Faculty of Education, Kyoto University of Education, 1 Fujinomori-cho, Fukakusa, Kyoto, 612-8522, Japan.
Faculty of Natural System, Institute of Science and Engineering, Kanazawa University, Kakuma, Ishikawa, 920-1192, Japan.
New Phytol. 2017 Aug;215(3):1210-1220. doi: 10.1111/nph.14630. Epub 2017 Jun 6.
Root apical meristem (RAM) organization in lycophytes could be a key to understanding the early evolution of roots, but this topic has been insufficiently explored. We examined the RAM organization of lycophytes in terms of cell division activities and anatomies, and compared RAMs among vascular plants. RAMs of 13 species of lycophytes were semi-thin-sectioned and observed under a light microscope. Furthermore, the frequency of cell division in the RAM of species was analyzed using thymidine analogs. RAMs of lycophytes exhibited four organization types: type I (Lycopodium and Diphasiastrum), II (Huperzia and Lycopodiella), III (Isoetes) and RAM with apical cell (Selaginella). The type I RAM found in Lycopodium had a region with a very low cell division frequency, reminiscent of the quiescent center (QC) in angiosperm roots. This is the first clear indication that a QC-like region is present in nonseed plants. At least four types of RAM are present in extant lycophytes, suggesting that RAM organization is more diverse than expected. Our results support the paleobotanical hypothesis that roots evolved several times in lycophytes, as well as in euphyllophytes.
石松类植物的根尖分生组织（RAM）的结构可能是理解根早期进化的关键，但这个课题尚未得到充分研究。我们从细胞分裂活动和解剖结构两方面研究了石松类植物的 RAM 组织，并比较了维管植物之间的 RAM。我们对 13 种石松类植物的 RAM 进行了半薄切片，并在光学显微镜下进行了观察。此外，我们还使用胸苷类似物分析了物种 RAM 中的细胞分裂频率。石松类植物的 RAM 表现出四种组织类型：I 型（石松属和卷柏属）、II 型（鳞木属和石松莱属）、III 型（水韭属）和具顶细胞的 RAM（卷柏属）。在石松属中发现的 I 型 RAM 具有一个细胞分裂频率非常低的区域，使人联想到被子植物根中的静止中心（QC）。这是第一个明确表明非种子植物中存在类似于 QC 的区域的证据。现存石松类植物至少存在四种类型的 RAM，这表明 RAM 组织比预期的更加多样化。我们的研究结果支持古植物学假说，即根在石松类植物和真叶植物中多次进化。
