MIZ1, an essential protein for root hydrotropism, is associated with the cytoplasmic face of the endoplasmic reticulum membrane in Arabidopsis root cells

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MIZ1 is encoded by a gene essential for root hydrotropism in Arabidopsis. To characterize the property of MIZ1, we used transgenic plants expressing GFP-tagged MIZ1 (MIZ1-GFP) and mutant MIZ1 (MIZ1G235E-GFP) in a miz1-1 mutant. Although both chimeric genes were transcribed, the translational products of MIZ1G235E-GFP did not accumulate in roots. Moreover, MIZ1-GFP complemented the mutant phenotype but not MIZ1G235E-GFP. The signal corresponding to MIZ1-GFP was detected at high levels in cortical cells and lateral root cap cells and accumulated in compartments in cortical cells. MIZ1-GFP was fractionated into a soluble protein fraction and an endoplasmic reticulum (ER) membrane fraction, where it was bound to the surface of the ER membrane at the cytosolic side.