Rhizosphere Bacteria Mobilize Zn for Hyperaccumulation by <i>Thlaspi</i> <i>caerulescens</i>

Thlaspi caerulescens has a remarkable ability to hyperaccumulate Zn from soils containing mostly nonlabile Zn. The present study shows that rhizosphere microbes play an important role in increasing the availability of water-soluble Zn in soil, thus enhancing Zn accumulation by T. caerulescens. The addition of bacteria to surface-sterilized seeds of T. caerulescens sown in autoclaved soil increased the Zn concentration in shoots 2-fold as compared to axenic controls; the total accumulation of Zn was enhanced 4-fold. When the same experiment was conducted with Thlaspi arvense, a nonaccumulator, bacteria had no effect on shoot Zn accumulation although they increased water-soluble Zn concentrations available to both Thlaspi species by 22-67% as compared to the axenic controls. Further evidence that bacteria increase the availability of water-soluble Zn in soil was obtained when liquid media that had supported bacterial growth mobilized 1.3-1.8-fold more Zn from soil as compared to axenic media. Other experiments with agar media showed that bacteria did not facilitate an increase in the rate of soluble Zn transport into the root nor did they enlarge the surface area of the roots of either Thlaspi species. Thus, the bacterially mediated increase in the dissolution of Zn from the nonlabile phase in soil may enhance Zn accumulation in T. caerulescens shoots.