Institute of Plant Science and Resources, Okayama University, Chuo 2-20-1, Kurashiki 710-0046, Japan.
Plant J. 2011 Apr;66(2):231-40. doi: 10.1111/j.1365-313X.2011.04483.x. Epub 2011 Feb 21.
A high accumulation of silicon (Si) is required for overcoming abiotic and biotic stresses, but the molecular mechanisms of Si uptake, especially in dicotyledonous species, is poorly understood. Herein, we report the identification of an influx transporter of Si in two Cucurbita moschata (pumpkin) cultivars greatly differing in Si accumulation, which are used for the rootstocks of bloom and bloomless Cucumis sativus (cucumber), respectively. Heterogeneous expression in both Xenopus oocytes and rice mutant defective in Si uptake showed that the influx transporter from the bloom pumpkin rootstock can transport Si, whereas that from the bloomless rootstock cannot. Analysis with site-directed mutagenesis showed that, among the two amino acid residues differing between the two types of rootstocks, only changing a proline to a leucine at position 242 results in the loss of Si transport activity. Furthermore, all pumpkin cultivars for bloomless rootstocks tested have this mutation. The transporter is localized in all cells of the roots, and investigation of the subcellular localization with different approaches consistently showed that the influx Si transporter from the bloom pumpkin rootstock was localized at the plasma membrane, whereas the one from the bloomless rootstock was localized at the endoplasmic reticulum. Taken together, our results indicate that the difference in Si uptake between two pumpkin cultivars is probably the result of allelic variation in one amino acid residue of the Si influx transporter, which affects the subcellular localization and subsequent transport of Si from the external solution to the root cells.
硅（Si）的大量积累对于克服非生物和生物胁迫是必需的，但 Si 吸收的分子机制，特别是在双子叶植物中，还知之甚少。在此，我们报道了在两个南瓜（Cucurbita moschata）品种中 Si 吸收的输入转运蛋白的鉴定，这两个品种在 Si 积累方面差异很大，分别用于开花和无花黄瓜（Cucumis sativus）的砧木。在非洲爪蟾卵母细胞和水稻 Si 吸收缺陷突变体中的异源表达表明，开花南瓜砧木的输入转运蛋白可以转运 Si，而无花砧木的则不能。定点突变分析表明，在两种砧木之间差异的两个氨基酸残基中，只有将位置 242 的脯氨酸突变为亮氨酸才会导致 Si 转运活性丧失。此外，所有测试的无花根砧南瓜品种都有这种突变。该转运蛋白定位于根的所有细胞中，通过不同方法进行的亚细胞定位研究一致表明，开花南瓜根砧的输入 Si 转运蛋白定位于质膜，而无花根砧的则定位于内质网。总之，我们的结果表明，两个南瓜品种之间 Si 吸收的差异可能是 Si 输入转运蛋白中一个氨基酸残基的等位变异的结果，这影响了 Si 从外部溶液到根细胞的亚细胞定位和随后的转运。
