# 7: Cell-cell communication in plants

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Index

 

 

 

 

cellular domains

symplasmic domains

transport phloem

 

 

plasmodesmal frequency

molecular size limit

molecular trafficking

 

 

plasmodesmograms

phloem loading

phloem unloading

sink phloem

Fig. 1

Fig. 2

Fig. 3

Fig. 4

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Photosynthesis in plants involves several processes which require either light and or chemical energy in order to facilitate the biochemical assembly of CO2 into more complex sugars and non-structural carbohydrates. Obviously, the process which results in carbohydrate synthesis requires that the molecules be transported away from synthesis sites, along recognizable transport pathways. These pathways must lead to the transport phloem in leaves, out of the leaf, and ultimately, to the sinks where these assimilated and transported compounds will be utilized in metabolism and growth..

So, the generalized procedure is one which involves:-
  1. Synthesis
  2. Accumulation
  3. Local transport
  4. Loading and transfer to the sieve tubes
  5. Movement away from the phloem in the leaf, to sites of utilization, where the unloading procedure will be the reverse of that involved in loading.

Our interest is thus in a thorough understanding of the transport pathway which is, for convenience, divided into the loading, transport and unloading phases.

In terms of intercellular communication in plants, there is still some difficulty in understanding the phloem loading and unloading processes. Clearly, if plasmodesmata are involved, then there may be no need to supply energy to drive the transport process. If plasmodesmata are involved, then how do we account for increased solute concentrations in the companion cell-sieve tube complex? What is the size limit of the compounds that may be transported through the plasmodesma? Examination of the literature indicates that loading and unloading, as well as cell-cell molecular trafficking can be either totally symplasmic, totally apoplasmic or a combination mixed symplasmic-apoplasmic process. The loading and unloading process, may in part, governed by the species, its location, and the ecophysiological niche occupied by the plant. As such, it could be argued that phloem loading may in part be ecophysiologically controlled.

Clearly, the process whereby solutes pass through plasmodesmata and the regulation of the cell-cell molecular trafficking, can therefore be considered as either being, a very simple system, governed to a large degree by the laws of thermodynamics. As an alternative, it could be an energy-demanding process, which involves a multiprogrammed, gatable pathway, with the key element within this pathway, being dynamic, 'regulatable' plasmodesmata. Plasmodesmata themselves may be the keepers of the programmable sequences involved in phloem loading in higher plants and as such may play a vital role in the strategies associated with phloem loading itself. The central questions which need to be addressed are:-
  • Are plasmodesmata functional throughout the life span of the plant?
  • Are they gatable and as such, can they regulate or control trafficking?
  • If plasmodesmata remain functional throughout the plant's life and they prove to be gatable, then does this imply that there are separate (programmable) regions (domains) along the loading pathway?

For answers to these questions, please look at the plasmodesmata factfile

 

In order to answer the above questions, it is necessary for us to examine the concepts involved in determining cell-cell transport systems viz., symplast domains, their visualization, Plasmodesmal functionality, connectivity within the transport phloem, connectivity within the sink phloem and finally, that we examine the interrelationships between frequency, connectivity communication (signalling) and transport.

The symplast and the concept of domains

In 1879, Eduard Tangl observed intercellular strands between the cotyledonary cells of Strychnos nux vomica which he interpreted as protoplasmic contacts . He pioneered the concept that cell-cell communication integrated the functioning of the cells from which higher plants are formed. He predicted that the protoplasmic strands enabled the plant to co-ordinate its overall activities and functioning, and secondly, he dissociated himself with the then popular view that cells were functional entities. Much later, the use of membrane-impermeable fluorescent dyes allowed Erwee and Goodwin to obtain direct evidence for cell-cell communication in Egeria densa in 1985. These authors also noted that the molecular size exclusion limits for intracellularly-injected dyes varied between the respective organs of Egeria. Erwee and Goodwin postulated that the symplast of plants could therefore not be considered to be one domain, but, instead, that it was (or could be) composed of many domains, which formed operational units in the plant. The tissue domains postulated by Erwee and Goodwin are thus a modern embodiment of the views expressed by Tangl 104 years earlier. Clearly, the presence of domains, must have great significance in the physiological functioning of the plant. Click HERE to go back to the Index

 

Identification of symplasmic domains

Attempts have been made to identify functional symplasmic domains in plants Intracellular injection of fluorescent dyes with membrane potential measurements have shown that cell clusters with similar cell potentials exist in plants, and these have been interpreted as being 'good' evidence for the presence of domains in widely differing plants such as Tomato ((Van der Schoot and van Bel, 1990) and Barley (Botha Cross and Hartley, 1995).

