Thallium (Tl) is one of the most toxic elements to ecosystems, even at very low concentrations. Monovalent Tl ions (Tl+) can be readily taken up by plants, thereby entering the food chain and posing risks to humans. However, the molecular mechanisms governing the uptake of Tl+ by plants remain unclear. In the present study, we investigated the role of OsHAK1, a high-affinity potassium ion (K+) transporter, in Tl+ uptake by rice roots. Compared with K+-sufficient treatment, K+-deficient treatment significantly increased Tl+ uptake by the roots of wild-type (WT) rice plants and its translocation to the aerial parts. In oshak1 mutants with OsHAK1 loss of function, root Tl+ uptake and its subsequent translocation to aerial parts remained markedly low across a range of external K+ concentrations. Short-term Tl+ root influx experiments demonstrated that OsHAK1 is the predominant transporter that mediates Tl+ uptake. A supply of K+ suppressed Tl+ uptake by roots and its subsequent translocation to aerial parts in WT plants, and when the external K+ concentration reached 0.5 mM or higher, there were no differences between the WT and the oshak1 mutant. Our findings suggest that OsHAK1 mediates Tl+ influx into roots and that supplying K+ is an effective way to suppress both Tl+ uptake and its subsequent translocation to aerial parts in rice.
Ishikawa et al. (Tue,) studied this question.