Rhizosphere bacteria mobilize Zn for hyperaccumulation by Thlaspi caerulescens

Thlaspi caerulescens has a remarkable ability to hyperaccumulate Zn from so ils containing mostly nonlabile Zn. The present study shows that rhizospher e microbes play an important role in increasing the availability of water-s oluble Zn in soil, thus enhancing Zn accumulation by T. caerulescens. The a ddition of bacteria to surf a ce-sterilized seeds of T. caerulescens sown i n autoclaved soil increased the Zn concentration in shoots 2-fold as compar ed to axenic controls; the total accumulation of Zn was enhanced 4-fold. Wh en the same experiment was conducted with Thlaspi arvense, a nonaccumulator , bacteria had no effect on shoot Zn accumulation although they increased w ater-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 medi a that had supported bacterial growth mobilized 1.3-1.8-fold more Zn from s oil as compared to axenic media. Other experiments with agar media showed t hat bacteria did not facilitate an increase in the rate of soluble Zn trans port into the root nor did they enlarge the surface area of the roots of ei ther Thlaspi species. Thus, the bacterially mediated increase in the dissol ution of Zn from the nonlabile phase in soil may enhance Zn accumulation in T. caerulescens shoots.