BOTANY VOLUME 2 - PLANT PHYSIOLOGY - 2007

6. METABOLIC PHYSIOLOGY

6.8. Transport of Assimilates in Plants

The delivery of Organic compounds synthesized by the plant (assimilates) from their Sites of production to their sites of consumption (broadly speaking, from source to sink Organs, see 6.8.3) is carried out in cormophytes almost entirely via the sieve elements of the phloem (see 3.2.4.1). To cover short distances, assimilates can also move from Cell to Cell symplastically or apoplastically, with diffusion serving as the underlying mechanism¹; the transfer and uptake of assimilates across the Plasmalemma is mediated by specialized translocators. Only exceptionally are assimilates transported via the xylem. This occurs in trees in the spring before the green leaves have developed (sap flow, see 6.3.5). In addition, Amino Acids synthesized during nitrate assimilation, such as glutamine and asparagine, and BIOLOGICALLY ACTIVE SUBSTANCES, such as phytohormones (see 7.6), are transported into the SHOOT via the xylem and are thus Components of the xylem sap (see 6.3.5).

¹ Both symplastic transport and cell-to-cell transport through the apoplast are active processes that require Energy Expenditure. For example, Vesicular Transport involving the Cytoskeleton plays a major role in symplastic transport; therefore, assimilate transport cannot be attributed solely to diffusion — Ed. note.

6.8.1. Composition of Phloem Sap

As a rule, all substances (or their respective precursors) that cannot be synthesized in non-autotrophic Cells must be delivered to them. The principal transport metabolites are sugars, along with amino acids detected in the sieve tube sap, as well as other nitrogenous compounds, NUCLEOTIDES (predominantly high concentrations of ATP), Vitamins, phytohormones, organic acids, and minerals. Of the more than 200 notable Proteins found within the contents of sieve tubes, the majority are specific to sieve cells and, consequently, to sieve tubes.

The analysis of sieve tube contents is carried out using the aphid technique. The aphid, which produces honeydew, inserts its stylet into individual sieve tubes; driven by turgor pressure, the phloem contents enter the insect's Digestive System, where certain nitrogenous compounds, vitamins, and minerals are assimilated by the insect Organism, while excess sugar is excreted as honeydew (a 10–15% aqueous sugar solution). The aphid's stylet is severed using a laser, thereby granting access to the pure sieve tube contents for subsequent analysis. By attaching a pressure-sensing probe to the stylet, the turgor pressure within the sieve tubes can also be determined.

Sugar typically accounts for more than 90% of the dry matter in "sieve tube sap." Based on the types of phloem transport sugars, plants can be divided into three main groups

1. Species that contain sucrose as the primary transport sugar. This includes the majority of studied species, for example, all currently known ferns, gymnosperms, and monocots, and among dicots, all known legumes.

2. Species that, alongside sucrose, contain significant amounts of Oligosaccharides of the raffinose family, such as raffinose, stachyose, verbascose, and ajugose (referring to sucrose galactosides, Fig. 6.91). This group also includes representatives of numerous plant families, such as Betulaceae (formerly Corylaceae), Malvaceae (formerly Tiliaceae), Ulmaceae, and Cucurbitaceae among native taxa.

3. Species that, alongside the aforementioned sugars, contain large amounts of sugar alcohols in their sieve tubes (see Fig. 6.91); for example, Oleaceae contain mannitol ("manna ash," with a high mannitol content, is obtained from Fraxinus ornus), plants from certain subfamilies of Rosaceae contain sorbitol, and Celastraceae contain dulcitol.

Reduced nitrogen is transported through the phloem predominantly in the form of Proteinogenic Amino Acids (mainly glutamine, glutamate, and aspartate). In Betulaceae and Juglandaceae, the most important transport form of nitrogen is the non-proteinogenic amino acid L-citrulline (see Fig. 6.91), which also serves for nitrogen storage.

Class="center">Fig. 6.91. Structure of certain additional transport assimilates present in specific plant groups (see text) alongside general transport metabolites (sucrose as a carbohydrate, proteinogenic amino acids, mainly glutamine, glutamate, aspartate)

6.8.2. Phloem Loading

Assimilates produced in the photosynthesizing leaf Tissues (mainly CARBOHYDRATES and amino acids) pass from mesophyll cells into the sieve elements of the finest leaf Veins, crossing the bundle sheath cells and phloem parenchyma that surround the vascular bundle; such cells are very few in number: 3 to 5. Transport occurs via diffusion through numerous plasmodesmata between these cells. The loading of sieve elements (sieve tubes or cells) starts from this point and takes place via two pathways (see Fig. 6.72): either apoplastically or symplastically. These two pathways probably can be combined.

