Department of Plant Sciences, University of Oxford, South Parks Road, Oxford, OX1 3RB, UK.
Institute of Molecular Plant Sciences, School of Biological Sciences, University of Edinburgh, Max Born Crescent, Edinburgh, EH9 3BF, UK.
Sci Rep. 2020 Dec 9;10(1):21547. doi: 10.1038/s41598-020-78171-y.
Rhizomorphic lycopsids are the land plant group that includes the first giant trees to grow on Earth and extant species in the genus Isoetes. Two mutually exclusive hypotheses account for the evolution of terminal rooting axes called rootlets among the rhizomorphic lycopsids. One hypothesis states that rootlets are true roots, like roots in other lycopsids. The other states that rootlets are modified leaves. Here we test predictions of each hypothesis by investigating gene expression in the leaves and rootlets of Isoetes echinospora. We assembled the de novo transcriptome of axenically cultured I. echinospora. Gene expression signatures of I. echinospora rootlets and leaves were different. Furthermore, gene expression signatures of I. echinospora rootlets were similar to gene expression signatures of true roots of Selaginella moellendorffii and Arabidopsis thaliana. RSL genes which positively regulate cell differentiation in roots were either exclusively or preferentially expressed in the I. echinospora rootlets, S. moellendorffii roots and A. thaliana roots compared to the leaves of each respective species. Taken together, gene expression data from the de-novo transcriptome of I. echinospora are consistent with the hypothesis that Isoetes rootlets are true roots and not modified leaves.
根状体型木贼类植物是陆生植物群，包括地球上第一批生长的巨型树木和现存的水韭属物种。有两个相互排斥的假说可以解释根状体型木贼类植物中终生根轴（称为根毛）的进化。一个假说是根毛是真正的根，就像其他木贼类植物的根一样。另一个假说是根毛是经过修饰的叶子。在这里，我们通过研究水韭属 Isoetes echinospora 的叶和根毛中的基因表达来检验这两个假说的预测。我们组装了水韭属 Isoetes echinospora 的无共生培养转录组。水韭属 Isoetes echinospora 根毛和叶子的基因表达特征不同。此外，水韭属 Isoetes echinospora 根毛的基因表达特征与卷柏属 Selaginella moellendorffii 和拟南芥 Arabidopsis thaliana 的真正根的基因表达特征相似。RSL 基因在根中正向调节细胞分化，与每个物种的叶片相比，在水韭属 Isoetes echinospora 根毛、卷柏属 Selaginella moellendorffii 根和拟南芥 Arabidopsis thaliana 根中要么是特异性表达，要么是优先表达。总的来说，水韭属 Isoetes echinospora 的从头转录组的基因表达数据与根状体型木贼类植物的根毛是真正的根而不是经过修饰的叶子的假说一致。
