Graduate School of Life Sciences, Tohoku University, Sendai 980-8577, Japan.
Planta. 2012 Oct;236(4):999-1012. doi: 10.1007/s00425-012-1655-5. Epub 2012 Apr 25.
Seedling roots display not only gravitropism but also hydrotropism, and the two tropisms interfere with one another. In Arabidopsis (Arabidopsis thaliana) roots, amyloplasts in columella cells are rapidly degraded during the hydrotropic response. Degradation of amyloplasts involved in gravisensing enhances the hydrotropic response by reducing the gravitropic response. However, the mechanism by which amyloplasts are degraded in hydrotropically responding roots remains unknown. In this study, the mechanistic aspects of the degradation of amyloplasts in columella cells during hydrotropic response were investigated by analyzing organellar morphology, cell polarity and changes in gene expression. The results showed that hydrotropic stimulation or systemic water stress caused dramatic changes in organellar form and positioning in columella cells. Specifically, the columella cells of hydrotropically responding or water-stressed roots lost polarity in the distribution of the endoplasmic reticulum (ER), and showed accelerated vacuolization and nuclear movement. Analysis of ER-localized GFP showed that ER redistributed around the developed vacuoles. Cells often showed decomposing amyloplasts in autophagosome-like structures. Both hydrotropic stimulation and water stress upregulated the expression of AtATG18a, which is required for autophagosome formation. Furthermore, analysis with GFP-AtATG8a revealed that both hydrotropic stimulation and water stress induced the formation of autophagosomes in the columella cells. In addition, expression of plastid marker, pt-GFP, in the columella cells dramatically decreased in response to both hydrotropic stimulation and water stress, but its decrease was much less in the autophagy mutant atg5. These results suggest that hydrotropic stimulation confers water stress in the roots, which triggers an autophagic response responsible for the degradation of amyloplasts in columella cells of Arabidopsis roots.
幼苗的根不仅表现出向重力性，还表现出向水性，这两种向性相互干扰。在拟南芥（Arabidopsis thaliana）根中，柱细胞中的淀粉体在向水性反应过程中迅速降解。参与重感的淀粉体的降解通过减少向重力性反应来增强向水性反应。然而，在向水性反应的根中淀粉体降解的机制仍然未知。在这项研究中，通过分析细胞器形态、细胞极性和基因表达变化，研究了柱细胞中淀粉体在向水性反应过程中降解的机制。结果表明，向水性刺激或系统水分胁迫导致柱细胞中细胞器形态和定位发生剧烈变化。具体而言，向水性反应或水分胁迫根的柱细胞失去了内质网（ER）分布的极性，并且表现出加速的液泡化和核运动。对 ER 定位 GFP 的分析表明，ER 围绕发育的液泡重新分布。细胞中常出现分解的淀粉体在自噬体样结构中。向水性刺激和水分胁迫均上调了 AtATG18a 的表达，AtATG18a 是自噬体形成所必需的。此外，用 GFP-AtATG8a 分析表明，向水性刺激和水分胁迫均诱导了柱细胞中自噬体的形成。此外，柱细胞中质体标记物 pt-GFP 的表达对向水性刺激和水分胁迫的反应显著降低，但在自噬突变体 atg5 中的降低幅度要小得多。这些结果表明，向水性刺激赋予了根中的水分胁迫，这触发了自噬反应，负责降解拟南芥根中柱细胞的淀粉体。
