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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

The Rice Aquaporin Lsi1 Mediates Uptake of Methylated Arsenic Species^1,[W]

Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, United Kingdom (R.-Y.L., W.-J.L., S.P.M., F.-J.Z.); Research Institute for Bioresources, Okayama University, Kurashiki 710–0046, Japan (Y.A., N.M., J.F.M.); and Department of Chemistry, University of Aberdeen, Meston Building, Aberdeen AB24 3UE, United Kingdom (J.F.)

Pentavalent methylated arsenic (As) species such as monomethylarsonic acid [MMA(V)] and dimethylarsinic acid [DMA(V)] are used as herbicides or pesticides, and can also be synthesized by soil microorganisms or algae through As methylation. The mechanism of MMA(V) and DMA(V) uptake remains unknown. Recent studies have shown that arsenite is taken up by rice (Oryza sativa) roots through two silicon transporters, Lsi1 (the aquaporin NIP2;1) and Lsi2 (an efflux carrier). Here we investigated whether these two transporters also mediate the uptake of MMA(V) and DMA(V). MMA(V) was partly reduced to trivalent MMA(III) in rice roots, but only MMA(V) was translocated to shoots. DMA(V) was stable in plants. The rice lsi1 mutant lost about 80% and 50% of the uptake capacity for MMA(V) and DMA(V), respectively, compared with the wild-type rice, whereas Lsi2 mutation had little effect. The short-term uptake kinetics of MMA(V) can be described by a Michaelis-Menten plus linear model, with the wild type having 3.5-fold higher V_max than the lsi1 mutant. The uptake kinetics of DMA(V) were linear with the slope being 2.8-fold higher in the wild type than the lsi1 mutant. Heterologous expression of Lsi1 in Xenopus laevis oocytes significantly increased the uptake of MMA(V) but not DMA(V), possibly because of a very limited uptake of the latter. Uptake of MMA(V) and DMA(V) by wild-type rice was increased as the pH of the medium decreased, consistent with an increasing proportion of the undissociated species. The results demonstrate that Lsi1 mediates the uptake of undissociated methylated As in rice roots.

² Present address: Nanjing University of Information Science and Technology, Nanjing 210044, China.

³ Present address: College of Resources and Environmental Science, Hebei Agricultural University, Baoding, Hebei Province 071001, China.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Fang-Jie Zhao (fangjie.zhao{at}bbsrc.ac.uk).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.140350

^* Corresponding author; e-mail fangjie.zhao{at}bbsrc.ac.uk.

Received April 24, 2009; accepted June 17, 2009; published June 19, 2009.

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