Phloem

Contents

Phloem transport

Leaves synthesise sugars, but most roots lack chlorophyll and cannot make their own sugars. Consequently, the leaves of most plants exist to produce sugars and other photosynthates, for their roots and other 'parasitic' tissues. Phloem is a transport system running alongside xylem that allows photosynthates to be redistributed in the plant.

A source organ has an excess of photosynthate; a sink organ has a deficit. Transport systems serve to speed up translocation from sources to sinks. Phloem transports photosynthates in both [gravitational] directions, depending on which organs are currently sinks, and which are sources. In deciduous trees, sugars are moved from storage in the roots and trunk to the buds during spring, and from the photosynthesising shoot and leaves to the roots in summer. If the phloem is removed from a deciduous tree in summer ('ring-barking'), photosynthates accumulate and swell the stem above.

Phloem transports photosynthate from sink to source in the same way as xylem transfers water from sources (soil) to sinks (leaves); and that arteries transfer oxygen from sources (lungs) to sinks (respiring cells); and that veins transfer sugars from sources (ileum) to sinks (liver). Xylem and phloem sometimes act like a circulatory system (water goes up; sugar comes down the trunk), but not always.

Most transport systems use water as a medium. The water is sometimes made to flow against its potential gradient, and this is done by active transport pumping solutes, after which the water follows passively by osmosis. There is no such thing as active transport of water, nor is there a 'water-ATPase'.

Phloem is the main food conducting tissue of the vascular plants. It consists of several cell types. The main two are:

Phloem cells.

Sieve elements are living cells and have thick, unlignified cellulose walls. Their cytoplasm is relatively empty, which allows free movement of photosynthate solutions. When mature, they have no nuclei and few organelles. They are connected by transverse sieve plates pierced by pores a few µm in diameter.

Companion cells sit alongside the sieve elements. They have a dense cytoplasm and retain their nuclei. They are connected to sieve elements by plasmodesmata, and act like life support units for the sieve elements. Companion cells and sieve elements work together as a single functional unit: longitudinal translocation occurs in the sieve elements. Companion cells keep them alive and load them with photosynthate for translocation.

In addition to these two basic cell types, phloem may contains various other cell types. Transfer cells are specialised companion cells with highly convoluted plasmalemmae. They serve to load photosynthate into sieve tubes. Intermediary cells are larger and plainer, but serve the same purpose in other species. Phloem may also contain parenchyma, and fibres.

Sugar enters the phloem, and water follows by osmosis, hugely increasing the internal pressure.

Sieve tubes contain a lot of sugar, consequently, they have a very negative osmotic potential (ψπ). Hence, water will flow into the sieve tubes by osmosis down its ψ gradient, generating a very high pressure potential (ψp = 2 to 3 MPa). This means that any injury is likely to lead to phloem 'haemorrhage'. Leakage in the event of injury is minimised by callose formation in the pores of the sieve plates. This takes just seconds after an injury. In some species, this is assisted by strands of P-protein loosely attached to the sides of the sieve tubes. The strands are dislodged and block the pores as the phloem sap surges towards the leak. P-protein acts much like fibrin, and callose much like platelets in the blood clotting response.

The rate of phloem transport can be measured in two ways.

  1. Linear velocity is measured by following the rate at which a radioactive tracer moves in the phloem. It varies from zero to about 1 m hr−1.
  2. Specific mass transfer is a measure of the amount of sugar transported. It is often measured by following the rate of increase in weight of a storage organ. Typical values are c. 5 g hr−1 cm−2 of sieve tube cross section.

In general, phloem transport rates are measured using radioactively labelled metabolites. Different radiolabels move at different rates. Tritium-labelled water is slowest. Sieve tubes act like chromatography columns with different substances adsorbed to different degrees and therefore moving at different rates. Water appears to move slowest because it leaks through the sieve tube membrane and equilibrates with the unlabelled water outside. The label moves (on average) more slowly than the bulk flow of water. Bulk flow rate was measured more accurately by Ziegler, who measured rate of movement of a pulse of heat in Heracleum phloem.