Apoplastic phloem loading predominates in species that use sucrose as their transport sugar. En route from the mesophyll to the phloem parenchyma, sucrose enters the apoplast. This occurs via passive diffusion because the concentration of sucrose in these cells is much higher than in the apoplast. The transport system is unknown. From the apoplast, sucrose enters the companion cells (or their functional equivalents) and subsequently the sieve cells with the help of a specialized translocator, the sucrose-proton symporter. The driving force for phloem sucrose uptake is provided by a P-type hydrogen ion-translocating ATPase (see 6.1.5, Fig. 6.5); the uptake of sucrose into the phloem is a secondary active process that leads to the concentration of sucrose within the sieve tubes. The ATP required to maintain the proton gradient is supplied by mitochondrial Respiration. As a result of respiratory poisons, the phloem loading process in plants with an apoplastic loading type is effectively inhibited. The sucrose-proton symporter has been successfully cloned (see Box 7.3) and localized in The Plasma Membrane of companion cells (e.g., in plantain) or sieve cells (in potato, tomato, tobacco) using specific Antibodies. Anucleate sieve elements often receive sucrose from companion cells, whose plasmalemma contains numerous molecules of sucrose transporters. Within the companion cells, the absorbed sucrose must reach the sieve tubes through plasmodesmata via diffusion¹.

¹ See note to subsection 6.8. — Ed. note.

Symplastic phloem loading is characteristic of species that transport significant amounts of raffinose-family oligosaccharides alongside sucrose (see Fig. 6.91). Cytological examination of these species reveals numerous plasmodesmata that symplastically connect all Cells of the transport pathway. How the increase in carbohydrate concentration observed in sieve cells is achieved remains unclear. According to a recent hypothesis, the synthesis of raffinose family oligosaccharides from sucrose and galactinol in these species occurs exclusively in the cells surrounding the sieve elements, thereby keeping the sucrose concentration in these cells at a low level, which drives its diffusion from the mesophyll. This model implies an exceptionally selective permeability of plasmodesmata (see 7.4.4.1); i.e., raffinose family sugars can diffuse into the sieve tubes but not back into the mesophyll¹. However, this has not yet been experimentally confirmed.

¹ It can also be postulated that selectivity is ensured by the compartmentalization of sucrose and raffinose family sugars in different PARTS OF THE ER. According to this model, sugar concentration is driven by vesicular transport involving the cytoskeleton. — Ed. note.

In plants with apoplastic phloem loading, amino acids probably also enter the sieve tubes via a secondary active amino acid-proton symporter. However, these translocators lack strict substrate Specificity, meaning that all amino acids synthesized locally end up in the phloem. Notably, plants with symplastic phloem loading also utilize specialized transport amino acids (Cucurbitaceae, for example, transport the non-proteinogenic amino acid citrulline (see Fig. 6.91), an intermediate in Arginine Biosynthesis). The reason for this lies in the fact that efficient symplastic phloem loading of amino acids, as in the case of carbohydrates, requires the targeted Synthesis of specific transport substances.

6.8.3. Phloem Transport of Assimilates

Within assimilating organs, osmotically active metabolites are produced in high concentrations (about 0.2–0.7 mol·L^-1 of carbohydrates and about 0.05 mol·L^-1 of amino acids). The passive influx of Water from surrounding tissues (including the xylem) generates high turgor at the site of phloem loading. Sieve cells can be subjected to plasmolysis in experimental settings, indicating that they possess an intact plasmalemma with selective permeability. On the other hand, at sites of assimilate consumption, unloading from the phloem takes place (see 6.8.4; Fig. 6.72), which causes a subsequent passive outflow of water and a corresponding drop in turgor. Water then enters the xylem. Thus, the close spatial association between xylem and phloem is entirely logical.

Ultimately, a pressure gradient is established within the sieve tubes (cells) between the sites of phloem loading and unloading. According to the pressure-flow hypothesis, originally formulated by Münch, this pressure gradient drives a mass flow of sieve tube contents from source to sink organs (English source-to-sink). Dissolved substances are carried along in this mass flow through the phloem. Flow velocities reach 0.5–1.5 m/h, enabling rapid long-distance delivery of assimilates (the contents of a single sieve tube element in linden, for example, are replaced 5 times per second!). At an average flow velocity of 0.6 m·h^-1 and 0.5 mol·L^-1 sucrose, the flux amounts to approximately 100 kg of sucrose per h^-1·m^-2 of sieve tube cross-section.

