduced the growth and survivals of both herbivores. Furthermore, plants grown on Cd-amended soil were more resistant to fungal infection. Growth of S. exigua and P. microspora on artificial media decreased with increasing Cd concentrations. In conclusion, we found that Cd accumulated in P. yunnanensis leaves could effectively reduce leaf herbivory and pathogen infection, which fully supported the Elemental defense hypothesis.

A Role for Zinc in Plant Defense Against Pathogens and Herbivores

Pests and diseases pose a threat to food security, which is nowadays aggravated by climate change and globalization. In this context, agricultural policies demand innovative approaches to more effectively manage resources and overcome the ecological issues raised by intensive farming. Optimization of plant mineral nutrition is a sustainable approach to ameliorate crop health and yield. Zinc is a micronutrient essential for all living organisms with a key role in growth, development, and defense. Competition for Zn affects the outcome of the host-attacker interaction in both plant and animal systems. In this review, we provide a clear framework of the different strategies involving low and high Zn concentrations launched by plants to fight their enemies. After briefly introducing the most relevant macro- and micronutrients for plant defense, the functions of Zn in plant protection are summarized with special emphasis on superoxide dismutases (SODs) and zinc finger proteins. Following, we cover recent meaningful studies identifying Zn-related passive and active mechanisms for plant protection. Finally, Zn-based strategies evolved by pathogens and pests to counteract plant defenses are discussed.

Heavy metal exposure through artificial diet reduces growth and survival of Spodoptera litura (Lepidoptera: Noctuidae)

Insect physiology is affected by the presence of toxins in the surrounding environment of insects as well as their food sources. The objective of this study was to determine the effect of heavy metal exposure to two low concentrations (50 μg/g and 150 μg/g) of lead (Pb) and zinc (Zn) through artificial diet to the larvae on biological parameters of Asian armyworm (Spodoptera litura Fabricius) (Lepidoptera: Noctuidae). Both Pb and Zn, even at low concentrations, had relatively high toxic effects on S. litura larvae (P < 0.01). S. litura larval weight and length suffered the maximum reduction when the larvae were fed on diet mixed with the high Pb concentration (150 μg/g) tested compared to the other treatments. At the same Pb concentration (150 μg/g), values of larva growth index, pupa growth index, immature growth index, standardized growth index, and fitness index were 4.66, 7.33, 7.82, 5.35, and 10.00 times lower, respectively, than those of control. At the same Zn concentration (150 μg/g), values of larval growth index, pupal growth index, immature growth index, standardized growth index, and fitness index were 5.61, 3.00, 3.04, 3.23, and 9.24 times lower, respectively, than those of control. The survival rate of S. litura larvae was also lower (12.5%) when the larvae were fed on diet mixed with Pb at 150 μg/g after 10 days of observation. Overall, the presence of those heavy metals in the environment, even at low concentrations, would exert an adverse impact on larvae development of this insect. From this point of view, findings could provide a basis for long-term evaluation of heavy metal risk and its impact on populations of important agricultural pests.

Trace element transfer from two contaminated soil series to Medicago sativa and one of its herbivores, Spodoptera exigua

Alfalfa was cultivated in two potted soil series obtained from two sandy soils contaminated by Cu (SM) and metal(loids)/PAH (CD). Shoot production was monitored for 8 weeks. Then, larvae of Spodoptera exigua were reared on alfalfa of both soil series for eight days. A biotest (using Phaseolus vulgaris) was used to assess the soil phytotoxicity. Increasing soil contamination reduced P. vulgaris growth, but alfalfa growth was only reduced on the SM soil series. Exposure to the SM soil was mirrored by shoot Cu and Cr concentrations of alfalfa (respectively, in mg kg ^-1 DW, Cu and Cr ranged from 11.9 and 0.4 in the CTRL soil to 98.5 and 1.2 in the SM one). Exposure to the CD soil series was mirrored by shoot Zn concentrations (i.e., 48-91.6 mg kg^-1 DW). Internal metal(loid) concentrations of S. exigua remained generally steady across both soil series (respectively Cd 0.05-0.16, Cr 0.5-3.3, Cu 5.8-98.5, Ni 0.6-1.6, Pb 0.4-1.3, and Zn 57-337 mg kg^-1 DW), and most of the associated transfer factors were lower than 1. Here, due to the excluder phenotype of alfalfa across our TE contamination gradients, S. exigua could cope with high total metal(loid) concentration in both contaminated soils.

