Chapter 10 Translocation In The Phloem PDF

Chapter 10 Translocation in the Phloem page 193-221 Dr. B Mdaka [email protected] 015 268 4935 1 P block, Office 1009 Learning Objectives Understand translocation process...

Chapter 10 Translocation in the Phloem page 193-221 Dr. B Mdaka [email protected] 015 268 4935 1 P block, Office 1009 Learning Objectives Understand translocation processes in the phloem considering the: Pathways and patterns of phloem translocation Characteristics of source and sink Materials translocated in the phloem. Mechanisms of phloem loading, unloading and translocation (The pressure flow model) of photo- assimilates and the driving forces for these processes. Allocation and partitioning of carbohydrates in plants 2 Materials, pathways and patterns of phloem translocation 1. Pathways in translocation The two long-distance transport pathways—the phloem and the xylem—extend throughout the plant body. The cells of the phloem that conduct sugars and other photo-assimilates throughout the plant are called sieve elements. Sieve element is a comprehensive term that includes both the highly differentiated sieve tube elements typical of the angiosperms and the relatively unspecialized sieve cells of gymnosperms. 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) and laticifers (latex-containing cells). The small veins of leaves and the primary vascular bundles of stems are often surrounded by a bundle sheath which consists of one or more layers of compactly arranged cells. 3  Materials, pathways and patterns of phloem translocation Mature sieve elements Mature sieve elements are unique among living plant cells. They lack many many structures normally found in living cells (lose their nuclei and tonoplasts during development). Microfilaments, microtubules, Golgi bodies, and ribosomes are also absent from the mature cells. In addition to the plasma membrane, organelles that are retained include somewhat modified mitochondria, plastids, and smooth endoplasmic reticulum. The walls are nonlignified, though they are secondarily thickened in some cases. Thus the sieve elements have a cellular structure different from that of tracheary elements of the xylem (which are dead at maturity), lack a plasma membrane, and have lignified secondary walls. 4  Materials, pathways and patterns of phloem translocation Sieve areas Sieve elements (sieve cells and sieve tube elements) have characteristic sieve areas in their cell walls, where pores interconnect the conducting cells. Sieve plates have larger pores than the other sieve areas in the cell and are generally found on the end walls of sieve tube elements, where the individual cells are joined together to form a longitudinal series called a sieve tube. Furthermore, the sieve plate pores of sieve tube elements are open channels that allow transport between cells. P-protein and Callose… The sieve tube elements of most angiosperms are rich in a phloem protein called P-protein It occurs in several different forms (tubular, fibrillar, granular, and crystalline) depending on the species and maturity of the cell. 5  Materials, pathways and patterns of phloem translocation …P-protein and Callose… In immature cells, P-protein is most evident as discrete bodies in the cytosol known as P-protein bodies. They may be spheroidal, spindle- shaped, or twisted and coiled. They generally disperse into tubular or fibrillar forms during cell maturation. P-protein appears to function in sealing off damaged sieve elements by plugging up the sieve plate pores. Sieve tubes are under very high internal turgor pressure, and the sieve elements in a sieve tube are connected through open sieve plate pores. A longer-term solution to sieve tube damage is the production of callose in the sieve pores. Callose, a β-1,3-glucan, is synthesized by an enzyme in the plasma membrane and is deposited between the plasma membrane and the cell wall. 6  Materials, pathways and patterns of phloem translocation …P-protein and Callose Callose is synthesized in functioning sieve elements in response to damage and other stresses, such as mechanical stimulation and high temperatures, or in preparation for normal developmental events, such as dormancy. The deposition of wound callose in the sieve pores efficiently seals off damaged sieve elements from surrounding intact tissue. Companion cells… Each sieve tube element is associated with one or more companion cells numerous plasmodesmata penetrate the walls between sieve tube elements and their companion cells, suggesting a close functional relationship and a ready exchange of solutes between the two cells. The plasmodesmata are often complex and branched on the companion cell side. 7  Materials, pathways and patterns of phloem translocation …Companion cells… Companion cells play a role in the transport of photosynthetic products from producing cells in mature leaves to the sieve elements in the minor (small) veins of the leaf. They are also thought to take over some of the critical metabolic functions, such as protein synthesis, that are reduced or lost during differentiation of the sieve elements. There are at least three different types of companion cells in the minor veins of mature leaves (dense cytoplasm and abundant mitochondria). i. Ordinary companion cells Have chloroplasts with well-developed thylakoids and a cell wall with a smooth inner surface. Of most significance, relatively few plasmodesmata connect this type of companion cell to any of the surrounding cells except its own sieve element. ii. Transfer cells Are similar to ordinary companion cells, except for the development of fingerlike wall ingrowths, particularly on the cell walls that face away from the sieve element. These wall ingrowths greatly increase the surface area of the plasma membrane, thus increasing the potential for solute transfer across the membrane. 