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# 8: The Ecophysiology of Phloem Loading

Revised August 07, 2006

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apoplasmic loading

apoplasmic phloem loading

proton motive force

symplasmic transport

symplasmic loading

 Loading type definition

symplasmic route

vein typology

 Selective pressure

Figs. 1 & 2

 

 

 

 

 

 

 

 

 

The widespread assumption has been that there there is a universal phloem loading mechanism which operated in all higher plants, has been a popular way of explaining phloem loading and phloem transport in general terms. In addition advocates suggested that an apoplasmic step would be required and that this apoplasmic 'discontinuity' would have to be quite close to the cc-se complex, in order to satisfactorily explain the accumulation of sugars in the sieve element. An additional argument was that the loading procedure was in some way connected (and correlated to) the relatively large proton motive force (Δm H+) which was known to exist at the cc-se complex, and this, it was thought was the driving force for sucrose loading. Interestingly proton motive force still attracts attention in experiments, as it is something really tangible that can be measured and demonstrated using glass microcapillary electrodes. Simply, the membranes will respond to the addition of sugars and to respiratory inhibitors. This is taken as direct evidence for the existence of proton co-transport mechanisms at the level of the plasmamembrane. Whilst this fairly rigid and somewhat thermodynamic approach to the concept of phloem loading has never been disproved (on the contrary -- the approach is really still alive and well!) it did in some way, stultify any innovative, new thinking and research concerning phloem loading, as most researchers felt that phloem loading or the 'problems' associated with phloem loading had been dealt with quite effectively.

There are some misconceptions however concerning the concept and term 'phloem loading' which need to be addressed and this is what is attempted in this particular factfile. Before determining loading type, it is important to understand that plasmodesmal frequency gives an indication of the potential loading pathway.

Plasmodesmal frequency definition: The number of plasmodesma, along or associated with a particular cell wall or cell wall interface, and expressed as plasmodesma per mm cell wall interface, or plasmodesma per mm wall contact area.  

Loading has been demonstrated to occur in the leaf. It has been shown to be associated with the minor veins of all species examined thus far. It is accordingly assumed that the pathway involved in the overall process ('the phloem loading pathway') is akin to a highway from mesophyll to the functional sieve tubes of the minor veins. If this simple view is acceptable, then there are a number of premises that must be tested.

  • The degree of intercellular connectivity.

  • The degree of (assumed) plasmodesmatal functionality.

  • The relative meaning/importance of frequency (in terms of number and assumed mass-transport capacity) must be determined.

The major assumption in this approach is indicated by the frequency or number of plasmodesmata that occur along this highway -- the premise is that the greater the number of plasmodesmata at an interface, then the greater the likelihood that transport may well follow a symplasmic route between the cells of that interface, and the converse, i.e., that the lower the frequency the greater the probability of apoplasmic phloem loading involvement, is also taken to be converse truism.

Many plasmodesmata have been shown to be structurally fairly simple, and many have been shown to posses a very complex substructure. So, the bottom line really remains, is there room for the molecules in question, to pass through the plasmodesmata?

In many species, there are specific ultrastructural characteristics which may be correlated to metabolic properties such as C3, C4 or CAM photosynthesis. The differences in cellular relationships within minor veins, may likewise suggest potentially different loading procedures. As an example, consider the the suberin lamella, which occurs in the interface between the bundle sheath and Kranz mesophyll cells, or between the mestome sheath and bundle sheath cells in many C4 and C3 grasses and sedges. This  is a good example of an ultrastructural characteristic which has a fairly conclusively-demonstrated function, in that the suberin lamella has been shown to act as an impermeable barrier to water, and, thus by inference, to solutes as well. This means that a  symplasmic route is forced at the interface which contains the suberin lamella.

In a series of papers in which 'plasmodesmatal frequencies' are published, van Bel and Gamalei have suggested that there are potentially four different phloem loading types. The conceptualization has led to various phrases such as 'vein loading typology'; 'ecophysiology of phloem loading'; 'phloem loading machinery' to name but three. Clearly, there is merit in examining the ultrastructure of the minor veins, and relating this to say, photosynthetic rate and in addition to determining the plasmodesmatal frequency along the phloem loading pathway. On this, many authors agree, but, it is in the derivation of frequency data within which a great deal of disagreement exists.

Clearly, there is need for some degree of uniformity -- we have to ensure that what is measured is in fact, representative of the plant species. All too often, 'frequency' is equated to the number of plasmodesmata that are present in one or two sections -- clearly; this is a dangerous

Correlation of Minor Vein Anatomy ('Typology') to Phloem Loading.