Both heavy metal-amendment of soil and aphid-infestation increase Cd and Zn concentrations in phloem exudates of a metal-hyperaccumulating plant

Plants that are able to hyperaccumulate heavy metals show increased concentrations of these metals in their leaf tissue. However, little is known about the concentrations of heavy metals and of organic defence metabolites in the phloem sap of these plants in response to either heavy metal-amendment of the soil or biotic challenges such as aphid-infestation. In this study, we investigated the effects of heavy metal-exposure and of aphid-infestation on phloem exudate composition of the metal hyperaccumulator species Arabidopsis halleri L. O'Kane & Al-Shehbaz (Brassicaceae). The concentrations of elements and of organic defence compounds, namely glucosinolates, were measured in phloem exudates of young and old (mature) leaves of plants challenged either by amendment of the soil with cadmium and zinc and/or by an infestation with the generalist aphid Myzus persicae. Metal-amendment of the soil led to increased concentrations of Cd and Zn, but also of two other elements and one indole glucosinolate, in phloem exudates. This enhanced defence in the phloem sap of heavy metal-hyperaccumulating plants can thus potentially act as effective protection against aphids, as predicted by the elemental defence hypothesis. Aphid-infestation also caused enhanced Cd and Zn concentrations in phloem exudates. This result provides first evidence that metal-hyperaccumulating plants can increase heavy metal concentrations tissue-specifically in response to an attack by phloem-sucking herbivores. Overall, the concentrations of most elements, including the heavy metals, and glucosinolates were higher in phloem exudates of young leaves than in those of old leaves. This defence distribution highlights that the optimal defence theory, which predicts more valuable tissue to be better defended, is applicable for both inorganic and organic defences.

Effects of single and combined heavy metals and their chelators on aphid performance and preferences

When present at elevated levels in the environment, heavy metals are toxic for most organisms. However, so-called hyperaccumulator plants tolerate heavy metals and use chelators for their internal long-distance transport. Thus, phloem-sucking insects may come in contact with the chelated metals. In the present study, the effects of individual and combined heavy metals, zinc (Zn) and cadmium (Cd), as well as of common chelators, nicotianamine and phytochelatin, were investigated on the performance, preferences, and metal accumulation of the generalist aphid Myzus persicae, using artificial diets. Added Zn increased aphid growth, whereas Cd reduced the survival of aphids. Chelators had neither protective nor negative effects on aphids. The combination of the 2 heavy metals in chelated or nonchelated form caused a potentiation effect that led to an extinction of the aphids within less than 2 wk, before they could reproduce. Both Cd and Zn accumulated in the aphids, indicating a possible biomagnification. In choice assays, aphids preferred diets amended with Zn with or without nicotianamine compared to a control diet. In contrast, a Cd-containing diet led to neither attraction nor aversion. The present study provides insight into how mixtures of heavy metals and their chelators influence the life history of a generalist aphid. The results have implications for the use of phytoremediation to remove heavy metals from contaminated soils. Environ Toxicol Chem 2016;35:3023-3030. © 2016 SETAC.

Copper Contamination Impairs Herbivore Initiation of Seaweed Inducible Defenses and Decreases Their Effectiveness

