Phloem Transport in Plants
Hypothesis:
The development of a highly specialised transport
system was essential in order to enable plant species
to develop, diversify and occupy the many different
niches that they do.
In Roots….
Phloem transport requires the
establishment of functional
SIEVE TUBES, which must connect
the SOURCE to the (local) SINK
In this onion root, the first formed
PROTOPHLOEM sieve tubes mature
quite close to the ROOT APEX. So,
within about 1000mm of the tip, carbon
skeletons are delivered to the rapidly
dividing and expanding cells within this
root.
In Stems...
Development of the vascular system
requires formation of VASCULAR
BUNDLES. Here in Cucurbita pepo, you
see SIEVE TUBES, with large SIEVE
PLATES and underlying these, is the PProtein (phloem proteins) associated with
many functional sieve tube members.
In Leaves…..
Vascular bundles develop
from PROCAMBIUM
Leaves may develop
specialised photosynthetic
layers such as the
PALISADE,
and SPONGY mesophyll
depicted here, as well as
PARAVEINAL MESOPHYLL is
highly specialised in
assimilation and solute
retrieval
Phloem Transport Mechanisms
What do we know?
Solutes move from source to sink
That sinks may be local or distant
That sink strength is a contributes to controlling
or is perhaps, the controlling factor in regulation
of transport capacity
•That the system could be symplastic,
apoplastic or mixed mode
Imperative to distinguish between
1. Phloem loading mechanisms
2. Phloem transport mechanisms
3. Phloem unloading mechanisms
The Loading Process:
Essentially, can follow 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 instance, ATP, NADPH+ would
be needed directly to drive co-transport
across membranes
Uphill….
The Transport process:
Phloem transport can be viewed as an
entirely passive process, which makes no
demands upon the energy cycles of the
plant, other than energy required for the
maintenance of plant membranes
The Transport process..
If transport is does not require energy input, then one
could envisage an entirely bulk (passive) 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
An Active Transport process..
The alternative, is a mechanism of phloem
transport which is an active process
This requires 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
NB. Metabolic inhibitors WOULD have an
effect upon the process
A Passive Transport process..
Metabolic inhibitors would/should not
have an effect upon the process
Conundrum!
But, there can be little argument that some energy has to
be expended along the way- else a “leaky” system would
develop, in which solute loss leads to << Yp, and hence,
turgor-related changes
The makings of the channel:
Phloem sieve tubes: Highly specialised Function under pressure (why?) therefore need
control and regulatory mechanisms. Callose is
one controlling mechanism
Callose formation on sieve plates
in the phloem of Saccharum
officinarum
This is fun!
Developmental Sequence
Complex interrelationship
during the early stages.
A MOTHER cell
differentiates,
to give rise to a sieve tube
member, and a companion
cell
Structural considerations of the
mature phloem
Long files of cells
are formed, joined
by their cross
walls. Cells
designed for rapid
longitudinal
transport.
Structural considerations
of the mature phloem
Sieve tubes are highly
specialised cells essentially devoid of
protoplasm at maturity
(everything is parietallylocated) - the end walls of
the cells are highly
modified, and contain a
number of sieve plate
pores, through which
substances travel from
cell to cell.
Phloem Functionality
Sieve tubes are composed
of files of sieve tube
members, joined end to
end via their cross walls
Sieve tube
Companion
cells
These cross walls are highly
specialised and form sieve
plates, each of which
contains many sieve plate
pores
Vascular Tissue in Roots
E
P
ST
X
In the Root, sieve tubes are larger in
diameter than their corresponding
companion cells. This is typified, in this
cross-section of a young Rannunculus
root. This section typifies UNLOADING
PHLOEM
Sieve
tube
members
Fibers
P-Protein
From: Raven, Evert and Eichhorn.
In Stems, the relationship of the sieve tube
members to their companion cells is clearer.
Here CC’s are narrower than their
corresponding STM as in this example of Tilia
americana
Note the inclined, compound sieve plates,
(stained blue) and large number of lateral sieve
area pores in the sieve tube member to the right
This view, typifies TRANSPORT PHLOEM,
where there may be many connections between
the companion cells and sieve tube members
Remember..
There is a requirement
for transport between
all organs within the
plant.
Here we see the
similarity between the
transport pathway in
salt glands, the leaf,
and the root
Phloem-related Transport
Phloem Loading…
Can follow an entirely
symplastic pathway
or have a specific apoplastic
disjunction
Plasmodesmata in short-distance
transport.
In actively-loading and unloading
systems, sugars are loaded at a
SOURCE, then transferred to the
loading phloem, then moved into the
long-distance transport phloem, and
are released at metabolically active
SINKS.
Local Sinks can occur along the way.
The potato tuber (Solanum tuberosum L.) acts as a
SOURCE and a SINK, depending on requirements
Mechanisms ?
Simple or Complex?
Clearly can be placed in one of two categories,
1. Those where OSMOTIC POTENTIAL is the
driving force
2. Those where ENERGY TRANSFORMATIONS
are necessary
Distinguish between loading, transport and
unloading parenchyma and the sieve elementcompanion cell complex?
Ultrastructural Investigations
Barley is one of the most studied crop plants, world-wide. Yet, it is only recently that
we have gained clear knowledge of cell structure, and the plasmodesmatal
frequencies, along the loading pathway from mesophyll to sieve tube. We now
recognise thick-walled (solid dots) and thin-walled (open circles) metaphloem.
Plasmodesmatal frequencies tell us a great deal about the cell-cell
pathway. Clearly, low frequencies at CC-ST interfaces, indicate that phloem loading
is apoplasmic.
Phloem transport - visualization
Phloem transport – Organization & mechanics
Tying things down…
From Ehlers, et al., 2000 Protoplasma 214: 80-92. Used with author’s permission
Variations in structure
young tissues
show this
‘good fixation shows this
not seen in any EM micrographs
Poor fixation
shows this
Phloem transport mechanisms
There are several,
some require energy inputs,
others do not.
Integrated Transport..
Xylem and phloem dependency.
Electro osmosis
Note ion gradient is necessary,
else system will not function
Trancellular strands
Strands “peristaltic” squeeze
substances along tubules
Facilitation of bi-directional transport
Sucrose, co-transport
So, what works?
It is simple… Pressure flow, regulated by a
difference in osmotic potential, along the
transport gradient. This will work, PROVIDED
accumulation does not attain equilibrium along
the gradient.
Well ain’t that
something!!
Finis