Set 14 Communication Channels Between Cells
and Their Origin in Higher Plants

Teaching Section Slide Program of the Botanical Society of America

Presented at UC Davis, California, by Katherine Esau, Professor of Botany, Emeritus, UCSB, on May 14, 1984, on the occasion of UCDís 75th Anniversary Celebration. Dept. of Botany symposium, "Integrating Plant Structure and Function."

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Slide 1. Example of Multicellular tissue. Young parenchyma tissue cut parallel with the epidermis. Euphorbia pulcherrima (poinsettia). Cells with c\vacuolated protoplasts, some sectioned through the nuclei. Chloroplasts in parietal cytoplasm contain starch grains (unstained bodies). The magnification is too low to show plasmodesmata but we know their location in this kind of tissue: in the parts of the cell walls that remained united between contiguous cells during development of intercellular spaces. 
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Slide 2. Example of parenchyma cells in phloem tissue. Cucurbita maxima leaf. In one view, the section of the cell wall exposes plasmodesmata in longitudinal views, in the other in transverse views. In the latter, the groups of plasmodesmata constitute primary pit fields. The profusion of plasmodesmata, as here illustrated, is typical of cells in regions of active intercellular transport of materials. 
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Slide 3. Details of plasmodesmatal structure in longitudinal section (negative magnifications 40,000 and 80,000). Phaseolus vulgaris root tip. A canal through the cell wall is lined with plasmalemma. The cytoplasm of contiguous cells is continuos through the canal and surrounds a conspicuous core. The latter extends to the lumina of endoplasmic reticulum (ER) cisternae (starred) located on both sides of the plasmodesmal canal. The core is often tubular in having clear space along the center; hence the term desmotubule for the core. Remarkable feature: two distinct communication systems, one between the lumina of the contiguous cells through the plasmodesmal canal, the other, between ER cisternae through the desmotubule. 
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Slide 4. Diagram by Robards illustrating features described in three plus additional detail: the plasmodesmal canal is constricted at both ends of the plasmodesma. Concept: that valve-like function is involved regulating volume and direction of transport through the plasmodesma. The ER cisternae appeared to be balloon-shaped in slide #3. They are elongated and shown only in part in slide #4. ER cisternae vary in form and constitute and interconnected system in the cytoplasm. 
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Slide 5. Plasmodesmata originate during cell division. Several aspects of the process are shown. First, mitosis in two cells side by side. Two stages of the process are depicted, metaphase and telophase. In the latter, accumulation of vesicles in the equatorial plane of the cell constitutes the cell plate (arrowheads), first stage in cell-wall formation. Nicotiana tabacum mesophyll. 
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Slide 6. Mitosis in longitudinal divisions of elongated cells, earlier stage in Nicotiana tabacum, later stage (more extended cell plate) in Sonchus oleraceus. Arrowheads delimit the cell plates. earlier and later stages of formation of cytoplasmic bridges making possible the advancement of cell plates to the lateral walls. 
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Slide 7. Late telophase (nuclei not shown). Microtubules involved in the direction of movement of dictyosome vesicles toward the cell plate are assembled into a phragmoplast. The cell plate "grows" centrifugally toward the lateral mother-cell walls. This growth follows the centrifugal spread of the phragmoplast. The vesicles are delimited by a membrane that becomes the plasmalemma after the vesicles fuse. Parenchyma from petiole of Echium judaeum
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Slide 8. Longitudinal and partial surface views of growing cell plates showing involvement of ER on formation of plasmodesmata. Larger arrowheads, ER tubules in position to be included in openings in the cell plate. Smaller arrowheads, transections of plasmodesmata clearly show the desmotubules and the plasmalemma. Phaseolus vulgaris root tip. 
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Slide 9. The cell plate has become the cell wall with fully formed plasmodesmata. Incomplete section made parallel with the surface of the cell wall. Phaseolus vulgaris root tip. 
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Slide 10. Young sieve element and companion cell, the latter narrow, with a narrow nucleus. Smaller arrowheads, cell wall between the two cells. Larger arrowheads, plasmodesmata in future sieve plates. Nicotiana tabacum root tip. 
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Slide 11. Two stages in sieve plate development: smaller arrowheads, plasmodesmal stage; larger arrowheads, plasmodesmata partly lined with callose. Nicotiana tabacum root tip. 
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Slide 12. Two stages in sieve plate development in longitudinal views: above, young wall with plasmodesmata; below, parts of sieve plate elements (left) and parts of companion cells (right). Larger arrowhead, early stage of callose accumulation at ends of a plasmodesma, differentiating pore site. In upper sieve element, part of plasmodesma between sieve element and companion cell (small arrowhead). Two small arrowheads to the right, ordinary plasmodesmata. Echium angustifolium and E. sabulicola petioles. 
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Slide 13. Further development of a pore site in longitudinal view. Callose nearly fused at middle lamella, plasmalemma and ER cisternae cover the callose, plasmodesma extends from ER to ER through future pore. Echium angustifolium petiole. 
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Slide 14. Two stages in sieve-plate pore development in longitudinal views. Above, continuous callose and plasmalemma in former plasmodesmal canal. Below, most of the callose was removed and the open pores were partly occluded with P-protein. Echium angustifolium petioles. 
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Slide 15. Earlier (left) and later (right) stages of pore development in near-surface views. Left, plugs of callose, penetrated by plasmodesmata, fill the pores. Right, pores open, lined with thin layer of callose, and most are occluded with P-protein. Some contain starch grains. Width of mature pores matches that of the callose plugs in immature pore sites. Echium wildpreti petioles. 
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Slide 16. P-protein plug in pore in longitudinal section. Compaction of P-protein within pore especially through middle lamella region. Echium judaeum petiole. 
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Slide 17. Cross sections through pores filled with P-protein and through masses of loose P-protein located at some distance from the sieve plate. Echium plantagineum petioles. 
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Slide 18. P-protein digestion with pepsin (left) and control (right). Only the compacted P-protein within the sieve plate pore became digested. Echium judaeum petiole. 
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Slide 19. Longitudinal section of cell wall including a plasmodesma connecting a sieve element (below) with a companion cell. The part of the plasmodesma located on the companion cell side of the wall is branched. Echium rosulatum petiole. 
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Slide 20. Light-microscope low-power view of primary xylem from the stem of Phaseolus vulgaris. Tracheary xylem elements with ring-like and helical secondary walls are close to the bottom of the slide. Immediately next to the narrow, densely stained sclerenchyma cells is a vessel showing the clear lumen typical of water-conducting cells. The middle part of this vessel is set off by two pairs of bumps. These are sections of rims of secondary wall surrounding the perforations at the two ends of one of the individual members making up the vessel. The perforations are formed by localized removal of the primary wall as is shown in the next slide.