Seaweed-herbivore interactions are often mediated by environmental conditions, yet the roles of emerging anthropogenic stressors on these interactions are poorly understood. For example, chemical contaminants have unknown consequences on seaweed inducible resistance and herbivore response to these defenses despite known deleterious effects of contaminants on animal inducible defenses. Here, we investigated the effect of copper contamination on the interactions between a snail herbivore and a brown seaweed that displays inducible resistance to grazing. We examined seaweed inducible resistance and its effectiveness for organisms exposed to copper at two time points, either during induction or after herbivores had already induced seaweed defenses. Under ambient conditions, non-grazed tissues were more palatable than grazed tissues. However, copper additions negated the preference for non-grazed tissues regardless of the timing of copper exposure, suggesting that copper decreased both how herbivores initiated these inducible defenses and their subsequent effectiveness. Copper decreased stimulation of defenses, at least in part, by suppressing snail grazing pressure—the cue that turns inducible defenses on. Copper decreased effectiveness of defenses by preventing snails from preferentially consuming non-grazed seaweed. Thus, contaminants can potentially stress communities by changing seaweed-herbivore interactions mediated via inducible defenses. Given the ubiquity of seaweed inducible resistance and their potential influence on herbivores, we hypothesize that copper contamination may change the impact of these resistant traits on herbivores.

Zinc and cadmium hyperaccumulation act as deterrents towards specialist herbivores and impede the performance of a generalist herbivore

Extraordinarily high leaf metal concentrations in metal hyperaccumulator plants may serve as an elemental defence against herbivores. However, mixed results have been reported and studies using comparative approaches are missing. We investigated the deterrent and toxic potential of metals employing the hyperaccumulator Arabidopsis halleri. Effects of zinc (Zn) and cadmium (Cd) on the preferences of three Brassicaceae specialists were tested in paired-choice experiments using differently treated plant material, including transgenic plants. In performance tests, we determined the toxicity and joint effects of both metals incorporated in an artificial diet on the survival of a generalist. Feeding by all specialists was significantly reduced by metal concentrations from above 1000 μg Zn g(-1) DW and 18 μg Cd g(-1) DW. By contrast, metals did not affect oviposition. Generalist survival decreased with increasing concentrations of individual metals, whereby the combination of Zn and Cd had an additive toxic effect even at the lowest applied concentrations of 100 μg Zn g(-1) and 2 μg Cd g(-1) . Metal hyperaccumulation protects plants from herbivory resulting from deterrence and toxicity against a wide range of herbivores. The combination of metals exacerbates toxicity through joint effects and enhances elemental defence. Thus, metal hyperaccumulation is ecologically beneficial for plants.

A Ni hyperaccumulator and a congeneric non-accumulator reveal equally effective defenses against herbivory

The defense hypothesis is commonly used to explain the adaptive role of metal hyperaccumulation. We tested this hypothesis using two Brassicaceae congeneric species: Alyssum pintodasilvae, a Ni hyperaccumulator, and the non-accumulator Alyssum simplex both growing on serpentine soils in Portugal. Artificial diet disks amended with powdered leaves from each plant species were used to compare the performance (mortality, biomass change) and feeding behavior of Tribolium castaneum in no-choice and choice tests. The performance of T. castaneum was not affected at several concentrations of A. pintodasilvae or A. simplex in no-choice tests. However, the consumption of plant-amended disks was significantly lower than that of control disks, irrespectively of the species fed. Accordingly, when insects were given an alternative food choice, disks of both plant species were significantly less consumed than control disks. Moreover, insects did not discriminate between disks in the combination "A. pintodasilvae+A. simplex". Contrary to our expectations, these results suggest that both plant species have equally effective defenses against herbivory. While Ni is believed to be part of the deterrence mechanism in the hyperaccumulator A. pintodasilvae, it seems likely that organic compounds, possibly glucosinolates, play an important role in the defense of A. simplex or in both species.

Do toxic ions induce hormesis in plants?

The concept of hormesis in plants is critically reviewed, taking growth stimulation by low concentrations of toxic trace elements as a reference. The importance of both non-adaptive and adaptive mechanisms underlying ion-induced hormetic growth responses is highlighted. The activation of defense mechanisms by metal ions and pathogenic elicitors and the cross talk between the signals induced by metal ions and biotic stressors are considered. The production of reactive oxygen species and, consequently, the induction of stress-induced antioxidants, are key mechanisms in metal ion-induced hormesis in plants. It is concluded that in the current scientific literature, hormesis is used as an "umbrella" term that includes a wide range of different mechanisms. It is recommended that the term hormesis be used in plant toxicology as a descriptive term for the stimulated phase in growth response curves that is induced by low concentrations of toxic metal ions without evidence of the underlying mechanisms. If the mechanisms underlying the stimulated growth phase have been identified, specific terms, such as amelioration, defense gene activation, priming or acclimation, should be used.

