COMPARATIVE ULTRASTRUCTURE OF PLASMODESMATA OF CHARA AND SELECTED BRYOPHYTES - TOWARD AN ELUCIDATION OF THE EVOLUTIONARY ORIGIN OF PLANT PLASMODESMATA

We have used transmission electron microscopy to examine plasmodesmata of the charophycean green alga Chara zeylanica, and of the putatively early divergent bryophytes Monoclea gottschei (liverwort), Notothylas orbicularis (hornwort), and Sphagnum fimbriatum (moss), in an attempt to learn when seed plant plasmodesmata may have originated. The three bryophytes examined have desmotubules. In addition, Monoclea was foun d to have branched plasmodesmata, and plasmodesmata of Sphagnum displa yed densely staining regions around the neck region, as well as ring-l ike wall specializations. In Chara, longitudinal sections revealed end oplasmic reticulum (ER) that sometimes appeared to be associated with plasmodesmata, but this was rare, despite abundant ER at the cell peri phery. Across all three fixation methods, cross-sectional views showed an internal central structure, which in some cases appeared to be con nected to the plasma membrane via spoke-like structures. Plasmodesmata were present even in the incompletely formed reticulum of forming cel l plates, from which we conclude that primary plasmodesmata are formed at cytokinesis in Chara zeylanica. Based on these results it appears that plasmodesmata of Chara may be less specialized than those of seed plants, and that complex plasmodesmata probably evolved in the ancest or of land plants before extant lineages of bryophytes diverged.