Note: Before the end of the discussion of slide #20 the first side of the tape ended and, to continue the taping, the cassette was turned over. This was done without warning so that there is an interruption in the talk covering the end of slide #20 and the beginning of slide #21. Users of the tape could fill the gap by reading the legends for slides #20 and #21.

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Slide 21. Removal of vessel end wall. Above, the end wall is intact. Its middle part consists of primary wall material, somewhat thickened at this stage and having a prominent middle lamella. Around the margin of the end wall, secondary wall material covers the part of the primary wall that remains unthickened. The exposed primary wall parts become hydrolyzed and disappear. This stage is shown below: the rim now surrounds a perforation. Phaseolus vulgaris stem. 
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Slide 22. Vessel wall parts that do not become perforated also undergo a change; the primary wall not covered by secondary wall layers undergoes partial hydrolysis. This slide shows an immature vessel member with sections of gyres of the helix covering the still intact primary wall. Beta vulgaris leaf. 
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Slide 23. Section of mature vessel element with a helical thickening. Right: wall between two contiguous vessel members with the primary wall partially hydrolyzed between the gyres of the secondary thickening. Noncellulosic wall components were removed, the cellulose remained as a loose network. Left: wall between vessel member and a xylem parenchyma cell. Hydrolysis has affected only the primary wall on the side of the vessel member. Capsella bursa pastoris leaf. 
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Slide 24. Scanning electron microscope picture of part of vessel member from Pelargonium leaf with perforations and pits. Micrograph, courtesy of Professor Peter B. Kaufman and Dr. P Dayanandan, Dept. of Botany, Univ. of Michigan, Ann Arbor, MI.