Graduate School of Life Sciences, Tohoku University, Aoba-ku, Sendai 980-8577, Japan.
Plant Physiol. 2012 Jan;158(1):239-51. doi: 10.1104/pp.111.188615. Epub 2011 Nov 7.
Cucumber (Cucumis sativus) seedlings grown in a horizontal position develop a specialized protuberance (or peg) on the lower side of the transition zone between the hypocotyl and the root. This occurs by suppressing peg formation on the upper side via a decrease in auxin resulting from a gravitational response. However, the gravity-stimulated mechanism of inducing asymmetric auxin distribution in the transition zone is poorly understood. The gravity-sensing tissue responsible for regulating auxin distribution in the transition zone is thought to be the endodermal cell. To characterize the gravity-stimulated mechanism, the auxin efflux facilitator PIN-FORMED1 (CsPIN1) in the endodermis was identified and the localization of CsPIN1 proteins during the gravimorphogenesis of cucumber seedlings was examined. Immunohistochemical analysis revealed that the accumulation pattern of CsPIN1 protein in the endodermal cells of the transition zone of cucumber seedlings grown horizontally differed from that of plants grown vertically. Gravistimulation for 30 min prompted changes in the accumulation pattern of CsPIN1 protein in the endodermis as well as the asymmetric distribution of auxin in the transition zone. Furthermore, 2,3,5-triiodobenzoic acid inhibited the differential distribution of auxin as well as changes in the accumulation pattern of CsPIN1 in the endodermis of the transition zone during gravistimulation. These results suggest that the altered pattern of CsPIN1 accumulation in the endodermis in response to gravistimulation influences lateral auxin transport through the endodermis, resulting in asymmetric auxin distribution in the transition zone.
黄瓜（Cucumis sativus）幼苗在水平位置生长时，在下胚轴和根之间的过渡区的下侧会形成一个特殊的突起（或钉）。这是通过抑制上侧的钉形成来实现的，这是由于重力反应导致生长素减少。然而，诱导过渡区中不对称生长素分布的重力刺激机制还知之甚少。负责调节过渡区中生长素分布的重力感应组织被认为是内皮层细胞。为了描述重力刺激机制，鉴定了内皮层中生长素外排促进因子 PIN-FORMED1（CsPIN1），并检查了 CsPIN1 蛋白在黄瓜幼苗重形态发生过程中的定位。免疫组织化学分析表明，水平生长的黄瓜幼苗过渡区的内皮层细胞中 CsPIN1 蛋白的积累模式与垂直生长的植物不同。30 分钟的重力刺激促使内皮层中 CsPIN1 蛋白的积累模式以及生长素在过渡区中的不对称分布发生变化。此外，2,3,5-三碘苯甲酸抑制了生长素在过渡区的内皮层中的差异分布以及 CsPIN1 蛋白积累模式的变化。这些结果表明，内皮层中 CsPIN1 积累模式的改变响应重力刺激影响侧向外源生长素运输，导致过渡区中生长素的不对称分布。
