Superfund Research Program

Molecular Mechanisms of Heavy Metal Detoxification and Accumulation in Plants

Project Leader: Julian I. Schroeder
Grant Number: P42ES010337
Funding Period: 2000-2017

Project-Specific Links

Final Progress Reports

Year:   2016  2009  2004 

One of the main goals of Superfund-supported research in the Schroeder laboratory is to characterize transporters of toxic metals. An Annual Review of Physiology was prepared and published on the subject of plant ion channels and transporters (Ward et al., Annual Rev. Physiology 2009). Researchers previously showed that the HKT family of metal transporters functions in controlling long distance transport of the toxic metal sodium between roots and shoots. The Arabidopsis AtHKT1 transporter removes sodium from the xylem of plants thus reducing toxic accumulation of sodium in shoots. These findings have led to research showing an important function of this class of HKT transporters in protecting plants from sodium over-accumulation in plant leaves and shoots and are being transferred in breeding research of others for field trials. A review on these recent advances was published (Horie et al., Trans in Pl. Sci. 2009).

Two HKT transporter/channel classes have been characterized that mediate either Na⁺ transport or Na⁺ and K⁺ transport when expressed in Xenopus oocytes and yeast. However, the Na⁺/K⁺ selectivities of the K⁺ permeable HKT transporters have not yet been studied in plant cells. A more recent study expressing 5’ UTR modified HKT constructs in yeast has questioned the relevance of cation selectivities found in heterologous systems for cation selectivity predictions in plant cells. In an new study, researchers therefore analyzed two highly homologous rice HKT transporters in plant cells, OsHKT2;1 and OsHKT2;2, that show differential K+ permeabilities in heterologous systems. When these transport proteins were stably expressed in cultured tobacco (Nicotiana tabacum L.) cv. Bright-Yellow 2 (BY2) cells, OsHKT2;1 mediated Na⁺ uptake, but little K⁺ or Rb⁺ uptake, consistent with findings in heterologous systems. In contrast, OsHKT2;2 mediated Na⁺-K⁺ co-transport in plant cells such that extracellular K⁺ stimulates OsHKT2;2-mediated Na⁺ influx and vice versa. Furthermore, at millimolar Na⁺ concentrations OsHKT2;2 mediated Na⁺ influx into plant cells without adding extracellular K⁺. The present study shows that the Na⁺/K⁺ selectivities of these HKT transporters in plant cells coincide closely with the selectivities in Xenopus oocytes and yeast (Yao et al., Plant Physiol. 2009). In addition, the presence of external K⁺ and Ca²⁺ down-regulated OsHKT2;1-mediated Na⁺ influx in plant systems BY2 cells and intact rice roots, and in Xenopus oocytes as well. Furthermore this study shows that the OsHKT2;2 selectivity in plant cells depends on the imposed cationic conditions, supporting the model that HKT transporters are multi-ion pores (Yao et al., Plant Physiol. 2009)