8  Materials, pathways and patterns of phloem translocation …Companion cells… iii. Intermediary cells Appear well suited for taking up solutes via cytoplasmic connections. They have numerous plasmodesmata connecting them to surrounding cells (bundle sheath cells). They are also distinctive in having numerous small vacuoles and poorly developed thylakoids and lack of starch grains in the chloroplasts. 2. Patterns of translocation: 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. Another type of source is a storage organ during the exporting phase of its development. 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. 9  Materials, pathways and patterns of phloem translocation The source-to-sink pathways Although the overall pattern of transport in the phloem can be stated simply as source-to-sink movement. The specific pathways involved are often more complex. Not all sources supply all sinks on a plant (certain sources preferentially supply specific sinks). Proximity. The proximity of the source to the sink is a significant factor. The upper mature leaves on a plant usually provide photosynthates to the growing shoot tip and young, immature leaves; the lower leaves supply predominantly the root system. Intermediate leaves export in both directions, bypassing the intervening mature leaves. Development. The importance of various sinks may shift during plant development. Whereas the root and shoot apices are usually the major sinks during vegetative growth, fruits generally become the dominant sinks during reproductive development, particularly for adjacent and other nearby leaves. Vascular connections. Source leaves preferentially supply sinks with which they have direct vascular connections. Modification of translocation pathways. Interference with a translocation pathway by wounding or pruning can alter the patterns established by proximity and vascular connections. In the absence of direct connections between source and sink, vascular interconnections, called anastomoses (singular; anastomosis), can provide an alternative pathway. 10  Materials, pathways and patterns of phloem translocation Materials translocated in the phloem Water is the most abundant substance transported in the phloem. Dissolved in the water are the translocated solutes (carbohydrates). Sucrose is the sugar most commonly transported in sieve elements (non-reducing form; ketone/aldehyde group reduced to -OH) Nitrogen is found in the phloem largely in amino acids and amides, especially glutamate and aspartate and their respective amides, glutamine and asparagine. Proteins found in the phloem include filamentous P proteins (which are involved in the sealing of wounded sieve elements), protein kinases (protein phosphorylation), thioredoxin (disulfide reduction), ubiquitin (protein turnover), chaperones (protein folding), and protease inhibitors (protection of phloem proteins from degradation and defence against phloem-feeding insects. Inorganic solutes that move in the phloem include potassium, magnesium, phosphate, and chloride *velocity, linear distance travelled per unit time or mass transfer rate, the quantity of material passing through per unit time (1 to 15 g h-1 cm-2 of sieve elements) 11  The mechanism of translocation in the phloem: Pressure-flow Model The mechanism of phloem translocation in angiosperms is best explained by the pressure-flow model, which accounts for most of the experimental and structural data currently available (Ernst Münch in 1930). It explains phloem translocation as a flow of solution (bulk flow) driven by an osmotically generated pressure gradient between source and sink. All theories, both active and passive, assume an energy requirement in both sources and sinks. In sources, energy is necessary to move photosynthate from producing cells into the sieve elements (phloem loading). In sinks, energy is essential for some aspects of movement from sieve elements to sink cells, which store or metabolize the sugar (phloem unloading). A pressure gradient drives translocation. 12  The mechanism of translocation in the phloem: Pressure-flow Model Predictions of the pressure-flow Model Some important predictions emerged from the pressure-flow model: The sieve plate pores must be unobstructed. True bidirectional transport (i.e., simultaneous transport in both directions) in a single sieve element cannot occur. Solutes within the phloem can move bidirectionally, but in different vascular bundles or in different sieve elements. 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. 13 The mechanism of translocation in the phloem: Pressure-flow Model 14 The mechanism of translocation in the phloem: Phloem loading: from chloroplasts to sieve elements Several transport steps are involved in the movement of photosynthate from the mesophyll chloroplasts to the sieve elements of mature leaves, which is called phloem loading. 1. Triose phosphate formed by photosynthesis during the day is transported from the chloroplast to the cytosol, where it is converted to sucrose. During the night, carbon from stored starch exits the chloroplast probably in the form of glucose and is converted to sucrose. 