title: Origin and development of the cryptic shoot meristem in Zeylanidium lichenoides (Podostemaceae).
abstract: OBJECTIVE
  The shoot apical meristem is the source of aerial shoot systems. In the aquatic eudicot family Podostemaceae, subfamily Podostemoideae, however, shoots develop in the absence of a distinct apical meristem. Previous studies suggest that the cryptic embryonic shoot meristem is involved in primary shoot development in some species (e.g., Zeylanidium lichenoides), although it is unclear whether the meristem maintains meristematic identity. Our aim was to determine how an embryonic shoot meristem is established during embryogenesis and how it is involved in plumular leaf development in the seedling.
  
  
  METHODS
  We observed anatomy and analyzed gene expression in Z. lichenoides, using an ortholog of the shoot meristem marker gene SHOOT MERISTEMLESS (STM).
  
  
  RESULTS
  Expression of the STM ortholog, ZlSTM, began in the apical part of the 16-cell embryo. By the heart-shaped embryo stage, its expression was restricted to the putative organizing center (OC) and the protodermal cells just above them, forming a cryptic embryonic shoot meristem without a typical stem cell (apical initials) layer. During seedling development, expression was not maintained in the meristem, but instead shifted to the adaxial bases of cotyledons where plumular leaves would form.
  
  
  CONCLUSIONS
  ZlSTM expression demonstrated that the meristematic identity is partly retained in the embryonic shoot apex. This cryptic embryonic shoot meristem has a putative OC, but no typical stem cell layer, and it is not maintained during primary shoot development. Modification of the regulatory mechanism between the OC and stem cells might be responsible for this ephemeral shoot meristem in Podostemaceae.
authors.affiliation: Division of Life Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan. nasu@staff.kanazawa-u.ac.jp
authors.forename: Natsu
authors.initials: N
authors.lastname: Katayama
authors.forename: Masahiro
authors.initials: M
authors.lastname: Kato
authors.forename: Toshihiro
authors.initials: T
authors.lastname: Yamada
mesh_terms: D000595:Amino Acid Sequence; D018525:Germination; D019684:Magnoliopsida / Q000737:chemistry / Q000196:embryology* / Q000502:physiology; D018519:Meristem / Q000196:embryology* / Q000254:growth & development; D008969:Molecular Sequence Data; D038081:Organogenesis; D018515:Plant Leaves / Q000254:growth & development; D010940:Plant Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D036226:Seedlings / Q000254:growth & development
chemical_list: D010940:Plant Proteins