# 5: The source-sink connection



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Apoplasm-symplasm concept

apoplasmic transport



extracellular transport

geographic plant distribution

Phloem loading physiology

plasmodesmal frequency



symplasmic transport

Transport evolution

Fig. 1

Fig. 2

The source-sink connection in plants relates to the connectivity between the source of assimilated material and the pathway that is followed by this material, to a sink, which can be defined as a (local) region where carbon-based material is metabolised and energy (usually in the form of ATP) is synthesised.

Assimilated material (photo-or other) is produced through a variety of complex biochemical reactions, and accumulates, usually in sufficient quantities, such that the material will commence movement and follows a diffusive pathway from a region of high concentration to a region of lower concentration, either nearby, or some distance removed from its region of origin (source), or site of production. Diffusion will continue to be the driving force, provided that a concentration gradient is maintained. However, there are few, truly diffusive pathways in plants, that are capable of providing or maintaining the necessary diffusion flow rates, to satisfy the demands made by general plant growth and metabolism. It follows that is sustained growth is to be satisfied and achieved, that a better way of mobilization of assimilated material becomes necessary. Clearly, such systems have evolved with time throughout the plant kingdom and the pinnacle of these evolutionary steps lies within the higher vascular plants.

How did successful transport systems evolve? What were the selection pressures that had to be overcome, if the system was truly to allow competitiveness between the species? What structural modifications were necessary, if the systems which evolved were to be in any way successful? What physiological constraints and related to this, structural constraints had to be overcome? These are but a few of the questions which we need to address if we are to come up with a reasonable answer to the central issue of transport in plants. So central to the dogma of the necessity of the co-evolution of the apoplasmic and symplasmic transport systems in higher plants, is a clear understanding of the interrelationships between the various components of the transport systems.

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The Apoplasm - Symplasm Concept

All vascular plants contain two potential avenues for the transport of substances -- a symplasmic and an apoplasmic system. For the sake of clarity and by way of explanation, the symplasm can be defined as the sum of all interconnected cells, that are delimited by the plasmalemma, and which are in direct symplasmic continuity, via plasmodesmata. In contrast, the apoplasm can be defined as the sum of all the cell walls, intercellular spaces and extracellular (extra-cytoplasmic) free space which makes up the plant body. The difference then, is that the one (symplasm) is theoretically isolated from the other (apoplasm), by the plasmamembrane system. Clearly, transfer between the two, must involve either diffusion or transmembrane trafficking either in a simple manner, along a concentration gradient, or if the Dy across the membrane becomes small, then increased energy demand (proton co-transport) will be demanded in order to satisfy the transport of substances against a concentration gradient. It follows that transport through the symplasmic system will require a degree of diffusivity and permeability of the substances to the symplasmic system as a whole. It also follows that the movement of the substance, will depend on its molecular mass as well as its own state of hydrophobia and lipophobia – (put another way, the relative degree of solubility in water or lipids) which must affect transport.

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Apoplasmic Transport

Apoplasmic transport is generally agreed to be transpiration driven, and is thus dependent to a large extent, upon the supply of water from the root systems, and its (related) loss from the leaf via evapotranspiration, particularly through the stomata. In terms of mass supply, the xylem is considered to be the principal pathway through which the majority of the solute flow takes place. However, we must not loose site of the definition of the apoplasmic, which includes intercellular spaces, cell walls and the free extracellular space within all living plants. Sustainable evapotranspirative loss will be dependent to a large extent, upon water availability and the level of droughting that exists at a particular time. Water availability (or the lack of it, i.e., a deficit) will influence all transport systems in vascular plants, where there is a need or an input requirement of water into the system. Apoplasmic transport is also generally agreed to be that extracellular transport which includes the xylem and its related free space. Apoplasmic transport represents an almost entirely unidirectional transport process, from the roots through the shoots, to the evaporative extracellular surfaces within leaves at the surface of the mesophyll. It is at the mesophyll cell walls, where evapotranspirative rate becomes governed by saturated vapour pressure gradients between the mesophyll cell walls, the intercellular spaces and the surrounding atmosphere.

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From this, it follows that apoplasmic water availability will in all probability govern both the geographical distribution of plants and may well have a major potential influence on the degree of hydromorphy, mesomorphy or xeromorphy exhibited by a plant and its ability to adapt to a particular environment. Transport systems and the relative success or failure of a plant in a particular environment, can thus be looked at in terms of the ecophysiological success with which a plant adapts to its environment. Click here to go back to the index.

