Some plants hyperaccumulate metals or metalloids to levels several orders of magnitude higher than other species. This intriguing phenomenon has received considerable attention in the past decade. While research has mostly focused on the above-ground organs, roots are the sole access point to below-ground trace elements and as such they play a vital role in hyperaccumulation. Here we highlight the role of the root as an effective trace element scavenger through interactions in the rhizosphere. We found that less than 10% of the known hyperaccumulator species have had their rhizospheres examined. When studied, researchers have focused on root physical characteristics, rhizosphere chemistry, and rhizosphere microbiology as central themes to understand plant hyperaccumulation. One physical characteristic often assumed about hyperaccumulators is that their roots are small, but this is not true for all species and many species remain unexamined. Transporters in root membranes provide avenues for root uptake, while root growth and morphology influence plant access to trace elements in the rhizosphere. Some hyperaccumulators exhibit unique root scavenging and direct their growth toward elements in soil. Studies on hyperaccumulator rhizosphere chemistry have examined the role of the root in altering elemental solubility through exudation and pH changes. Different interpretations have been reported for mobilization of non-labile trace element pools by hyperaccumulators. However, there is a lack of evidence for a novel role for rhizosphere acidification in hyperaccumulation. As for microbiological studies, researchers have shown that bacteria and fungi in the hyperaccumulator rhizosphere may exhibit increased metal tolerance, act as plant growth promoting microorganisms, alter elemental solubility, and have significant effects on plant trace element concentrations. New evidence suggests that symbiosis with arbuscular mycorrhizae may not be rare in hyperaccumulator taxa, even in some members of the Brassicaceae. Although there are several reports on the presence of mycorrhizae, a cohesive interpretation of their role in hyperaccumulation remains elusive. In summary, we present the current state of knowledge about how roots hyperaccumulate and we suggest ways in which this knowledge can be applied and improved.

Invasions of North American grasslands by Spotted knapweed (Centaurea maculosa Lam.) are mediated in part by Spotted knapweed root exudation of (±)-catechin, a potent phytotoxin. Residual soil (±)-catechin may interfere with reestablishment of native grassland species even after Spotted knapweed populations are controlled. Grassland species that are resistant to (±)-catechin may be more successful for restoration of areas infested by Spotted knapweed. We evaluated the (±)-catechin resistance of 23 grassland species by measuring the effects of seven (±)-catechin concentrations (0-4.0 mg/mL) on seed germination, seedling root and shoot elongation, and seedling mortality. (±)-Catechin treatments were chosen to reflect the range of observed Spotted knapweed field soil (±)-catechin concentrations. Inhibition of root elongation was the strongest and most common effect of (±)-catechin treatment. High (±)-catechin concentrations reduced mean root lengths of 5 of the species by more than 75% and another 10 species by more than 55%. Experimentally derived concentrations needed to reduce root length by 50% (EC50), an indicator of (±)-catechin resistance, ranged from 0.43 mg/mL ± 0.30 SE to greater than 4.0 mg/mL among species. Eight species with EC50s greater than 3.0 mg/mL were identified as resistant to (±)-catechin and are likely suitable for revegetation of Spotted knapweed-infested areas. (±)-Catechin resistance was positively correlated with mean seed mass, suggesting that seed carbohydrate reserves may allow seedlings to detoxify (±)-catechin, develop barriers to (±)-catechin exposure, or sustain a positive growth rate, despite (±)-catechin-induced cell death. Future efforts to identify allelochemical-resistant grassland species should focus on large-seeded species.

The goal of this study was to investigate how plant selenium (Se) hyperaccumulation may affect ecological interactions and whether associated partners may affect Se hyperaccumulation. The Se hyperaccumulator Astragalus bisulcatus was collected in its natural seleniferous habitat, and x-ray fluorescence mapping and x-ray absorption near-edge structure spectroscopy were used to characterize Se distribution and speciation in all organs as well as in encountered microbial symbionts and herbivores. Se was present at high levels (704–4,661 mg kg−1 dry weight) in all organs, mainly as organic C-Se-C compounds (i.e. Se bonded to two carbon atoms, e.g. methylselenocysteine). In nodule, root, and stem, up to 34% of Se was found as elemental Se, which was potentially due to microbial activity. In addition to a nitrogen-fixing symbiont, the plants harbored an endophytic fungus that produced elemental Se. Furthermore, two Se-resistant herbivorous moths were discovered on A. bisulcatus, one of which was parasitized by a wasp. Adult moths, larvae, and wasps all accumulated predominantly C-Se-C compounds. In conclusion, hyperaccumulators live in association with a variety of Se-resistant ecological partners. Among these partners, microbial endosymbionts may affect Se speciation in hyperaccumulators. Hyperaccumulators have been shown earlier to negatively affect Se-sensitive ecological partners while apparently offering a niche for Se-resistant partners. Through their positive and negative effects on different ecological partners, hyperaccumulators may influence species composition and Se cycling in seleniferous ecosystems.

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