Positive responses to Zn and Cd by roots of the Zn and Cd hyperaccumulator Thlaspi caerulescens

Abstract

The effects of localized zinc (ZnO) and cadmium (CdS) enrichment on the allocation of root biomass, root length and partitioning of current assimilate within root systems of the Zn hyperaccumulator Thlaspi caerulescens were investigated using a rhizobox system. The rhizoboxes contained either homogeneous soil or juxtaposed control and metal-enriched soil. In the heterogeneous treatments the Zn-enriched soil contained 250, 500 or 1000 mg Zn kg −" . The plants consistently allocated c. 70% of their total root biomass and length into the metal-enriched soil. Moreover, 70% of the current assimilate ("%C) allocated to the roots was localized in the Zn-enriched soil of the heterogeneous treatments. The concentration of Zn (250-1000 mg kg −" ) in the enriched soil had no effect on these patterns of root allocation, nor were there significant effects of the Zn treatments on the total root or shoot biomass of the plants. The positive responses to the localized metal treatments were therefore characterized by a concomitant reduction in root allocation into the control soil. In contrast to T. caerulescens, when grown with juxtaposed control and Zn-enriched soil the non-accumulator Thlaspi arvense showed reduced root allocation into a patch enriched with 500 mg kg −" Zn. In a further experiment, two populations of T. caerulescens that differ in their abilities to accumulate Cd were grown with juxtaposed control and Cd-enriched soil. The plants from a population that accumulated Cd also showed increased root biomass and root length allocation into the Cdenriched soil. Plants from the population that did not accumulate Cd showed no such increase. The possibility that T. caerulescens forages for metals, and the precision of its root allocation with respect to localized metal enrichment is highlighted. The significance of these findings for the selection of hyperaccumulator plants for use in the phytoextraction of Zn and Cd from mine spoils (phytomining) and the phytoremediation of heterogeneous contaminated soils are discussed.

The nutritional control of root development

“…When one-half of a split-root system of the Zn hyperaccumulator species Thlaspi caerulescens was exposed to soil enriched for Zn (at 250-1000 mg kg −1 soil), that portion of the root system showed increased biomass and length (Whiting et al, 2000). These experiments showed that hyperaccumulator species have the ability to forage for metals at concentrations that are inhibitory to root growth in non-accumulator species from the same genus.…”

Analyzing Lateral Root Development: How to Move Forward

“…These include growth on moistened germination paper rolls or pouches, sand rhizotrons, rhizoboxes, in compost followed by washing, soil columns and gelbased systems where phenotypic effects can be imaged using flatbed scanners, digital cameras, lasers, or even x-ray computed tomography (CT) (Hetz et al, 1996;Whiting et al, 2000;Bengough et al, 2004;Fang et al, 2009;French et al, 2009;Gregory et al, 2009;Hammond et al, 2009;Iyer-Pascuzzi et al, 2010;Trachsel et al, 2010;Tracy et al, 2010Tracy et al, , 2011Chapman et al, 2011;Lobet et al, 2011;Lucas et al, 2011). Magnetic resonance imaging (for noninvasive analysis of root structures) and positron emission tomography (for analysis of carbon transport and accumulation) can be combined to study the dynamics of structure-function relationships of roots in real soils in a noninvasive manner .…”

Plants as extreme environments? Ni-resistant bacteria and Ni-hyperaccumulators of serpentine flora

“…Also, the ability of hyperaccumulators to mobilize more nickel or zinc from soils or rocks does not seem to be higher than in normal plants (Bernal et al 1994;McNear et al 2007;Puschenreiter et al 2005;Shallari et al 2001). A possible hypothesis for the presence of highly tolerant bacteria near the roots of metal hyperaccumulators could be related to the specific tropism, shown in controlled experiments, of roots of hyperaccumulating plants toward soil patches rich in metals, a phenomenon known as "root metal foraging" (Haines 2002;Liu et al 2009;Schwartz et al 1999;Whiting et al 2000). Consequently, the presence of highly tolerant bacteria near plants roots may not be due to plant activity but simply to the chemical parameters of the soil patch that already selected a highly tolerant bacterial flora (Fig.…”