2. Sucrose moves from the mesophyll cell to the vicinity of the sieve elements in the smallest veins of the leaf. This short-distance transport pathway usually covers a distance of only two or three cell diameters. 3. In a process called sieve element loading, sugars are transported into the sieve elements and companion cells. In most of the species studied so far, sugars become more concentrated in the sieve elements and companion cells than in the mesophyll. Note that with respect to loading, the sieve elements and companion cells are often considered a functional unit, called the sieve element–companion cell complex. Once inside the sieve elements, sucrose and other solutes are translocated away from the source, a process known as export. Translocation through the vascular system to the sink is referred to as long-distance transport 15 The mechanism of translocation in the phloem: 16 The mechanism of translocation in the phloem: 17 The mechanism of translocation in the phloem: Phloem unloading In many ways the events in sink tissues are simply the reverse of the events in sources. Transport into sink organs, such as developing roots, tubers, and reproductive structures, is termed import. The following steps are involved in the import of sugars into sink cells. 1. Sieve element unloading. This is the process by which imported sugars leave the sieve elements of sink tissues. 2. Short-distance transport. After sieve element unloading, the sugars are transported to cells in the sink by means of a short-distance transport pathway. This pathway has also been called post–sieve element transport. 3. Storage and metabolism. In the final step, sugars are stored or metabolized in sink cells. These three transport steps together constitute phloem unloading, the movement of photosynthates from the sieve elements and their distribution to the sink cells that store or metabolize them. 18 Photosynthate allocation & partitioning The regulation of the diversion of fixed carbon into the various metabolic pathways is termed allocation. The differential distribution of photosynthates within the plant is termed partitioning. Allocation: i. Synthesis of storage compounds. Starch is synthesized and stored within chloroplasts and, in most species, is the primary storage form that is mobilized for translocation during the night. Plants that store carbon primarily as starch are called starch storers. ii. Metabolic utilization. Fixed carbon can be utilized within various compartments of the photosynthesizing cell to meet the energy needs of the cell or to provide carbon skeletons for the synthesis of other compounds required by the cell. iii. Synthesis of transport compounds. Fixed carbon can be incorporated into transport sugars for export to various sink tissues. A portion of the transport sugar can also be stored temporarily in the vacuole. 19 Photosynthate allocation & partitioning The regulation of the diversion of fixed carbon into the various metabolic pathways is termed allocation. The differential distribution of photosynthates within the plant is termed partitioning. Allocation: i. Synthesis of storage compounds. Starch is synthesized and stored within chloroplasts and, in most species, is the primary storage form that is mobilized for translocation during the night. Plants that store carbon primarily as starch are called starch storers. ii. Metabolic utilization. Fixed carbon can be utilized within various compartments of the photosynthesizing cell to meet the energy needs of the cell or to provide carbon skeletons for the synthesis of other compounds required by the cell. iii. Synthesis of transport compounds. Fixed carbon can be incorporated into transport sugars for export to various sink tissues. A portion of the transport sugar can also be stored temporarily in the vacuole. 20 Photosynthate allocation & partitioning Partitioning of transport sugars in sink tissues: The greater the ability of a sink to store or metabolize imported sugars (the process of allocation), the greater its ability to compete for photosynthate being exported by the sources. Such competition determines the distribution of transport sugars among the various sink tissues of the plant (photosynthate partitioning). Partitioning determines the patterns of growth, and must be balanced between shoot growth (photosynthetic productivity) and root growth (water and mineral uptake). Turgor pressure in the sieve elements could be an important means of communication between sources and sinks, acting to coordinate rates of loading and unloading. Chemical messengers (plant hormones & nutrients) are also important in signaling to one organ the status of the other. Allocation and partitioning in the whole plant must be coordinated such that increased transport to edible tissues does not occur at the expense of other essential processes and structures. 