Water appears to travel slowest because individual molecules equilibrate with water outside the sieve tubes.

Phloem sap can be analysed to see what is being translocated. Samples are difficult to obtain, because cutting the phloem usually causes immediate blockage. Ricinus lacks a blocking mechanism and bleeds freely when cut. Aphid stylets may also be used for other species: aphids are allowed to feed on the plant, because they insert their stylets into sieve tubes without triggering blockage. The sap exudes from the stylets if the rest of the aphid is cut off.

The phloem sap is c. 30% dry matter, of which c. 80% is sugar.  Reducing sugars (glucose, fructose) are not found in phloem sap. Sucrose (Glc-Frc) and raffinose (Gal-Glc-Frc) are most common. Sorbitol (a polyol or sugar-alcohol) is transported in cherries and plums.

Sucrose.    Raffinose.   Sorbitol.
Sucrose, raffinose and sorbitol.

Amino acids and other nitrogenous species (e.g. allantoin, below), hormones, viruses and RNA also ride in the phloem. There is so much sugar compared to nitrogen, that aphids excrete most of the sugar as honeydew, to filter out nitrogen for protein synthesis and growth.

Allantoin.

Tracer experiments show that materials move simultaneously upwards and downwards in the phloem, even in the same region of stem. Can they move in both directions in the same sieve tube? Very likely not, but, sieve tubes contain SER, which could allow the conduction of materials in the opposite direction to the bulk-flow through the rest of the tube. However, there is very little evidence that this is the case.

Many mechanisms have been proposed for phloem transport. The simplest would be diffusion of sugars along their concentration gradients from source to sink. Actual transport rates are 1 m hr−1, which is vastly faster than diffusion can account for. There must be something cleverer going on…

The pressure flow hypothesis (Münch, 1930) is the most widely accepted theory of phloem transport. A high pressure is generated in the phloem of the source, and a low pressure in the phloem of the sink. This forces phloem sap along the sieve tubes in a bulk-flow (like a hosepipe). The flow of water is passive, i.e. water is not pumped, it merely follows solutes osmotically.

Phloem loading occurs when solutes are actively transported into the phloem in sources. This generates a low osmotic potential in the sieve tubes. Water flows in from xylem down its potential gradient, making the sieve cells very turgid. This generates a high pressure potential that pushes the phloem sap along.

Solutes are actively transported out of the phloem in sinks. This generates a higher osmotic potential in the sieve tubes. Water flows out of the phloem down its potential gradient, making the sieve cells flaccid. This generates a low pressure potential that sucks the phloem sap along.

If there was a continuous pathway from source to sink, pressure equilibrium would be achieved rapidly and flow would stop. The sieve plates increase the resistance to water flow, and allow a pressure gradient to be maintained.

The water in sieve tubes actually moves against the overall potential gradient. This is because the water is pushed along by bulk-flow. This is powered by the spontaneous or ATP-driven loading and unloading of solutes. The non-spontaneous flow of water in the phloem is coupled to the spontaneous flow of water up the xylem and between the two vascular elements.  No-one escapes the laws of thermodynamics.

Water potentials in xylem and phloem.

Predictions of pressure flow

Gymnosperm phloem elements are somewhat different. The pressure-flow hypothesis may not be valid for these plants.

Phloem loading

The pressure gradient for pressure-flow in angiosperms is generated by an osmotic gradient. This is generated by loading and unloading the phloem. Loading may be done in two ways: either apoplastically, or symplastically.

In Cucurbita, a symplastic loader, the phloem is connected to the leaf mesophyll cells by plasmodesmata. The high concentrations of sugars 'suck' water into the mesophyll by osmosis. This increases their turgor and forces the sugar solution through the plasmodesmata into the phloem. This is called symplastic loading, and involves intermediary cells. However, it isn't immediately obvious why sugars should accumulate in the intermediary cells, given that the phloem and intermediary cells contains far more sugar than the surrounding mesophyll. The model proposed to solve this problem is termed the polymer-trapping model. The important point to note is that the phloem and mesophyll use different sugars.