Symplasmic Transport

Cytoplasmic Domains

Domain: “A region of interconnected cells, which have similar transport patterns, similar physiological inputs and outputs and which are electrophysiologically coupled.”

In terms of the definition of the symplast outlined earlier, we can think of an interconnected series of cells, where the major unknown is the actual extent of the symplasmic domain in terms of its distribution and cell number. Since symplasmic transport is delimited and bounded by plasmamembranes, it follows that it must be a more complex process in the higher plant, than is apoplasmic transport. Clearly, the limiting factor is the number of cells which remain interconnected via plasmodesmata. Interconnectivity of a group of cells suggests the use of the term domain to describe these connected cells. Clearly, it is entirely feasible that there could be many domains within the higher plant. In fact, there are times when it would be useful to suggest or postulate the presence of such domains, even if there was no evidence of their existence. Until recently it has been very difficult prove that domains existed in plants.

Many studies have been carried out in the past, in which the frequency of plasmodesmata along a loading pathway have been determined. Clearly, such studies have given us an indication of the potential connectivity between the cells -- (see Factfile #1, vegetative apical growth; )but, there is always the possibility that many of the plasmodesmata as seen at the electron microscope level and counted (frequency determination) may be non-functional. Additionally, there exists the potential for plasmodesmata to be functional only under certain circumstances, and therefore, we need to be aware of the potential of gated transport through selectively functional transport systems. So frequency, no matter how it is expressed, serves only as an indication of a potential pathway of cell-cell communication and transport. One of the most visual ways of expressing plasmodesmal frequency, is by use of a plasmodesmogram, which immediately conveys a visual impression of the frequency with which plasmodesmata occur along interfaces involved in the cell-cell transport/transfer of assimilates to the functional phloem. Plasmodesmatal frequency, as determined from the observation of statistically-reliable dataset, gives an indication of the relative state of symplasmic continuity and also of the potential symplasmic domains and discontinuities, as illustrated in the Figure below.

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 Fig. 1. Shows the relationship between two separate domains, in which that on the left is composed of cells which are linked by plasmodesmata, and is separated for the domain on the right, as there is no direct symplasmic connection. Transport between the left and right domains, must therefore be energetically dependent.

(Diagram based upon the concept of plasmodesmograms first used by AJE van Bel

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However, frequency remains a theoretical measurement, until substantiated by other experimental evidence. As the technology has become available (and often adapted from the medical sciences) it has become possible to inject substances into living cells, and to watch the progress of these substances from cell to cell. The techniques involved in microinjection are not that difficult, the equipment is expensive and may, if coupled to electrophysiological systems, become very expensive, and therefore self-limiting. One of the newer, exciting ways in which cell to cell transport may be monitored at the microscope level in real time, is to inject a substance via a microcapillary needle (usually a glass needle with a tip with a diameter of < 0.5m m) filled with a single or double chloride ion salt solution (to enable simultaneous electrical y measurement) and to serve as the channel through which the dye substance can be injected, either by applying a very small electrical current, or by using a very slight positive backpressure. Injection is carried out once the cell of interest has been impaled and the plasmamembrane has recovered and sealed itself to the capillary needle, one may proceed to inject. By using techniques such as microiontophoresis, it is possible to measure electrical y, and to monitor the movement of substances which do not have the ability to cross the plasmamembrane. Such membrane-impermeable substances (like Lucifer Yellow) must therefore, pass from cell to cell, via functional plasmodesmata. Plasmodesmatal frequencies need to be established, before relying on the microiontophoretic technique (i.e., know what you are up against first!).

 Another major recognizable symplasmic domain in vascular plants is represented by the phloem tissue. Here, one finds a very close relationship between the conduits of long-distance sugar transport (the sieve tube members which, when joined end to end, form sieve tubes), their associated parenchymatous elements, including companion cells, and any associated mechanical supportive tissues. Symplasmic transport conjures up the vision of molecules travelling from cell to cell, via plasmodesmata, along concentration gradients and moving from regions of high solute concentration, to regions of lower solute concentration. In essence, this is what it is, but, the system is not as simple as this in most higher vascular plants. Clearly, if translocation were to rely solely on the relative difference (D) between the point at which insertion (loading) is carried out and the point at which offloading or unloading takes place, then we would find that in most plants, symplasmic continuity between the loading, the transport and the unloading phloem is not only undesirable, but is also impractical possibly, under normal conditions, as this would impede  rapid uptake and high rates of transport.

