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 found to have branched plasmodesmata, and plasmodesmata of Sphagnum displayed densely staining regions around the neck region, as well as ring-like wall specializations. In Chara, longitudinal sections revealed endoplasmic reticulum (ER) that sometimes appeared to be associated with plasmodesmata, but this was rare, despite abundant ER at the cell periphery. Across all three fixation methods, cross-sectional views showed an internal central structure, which in some cases appeared to be connected to the plasma membrane via spoke-like structures. Plasmodesmata were present even in the incompletely formed reticulum of forming cell 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 ancestor of land plants before extant lineages of bryophytes diverged. num. Plants are divided into cells that remain cytoplasmically connected via plasmodesmata. Several workers have emphasized the unity of the plant body, and have stressed the importance of plasmodesmata in maintaining cell-to-cell communication within the plant as an integrated organism (Niklas, 1989; Kaplan and Hagemann, 1991; Lucas and Wolf, 1993). It has been suggested that plasmodesmata may serve as channels for simple diffusion and complex molecular trafficking, and that plant development at the cell, tissue, and organ levels may be regulated by means of plasmodesmata in