Symplastic loading.

Sucrose (and glucose) diffuses from the mesophyll into intermediary cells down its concentration gradient. The intermediary cells synthesise oligosaccharides such as raffinose and stachyose from sucrose. Raffinose (and other oligosaccharides) is too big to flow back into the mesophyll, so can only equilibrate with the raffinose in the sieve element.

Symplastic loading plants characteristically have intermediary cells with abundant plasmodesmata connecting mesophyll to companion cells/sieve tubes. The pressure generated in the phloem of these plants is relatively low, so they have wider sieve tubes to reduce the resistance to flow. They are generally trees and shrubs, and generally tropical or subtropical.

Other plants (e.g. Ricinus, the castor oil plant) have a different method of phloem loading, termed apoplastic loading. Sugar is released from the mesophyll cells into their cell walls (i.e. the apoplast). It is then actively pumped into the companion cells by symports in the companion cell membranes. The companion cells ('transfer cells') are the initial sites of absorption: if radioactive sugar is supplied to the mesophyll, it enters these cells and then moves into the sieve tubes via the plasmodesmata. Proton-symports drive sugar uptake into the phloem (secondary active transport, involving an H+-ATPase). Transfer cells are specialised companion cells with highly convoluted membranes and mitochondria.

Apoplastic loading.

Apoplastic loading plants characteristically have transfer cells with highly convoluted membranes. The pressures generated in the phloem of these plants are much higher than for symplastic loaders, so the sieve tubes are narrower than symplastic-loaders. They are generally herbaceous, and generally temperate or arid.

It is not clear how the sugars get from the mesophyll to the companion cells in plants with apoplastic loading. Pressure flow within the symplast, is more efficient than diffusion along cell walls. The location of the plasmodesmata in relation to the phloem strands in wheat leaves suggests that the symplast may be the favoured route.

The rate of phloem loading is controlled by the pressure in the sieve tubes: cutting the phloem (which releases the pressure) increases the rate of loading.  In nature, phloem pressure would act like a hydraulic control system and be a good way of instantaneously regulating phloem loading to compensate for the release of materials in the importing region.

Unloading of sugars goes down a concentration gradient. There is no obvious need for an active pumping mechanism; however, translocated materials are only released where they are needed, so this may be under hormonal control. The inhibitor PCMBS prevents sucrose transport across membranes. It does not inhibit phloem unloading in growing shoots and roots, which implies the unloading is primarily symplastic in these organs. However, apoplastic unloading occurs in developing seeds and storage organs.

Well, that about covers this whirlwind trip round plant physiology. I hope you have realised that the stuff that goes on inside plants is at least as interesting as what goes on inside animals. To bang my little botany drum for just a moment, animals are biochemically less than half as interesting as plants (since they generally don't photosynthesise or produce exciting secondary chemicals); and as far as I'm concerned, the polymer trapping model that generates the pressure in the phloem is far more elegant than that tiresome little pump that animals use to shunt their fluids about. Although the animal nervous system is fascinating in its complexity on the large scale, it doesn't do anything on the cellular level that isn't done by plants. It's not that I want to do down animals, but I get terribly annoyed by the 'plant are dull' brigade, who then proceed to force you to memorise the endless mediaeval Graeco-Latin vocabulary of human anatomy. I'd like so see anybody try to convince me that the loop of Henlé is more interesting than water potentials in xylem. They're both dull, and just because one is inside a fluffy bunny should have no bearing on how boring the topic is. The fact the fluffy bunny is being slowly poisoned by cyanogenic glycosides is much more interesting…

Test yourself

  1. Explain fully why phloem contains P-proteins and callose-synthesis enzymes?
  2. Why do sieve tubes need companion cells?
  3. How could you test these predictions of the pressure flow hypothesis?
    • Sieve plates unobstructed.
    • Bidirectional transport cannot occur.
    • ATP is not required.
    • Turgor in sources > turgor in sinks.
    • Phloem transport rates.
  4. What predications might you make from the polymer-trapping model of symplastic loading?

Answers

Bibliography

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