21 Photosynthate allocation & partitioning Allocation in source leaves Increases in the rate of photosynthesis in a source leaf generally result in an increase in the rate of translocation from the source. Control points for the allocation of photosynthate include the allocation of triose phosphates to the following processes: Regeneration of intermediates in the C3 photosynthetic carbon reduction cycle (the Calvin cycle) Starch synthesis Sucrose synthesis, as well as distribution of sucrose between transport and temporary storage pools 22 Photosynthate allocation & partitioning Competition for translocated photosynthate in sink tissues: Translocation to sink tissues depends on the position of the sink in relation to the source and on the vascular connections between source and sink.. The pattern of transport may be determined by competition between sinks [reproductive tissues (seeds) might compete with growing vegetative tissues (young eaves and roots) for photosynthates]. Removal of a sink tissue from a plant generally results in increased translocation to alternative, and hence competing, sinks. In the reverse type of experiment (page 216), the source supply can be altered while the sink tissues are left intact. The roots receive less sugar from the single source, while the young leaves (stronger sinks) receive relatively more. A stronger sink can deplete the sugar content of the sieve elements more readily and increase the pressure gradient and the rate of translocation toward itself. 23 Photosynthate allocation & partitioning The ability of a sink to mobilize photosynthate toward itself, the sink strength, depends on two factors— sink size and sink activity—as follows: Sink strength = sink size × sink activity Sink size is the total weight of the sink tissue, and sink activity is the rate of uptake of photosynthates per unit weight of sink tissue. Changes in sink activity can be complex because various activities in sink tissues can potentially limit the rate of uptake by the sink. Changes in the source-to-sink ratio cause long term alterations in the sources. These changes include a decrease in starch concentration and increases in photosynthetic rate, rubisco activity, sucrose concentration, transport from the source, and orthophosphate concentration. Photosynthetic rate (the net amount of carbon fixed per unit leaf area per unit time) often increases over several days when sink demand increases, and it decreases when sink demand decreases. Photosynthesis is most strongly inhibited under conditions of reduced sink demand in plants that normally store starch, rather than sucrose, during the day. 24 Photosynthate allocation & partitioning Long-Distance Signals May Coordinate the Activities of Sources and Sinks The phloem is a conduit for the transport of signal molecules from one part of the organism to another. Signals between sources and sinks might be physical (turgor pressure) or chemical (plant hormones and carbohydrates). Signals indicating turgor change could be transmitted rapidly via the interconnecting system of sieve elements. Some data suggest that cell turgor can modify the activity of the proton- pumping ATPase at the plasma membrane and alter transport rates. Shoots produce growth regulators such as auxin, which can be rapidly transported to the roots via the phloem; and roots produce cytokinins which move to the shoots through the xylem. Gibberellins (GA) and abscisic acid (ABA) are also transported throughout the plant in the vascular system. Plant hormones play a role in regulating source–sink relationships. They affect photosynthate partitioning by controlling sink growth, leaf senescence, and other developmental processes. 25 Photosynthate allocation & partitioning Long-Distance Signals May Coordinate the Activities of Sources and Sinks The phloem is a conduit for the transport of signal molecules from one part of the organism to another. Signals between sources and sinks might be physical (turgor pressure) or chemical (plant hormones and carbohydrates). Signals indicating turgor change could be transmitted rapidly via the interconnecting system of sieve elements. Some data suggest that cell turgor can modify the activity of the proton- pumping ATPase at the plasma membrane and alter transport rates. Shoots produce growth regulators such as auxin, which can be rapidly transported to the roots via the phloem; and roots produce cytokinins which move to the shoots through the xylem. Gibberellins (GA) and abscisic acid (ABA) are also transported throughout the plant in the vascular system. Plant hormones play a role in regulating source–sink relationships. They affect photosynthate partitioning by controlling sink growth, leaf senescence, and other developmental processes. 26 Photosynthate allocation & partitioning Long-Distance Signals seem to also regulate plant growth and development. Endogenous mRNA molecules and proteins have been found in phloem sap, and at least some of these are thought to be signal molecules. This pathway seems to be open to the movement of macromolecules over long distances: from companion cells of sources to source sieve elements, through the path to sink sieve elements, to companion cells of the sink, and finally to cells of the sink itself. Proteins synthesized in companion cells can clearly enter the sieve elements through the plasmodesmata that connect the two cell types. Some of the proteins that enter sieve elements may simply diffuse through the plasmodesmata into the sieve elements, others may mediate their own cell-to-cell transport, and yet others may be aided by specific control proteins. Once in the sieve elements, some proteins are targeted to the plasma membrane or other cellular locations, while other proteins move with the translocation stream to sink tissues Clearly, proteins can be transported from the companion cells in the source through the intervening sieve elements to sink companion cells. Plasmodesmata can exercise dynamic control of the intercellular diffusion of small molecules. 27