Symplasmic domains have been "visualized" in various ways (see Gamalei 1985; Russin and Evert, 1985; van Bel et al., 1988; Robinson-Beers and Evert, 1991; Botha and van Bel, 1992). The quickest spatial interpretation is obtained using plasmodesmograms (Botha and van Bel, 1992; van Bel et al., 1988). The rationale behind the interpretation of plasmodesmograms, is simply that the higher the frequency of plasmodesmata at any particular cell-cell interface, the higher the probability of symplasmic transfer between the cells, and, conversely, the lower the frequency at any particular cell-cell interface, the lower the probability of symplasmic cell-cell transfer. In other words, the latter case indicates the potential for two cell domains, where plasmodesmal frequencies are low. Clearly, this is a simplistic overview, and does not take into account many of the potential problems and pitfalls usually associated with such overviews. We do not know the functional state of the plasmodesmata, we do not know how many plasmodesmata are required to ensure rapid (efficient) cell transport, we do not know the molecular size cut off limit, and we do not know if the images that we see with the electron microscope are artefact, or if the represent functional plasmodesmata. Reliance on plasmodesmograms, or indeed, any other single means of expressing plasmodesmal frequency, has potential drawbacks. The validity of plasmodesmograms (at least heavy reliance upon the results which they visualize) can and may thus be disputed.

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plasmodesmal functionality

Fig. 1.

Fig. 1. Transections of plasmodesmata in the algae, Chara.

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There is no 'universal' structure for a plasmodesma, nor is there a 'universal' plasmodesmal diameter. Clearly, the substructural details firs presented by Robards (see Fig. 3) still stand and the subsequent years of electron microscopy have done very little to increase our knowledge of the functional substructure of plasmodesmata. Take for example, the electron micrographs of plasmodesmata in Chara shown in Fig. 1 above. In essence, these plasmodesma are simpler than those of higher plants. -there does not seem to be much evidence for the same level of 'complex' sub-structure as is seen in higher plant plasmodesmata - yet there is a similarity which demonstrates the lineage of the higher plants, which suggests the early evolution of the involvement of plasmodesmata in cell to cell transport. Much has been written in the literature concerning diameter in relation to cross-sectional area of plasmodesmal contacts between various interfaces (see Overall and Gunning, 1982). The argument is that transport capacity is dependent upon the number of functional plasmodesmata, as well as their functional diameter. Unfortunately, there is no 'universal' plasmodesmal diameter (van Bel and Oparka, 1995). Diversity in plasmodesmal ultrastructure is apparent from the results described by Robinson-Beers and Evert (1991) for Saccharum officinarum and by Botha, Hartley and Cross (1993) for Themeda triandra, also hints at a potential variety of molecular exclusion. size limits, unlike that reported for animal gap junctions (800Da).

Fig. 2.

Fig. 2. Diagram of a pore plasmodesmata between a sieve tube (left) and a companion cell (right). Sometimes called 'branched plasmodesmata' in the literature, these structures have enlarged pores on the sieve tube side of the wall, and plasmodesmal pores on the companion cell side. These pore plasmodesmal units (PPU's) are recognised as having a significant role in phloem loading, but also in regulating the trafficking processes that occur between the sieve tube member and its associated companion cell. Click HERE to go back to the Index

There is a growing body of evidence which suggests that plasmodesmata have the ability to "gate" their diameters. Gating is  presumably associated with the neck regions of plasmodesmata -- in the zones termed "sphincters" by Gunning and Robards (1976). The ability to regulate or to even control aperture size, would nowhere be more important than within pore-plasmodesmata in the common wall between sieve tube and companion cell as illustrated in Fig. 2 on the right. Evidence for the direct implication of pressure on the opening and closure of plasmodesmata has been provided by Oparka and Prior ((1987). Thus, plasmodesmal regulation may be induced by a number of stimuli. Virus infection and in particular the effects of the coat-associated movement protein has been well documented recently by Lucas et al., (1993). Recently evidence for a 'set point gate' has been provided by Cleland et al., (1994).

Cleland et al., (1984) suggested that the process of 'set point gating' requires an ATP-mediated step. In addition, there is evidence that plasmodesmal gating may also be light-mediated (Epel and Erlanger, 1991), and to be able to close in response to turgor gradients and elevation of internal calcium  Closure of plasmodesmata has been confirmed by the absence of dye-coupling or by very low coupling ratios, especially near cut or wounded tissues (van der Schoot and van Bel, 1990). Closure or down-regulation, may well be due to changes in osmotic potential and as a result, the stimulation of the formation of callose material ((1-3) b-D glucan)). It has also been suggested that 1-3) b-D glucan synthase is located in the cell wall, near and/or associated with the plasmodesmal orifices or 'sphincters' (Lucas et al., 1993).

