Translocation in the Phloem PDF

Translocation in the Phloem  The phloem is the tissue that translocates the products of photosynthesis from mature leaves to areas of growth and storage, including the roots. the phloem also redistributes water and various compounds throughout the plant body.  The phloem is generally found o...

Translocation in the Phloem  The phloem is the tissue that translocates the products of photosynthesis from mature leaves to areas of growth and storage, including the roots. the phloem also redistributes water and various compounds throughout the plant body.  The phloem is generally found on the outer side of vascular tissues.   The cells of the phloem that conduct sugars and other organic materials throughout the plant are called sieve elements  In addition to sieve elements, the phloem tissue contains companion cells and parenchyma cells (which store and release food molecules). In some cases the phloem tissue also includes fibers and sclereids (for protection and strengthening of the tissue).  Mature sieve elements are unique among living plant cells.  Sieve elements lack many structures normally found in living cells, For example, lose their nuclei, tonoplasts (vacuolar membrane), Microfilaments, microtubules, Golgi bodies, and ribosomes during development.  In addition to the plasma membrane, organelles that are retained include somewhat modified mitochondria, plastids, and smooth endoplasmic reticulum.  The walls of sieve elements are nonlignified, though they are secondarily thickened in some cases.  Sieve tube elements joined together to form a sieve tube. The pores in the sieve plates between the sieve tube elements are open channels for transport through the sieve tube. The plasma membrane of a sieve tube element is continuous with that of its neighboring sieve tube element.  Each sieve tube element is associated with one or more companion cells, which take over some of the essential metabolic functions that are reduced or lost during differentiation of the sieve tube elements and supply energy as ATP to the sieve elements.  The companion cell has many cytoplasmic organelles, whereas the sieve tube element has relatively few organelles.  Tracheary elements of the xylem are dead at maturity, lack a plasma membrane, and have lignified secondary walls, while sieve elements of phloem are living cells, retained with plasma membrane.  Patterns Of Translocation: From Source To Sink  Sap in the phloem is not translocated exclusively in either an upward or a downward direction, and translocation in the phloem is not defined with respect to gravity.  Sap is translocated from areas of supply, called sources, to areas of metabolism or storage, called sinks.  Sources include any exporting organs, typically mature leaves, that are capable of producing photosynthate in excess of their own needs.  Sinks include any nonphotosynthetic organs of the plant and organs that do not produce enough photosynthetic products to support their own growth or storage needs. Roots, tubers, developing fruits, and immature leaves, which must import carbohydrate for normal development, are all examples of sink tissues. THE MECHANISM OF TRANSLOCATION IN THE PHLOEM: THE PRESSURE-FLOW MODEL  In early research on phloem translocation, both active and passive mechanisms were considered.  In sources, energy is necessary to move photosynthate from producing cells into the sieve elements. This movement of photosynthate is called phloem loading.  In sinks, energy is essential for some aspects of movement from sieve elements to sink cells, which store or metabolize the sugar. This movement of photosynthate from sieve elements to sink cells is called phloem unloading.  In source tissues, energy-driven phloem loading leads to an accumulation of sugars in the sieve elements, generating a low (negative) solute potential (ΔYs) and causing a steep drop in the water potential (ΔYw). In response to the water potential gradient, water enters the sieve elements and causes the turgor pressure (Yp) to increase.  At the receiving end of the translocation pathway, phloem unloading leads to a lower sugar concentration in the sieve elements, generating a higher (more positive) solute potential in the sieve elements of sink tissues. As the water potential of the phloem rises above that of the xylem, water tends to leave the phloem in response to the water potential gradient, causing a decrease in turgor pressure in the sieve elements of the sink.  If no cross-walls were present in the translocation pathway— that is, if the entire pathway were a single membrane-enclosed compartment—the different pressures at the source and sink would rapidly equilibrate. The presence of sieve plates greatly increases the resistance along the pathway and results in the generation and maintenance of a substantial pressure gradient in the sieve elements between source and sink. The sieve element contents are physically pushed along the translocation pathway as a bulk flow, much like water flowing through a garden hose.  Close inspection of the water potential values shown in the previous Figure shows that water in the phloem is moving against a water potential gradient from source to sink. Such water movement does not violate the laws of thermodynamics because the water is moving by bulk flow rather than by osmosis. Solutes are moving at the same rate as the water molecules.  Under these conditions, the solute potential, Ys, cannot contribute to the driving force for water movement, although it still influences the water potential. Water movement in the translocation pathway is therefore driven by the pressure gradient rather than by the water potential gradient.  The passive, pressure-driven, long-distance translocation in the sieve tubes ultimately depends on the active, short-distance transport mechanisms involved in phloem loading and unloading. These active mechanisms are responsible for setting up the pressure gradient.  Materials translocated in the phloem: Sucrose, amino acids, hormones, and some inorganic ions.  Some important predictions emerge from the pressure-flow model: The sieve plate pores must be unobstructed. If any materials blocked the pores, the resistance to flow of the sieve element sap would be too great. True bidirectional transport (i.e., simultaneous transport in both directions) in a single sieve element cannot occur. Amass flow of solution precludes such bidirectional movement because a solution can flow in only one direction in a pipe at any one time. Solutes within the phloem can move bidirectionally, but in different sieve elements or at different times. Great expenditures of energy are not required in order to drive translocation in the tissues along the path, although energy is required to maintain the structure of the sieve elements and to reload any sugars lost to the apoplast by leakage. The pressure-flow hypothesis demands the presence of a positive pressure gradient. Turgor pressure must be higher in sieve elements of sources than in sieve elements of sinks, and the pressure difference must be large enough to overcome the resistance of the pathway and to maintain flow at the observed velocities. Movement of Photosynthate from Mesophyll Cells to the Sieve Elements  Sugars might move entirely through the symplast (cytoplasm) via the plasmodesmata, or they might enter the apoplast at some point en route to the phloem.  The sugars are actively loaded from the apoplast into the sieve elements and companion cells by an energy-driven, selective transporter located in the plasma membranes of these cells. In fact, the apoplastic and symplastic routes are used in different species.  the plasma membrane ATPase pumps protons out of the cell into the apoplast, establishing a high proton concentration there. The energy in this proton gradient is then used to drive the transport of sucrose into the symplast of the sieve element–companion cell complex through a sucrose–H+ symporter.