Me. Cook et al., COMPARATIVE ULTRASTRUCTURE OF PLASMODESMATA OF CHARA AND SELECTED BRYOPHYTES - TOWARD AN ELUCIDATION OF THE EVOLUTIONARY ORIGIN OF PLANT PLASMODESMATA, American journal of botany, 84(9), 1997, pp. 1169-1178
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.