The current status of the elemental defense hypothesis in relation to pathogens

Metal hyperaccumulating plants are able to accumulate exceptionally high concentrations of metals, such as zinc, nickel, or cadmium, in their aerial tissues. These metals reach concentrations that would be toxic to most other plant species. This trait has evolved multiple times independently in the plant kingdom. Recent studies have provided new insight into the ecological and evolutionary significance of this trait, by showing that some metal hyperaccumulating plants can use high concentrations of accumulated metals to defend themselves against attack by pathogenic microorganisms and herbivores. Here, we review the evidence that metal hyperaccumulation acts as a defensive trait in plants, with particular emphasis on plant-pathogen interactions. We discuss the mechanisms by which defense against pathogens might have driven the evolution of metal hyperaccumulation, including the interaction of this trait with other forms of defense. In particular, we consider how physiological adaptations and fitness costs associated with metal hyperaccumulation could have resulted in trade-offs between metal hyperaccumulation and other defenses. Drawing on current understanding of the population ecology of metal hyperaccumulator plants, we consider the conditions that might have been necessary for metal hyperaccumulation to be selected as a defensive trait, and discuss the likelihood that these were fulfilled. Based on these conditions, we propose a possible scenario for the evolution of metal hyperaccumulation, in which selective pressure for resistance to pathogens or herbivores, combined with gene flow from non-metallicolous populations, increases the likelihood that the metal hyperaccumulating trait becomes established in plant populations.

Exploring lower limits of plant elemental defense by cobalt, copper, nickel, and zinc

Elemental defense is a relatively newly recognized phenomenon in which plants use elements present in their tissue to reduce damage by herbivores or pathogens. In the present study, neonates of the generalist herbivore, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), were fed artificial diets amended with varying concentrations of Co, Cu, Ni, and Zn that are hyperaccumulated by plants to determine minimum lethal concentrations (MLC) and minimum sublethal concentrations (MSC) for each metal. MLC values (dry mass) for Co (45 μg/g), Ni (230 μg/g), and Zn (280 μg/g) were below published minimum hyperaccumulator levels. MSC levels (dry mass) for Co (15 μg/g), Ni (140 μg/g), and Zn (200 μg/g) were at concentrations lower than published minimum accumulator levels. Furthermore, both MLC and MSC values for Zn were within normal tissue concentrations. These results indicate that elemental defense for Co, Ni, and Zn may be effective at concentrations lower than hyperaccumulator levels and so may be more widespread than previously believed.

Plant defense using toxic inorganic ions: conceptual models of the defensive enhancement and joint effects hypotheses

The concept of plant defense using toxic mineral elements originated as an explanation for extremely elevated concentrations of some elements (termed hyperaccumulation) in some plant tissues. The Defensive Enhancement Hypothesis suggests that hyperaccumulation evolved because, after an initial defensive benefit accrued from a relatively low initial concentration, increased concentration of an element provided increased plant fitness and drove evolution of higher element concentrations until hyperaccumulation was achieved. The Joint Effects Hypothesis postulates that additive or synergistic effects between element-based defenses, or between toxic element and organic chemical defenses, may have contributed to the evolution of hyperaccumulation. By lessening the concentration of an element necessary to provide an initial defensive benefit to a plant, joint effects could decrease the level of an element that provides an initial defensive benefit, allowing additive or synergistic defensive enhancement to take effect. Recent experimental tests have demonstrated defense at relatively low element concentrations, and tests of metal/metal and metal/organic compound combinations have shown joint effects. These hypotheses suggest how hyperaccumulator plants may have evolved in response to plant-herbivore interactions, and suggest that toxic element levels below those used to define hyperaccumulation may be ecologically effective.

