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Comparative anatomy of embryogenesis in three species of Podostemaceae and evolution of the loss of embryonic shoot and root meristems

Authors

  • Natsu Katayama,

    Corresponding author
    1. Division of Life Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
      *Author for correspondence (email: nuts@stu.kanazawa-u.ac.jp)
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  • Masahiro Kato,

    1. Department of Botany, National Museum of Nature and Science, Amakubo, Tsukuba 305-0005, Japan
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  • Takumi Nishiuchi,

    1. Division of Life Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
    2. Advanced Science Research Center, Division of Functional Genomics, Kanazawa University, Takaramachi, Kanazawa 920-0934, Japan
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  • Toshihiro Yamada

    1. Division of Life Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
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*Author for correspondence (email: nuts@stu.kanazawa-u.ac.jp)

Abstract

SUMMARY During embryogenesis in angiosperms, the embryonic shoot and root meristems are created at opposite poles of the embryo, establishing a vertical body plan. However, the aquatic eudicot family Podostemaceae exhibits an unusual horizontal body plan, which is attributed to the loss of embryonic shoot and root meristems. To infer the embryogenetic changes responsible for the loss of these meristems, we examined the embryogenesis of three podostemads with different meristem characters, that is, Terniopsis brevis with distinct shoot and root meristems, Zeylanidium lichenoides with reduced shoot and no root meristems, and Hydrobryum japonicum with no shoot and no root meristems. In T. brevis, as in other eudicots, the putative organizing center (OC) and L1 layer (=the epidermal cell layer) arose to generate a distinct shoot meristem initial, and the hypophysis formed the putative quiescent center (QC) of a root meristem. Z. lichenoides had a morphologically unrecognizable shoot meristem, because a distinct L1 layer did not develop, whereas the putative OC precursor arose normally. In H. japonicum, the vertical divisions of the apical cells of eight-cell embryo prevented putative OC initiation. In Z. lichenoides and H. japonicum, the putative QC failed to initiate because the hypophysis repeated longitudinal divisions during early embryogenesis. Based on their phylogenetic relationships, we infer that the conventional embryonic shoot meristem was lost in Podostemaceae via two steps, that is, the loss of a distinct L1 layer and the loss of the OC, whereas the loss of the embryonic root meristem occurred once by misspecification of the hypophysis.

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