The turgor gradient in sieve tubes along the direction of assimilates transport can be determined through various experimental approaches. Overcoming the flow resistance within a sieve tube requires a pressure gradient of approximately -0.04 MPa • m^-1 (based on typical cell dimensions and the viscosity of the sieve tube contents). Roughly half of this flow resistance is attributed to the sieve plates—the inclined, porous cross-walls of the sieve elements (see 3.2.4.1). The resistance provided by the sieve plates and transverse walls is believed to help stabilize the turgor gradient that drives mass flow through the phloem, as pressure gradients in a continuous liquid Column would otherwise dissipate very rapidly. The driving force behind phloem mass flow can be provided by numerous local osmotic gradients established between the sieve elements and the surrounding assimilate sources and sinks.

According to the pressure-flow hypothesis, the direction of substance transport in the phloem is determined by osmotic gradients in assimilate concentrations (coupled with the turgor gradient) running from sources to sinks. Source organs include, for example, photosynthetically active mature leaves or storage organs undergoing the mobilization of reserves (such as stems or roots during leaf unfolding; cotyledons or endosperm during

seed germination; and tubers, bulbs, or ROOT crops during shoot elongation). An exceptionally high export of nitrogenous compounds begins in the leaves of perennial plants prior to leaf fall; this process returns a major portion of the leaf protein nitrogen—after its Hydrolysis into amino acids—back to the overwintering perennial organs. During this phase, the total amino acid concentration in the phloem can reach 0.5 mol l^-1, whereas carbohydrates are almost entirely absent from the phloem sap at this time.

Sink organs include all actively developing parts of the plant (such as the apical Meristems of shoots and roots; the cambium; young, growing leaves up to half their final size; and ripening fruits). A single large plant may contain multiple source and sink organs whose roles shift over time. For instance, lower leaves often supply the roots, whereas upper leaves supply the shoot apex, flowers, and fruits. Consequently, oppositely directed transport streams can be observed within the same segment of a stem, but never within the same individual sieve tube.

6.8.4. Phloem Unloading

Phloem unloading can occur via either a symplastic or an apoplastic pathway (see Fig. 6.72). In the former case, assimilates move from the sieve elements into the cells of the sink organ via plasmodesmata. This mechanism predominantly operates in non-storage tissues, such as growing roots and shoots. Metabolic processes within these cells presumably maintain the concentration gradient required for continuous phloem unloading. In apoplastic unloading, which is critical for storage tissues, assimilates first enter the apoplast and are subsequently taken up by storage cells. Although specific transport proteins have not yet been fully characterized, proton-coupled symporters for metabolites likely play a key role in the uptake of assimilates by storage parenchyma cells.

On the one hand, sucrose enters metabolic pathways within the cells following the reaction sequence illustrated in Fig. 6.72 (initiating with the sucrose synthase reaction). The resulting glucose-6-phosphate (or glucose-1-phosphate in certain species) is transported into leucoplasts via a phosphate translocator, where it is channeled into the synthesis of reserve starch. On the other hand, sucrose is hydrolyzed by the enzyme invertase, meaning it is broken down in the apoplast into glucose and fructose. The resulting hexoses are then taken up into the cells via a hexose-proton symporter, phosphorylated, and utilized for reserve starch synthesis (see Fig. 6.72).

Certain plant species store carbohydrates in the form of sucrose (sugar beet, sugarcane) or glucose (certain fruits, such as grapes). The storage of these soluble sugars takes place within vacuoles. In addition to starch, various other Polysaccharides function as reserve carbohydrates (see 6.17.1.2).

Many metabolic processes that consume and mobilize assimilates are regulated by growth Hormones and inhibitors. Therefore, it is unsurprising that the spatial distribution of assimilate sources and sinks is closely linked to the local activity of growth regulators. For example, phytohormones can stimulate cambial division and dictate its function as a sink tissue. Cytokinins (a class of phytohormones, see 7.6.2) promote the synthesis of apoplastic invertase, which enhances sucrose hydrolysis in the apoplast of surrounding cells and facilitates the withdrawal of sucrose from the phloem. Consequently, cytokinins regulate the "sink strength" of a given organ or tissue.

Transgenic Plants (see Box 7.3) that accumulate higher levels of invertase in the apoplast of their storage organs exhibit an enhanced capacity for starch storage in those tissues. This demonstrates that invertase activity within a sink organ can act as a rate-limiting factor for phloem unloading.