Plasmodesmal gating and the implied potential down-regulation of symplasmic transport, certainly casts some doubt on the validity of plasmodesmograms, or any other means of frequency determination, without related dye-coupling and concomitant membrane electrical potential measurements. As examples the sugar-secreting trichomes in Abutilon contain more than 10 plasmodesmata/ mm2 cell wall interface, yet, dye-coupling studies indicate little or no transfer of probes across this plasmodesmata-containing interface (Terry and Robards, 1987). Similar domains have been reported in different tissues in Egeria (Erwee and Goodwin, 1985) and partial symplasmic barriers to carboxyfluorescine have been reported by Epel and Bandurski (1990) between the stele and cortex in the mesocotyl of Zea mays, which suggests either low plasmodesmal frequencies, or a down-regulation in functionality of the plasmodesmata.

 

connectivity within the transport phloem

Many of the studies which have been made in the past, give credence to plasmodesmograms. In other words, they are useful, but care must be taken to interpret them as indicators of potential transport and not absolute transporters. Clearly, there are indicators of families that are apparently entirely symplasmic loaders, others that follow a mixed apoplasmic-symplasmic pathway, and yet others, where phloem loading is exclusively apoplasmic. Experimentally, some support and elegant evidence comes from the work done using PCMBS, to separate those that require metabolic energy to mediate uptake (apoplasmic loaders) from those that do not (symplasmic loaders) -- (see van Bel et al., 1995 and literature cited).

The symplasmic isolation of the cc-se complex from other vascular parenchyma, is of great physiological significance, as it will allow a delicate balance between loss and recovery via carrier-mediated retrieval systems. Generally, the evidence for isolation is good, coming from various papers in which low plasmodesmal frequencies, low dye-coupling, and relatively high electrical resistances, are taken as evidence for the isolation of the cc-se complex. This is particularly true of transport phloem . Low frequencies or sporadic occurrence at the cc-st interface, suggest that these plasmodesmata are ideally suited to gating.

 

connectivity within the sink phloem

Plasmodesmograms indicate a variety of unloading options in sinks (see van Bel and Oparka 1995 and literature cited). In many terminal sinks, there is evidence for symplasmic unloading of the sieve elements. Plasmodesmal frequencies in barley, Zea mays and Cucurbita pepo suggests that there is a continuous symplasmic domain from the sieve tubes, to all other cells within the roots (Warmbrodt, 1986, and literature cited). However in sink leaves such as those in young developing Zea mays plants, there is little evidence to support symplasmic unloading (Evert and Russin, 1993). Similar non-symplasmic unloading pathways have been suggested in Rice (Oparka and Gates, 1981) and in bean (Offler and Patrick, 1984).

However, a great deal more interconnective complexity is offered where radial and axial components are present. In these tissues is entirely possible that parts of the phloem will be involved with retrieval, whilst other parts may be actively engaged in solute transfer to parenchymatic cells other than companion cells.

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frequency, connectivity, communication and transport

 


Fig. 3.

 

Fig. 3. Diagram illustrating the component parts of a plasmodesma. Click HERE to go back to the Index
An assessment of the  transport capacity between cells may be made by determining the plasmodesmal frequencies between all cells in the pathway of interest. As mentioned, high plasmodesmal frequencies such as those that exist between the Kranz mesophyll cells and bundle sheath cells in virtually all C4 grass leaf blade bundles, is indicative of a potential for high volume cell-cell symplasmic transfer pathway. Concomitantly, low frequencies is indicative of a low potential for symplasmic transfer between cells. However, potentials are just that -- they are simply indicators of a probable pathway. Without other more direct experiments, such as iontophoretic dye-coupling studies, in which membrane-impermeant dyes are injected intracellularly, or experiments in which the electrical potential of the cells along the pathway of interest are determined, then frequency is without doubt, simply an estimate. Frequency, and indeed, electron microscope-based studies -no matter how detailed- simply give a snapshot in time and do not in any way, reflect the potential gating that may well have to occur on occasion in order to enable regulation of the physiological processes necessary for normal plant growth. Alone, frequency is thus simply one measure of what could potentially occur within the plant part of interest. The data presented in Table 1 below, illustrates cell-cell potentials as measured in Hordeum vulgare leaf blade bundles.

 Clearly, there seems to be an electrical potential difference, between the cells exarch to the companion cell-sieve tube complex and the thick-walled sieve tubes. Even though the average (-52mV) potential from the mesophyll to the vascular parenchyma, is low, the indication is that there may well be one symplasmic domain from mesophyll through bundle-sheath -> mestome sheath -> vascular parenchyma. Endarch to the vascular parenchyma, the electrical potential increases 2.3 x, which suggests a second domain, and a apoplasmic disjunction. Examination of the plasmodesmal frequencies, (Fig.4) confirms this.