The challenge is to link structure with the physiological processes that govern and determine the rate of phloem loading from source leaves. There are a number of problems associated with the determination of phloem loading physiology however, chiefly governed by the relatively small size of the cells concerned, in addition to the difficulty experienced in their accurate identification. As mentioned an abundance of plasmodesmata is taken as an indication of a continuous symplasmic domain, whereas a paucity of plasmodesmata, is taken as indicating that there is a break in the continuous domain, and that it in fact, is divided into two separate domains. Some authors insist that the second domain, occurs at the companion cell-sieve element interface, yet there is strong evidence that this is in fact not so, that it can occur at the interface between the vascular parenchyma, and the cc-se interface.

Without too much difficulty then, at least two types of loaders can be predicted, based entirely on the plasmodesmatal frequency.

Symplasmic phloem loaders (one-domain structure) - Tree families and some shrubs - includes many tropical species.

Apoplasmic phloem loaders (two-domain structure).  This type is apparently dominated by herbaceous plants. Families include Asteraceae, Brassicaceae, Chenopodiaceae, Amaranthaceae, Asclepiadaceae, Cyperaceae, Euphorbiaceae, Fabaceae, Polygonaceae, Portulacaceae, Poaceae, Liliaceae and Solanaceae. These species usually have a symplasmic disjunction between VP and the CC-SE complex.

 

 

There have been a number of publications which have elaborated this concept, and in which the authors (van Bel and Gamalei) recognize four 'subgroups' or subdivisions amongst the two major groupings listed above. The speculation, based upon examination of Takhtajan's classification of the origins of flowering plants, suggests an ancient origin for the symplasmic loaders, and a more advanced and recent origin for the apoplasmic loaders.

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The various types of phloem loading processes that have been identified to date, are summarized in the table below.

 

Definitions of phloem loading types

TYPES

PLASMODESMALAL ABUNDANCE

COMPANION CELL ULTRASTRUCTURE

LOADING METHOD

1

Many plasmodesma at all cell interfaces between Mesophyll and cc-se complex

Intermediary cells

symplasmic

2a

Few plasmodesma

Companion cells may have extensive vesicular substructure

apoplasmic

2b

cc-se is apparently apoplasmically  isolated

Companion cells modified with wall ingrowths (transfer cells)

apoplasmic

2c

High frequencies of plasmodesmata to bundle sheath, few internal of this and the cc-se may be isolated

Companion cells

apoplasmic

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The selective pressures for evolution of minor vein configuration.

There has been a great deal of speculation as to what has driven the evolution of the two basic phloem loading strategies -- amongst those most often touted, are water stress (drought stress) and low temperature. Temperatures below 10oC may induce the selective storage of photoassimilates within the mesophyll, in symplasmic loaders, and may well be absent in apoplasmic loaders. Clearly, if this is universal, this would confirm the ability of apoplasmic phloem loaders to assimilate carbon skeletons, as well as to transport them under low temperature conditions. Clearly, as more evidence emerges for the distinction of two basic loading types, so the coincidence of minor vein typology and mode of phloem loading will become more clearly understood. There is evidence for example, that typically, tropical plants probably load mainly raffinose-related sugars via the symplasmic pathway, and again, typically, apoplasmic loaders tend to load sucrose via the apoplasmic route.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

From the above diagram, it should possible to position a large number of species with respect to the mode of phloem loading, along two major ecophysiological gradients – temperature and drought, which will impart particular stresses upon plants growing under these stresses individually, as well as in concert.

 

 

 

 

 

 

 

 

 

 

 

 

Figs. 1 and 2. Diagrams showing the relationships between environment and plasmodesmatal frequency (Fig. 1) and the suggested relationships between the types of phloem loading, as suggested by van Bel and Gamalei. Fig. 2. Demonstrates the origins and vein loading typologies amongst higher plants. Diagrams redrawn from van Bel and Gamalei (1992) Plant Cell and Environment 15: 265-270

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Concluding Remarks.

It is clear that there are potentially two options involved in phloem loading, namely apoplasmic and symplasmic loading, and that cannot be disputed. What is not clear, is how many variations exist within these two basic symplasmic and apoplasmic phloem loading systems - How do we define the 'process' of phloem loading, in relation to plasmodesmatal frequency, or the 'process' of phloem loading in relation to minor vein typology? What is important is that we need to take careful stock of what has been published, in relation to what is known about the ultrastructure of minor veins, in the families that have already been classified.

Turgeon, Medville, and Nixon (2001) recently reported on the evolution of minor vein phloem and phloem loading. They reported that phylogenetic analysis provides a rational basis for comparative studies of phloem structure and phloem loading. Turgeon at al state that although several types of minor vein companion cell have been identified, and progress has been made in correlating structural features of these cells with loading mechanisms, little is known about the phylogenetic relationships of the different types. Turgeon and his co-workers, have added significantly to the available data on companion cells, in their data analysis of the ultrastructure of minor veins in Euonymus fortunei and Celastrus orbiculatis (Celastraceae) leaves. They have determined that in these species, the companion cells are specialized as intermediary cells. This cell type is implicated in symplasmic phloem loading, via a polymer trap process. 

