Muro Keita, Kamiyo Jio, Wang Sheliang, Geldner Niko, Takano Junpei
Department of Agricultural Biology, Graduate School of Agriculture, Osaka Metropolitan University, Sakai, Japan.
Division of Applied Life Sciences, Graduate School of Life and Environmental Sciences, Osaka Prefecture University, Sakai, Japan.
Front Plant Sci. 2023 Dec 14;14:988419. doi: 10.3389/fpls.2023.988419. eCollection 2023.
Casparian strips are ring-like structures consisting of lignin, sealing the apoplastic space between endodermal cells. They are thought to have important functions in controlling radial transport of nutrients and toxic elements in roots. However, mutants with a defective Casparian strip structure have been found to maintain nutrient homeostasis in ranges supportive of growth under standard laboratory conditions. In this study, we investigated the function of Casparian strips under excess boron (B) conditions using and mutants with defective Casparian strip development but which do not exhibit excessive deposition of suberin, another endodermal diffusion barrier. The growth of and mutants did not differ significantly from that of wild-type (WT) plants under different B conditions in plate cultures; however, they were highly sensitive to B excess in hydroponic culture, where transpiration drives the translocation of boric acid toward the shoot. In hydroponic culture with sufficient to excess boric acid, B accumulation in shoots of the and mutants was higher than that in the WT. A time-course tracer study using B-enriched boric acid at a sufficient or slightly excessive concentration showed higher translocation of B into shoots of the and mutants. Furthermore, a genetically encoded biosensor for boric acid expressed under a stele-specific promoter (') visualized faster boric acid flux into the mutant steles. Collectively, our results demonstrate the importance of Casparian strips in preventing apoplastic diffusion of boric acid into the stele under excess supply.
凯氏带是由木质素组成的环状结构，封闭了内皮层细胞之间的质外体空间。它们被认为在控制根中养分和有毒元素的径向运输方面具有重要功能。然而，已发现具有缺陷凯氏带结构的突变体在标准实验室条件下能在支持生长的范围内维持养分稳态。在本研究中，我们使用凯氏带发育缺陷但未表现出木栓质（另一种内皮层扩散屏障）过度沉积的突变体，研究了过量硼（B）条件下凯氏带的功能。在平板培养中，突变体在不同硼条件下的生长与野生型（WT）植物没有显著差异；然而，在水培中，它们对硼过量高度敏感，在水培中蒸腾作用驱动硼酸向地上部运输。在含有充足至过量硼酸的水培中，突变体地上部的硼积累高于野生型。使用充足或略过量浓度的富含硼的硼酸进行的时间进程示踪研究表明，硼向突变体地上部的转运更高。此外，在中柱特异性启动子（）控制下表达的硼酸基因编码生物传感器显示硼酸更快地流入突变体中柱。总体而言，我们的结果证明了在硼供应过量时，凯氏带在防止硼酸通过质外体扩散进入中柱方面的重要性。
