Metallophytes—a view from the rhizosphere

Abstract

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.

Biodiversity in Metal-Contaminated Sites – Problem and Perspective – A Case Study

“…Though roots are the only organ directly interacting with soil trace elements, most of the studies on hyperaccumulation by plants have focused on above-ground organs. Less than 10% of the known hyperaccumulator species have been investigated at the rhizosphere level [55].…”

“…Bacteria and fungi in the hyperaccumulator rhizosphere may exhibit increased metal tolerance by i) acting as a plant growth promoting microorganisms; ii) modifying metal speciation and solubility; iii) influencing plant trace element concentrations [55,[56][57][58].…”

Root system morphology and primary root anatomy in natural non-metallicolous and metallicolous populations of three Arabidopsis species differing in heavy metal tolerance

“…Hyperaccumulators such as Thlaspi caerulescens or T. goesingense may have shallow root systems (Keller et al 2003;Himmelbauer et al 2005), and develop diverse types of morphology and architecture according to the presence of Zn and possibly Cd in the soil (Schwartz et al 1999;Haines 2002). Variability of root system morphology exists not only between hyperaccumulator species but also between ecotypes and populations (Haines 2002;Loudet et al 2005;Alford et al 2010).…”

“…Further work focusing on root morphology (Alford et al 2010) and full biological diversity of non-model species (Clauss & Koch 2006) is needed. This work extends our knowledge on root morphological traits of three Arabidopsis species differing in heavy metal tolerance, and of their nonmetallicolous (NM) and metallicolous (M) populations originating from natural sites in Slovakia.…”

Heavy Metals in Vineyards and Orchard Soils

“…For example, the carriers of heavy metals in plants fall into four different families: as the CPx-ATPases, Nramps, CDF (facilitator of diffusion of cations), and ZIP, which are involved in the absorption of metals and homeostasis in general, playing a key role in tolerance to these elements (SETH, 2012). Non-essential heavy metals can effectively compete with essential metals by the same transmembrane transporters having similar ionic radii oxidation states (ALFORD et al, 2010). This relative lack of selectivity for the transport of trans-membrane ions may partially explain why non-essential heavy metal enter the cells of plants root system, even against a concentration gradient (SETH, 2012).…”