To clarify the mechanism of gravity-controlled polar auxin transport, we conducted the International Space Station (ISS) experiment “Auxin Transport” (identified by NASA's operation nomenclature) in 2016 and 2017, focusing on the expression of genes related to auxin efflux carrier protein PsPIN1 and its localization in the hook and epicotyl cells of etiolated Alaska pea seedlings grown for three days in the dark under microgravity (μg) and artificial 1 g conditions on a centrifuge in the Cell Biology Experiment Facility (CBEF) in the ISS, and under 1 g conditions on Earth. Regardless of gravity conditions, the accumulation of PsPIN1 mRNA in the proximal side of epicotyls of the seedlings was not different, but tended to be slightly higher as compared with that in the distal side. 2,3,5-Triiodobenzoic acid (TIBA) also did not affect the accumulation of PsPIN1 mRNA in the proximal and distal sides of epicotyls. However, in the apical hook region, TIBA increased the accumulation of PsPIN1 mRNA under μg conditions as compared with that under artificial 1 g conditions in the ISS. The accumulation of PsPIN1 proteins in epicotyls determined by western blotting was almost parallel to that of mRNA of PsPIN1. Immunohistochemical analysis with a specific polyclonal antibody of PsPIN1 revealed that a majority of PsPIN1 in the apical hook and subapical regions of the seedlings grown under artificial 1 g conditions in the ISS localized in the basal side (rootward) of the plasma membrane of the endodermal tissues. Conversely, in the seedlings grown under μg conditions, localization of PsPIN1 was greatly disarrayed. TIBA substantially altered the cellular localization pattern of PsPIN1, especially under μg conditions. These results strongly suggest that the mechanisms by which gravity controls polar auxin transport are more likely to be due to the membrane localization of PsPIN1. This physiologically valuable report describes a close relationship between gravity-controlled polar auxin transport and the localization of auxin efflux carrier PsPIN1 in etiolated pea seedlings based on the μg experiment conducted in space.

为了阐明重力控制的极地生长素运输机制，我们于2016年和2017年进行了国际空间站（ISS）实验“生长素运输”（由NASA的操作命名法确定），重点研究与生长素流出载体蛋白相关的基因的表达PsPIN1 及其在黄化阿拉斯加豌豆幼苗钩细胞和上胚轴细胞中的定位，该幼苗在国际空间站细胞生物学实验设施 (CBEF) 的离心机上、在微重力 (μ g ) 和人工 1 g条件下在黑暗中生长三天，以及在地球上 1 g条件下。无论重力条件如何，幼苗上胚轴近侧的PsPIN1 mRNA积累量没有差异，但与远侧相比略高。 2,3,5-三碘苯甲酸 (TIBA) 也不影响上胚轴近端和远端PsPIN1 mRNA 的积累。然而，在顶端钩区，与 ISS 中人工 1 g条件下相比，TIBA 在 μ g条件下增加了PsPIN1 mRNA 的积累。 Western blotting检测上胚轴中PsPIN1蛋白的积累与PsPIN1 mRNA的积累几乎平行。使用 PsPIN1 特异性多克隆抗体进行的免疫组织化学分析表明，在 ISS 中人工 1 g条件下生长的幼苗的顶端钩和顶端下区域中的大部分 PsPIN1 定位于内胚层组织质膜的基侧（根侧） 。相反，在μg条件下生长的幼苗中，PsPIN1的定位严重混乱。 TIBA 显着改变了 PsPIN1 的细胞定位模式，尤其是在μg条件下。这些结果强烈表明重力控制极性生长素运输的机制更有可能是由于 PsPIN1 的膜定位。这份具有生理价值的报告基于在太空进行的μg实验描述了重力控制的极性生长素运输与黄化豌豆幼苗中生长素外流载体PsPIN1的定位之间的密切关系。