Department of Botany, National Museum of Nature and Science, Tsukuba, Japan.
Plant Biol (Stuttg). 2010 Sep 1;12(5):794-9. doi: 10.1111/j.1438-8677.2009.00275.x.
In Tristichoideae, aquatic angiosperms in the family Podostemaceae, Terniopsis, Tristicha, Indotristicha and Cussetia have creeping roots with flanking (sub)cylindrical shoots, while Dalzellia is rootless and has crustose shoots. Indodalzellia gracilis, sister to a clade of Dalzellia zeylanica and Indotristicha ramosissima, has subcrustose shoots on the side of creeping roots, suggesting that I. gracilis may be a key species to reveal how saltational evolution of the body plan occurred in these three species. We investigated developmental morphology of I. gracilis seedlings grown in culture, using scanning electron microscopy and semi-thin serial sections. As in D. zeylanica, the plumular apical meristem in the seedling gives rise to two shoot apical meristems, which develop into horizontal subcrustose shoots with dorsal and marginal leaves. Neither radicle nor adventitious root is produced from the hypocotyl, but an adventitious root arises endogenously from the juvenile shoot and from some shoots of adult plants. These results, together with the phylogenetic relationships, suggest that the Indodalzellia seedling evolved by loss of the adventitious root derived from the hypocotyl, appearance of shoots in the axil of cotyledons, and appearance of adventitious roots from adventitious shoots. The difference in place of origin of the root between Indodalzellia and I. ramosissima suggests differing evolutionary origin of the root in Tristichoideae.
在水龙骨科的胎生水生被子植物中，扭叶苣苔族的 Terniopsis、Tristicha、Indotristicha 和 Cussetia 具有匍匐根和侧生（亚）圆柱形芽，而 Dalzellia 无根，具壳状芽。Indodalzellia gracilis 是 Dalzellia zeylanica 和 Indotristicha ramosissima 分支的姊妹群，在匍匐根的侧面具有亚壳状芽，表明 I. gracilis 可能是揭示这三个物种中体式跳跃进化的关键物种。我们使用扫描电子显微镜和半薄连续切片研究了在培养中生长的 I. gracilis 幼苗的发育形态。与 D. zeylanica 一样，幼苗的胚轴顶端分生组织产生两个茎尖分生组织，它们发育成具有背侧和边缘叶的水平亚壳状芽。下胚轴既不产生不定根也不产生不定根，而是从幼茎和一些成年植株的芽中内源产生不定根。这些结果以及系统发育关系表明，Indodalzellia 幼苗是通过失去来自下胚轴的不定根、子叶腋芽的出现以及不定芽的不定根的出现而进化的。Indodalzellia 和 I. ramosissima 之间根的起源位置的差异表明扭叶苣苔族的根具有不同的进化起源。
