Robert S. Boyd

The defense hypothesis of elemental hyperaccumulation: status, challenges and new directions

Elemental hyperaccumulation may have several functions, including plant defense against natural enemies. A total of 34 studies, including 72 experimental tests, have been conducted to date. At least some tests have demonstrated defense by hyperaccumulated As, Cd, Ni, Se and Zn, but relatively few plant taxa and natural enemies have been investigated. Defense by hyperaccumulated Ni has been shown for most leaf/root chewing herbivores and pathogens tested (20 of 26 tests) but not for herbivores of other feeding modes (1 of 8 tests). Most tests (5 of 6) using Ni concentrations below accumulator levels found no defensive effect, and the single test using plants in the accumulator range also found no effect. For Zn, mixed results have been reported for both hyperaccumulator (3 of 6 tests showed defense) and accumulator levels (3 of 4 tests showed defense). These tests have focused exclusively on leaf chewing/scraping herbivores: no herbivores of other feeding modes, or pathogens, have been tested. Both hyperaccumulator and accumulator concentrations of Se generally have shown defensive effects (12 of 14 tests). Most (75%) of these positive results used plants with accumulator Se concentrations. The three tests of Cd showed defensive effects in two cases, one for hyperaccumulator and one for sub-accumulator Cd concentrations. Arsenic has been tested only once, and was found effective against a leaf-chewing herbivore at a concentration much less than the hyperaccumulator level. Defense studies have used a variety of experimental approaches, including choice and no-choice experiments as well as experiments that use artificial diet or growth media. Investigations of hyperaccumulation as a defense against natural enemies have led to two emerging questions. First, what is the minimum concentration of an element sufficient for defense? Evidence suggests that plants other than hyperaccumulators (such as accumulators) may be defended by elements against some natural enemies. Second, do the effects of an element combine with the effects of organic defensive compounds in plants to produce enhanced joint defensive effects? Recent investigation of this “joint effects hypothesis,” using Ni and secondary plant compounds in artificial insect diet, has demonstrated joint effects. Initial answers to both these questions suggest that defensive effects of elements in plants are more widespread than previously believed. These results also suggest an evolutionary pathway by which elemental hyperaccumulation may have evolved from accumulation. In this “defensive enhancement” scenario, defensive benefits of elevated levels of elements may have led to stepwise increases in element concentrations that further magnified these benefits. This series of steps could have led to increased accumulation, and ultimately hyperaccumulation, of elements by plants.

The ecological significance of nickel hyperaccumulation: a plant chemical defense

Nickel hyperaccumulating plants have more than 1000 mg Ni kg−1 dry weight when grown on nickel-bearing soils. We hypothesized that Ni hyperaccumulation could serve as a chemical defense against herbivores In feeding experiments with potential insect herbivores and Ni hyperaccumulating plants, only those inseets fed leaves from plants grown on non-nickel-bearing soil survived or showed a weight gain. Among chemical parameters measured, only Ni content of plants was sufficient to explain this result. When subjected to herbivory by lepidopteran larvae, plants grown on Ni-amended soil showed greater survival and yield than plants on unamended soil. Ni hyperaccumulation may be an effective plant chemical defense against herbivores because of its high lethality, apparent low cost, and broad spectrum of toxicity.

Heavy Metal Pollutants and Chemical Ecology: Exploring New Frontiers

Heavy metals are an important class of pollutants with both lethal and sublethal effects on organisms. The latter are receiving increased attention, as these may have harmful ecological outcomes. For example, recent explorations of heavy metals in freshwater habitats reveal that they can modify chemical communication between individuals, resulting in “info-disruption” that can impact ecological relationships within and between species. Info-disruption can affect animal behavior and social structure, which in turn can modify both intraspecies and interspecies interactions. In terrestrial habitats, info-disruption by metals is not well studied, but recent demonstrations of chemical signaling between plants via both roots and volatile organic molecules provide potential opportunities for info-disruption. Metals in terrestrial habitats also can form elemental plant defenses, in which they can defend a plant against natural enemies. For example, hyperaccumulation of metals by terrestrial plants has been shown to provide defensive benefits, although in almost all known cases the metals are not anthropogenic pollutants but are naturally present in soils inhabited by these plants. Info-disruption among microbes is another arena in which metal pollutants may have ecological effects, as recent discoveries regarding quorum sensing in bacteria provide an avenue for metals to affect interactions among bacteria or between bacteria and other organisms. Metal pollutants also may influence immune responses of organisms, and thus affect pathogen/host relationships. Immunomodulation (modification of immune system function) has been tied to some metal pollutants, although specific metals may boost or reduce immune system function depending on dose. Finally, the study of metal pollutants is complicated by their frequent occurrence as mixtures, either with other metals or with organic pollutants. Most studies of metal pollutants focus on single metals and therefore oversimplify complex field conditions. Study of pollutant impacts on chemical ecology also are difficult due to the necessity of studying effects at varying ecological scales: “dynamic scaling” of chemical ecology studies is rarely done completely. It is clear that much remains to be learned about how heavy metal pollution impacts organisms, and that exciting new research frontiers are available for experimental exploration.

