Developmental anatomy of the reproductive shoot in Hydrobryum japonicum (Podostemaceae)

Abstract

Podostemaceae are unusual aquatic angiosperms adapting to extreme habitats, i.e., rapids and waterfalls, and have unique morphologies. We investigated the developmental anatomy of reproductive shoots scattered on crustose roots of Hydrobryum japonicum by scanning electron microscopy and using semi-thin serial sections. Two developmental patterns were observed: bracts arise either continuously from an area of meristematic cells that has produced leaves, or within differentiated root ground tissue beneath, and internal to, leaf base scars after an interruption. In both patterns, the bract primordia arise endogenously at the base of youngest bracts in the absence of shoot apical meristem, involving vacuolated-cell detachment to each bract separately. The different transition patterns of reproductive shoot development may be caused by different stages of parental vegetative shoots. The floral meristem arises between the two youngest bracts, and is similarly accompanied by cell degeneration. In contrast, the floral organs, including the spathella, arise exogenously from the meristem. Bract development, like vegetative leaf development, is unique to this podostemad, while floral-organ development is conserved.

Expression ofSHOOT MERISTEMLESS,WUSCHEL, andASYMMETRIC LEAVES1Homologs in the Shoots of Podostemaceae: Implications for the Evolution of Novel Shoot Organogenesis

“…Such a leaf/bract is not a leaf and bract in a strict sense, but comparable to a sympodial unit composed of a meristematic shoot zone and a single apical leaf and bract ( Figure 7C). By contrast, the expression patterns of the three genes support the interpretation that the spathella, like the tepal and stamen, is a typical floral organ of leaf homology, while the flower as a whole is a typical shoot homolog, as suggested from morphological evidence (Katayama et al, 2008).…”

“…The new leaf is histologically connected to its parental leaf, and the two leaves only become separated by an abscission-like process, in which vacuolated cells between the two leaves detach from the surrounding tissue (endogenous origin; Imaichi et al, 2005;Koi et al, 2005). This developmental process is also observed in reproductive shoots in Hydrobryum japonicum ( Figure 1C; Katayama et al, 2008). The new bracts arise below the base of the second youngest bract in the same manner as vegetative leaf development ( Figure 1E).…”

“…Likewise, the floral meristem arises between and under the two youngest bracts endogenously and involves vacuolated cell detachment ( Figure 1F). However, a cupular envelope (spathella: a floral organ unique to Podostemoideae) and other floral organs (tepals, stamens, and pistil) develop in an ordinary (exogenous) manner from a conventional floral meristem (Figures 1C and 1F;Katayama et al, 2008;Sehgal et al, 2009).…”

Developmental morphology of seedling and shoot and phylogenetic relationship of Diplobryum koyamae (Podostemaceae)

“…Its leaves arise endogenously from the young leaves, accompanying cell separation, and there is no obvious shoot apical meristem through leaf morphogenesis. This developmental pattern is the same as that of other Asian Podostemoideae species examined, e.g., Cladopus queenslandicus (Domin) C. D. K. Cook & Rutish., Hydrobryum japonicum Imamura, Zeylanidium subulatum (Gardner) C. Cusset (Imaichi et al, 2005; Koi et al, 2005; Katayama et al, 2008), suggesting that H. koyamae is devoid of a conventional shoot apical meristem. From gene expression analyses, N. Katayama et al (unpublished manuscript) suggest that an incipient leaf primordium in H. japonicum is genetically comparable to the shoot apical meristem of other angiosperms.…”

Origin and development of the cryptic shoot meristem in Zeylanidium lichenoides (Podostemaceae)

“…In the Tristichoideae shoots, which have a distinct shoot meristem, STM ortholog expression is similar to that seen in A. thaliana. Similarly, in the floral meristem of H. japonicum (Podostemoideae), which possess a conventional morphology (Katayama et al, 2008), STM ortholog expression closely resembles that seen in A. thaliana.…”