COOK, MARTHA E.* AND COLLEEN A. LAVIN. Department of Biological Sciences, Illinois State University, Normal IL, 61790-4120 and University of Wisconsin Microscopy Resource, Biological Imaging Center, Madison, WI 53706. - Plasmodesmal ultrastructure in the green alga Coleochaete orbicularis (Charophyceae): implications for the evolution of plant plasmodesmata.
Based on molecular, morphological, and biochemical data, the
charophycean green algae are considered to be the closest living
relatives of plants. Hence study of these algae may reveal how or when
various features of plants evolved. Transmission electron microscopy
was used to examine plasmodesmal ultrastructure in the advanced
charophycean alga Coleochaete orbicularis. Three methods of
preservation were employed: standard chemical fixation,
microwave-enhanced chemical fixation, and high pressure freezing
followed by freeze substitution. With all three methods, similar
features were observed in plasmodesmata of Coleochaete. A
central structure is seen in cross-sectional views of some
plasmodesmata, and appears to be connected to the plasma membrane via
spoke-like structures. In longitudinal section, connections between
the endoplasmic reticulum and the plasmodesmata were sometimes
observed. Spoke-like structures radiate away from the plasma membrane
and in some cases connect with ring-like wall specializations. Wall
specializations are found in some plant plasmodesmata, but were not
observed in a previous study of plasmodesmata in another advanced
charophycean alga, Chara zeylanica. Generally however, the
plasmodesmata of Coleochaete and Chara are similar to
each other and to the plasmodesmata of plants. Given that the central
structure of charophycean plasmodesmata appears to be less commonly
observed than is the desmotubule of plant plasmodesmata, it is
possible that the plasmodesmata of charophyceans are less specialized
than those of plants, or that they have been modified after
charophyceans diverged from a common ancestor with plants.
Key words: charophycean algae, Coleochaete, evolution, plasmodesmata, ultrastructure