Nickel hyperaccumulated by Thlaspi montanum var. montanum is acutely toxic to an insect herbivore

Some plants growing on serpentine soils sequester (hyperaccumulate) nickel from those soils in their tissues. Several ecological functions for metal hyperaccumulation have been suggested, including defense against herbivores. This study tests the herbivore defense hypothesis using the Ni hyperaccumulator Thlaspi montanum var. montanum. Leaves differing 167-fold in Ni content (3,000 vs 18 ppm) were obtained by growing plants on high- and low-Ni soils. Leaves were fed to larvae of Pieris rapae, a generalist folivore. Larvae fed high-Ni leaves did not grow and suffered 100 % mortality after 12 d, whereas those fed low-Ni leaves quadrupled in weight with a corresponding mortality of only 21 % (...)

The significance of metal hyperaccumulation for biotic interactions

Abstract. Metal hyperaccumulating plants contain very high metal contents. Because of the general toxicity of metals, chemically-mediated biotic interactions involving hyperaccumulating plants may differ greatly from those of non-hyperaccumulators. Recent research has demonstrated a defensive function for hyperaccumulated metals against herbivores and pathogens. We predict that some herbivore/pathogen species have evolved metal tolerance, and suggest that resulting high metal levels in herbivores/pathogens may defend them against their own predators. Little is known regarding interference and commensal interactions involving hyperaccumulating plants. Decreased competition may occur through an interference interaction similar to allelopathy, in which enrichment of metal in the soil under a hyperaccumulator plants canopy may inhibit another plant species, thus resulting in “elemental allelopathy”. Metal enrichment of soil under hyperaccumulators also may result in commensalism if another plant species (possibly another hyperaccumulator) derives a benefit from growing in the metal-enriched soil under the canopy of a hyperaccumulating overstory plant. It seems likely that high-metal plant litter will host a specialized microflora of decomposers and may affect nutrient cycling rates. Mutualist biotic interactions also may be affected by the elevated metal contents of hyperaccumulating species. Mycorrhizal fungi may form mutualisms with hyperaccumulators, but the phenomenon is poorly-explored. The few cases investigated to date have not detected mycorrhizae. Pollination and seed dispersal mechanisms may require biotic vectors that might be affected by plant metal content. Hyperaccumulating plants may have solved this dilemma in three ways. First, some may rely on abiotic vectors for pollen or seed dispersal. Second, biotic vectors used by these species may have varied diets and thus dilute metal intake to non-toxic levels. Finally, biotic vectors may have evolved tolerance of elevated dietary levels of metals, and perhaps have become specialists on hyperaccumulator species.

Transfer of heavy metals through terrestrial food webs: a review

Heavy metals are released into the environment by both anthropogenic and natural sources. Highly reactive and often toxic at low concentrations, they may enter soils and groundwater, bioaccumulate in food webs, and adversely affect biota. Heavy metals also may remain in the environment for years, posing long-term risks to life well after point sources of heavy metal pollution have been removed. In this review, we compile studies of the community-level effects of heavy metal pollution, including heavy metal transfer from soils to plants, microbes, invertebrates, and to both small and large mammals (including humans). Many factors contribute to heavy metal accumulation in animals including behavior, physiology, and diet. Biotic effects of heavy metals are often quite different for essential and non-essential heavy metals, and vary depending on the specific metal involved. They also differ for adapted organisms, including metallophyte plants and heavy metal-tolerant insects, which occur in naturally high-metal habitats (such as serpentine soils) and have adaptations that allow them to tolerate exposure to relatively high concentrations of some heavy metals. Some metallophyte plants are hyperaccumulators of certain heavy metals and new technologies using them to clean metal-contaminated soil (phytoextraction) may offer economically attractive solutions to some metal pollution challenges. These new technologies provide incentive to catalog and protect the unique biodiversity of habitats that have naturally high levels of heavy metals.

Nickel hyperaccumulation by Thlaspi montanum var. montanum (Brassicaceae): a constitutive trait.

Adaptations to particular stresses may occur only in populations experiencing those stresses or may be widespread within a species. Nickel hyperaccumulation is viewed as an adaptation to high-Ni (serpentine) soils, but few studies have determined if hyperaccumulation ability is restricted to populations from high-Ni soils or if it is a constitutive trait found in populations on both high- and low-Ni soils. We compared mineral element concentrations of Thlaspi montanum var. montanum plants grown on normal and high-Ni greenhouse soils to address this question. Seed sources were from four populations (two serpentine, two non-serpentine) in Oregon and northern California, USA. Plants from all populations were able to hyperaccumulate Ni, showing Ni hyperaccumulation to be a constitutive trait in this species. Populations differed in their ability to extract some elements (e.g., Ca, Mg, P) from greenhouse soils. We noted a negative correlation between tissue concentrations of Ni and Zn. We suggest that the ability to hyperaccumulate Ni has adaptive value to populations growing on non- serpentine soil. This adaptive value may be a consequence of metal-based plant defense against herbivores/pathogens, metal- based interference against neighboring plant species, or an efficient nutrient scavenging system. We suggest that the Ni hyperaccumulation ability of T. montanum var. montanum may be an inadvertent consequence of an efficient nutrient (possibly Zn or Ca) uptake system.

