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The Phloem
Introduction:
The plant vascular system plays a pivotal role in the delivery of nutrients to distantly
located organs. Recent discoveries have provided new insight into a novel role for
plasmodesmata and the phloem in terms of the transport and delivery of information
macromolecules including proteins and ribonucleoprotein complexes. It has been
suggested that the phloem may function as a conduit for inter-organ communication. A
supracellular highway complex, like an Interstate system, along which a great deal of
assimilate is transported through the axial system, to local or distant sinks. Fig 1
illustrates the difference in size between loading, transport and unloading phloem in
Nymphoides. Within the transport phloem, sieve tubes are between 10 and 25 µm in
diameter, and their associated companion cells between 15 and 38 µm in diameter. The
sieve tubes in transport phloem are larger in diameter than corresponding sieve tubes in
loading phloem in this species.
Fig. 1 Micrographs showing the size change relationships between the companion cell and the sieve tube
cell in loading, transport and unloading phloem, in Nymphoides. Left; minor vein in leaf lamina, Middle;
phloem in central vascular bundle in the submerged petiole (STEM), Right; phloem strand from a root.
Scale bars: = 10 µm.
In Dicotyledons:
Phloem tissue in dicotyledons is composed of assimilate transport cells, called sieve tube
members, and their associated companion cells. Companion cells are described as being
parenchymatic, but, the origin (ontogeny) is from the same mother cell that divides to
form the sieve tube member. The companion cells in angiosperms are important, as they
serve as ‘traffic control centres’ and regulate movement of all substances that either enter
or leave the companion cell coming from, or going to, the sieve tube members. Given that
mature sieve tube members are enucleate, (that is, they lack a functional nucleus at
maturity) all the small proteins that are required on an on-going basis by the sieve tube
members, must be synthesised in the sieve tube member under the control of the nucleus
in the companion cell. This is a tricky business, and one could liken the sieve tubecompanion cell complex to a comatose patient and an hyperactive nurse. The companion
cell provides all essential control proteins, and takes care of the complex biochemistry
that is involved in the loading transport and unloading process that occur in the sieve tube
members. It also takes care of membrane maintenance in its associated sieve tube
members.
In dicots and monocots, the sieve tube members are joined end to end by their common
cross walls. In this way, the cells form long sieve tubes, through which carbohydrates, as
well as a host of other materials including viruses, can move. These cross walls are
perforate, and allow rapid transport of materials through the sieve tubes.
In Monocotyledons:
Monocotyledonous phloem is very similar to that in the dicotyledonous plants, in that it is
composed of sieve tube members, and associated companion cells. We assume that the
companion cell has a similar function in the monocotyledons, to that in the dicotyledons.
Like the dicots, monocot phloem also contains parenchymatic elements, as well as
fibres. The course of vascular systems in monocotyledon stems has been studied for
many years and is under active investigation at present. Modern microscopy techniques,
including fluorescence and confocal microscopy, and stack frame imaging of whole and
sectioned material have enabled researchers to understand for the first time the true
complexity of many stems, including palms and Pandanaceae. With newer and more
powerful techniques becoming available, a new area of comparative anatomy is emerging
the study of whole vascular systems. The results of this study might well show basic
types which underlie the major phylogenetic divisions in the plant kingdom.
Within the vascular bundles of monocots, the primary transport system (phloem and
xylem) is composed of an axial system only. Rays are a feature of secondary
development, and are associated with gymnospermous and dicotyledonous plants only.
In Gymnosperms:
The phloem in gymnosperms is considered to be less specialized, and the functional
phloem consists of well developed sieve cells, with sieve areas (no sieve plates, as in
dicots and monocots) and their associated albuminous cells. In angiosperms the
albuminous cells are replaced by companion cells. It is thought by those who study
phloem that an evolutionary sequence can be observed, from systems in which
companion cells are poorly defined and the sieve tube elements communicate by rather
scattered sieve areas on oblique walls to the most advanced in which sieve plates are very
well defined and constitute the transverse end wall between elements in a sieve tube, and
in which the companion cells are very well developed. Since the advanced sieve tube
member has no nucleus, the organization of the element is carried out by the nucleate
companion cell adjacent to it. Damage to the companion cell in this system may bring
about failure of the element which it directs.
How does phloem work?
This has been the subject of debate for a very long time, with many schools of thought
emerging over the past 50 years. For the moment, let us accept that the phloem is
involved in the movement of assimilated material (mostly photoassimilate) from leaves to
other parts of the plant that are in need of carbohydrate. The movement is described as
being from source to sink, and is therefore source-sink driven, as a result of
photosynthesis. So, the transport of the various food substances from a region of
manufacture (source) to a region of biochemical utilization (sink) is thus an essential
process which all plants must complete successfully if they are to increase there biomass
significantly. It is widely accepted that the sieve tube constitutes a highly specialized
phase of the symplastic transport pathway in plants.