Their data were added to published data sets on minor vein phloem characteristics, and  mapped to a well-supported molecular tree. The analysis indicates that extensive plasmodesmatal continuity between minor vein phloem and surrounding cells is ancestral in the angiosperms. Reduction in plasmodesmatal frequency at this interface is apparently a general evolutionary trend, punctuated by instances of the reverse. This is especially true in the case of intermediary cells that have many plasmodesmata, but other distinguishing characteristics as well, and have arisen independently at least four, and probably six, times in derived lineages. Thus the character of highly reduced plasmodesmatal frequency in minor vein phloem, common in crop plants, has several points of origin in the tree

Turgeon et al make a point that caution should be exercised in generalizing results relating to apoplasmic phloem loading obtained from model species. Their data supports the suggestion that transfer cells have many independent points of origin, and intriguingly, not always from lineages with reduced plasmodesmatal frequency.

The literature remains full of controversial issues related to phloem loading - indeed, that is what makes it so interesting! There are suggestions in the literature that 'symplasmic' and 'apoplasmic' phloem loaders may show different membrane depolarization effects, dependent upon sucrose-raffinose gradients (van Bel et al 1996), and that the ER may undergo conformational changes, dependent upon temperature (Gamalei et al 1994).

 

 
Finally, a word about grasses. These do not really ‘fit’ very well with the Gamalei model as it stands now. In broad terms, one could apply ‘Type 2c’ to their loading system  -- “High frequencies of plasmodesmata to bundle sheath, few internal of this and the cc-se appear (based on pd frequency studies) to be isolated”. This does not take into consideration the variations seen in ultrastructure, typified here in this electron micrograph of Hordeum vulgare which has a bundle sheath, surrounding a mestome sheath which may be in direct contact with either vascular parenchyma cells or companion cells. 

(See Factfile # 5:The Source-Sink Connection, for further detail relating to phloem loading and unloading).

Some useful References

Geiger, D. R. Malone, J. Cataldo, D. A. (1971) Structural evidence for a theory of vein loading of translocation. Amer. J. Bot. 58, 672-675
Turgeon, R. (1984) Termination of nutrient import and development of vein loading capacity in albino tobacco leaves. Plant Physiol. 76, 45-48
Turgeon, R. (1984) Termination of nutrient import and development of vein loading in albino tobacco leaves. Plant Physiol. 76, 45-48
Madore, M. Webb, J. A. (1982) Leaf free space analysis and vein loading in Cucurbita pepo. Can. J. Bot. 59, 2550-2557
Malek, F. Baker, D. A. (1977) Proton co-transport of sugars in vein loading. Plant Physiol. 135, 297-299
van Oene, M. A. Terlou, M. Wolfswinkel, P. (1992) Effect of reduction of sink strength in developing seeds on vein loading patterns of photoassimilate in source leaves of Pisum sativum L.. J. Exp. Botany 43, 695-702
Van Bel, A. J. E. Gamalei, Y. V. (1990) Multiprogrammed phloem loading. , 1-13 (Click here1 to return to your place).
Gamalei, Y. (1989) Structure and function of leaf minor veins in trees and herbs. A taxonomic review. Trees 3, 96-110
Gamalei, Y. (1991) Phloem Loading and Its Development Related to Plant Evolution from Trees to Herbs. Trees-Structure and Function Trees 5, 50-64

Van Bel, A. J. E. Gamalei, Y. V. (1992) Ecophysiology of phloem loading in source leaves. Plant Cell and Environ., 15-265
Gamalei, Y. V. Van Bel, A. J. E. Pakhomova, M. V. Sjutkina, A. V. (1994) Effects of temperature on the conformation of the endoplasmic reticulum and on starch accumulation in leaves with the symplasmic minor-vein configuration. Planta 194, 443-453
Turgeon Robert, Medville Richard, Nixon Kevin C. (2001) The evolution of minor vein phloem and phloem loading Amer. J. Bot. 88 1331-1339.
Van Bel, A. J. E. Hendriks, J. H. M. Boon, E. J. M. C. Gamalei, Y. V. Van de Merwe, A. P. (1996) Different ratios of sucrose/raffinose-induced membrane depolarizations in the mesophyll of species with symplasmic (Catharanthus roseus, Ocimum basilicum) or apoplasmic (Impatiens walleriana, Vicia faba) minor-vein configurations. Planta 199, 185-192

 

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