 

Table 1. Data from an averaged potential measurement of cell types in intermediate and small leaf blade bundles of H. vulgare c.v. Dyan.

Cell Type

# Cells

Y (mV)

(Low/High)

1. Mesophyll

23

- 42

- 35 -> - 48

2. Bundle Sheath

19

- 48

- 42 -> - 54

3. Mestome Sheath

13

- 48

- 40 -> - 50

4. Vascular Parenchyma

15

- 70

- 65 -> - 90

 

 

-52

 

5. Thick-Walled Sieve Tube

****

 

****

6. Companion Cell

****

 

****

7. Thin-Walled Sieve Tube Companion Cell Complex

11

-122.5

- 90 -> -155

****:- Cell association could not be clearly identified within whole vascular bundles in longitudinal view, due to the compact nature of the CC-ST complex, and that the thick-walled sieve tube occurs adjacent to and partly obscured by the metaxylem vessels.

 

Fig. 4. Plasmodesmograms of intermediate (left) and small (right) vascular bundle of Hordeum vulgare c.v. Dyan, showing distribution of plasmodesmata, calculated as % plasmodesmata/m m vein . Note symplasmic discontinuity between the VP and ST (open circles) and lack of plasmodesmal connections between thick (closed circles) and thin-walled sieve tubes. Based upon the data shown here, the companion cell-sieve tube complex and thick-walled sieve tubes are symplasmically isolated. Click HERE to go back to the Index

concluding remarks

Plasmodesmal functionality and frequency are two important areas within the broad framework of studies aimed at unravelling the processes and controls which facilitate and regulate the movement of substances from one cell to another. As yet, we are unsure of the exact functions of the subunits within plasmodesmata within higher plants. The presence of 'complicated' substructures may well mean that the plasmodesma are more complex in their own right. Increased complexity may well be an indicator of enhanced functional control, such as through gating mechanisms. We do not yet know how many functional plasmodesmata are required to facilitate the movement of substances such as photoassimilated material, from cell to cell from the mesophyll to the sieve tubes. We still do not know if plasmodesmata are functional throughout the life of the plant. Do they cease functioning, at the onset of senescence, or at some stage before senescence commences? We still do not really know how viruses modify plasmodesmata to allow passage of the virus. We still do not know if all plasmodesmata may be modified, or if only some can be modified.

Clearly there are many questions - questions to which no single, acceptable and plausible answer exists.

Some selected references:

Botha CEJ, Cross RHM (1997) Plasmodesmatal frequency in relation to short-distance transport and phloem loading in leaves of barley (Hordeum vulgare). Phloem is not loaded directly from the symplast Physiologia Plantarum 99: 355-362

Erwee M G, Goodwin P B . Symplast domains in extrastelar tissues of Egeria densa Planch. Planta 163, 9-19. 1985.

Erwee M G, Goodwin P B, Van Bel A J E . Cell-cell communication in the leaves of Commelina cyanea and other plants. Plant Cell and Environment 8, 173-178. 1985.

Warmbrodt R D . Structural aspects of the primary tissues of the Cucurbita pepo L. root with special reference to the phloem. New Phytol. 102, 175-192. 1986.

Evert RF, Russin WA (1993) Structurally, phloem unloading in the maize leaf cannot be symplastic abstract no. 11). American Journal of Botany 80: 1310-1317

Epel B L, Bandurski R S . Tissue to tissue symplastic communication in the shoots of etiolated corn seedlings. Physiologia Plantarum 79, 604-609. 1990.

Gunning B E S, Robards A W . Eds. Intercellular Communication in Plants: Studies on Plasmodesmata, Spinger Berlin Heidelberg

Offler CE, Patrick JW (1984) Cellular structures, plasma membrane surface areas and plasmodesmatal frequencies of seed coats of Phaseolus vulgaris L. in relation to photosynthate transfer Aust. J. Plant Physiol. 11: 79-99

Oparka K J, Gates P . Transport of assimilates in the developing caryopsis of rice (Oryza sativa L.). Planta 151, 561-573. 1981.

Tucker E B, Spanswick R M . Translocation in the staminal hairs of Setcreasea purpurea. II. Kinetics of intracellular transport. Protoplasma 128, 167-172. 1985.

Tucker E (1998) Inositol bisphospate and inositol trisphosphate inhibit cell-to-cell passage of carboxyfluorescine in staminal hairs of Setcreasea purpurea. Planta 174: 358-363

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