Are unusual plasmodesmata in the embryo-suspensor restricted to species from the genus Sedum among Crassulaceae?

The Crassulaceae family comprises mainly herbaceous leaf succulents, some of which have an ornamental value. During embryogenesis, they produce a suspensor with a giant polyploid basal cell. It has recently been shown that in Sedum acre and S. hispanicum this cell has compound plasmodesmata with an unusual dome of electron-dense material associated on the cell's side. These compound plasmodesmata differ from the typical ones occurring in other angiosperms. In this study, the hypothesis was tested that the unusual plasmodesmata in the embryo-suspensor are a feature not only restricted to species from the genus Sedum, but are also found in other Crassulaceae genera. Suspensors of example species from the genera Sempervivum and Jovibarba, which have vegetative morphologies quite different from Sedum and which are placed in the traditional classification into another subfamily, were first examined using an electron microscope. It was found that the unusual compound plasmodesmata in the suspensor are not only restricted to species from the genus Sedum but are also found in species from other Crassulaceae genera (Sempervivum arachnoideum and Jovibarba sobolifera). It should be noted that some ultrastructural features of compound plasmodesmata in the analyzed genera (e.g. the character of the wall with plasmodesmata, plasmodesmata diameter or occurrence of the electron-dense material) are different from the suspensor plasmodesmata recorded in species from the Sedum genus. We found that in Sempervivum arachnoideum the size of the plasmodesmata diameter varies according to the micropylar-chalazal axis of the embryo. This is the first report of variation in the diameter of the plasmodesmata within the embryo of angiosperms. Further study will be needed to show whether compound plasmodesmata occur in other Crassulaceae clades, whether they are a stable feature at the genus level in this family, and also whether they have evolved several times or only once in Crassulaceae.