Sucrose phloem

Braun DM, Wang L, Ruan YL. Understanding and manipulating sucrose phloem loading, unloading, metabolism, and signalling to enhance crop yield and food security. J Exp Bot. 65(7) (2014) 1713-1735. 

Cakmak, C. Hengeler, and H. Manschner, Changes in phloem export of sucrose in leaves in response to phosphorus, potassium and magnesium deficiency in beau plants. J. Exp. Bot. 45 1251 (1994). 

The cambium layer of plant stems (Fig. 1-16) differentiates continuously to form phloem on the outside of the cambium and xylem on the inside. At the same time, cambium cells are retained. Thus, at each cell division one daughter cell becomes a differentiated cell, while another remains the less differentiated cambium. This pattern of continuous differentiation from a line of stem cells with constant properties is found in animals as well as in plants. In the differentiation of cambium it appears that chemical signals obtained from the surrounding cells on either the inside or the outside of the cambium layer determine whether the differentiated cell becomes phloem or xylem. Sucrose, auxin, and cytokinins are all involved. 

Yang, N.-S. Russell, D. (1990). Maize sucrose synthase-1 promoter directs phloem cell-specific expression of GUS-gene in transgenic tobacco plants. Proceedings of the National Academy of Sciences (USA) 87, 4144-8. 

Riesmeier, J.W., Willmitzer, L., and Frommer, W.B., Evidence for an essential role of the sucrose transporter in phloem loading and assimilate partitioning, EMBO J., 13, 1-7, 1994. 

Foreign enzymes—that is, with no plant equivalent, have been introduced into plants one of the uses of this approach was to address the nature of sucrose transport into the phloem. An invertase derived from the yeast enzyme was targetted to the cell wall of tobacco, potato, tomato, and A. thaliana (Sonnewald et al., 1994). The introduction of invertase decreased yield, presumably through the inhibition of sucrose transport. 

Phloem Loading. How does sucrose which is produced in the mesophyll cells of source leaves enter the translocation stream and how does this sucrose exit from the translocation stream in the sink regions More importantly, can this tell us anything about agrichemical transport Figure 1A shows an autoradiograph of a source leaf following the accumulation of C-sucrose ( C-label is in white). The label is accumulated markedly into the vein network com-... 

Figure 1. Source leaf minor vein phloem. (A) Autoradiograph of leaf tissues following l C-sucrose accumulation showing radioactivity (white) in veins. (B) Tracing of an electron micrograph of a cross section of minor vein, x, xylem, vp, vascular parenchyma cc, companion cell se, sieve element pp, phloem parenchyma, me, mesophyll cell. Reproduced with permission from Ref. 6. Copyright 1983. Annual Reviews.

The characteristics of the phloem loading system can be summarized as follows. Sucrose loading is (1) dependent on metabolism (2) carrier-mediated (3) selective for sucrose (4) maintains a high concentration inside the phloem which is the basis for the osmotically-driven mass flow of solutions and (5) dependent on the factors which control assimilate supply to the loading sites (e.g., photosynthesis, sucrose synthesis, and sucrose movement between leaf cells, and within subcellular compartments such as the cytoplasm and vacuole) ((>, 7 ). 

Figure 2. Model for phloem loading of sucrose. See text for details. Reproduced with permission from Ref. 8. Copyright 1980. Academic Press.

The movement of most organic compounds throughout the plant takes place in the other vascular tissue, the phloem. A portion of the photosynthetic products made in the mesophyll cells of the leaf diffuses or is actively transported across cellular membranes until it reaches the conducting cells of the leaf phloem. By means of the phloem, the photosynthetic products— which then are often mainly in the form of sucrose—are distributed throughout the plant. The carbohydrates produced by photosynthesis and certain other substances generally move in the phloem toward regions of lower... 

Rather than the solute speed in the phloem, we are sometimes more interested in how much matter is translocated. For example, if the sieve elements contain 0.5 m (500 mol m-3) sucrose moving at an average speed of 0.6 m hour-1, what is the transfer rate of sucrose in kg m-2 hour-1 By Equation 3.7 (Jj = vjcj), the flux density of sucrose is... 

Because sucrose has a mass of 0.342 kg mol-1, this flux density corresponds to (300 mol m-2 hour-1)(0.342 kg mol-1) or 100 kg m-2 hour-1. In the current example, the flow is per m2 of sieve-tube lumens the rate of flow per unit area of phloem tissue is less by the ratio of the lumen cross-sectional area to the total phloem cross-sectional area, which is usually 0.2 to 0.5. 

Next we will estimate the various components of the water potential at the upper end of the phloem tissue under consideration (Fig. 9-18). The osmotic pressure in a sieve tube in the phloem of a leaf that is 10 m above the ground, nphloemi0m, might be due to the following solutes 0.5 m sucrose, 0.1 m other sugars, 0.05 m amino acids, and 0.05 m inorganic ions. Thus the... 

Such a large osmotic pressure, caused by the high concentrations of sucrose and other solutes, suggests that active transport is necessary at some stage to move certain photosynthetic products from leaf mesophyll cells to the sieve elements of the phloem. From the definition of water potential, = P — II + pwgh (Eq. 2.13a), we conclude that the hydrostatic pressure in the phloem of a leaf that is 10 m above the ground is... 

The known involvement of metabolism in translocation in the phloem could be due to active transport of solutes into the phloem of a leaf or other source, which is often referred to as loading, and/or to their removal, or unloading, in a root or other sink, such as a fruit (Fig. 9-l7a). Indeed, loading often involves proton-sucrose cotransport (Fig. 3-l4a) via a carrier located in... 

Sucrose is transported from the leaf to the rest of the plant through the phloem. 

Sucrose is loaded into the phloem by active transport. 

Phloem loading of sucrose is decisive for the speed of mass flow, because sucrose is the dominant solute in the sieve tube sap of nearly all plant species. The interaction of sucrose metabolism, starch storage and phloem export in source leaves and the effects on sugar status in phloem was presented. The sucrose concentrations was revealed by a NMR imaging method. 

Sucrose is a key carbohydrate in plant metaholism. The concentration of. sucrose in space and time is an important parameter in plant growth and morphogenesis, which can be determined by spatially resolved NMR [Metl, TselJ. The sucrose distribution, for example, has been examined quantitatively in Ricinus communis seedlings Metl ]. Until now, information on the sucrose concentration in the phloem has been obtained only by several destructive methods. They all require the opening of the sieve tubes, which in turn may modify the water flow in the plant, so that it cannot be knowm whether or not... 

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