The phytohormone auxin is involved in the regulation of a variety of developmental processes. In this report, we describe how the processes of lateral root and root hair formations and root gravity response in rice are controlled by auxin. We use a rice mutant aem1 (auxin efflux mutant) because the mutant is defective in these characters. The aem1 line was originally isolated as a short lateral root mutant, but we found that the mutant has a defect in auxin efflux in roots. The acropetal and basipetal indole-3-acetic acid (IAA) transports were reduced in aem1 roots compared to wild type (WT). Furthermore, gravitropic bending as well as efflux of radioactive IAA was impaired in the mutant roots. We also propose a unique distribution of endogenous IAA in aem1 roots. An immunoassay revealed a 4-fold-endogenous IAA content in the aem1 roots compared to WT, and the application of IAA to the shoot of WT seedlings mimicked the short lateral root phenotype of aem1, suggesting that the high content of IAA in aem1 roots impaired the elongation of lateral roots. However, the high level of IAA in aem1 roots contradicts the auxin requirement for root hair formation in the epidermis of mutant roots. Since the reduced development in root hairs of aem1 roots was rescued by exogenous auxin, the auxin level in the epidermis is likely to be sub-optimum in aem1 roots. This discrepancy can be solved by the ideas that IAA level is higher in the stele and lower in the epidermis of aem1 roots compared to WT and that the unique distribution of IAA in aem1 roots is induced by the defect in auxin efflux. All these results suggest that AEM1 may encode a component of auxin efflux carrier in rice and that the defects in lateral roots, root hair formation and root gravity response in aem1 mutant are due to the altered auxin efflux in roots.

中文翻译：

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水稻 aem1 突变体根系发育和重力响应缺陷与生长素外排减少有关

植物激素生长素参与多种发育过程的调节。在本报告中，我们描述了水稻侧根和根毛形成过程以及根系重力响应是如何受生长素控制的。我们使用水稻突变体 aem1（生长素外排突变体），因为该突变体在这些特征上存在缺陷。aem1 系最初是作为短侧根突变体分离出来的，但我们发现该突变体在根部的生长素外排方面存在缺陷。与野生型 (WT) 相比，aem1 根中的顶叶和顶叶吲哚-3-乙酸 (IAA) 转运减少。此外，在突变体根中，向地弯曲以及放射性 IAA 的流出受到损害。我们还提出了 aem1 根中内源性 IAA 的独特分布。免疫分析显示，与 WT 相比，aem1 根中的内源 IAA 含量是 WT 的 4 倍，并且将 IAA 应用于 WT 幼苗的芽模拟了 aem1 的短侧根表型，表明 aem1 根中的高 IAA 含量受损侧根的伸长。然而，aem1 根中高水平的 IAA 与突变根表皮中根毛形成所需的生长素相矛盾。由于 aem1 根的根毛发育减少是由外源生长素挽救的，因此 aem1 根中表皮中的生长素水平可能不是最佳的。这种差异可以通过以下观点来解决，即与 WT 相比，aem1 根的石碑中 IAA 水平更高，而 aem1 根的表皮中的水平更低，并且 IAA 在 aem1 根中的独特分布是由生长素外排缺陷引起的。