 There are several models that attempt to explain phloem loading and unloading for which there is adequate experimental proof of existence. Whilst there are many plants in which phloem loading is an entirely symplasmic process, and there are many plants in which phloem loading is an apoplasmic process. Equally, there are many in which it can be mixed symplasmic-apoplasmic. It follows that the unloading process must mirror these -- it could be symplasmic; mixed symplasmic-apoplasmic, or apoplasmic. The possibility that a break, or discontinuity could exist somewhere along the loading or the unloading pathway, in which the process could in broad terms, be, described as apoplasmic, is a real one, and one which has some merit, even though there is of necessity expenditure of energy (ATP) in order to facilitate loading or unloading.

Close examination of the plasmodesmogram above, suggest that there are very few plasmodesmal connections between the vascular parenchyma (VP) and the companion cell/sieve element complex (CC-SE). Sieve tubes are shown with open circles. Clearly, there appears to be a high degree of symplasmic isolation between the CC-SE and the rest of the phloem loading pathway. Phloem loading then seems, on the basis of the plasmodesmogram, to be apoplasmic.

Examples of symplasmic and apoplasmic phloem loading and unloading pathways are illustrated in Fig. 3 below.

Fig. 2

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Fig.2. Diagrams, showing the potential symplasmic and apoplasmic phloem loading and unloading ((un)loading) pathways in higher plants. Note that the term loading or unloading ((Un)loading can apply to the pathway along the mesophyll to sieve tube route, OR to the short-distance between companion cells and vascular parenchyma cells. In A, total symplasmic continuity exist, whilst in B- D, there are varying degrees of symplasmic isolation. Click here to go back to the index. Adapted from: Van Bel, A. J. E. Gamalei, Y. V. (1992).

Intercellular symplasmic trafficking retrieval of solutes and phloem loading

Using fluorochromes such as 5,6-carboxyfluorescine (5,6-CF) may help unlock some of the mystery associated with phloem retrieval and loading systems as well as the pathways that these processes follow in the monocotyledons. In its diacetate form, 5,6-CF is membrane-permeant and will move with ease through cells (symplasmically) can cross membranes and may be delivered (apoplasmically) for example via the xylem. We have been involved in some fundamental research related to the movement and localization of 5,6-CF in a number of grass species, for which plasmodesmal frequency data are well-documented.

Fig 3 above, illustrates results from an experiment in which 5,6-carboxyfluorescine-diacetate was introduced via the cut end of a leaf into the transpiration stream. After several hours this was followed by the apoplasmic tracer, Texas Red. The leaf was removed from the Texas red solution and was sectioned to localize the distribution of both fluorochrome species. The composite image shows patches of 5,6-CF (green fluorescence) in the mesophyll of this young maize leaf, and red-orange areas localize the distribution of Texas Red. More interestingly, the areas of bright yellow fluorescence adjacent to the large metaxylem vessels and below the xylem, in the phloem, show co-localization of apoplasmically-transported Texas Red, in conjunction with symplasmically offloaded 5,6CF which is in the vascular parenchyma!

5,6-CFDA was unloaded from the xylem vessels, into the xylem parenchyma, where is was metabolized 5,6-CF was then transported symplasmically via plasmodesmata, towards the phloem. The early experiments by Fritz  et al (1983) which dealt with the retrieval of solutes via the xylem, is confirmed using fluorochromes for  Zea mays.


There has been logical (and required?) evolution of the symplasmic system, in order to first facilitate and second, to ensure that efficient compartmentation of the phloem system occurs. This  in turn must have resulted in improved efficiency of the loading, transport and unloading systems within the plant as a whole. In terms of the physiology of phloem loading and unloading, there are apparent advantages to a system in which there is at least one cytoplasmic discontinuity, or domain change exists.  Simply, this will allow for an effective ‘pumping’ procedure at some point, during which the Δψp between the one side and the other, may be maintained, even though this may require the expenditure of energy, (usually in the form of ATP-mediated and driven proton co-transport symporters). Fig 3 provided exciting support for the intimate relationship between xylem and phloem, and the co-existence requirement required to facilitate

Must read references:

Botha CEJ and AJE van Bel (1992) Quantification of symplastic continuity as visualised by plasmodesmograms: diagnostic value for phloem-loading pathways Planta 187 359-366.

Fritz E, Evert RF, Heyser W . (1983) Microautoradiographic studies of phloem loading in the leaf of Zea mays L. Planta 159 193-206.

Van Bel, AJE Gamalei, YV (1992) Ecophysiology of phloem loading in source leaves. Plant Cell and Environ., 15-265