Accumulation of Cu, Zn, Pb, and Cd in edible parts of four commonly grown crops in two contaminated soils

Soil heavy metal pollution resulting from human activities is causing major concern due to its potential risk. In this study, four crop species with different cultivars were planted in 2 levels (heavily and slightly) of heavy metal contaminated soils, and the accumulation of Cu, Zn, Pb, and Cd in the edible parts of the crops were investigated. Metal concentrations in sesame seeds grown in both soils exceeded both the Chinese Food Hygiene Standard (CFHS) and Codex Alimentarius Commission Standard (CACS), while the metal concentrations in all pepper cultivars in the slightly contaminated soil were below the CFHS and CACS. Other crops were generally in between in both soils. Among the tested crops, the order of soil-plant transfer factor (TF) was: sesame > green soybean > cowpea > pepper. Additionally, old fruit of cowpea contained larger amounts of metals than young fruit. It suggests that sesame should not be planted in the metal contaminated area, while pepper cultivar "Chaobianjiao No.1" may be an alternative to be grown in the slightly contaminated soil. There were differences in individual human susceptibilities to metals. Therefore, a comprehensive risk assessment should consider the frequency, amount and species consumed by human besides metal concentrations in crops.

Heavy metal hyperaccumulating plants: how and why do they do it? And what makes them so interesting?

The term "hyperaccumulator" describes a number of plants that belong to distantly related families, but share the ability to grow on metalliferous soils and to accumulate extraordinarily high amounts of heavy metals in the aerial organs, far in excess of the levels found in the majority of species, without suffering phytotoxic effects. Three basic hallmarks distinguish hyperaccumulators from related non-hyperaccumulating taxa: a strongly enhanced rate of heavy metal uptake, a faster root-to-shoot translocation and a greater ability to detoxify and sequester heavy metals in leaves. An interesting breakthrough that has emerged from comparative physiological and molecular analyses of hyperaccumulators and related non-hyperaccumulators is that most key steps of hyperaccumulation rely on different regulation and expression of genes found in both kinds of plants. In particular, a determinant role in driving the uptake, translocation to leaves and, finally, sequestration in vacuoles or cell walls of great amounts of heavy metals, is played in hyperaccumulators by constitutive overexpression of genes encoding transmembrane transporters, such as members of ZIP, HMA, MATE, YSL and MTP families. Among the hypotheses proposed to explain the function of hyperaccumulation, most evidence has supported the "elemental defence" hypothesis, which states that plants hyperaccumulate heavy metals as a defence mechanism against natural enemies, such as herbivores. According to the more recent hypothesis of "joint effects", heavy metals can operate in concert with organic defensive compounds leading to enhanced plant defence overall. Heavy metal contaminated soils pose an increasing problem to human and animal health. Using plants that hyperaccumulate specific metals in cleanup efforts appeared over the last 20 years. Metal accumulating species can be used for phytoremediation (removal of contaminant from soils) or phytomining (growing plants to harvest the metals). In addition, as many of the metals that can be hyperaccumulated are also essential nutrients, food fortification and phytoremediation might be considered two sides of the same coin. An overview of literature discussing the phytoremediation capacity of hyperaccumulators to clean up soils contaminated with heavy metals and the possibility of using these plants in phytomining is presented.

Effects of pollutants on bottom-up and top-down processes in insect-plant interactions

Bottom-up (host plant quality) and top-down (natural enemies) forces both influence the fitness and population dynamics of herbivores. However, the impact of pollutants acting on these forces has not been examined, which prompted us to review the literature to test hypotheses regarding this area of research. A comprehensive literature search found 126 references which examined fitness components and population dynamics of 203 insect herbivores. One hundred and fifty-three of the 203 herbivores (75.4%) had fitness impacted due to bottom-up factors in polluted environments. In contrast, only 20 of the 203 (9.9%) had fitness significantly impacted due to top-down factors in polluted environments. The paucity of results for top-down factors impacting fitness does not necessarily mean that top-down factors are less important, but rather that fewer studies include natural enemies. We provide a synthesis of available data by pollution type and herbivore guild, and suggest future research to address this issue.