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.

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The structure and transport properties of pit membranes at the interface between the metaxylem and xylem parenchyma cells and the possible role of these pit membranes in solute transfer to the phloem were investigated. Electron microscopy revealed a fibrillar, almost tubular matrix within the pit membrane structure between the xylem vessels and xylem parenchyma of leaf blade bundles in rice (Oryza sativa). These pits are involved primarily with regulating water flux to the surrounding xylem parenchyma cells. Vascular parenchyma cells contain large mitochondrial populations, numerous dictyosomes, endomembrane complexes, and vesicles in close proximity to the pit membrane. Taken collectively, this suggests that endocytosis may occur at this interface. A weak solution of 5,6-carboxyfluorescein diacetate (5,6-CFDA) was applied to cut ends of leaves and, after a minimum of 30 min, the distribution of the fluorescent cleavage product, 5,6-carboxyfluorescein (5,6-CF), was observed using confocal microscopy. Cleavage of 5,6-CFDA occurred within the xylem parenchyma cells, and the non-polar 5,6-CF was then symplasmically transported to other parenchyma elements and ultimately, via numerous pore plasmodesmata, to adjacent thick-walled sieve tubes. Application of Lucifer Yellow, and, separately, Texas Red-labelled dextran (10 kDa) to the transpiration stream, confirmed that these membrane-impermeant probes could only have been offloaded from the xylem via the xylem vessel–xylem parenchyma pit membranes, suggesting endocytotic transmembrane transfer of these membrane-impermeant fluorophores. Accumulation within the thick-walled sieve tubes, but not in thin-walled sieve tubes, confirms the presence of a symplasmic phloem loading pathway, via pore plasmodesmata between xylem parenchyma and thick-walled sieve tubes, but not thin-walled sieve tubes.

This paper addresses plasmodesmatal distribution and frequency in the leaf-blade bundles of four southern African grasses - one C3, and one each of the NAPD-malic-enzyme (ME), NAD-ME and phosphoenolpyruvate carboxykinase (PCK) C4 photosynthetic subtypes and, additionally, relates the calculated plasmodesmatal frequencies to the photosynthetic carbon-fixation rate. Plasmodesmata are, in all instances, constricted where they traverse suberin lamellae at the Kranz mesophyll-bundle sheath (KMS-BS), the bundle-sheath-vascular parenchyma (BS-VP), or the bundle-sheath-mestome-sheath interfaces (BS-MS). Frequency studies clearly show that plasmodesmata are most numerous at the KMS-BS, BS-MS and BS-VP interfaces, (31.9-76.8% of the total) and that their numbers decrease rapidly with increasing proximity to both thin- and thickwalled sieve tubes. In Themeda triandra var. imberbis (Retz.) A. Camus and Bromus unioloides H.B.K. thickwalled sieve tubes have few connections with vascular parenchyma cells and are, to all intents and purposes, almost totally isolated from the rest of the vascular tissue, indicating that the loading pathways for these sieve tubes are predominantly apoplastic. Although decreasing plasmodesmatal frequencies indicate that loading of assimilate may become progressively more apoplastic with increasing proximity to the sieve tubes, a symplastic route to the thin-walled sieve tubes cannot be ruled out. Studies of net assimilation rate indicate a good correlation of photosynthetic rate with the photosynthetic type (C3, C4 NADP-ME, C4 PCK, and C4 NAD-ME); furthermore, the lowest plasmodesmatal frequencies were associated with C3, and the highest with C4 NAD-ME types.

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