Functional physiology of mature sieve elements and sieve tubes:
In order to be efficient carbohydrate transporters, phloem has evolved in a way that is
described as resulting in a highly differentiated, enucleate state. There are no vacuoles,
and the remaining cytoplasmic matrix of protein-containing plastids, mitochondria and
endoplasmic reticulum is ‘zip locked’ via connections across the outer leaflet of their
encompassing membranes, to the outer leaflet of the plasmamembrane. to the
plasmamembrane, to keep these components in a parietal location. This is an exciting
discovery, made some years ago, quite by chance! The principal features of their
structural and functional specialization are the loss of nuclei at maturity, their continuity,
loss of tonoplast, parietal cytoplasm and other plastids. The xylem on the other hand, may
be considered to be a specialized phase of the apoplast, allowing for the rapid, longdistance transport of water and dissolved inorganic and organic solutes, from roots to
shoots.. In its specialization, we witness the fact that the conduits have lost their
protoplasts at maturity, and that the end walls become highly specialized - in fact, most of
the wall is hydrolyzed, to allow for unimpeded flow.
It is important to remember that there is a close spatial relationship between the xylem
and phloem throughout the symplasm/apoplasm of the living plant. One should also bear
in mind the fact that the functioning of the phloem is very dependent on an adequate
supply of water. This water in turn, will affect the osmotic potential of the contents of
the sieve tubes, driving up the pressure, and forcing the accumulated solutes to move
away, carried by the solvent, and translocation occurs.
There is a widespread assumption that a universal phloem loading mechanism exists,
which operates in all higher plants. This belief remains popular, as it is easier to explain
phloem loading and phloem transport in general terms, rather than to get bogged down in
detail. 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 build up a partial pressure, commence movement and follow a diffusive
pathway from a region of high concentration (the source) to a region of lower
concentration, (the sink) either very 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.
The need to sustain growth through carbohydrate supply
If sustained growth is to be satisfied and maintained, then an efficient system is required.
Such a system (the phloem) has evolved with time throughout the plant kingdom, and the
pinnacle of these evolutionary steps lies within the higher vascular plants.
When we study the phloem, it is very important to realize that there is a great difference
in the physiological requirements for efficient function at the source, compared with the
sink. At the source, assimilates are loaded into the system, and at the sink, they are
unloaded from the system. The bit in the middle is purely involved in transport, with an
unloading component and a retrieval component ensuring efficient and adequate supply
of nutrient to living stem cells.
The process is composed of three parts; phloem loading; phloem transport; and phloem
unloading. Refer back to the micrographs, showing the different diameters of the cells
(sieve tube and companion cells) involved in the overall process. Each of these phases of
phloem function, has a mechanism that kept the system operational, but the details of
these are not important at this time. Enough said that the loading process can essentially
follow either a passive pathway, or could involve an active (accumulating) step.
In the first instance, there may be no energy or thermodynamic demands placed upon the
system. In the second, ATP & NADPH would be needed directly to drive co-transport
across membranes.
The transport process can be viewed as an entirely passive process, that makes no
demands upon the energy cycles of the plant other than energy required for the
maintenance of plant membranes. If transport is passive then one could envisage an
entirely bulk flow system, driven by concentration gradients established and
maintained between the source and the sink. Transport would thus be along or down a
concentration gradient. If transport is passive, then metabolic inhibitors would and
should have no effect upon the process.
Alternatively one could argue that phloem transport is an active process, and one
requiring energy (physiological or thermodynamic) in order to drive and maintain it. Here
one would envisage ATP NADPH or H+ K+ ion exchange as the driving force.
Metabolic inhibitors would severely impede transport, and a velocity decrease would be
measurable.
In any event, there is little argument that some energy has to be expended upon the wayelse a “leaky” system would develop, in which solute loss leads to Ψp and hence, turgorrelated changes.
The unloading process is essentially the opposite of the loading process, as conversion
of the soluble carbohydrate into a less osmotically-active form will be necessary in order
to set up the driving force of the unloading process.
References
‘The Virtual Plant’ Factfiles on phloem loading and phloem transport mechanisms. Please follow the
hyperlink:http://anubis.tu.ac.za/virtualplant.htm Follow the links to the ‘Factfiles’. Note: This information will only
be accessible until the end of June, after which, its publication and dissemination is controlled by
Blackwell’s Scientific, who have contracted for the publication rights of The Virtual Plant.
Esau, K. Plant Anatomy Wiley Call Number 581.4 ESA.
Esau, K Anatomy of Seed Plants Call Number 581.4 ESA
Both these texts describe and illustrate the structure of phloem tissue, in simple terms. Both are well
illustrated with beautiful drawings, light and electron micrographs.
Transport of photoassimilates / editors, D.A. Baker and J.A. Milburn.
Publish London : Longman, 1989. Call Number 581.11 TRA
Although old, this book is delightfully readable, and contains a wealth of solid information on this complex
subject.