The defensive function of Ni in plants: response of the polyphagous herbivore Spodoptera exigua (Lepidoptera: Noctuidae) to hyperaccumulator and accumulator species of Streptanthus (Brassicaceae)

Abstract Metals sequestered by plants may defend them against herbivores and/or pathogens. We explored the effect of plant metal content on a polyphagous herbivore, Spodoptera exigua. Plant experiments used a Ni hyperaccumulator (Streptanthus polygaloides) and two Ni accumulator species (S. breweri and S. tortuosus). High- and low-Ni plants of each species were produced by growing plants on either Ni-amended or unamended soil. Mean leaf Ni contents for plants grown on Ni-amended soil and control soil, respectively, were: 1500 and 20 mg Ni kg−1 for S. polygaloides, 40 and 9 mg kg−1 for S. breweri, and 93 and 0.5 mg kg−1 for S. tortuosus. Neonate or second-instar Sp. exigua larvae were fed high- or low-metal leaves of each plant species, and survival and other parameters were monitored. High-Ni leaves of S. polygaloides were acutely toxic, resulting in 96% mortality within 10 days, whereas only 48% of larvae fed low-Ni leaves died. Low- and high-Ni leaves of S. breweri did not differ in their effects on larval survival, larval weight, adult weight, and duration of pupation. Leaves of S. tortuosus from high-Ni soil did not significantly affect larval survival relative to low-Ni leaves. However, larvae eating high-Ni leaves weighed significantly less and pupation was significantly delayed. Larval feeding experiments using artificial diet amended with Ni demonstrated a toxic threshold at 963 mg Ni kg−1 and a sublethal threshold at 535 mg Ni kg−1. Because plant material containing less Ni had detectable sublethal effects, we suggest that Ni interacts with other plant qualities (including secondary defensive compounds) to produce those effects. We conclude that hyperaccumulated Ni is a potent defense against polyphagous folivorous insects, but suggest that the sublethal impacts of the lesser Ni levels found in accumulator plant species may play only a minor defensive role against herbivores.

The defensive role of Ni hyperaccumulation by plants: a field experiment

Hyperaccumulation of Ni by plants is hypothesized to function as an elemental defense against herbivores and pathogens. Laboratory experiments have documented toxic effects to herbivores consuming high-Ni plant tissues, but this paper reports the first experiment to examine the defensive effectiveness of Ni hyperaccumulation under field conditions. The experiment was conducted at an ultramafic soil site naturally inhabited by the Ni hyperaccumulator Streptanthus polygaloides (Brassicaceae). Experimental treatments examined the response of herbivores to hyperaccumulated Ni, using exclosure and insecticide treatments to divide herbivores into groups based primarily upon herbivore size. Three soils (Ni-amended greenhouse soil, unamended greenhouse soil, ultramafic soil), three exclosure treatments (exclosure, control exclosure, no exclosure), and a systemic insecticide treatment were combined in a fractional factorial experimental design. Streptanthus polygaloides plants were grown in a greenhouse for 2 mo, transplanted into the field by inserting potted plants into holes dug on the experimental site, and periodically examined for herbivore damage during a 41-d period. Initial surveys showed greater amounts of insect damage to plants with low tissue Ni levels, confirming the defensive effect of Ni against some insect herbivores, but large herbivores (probably vertebrates) later consumed entire plants regardless of plant Ni status. We concluded that Ni was not an effective defense against these large herbivores, probably because their diets mix high-Ni S. polygaloides foliage with that of associated non-hyperaccumulating species. We suggest that such dietary dilution is one mechanism whereby some herbivores can circumvent elemental plant defenses.

Nickel defends the South African hyperaccumulator Senecio coronatus (Asteraceae) against Helix aspersa (Mollusca: Pulmonidae).

SummaryThe elevated Ni concentration of Ni hyperaccumulator plants has been proposed to be an effective chemical defence against herbivores. To test this hypothesis, we fed leaves from hyperaccumulator and non-hyperaccumulator populations of South African Senecio coronatus to a generalist herbivore species, the brown garden snail (Helix aspersa). Snails fed hyperaccumulator leaves experienced significantly greater mortality than those fed non-hyperaccumulator leaves and also contained 10-fold greater concentrations of Ni. A choice experiment showed snails preferred non-hyperaccumulator leaves in two of three trials. Snails fed cornmeal diet amended with Ni had significantly reduced mass for diets containing as little as 140 μg Ni g-1, and significantly greater mortality occurred for snails consuming diets containing 830 μg Ni g-1 and greater. Because hyperaccumulator S. coronatus leaves contained far more Ni (12,100 μg Ni g-1) than the toxic threshold shown in the diet experiment, we concluded that the Ni concentration of hyperaccumulator leaves was sufficient to cause the elevated mortality of snails fed those leaves. This research adds another example to the growing literature showing the toxicity of hyperaccumulated Ni to generalist folivores.