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NIP6;1 Is a Boric Acid Channel for Preferential Transport of Boron to Growing Shoot Tissues in Arabidopsis^[W]

^a Graduate School of Agricultural and Life Science, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
^b Biotechnology Research Center, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
^c Department of Biochemistry, Cellular, and Molecular Biology and Center of Excellence in Structural Biology, University of Tennessee, Knoxville, Tennessee 37996
^d Solution Oriented Research for Science and Technology, Japan Science and Technology Agency, 3-4-15 Nihonbashi, Tokyo 103-0027, Japan

¹ Address correspondence to atorufu{at}mail.ecc.u-tokyo.ac.jp.

Boron (B) in soil is taken up by roots through NIP5;1, a boric acid channel, and is loaded into the xylem by BOR1, a borate exporter. Here, the function of Arabidopsis thaliana NIP6;1, the most similar gene to NIP5;1, was studied. NIP6;1 facilitates the rapid permeation of boric acid across the membrane but is completely impermeable to water. NIP6;1 transcript accumulation is elevated in response to B deprivation in shoots but not in roots. NIP6;1 promoter–β-glucuronidase is predominantly expressed in nodal regions of shoots, especially the phloem region of vascular tissues. Three independently identified T-DNA insertion lines for the NIP6;1 gene exhibited reduced expansion of young rosette leaves only under low-B conditions. B concentrations are reduced in young rosette leaves but not in the old leaves of these mutants. Taken together, these data strongly suggest that NIP6;1 is a boric acid channel required for proper distribution of boric acid, particularly among young developing shoot tissues. We propose that NIP6;1 is involved in xylem–phloem transfer of boric acid at the nodal regions and that the water-tight property of NIP6;1 is important for this function. It is proposed that during evolution, NIP5;1 and NIP6;1 were diversified in terms of both the specificity of their expression in plant tissues and their water permeation properties, while maintaining their ability to be induced under low B and their boric acid transport activities.

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