Translocation

Pathway of Translocation

The movement of sugars and other molecules generally follows a path that originates in plant organs where sugars (the primary solute) are made and terminates in regions where these nutrients are utilized. The organs where the pathway begins are called source regions, or sources, and the ends of the pathway are referred to as sink regions, or sinks. The predominant organ for the manufacture of sugars is the leaf, which can take in carbon dioxide and light energy to produce sugars through the process of photosynthesis. These sugars can be used locally by the leaf or can be translocated to the rest of the plant. Leaves are generally considered the primary source regions, but it should be noted that only fully expanded, mature leaves can act as sources. Newly emerging leaves are unable to fully nourish themselves with their own sugar production, and thus they act as sink regions until they reach full maturity.

Other sink tissues include root systems, which cannot carry out the process of photosynthesis and must be fed by the leaves, and developing reproductive tissues, such as seeds and fruit, which store nutrients for future use. Additional storage organs that are translocation sinks and which are important human food crops include tubers (e.g., potatoes and yams) and tap roots (e.g., carrots and beets).

Plant structures that lie between terminal source and sink tissues, such as the stem of an herbaceous plant, the trunk and branches of a tree, or the petiole of a leaf, make up the translocation pathway. All of these structures contain numerous living cells that require nourishment and, thus, these pathway tissues can also function as sinks. In certain cases, however, they serve dual roles, because in some plants (e.g., cereals such as rice and wheat) the stems act as temporary storage organs for nutrients. At late stages in the plant's life cycle, these stems are converted to source regions that provide nutrients for the developing seeds. Various non-leafy green tissues that can conduct photosynthesis also can serve as sources; pea pods, for instance, can translocate sugars and other nutrients to the developing pea seeds.

Structure of Phloem Cells

The translocation of molecules via the phloem pathway is dependent on the functioning of specialized cells that are distributed in an organized manner throughout the plant. The cells that conduct nutrients over long distances are called sieve elements, of which there are two types: sieve cells, which are found in gymnosperms (e.g., conifers and cycads), and sieve-tube members, which are found in angiosperms (i.e., monocots and dicots). Sieve elements are narrow, elongated cells that are aligned in long columns that extend from source to sink regions within the plant. Sieve elements are living cells and thus possess a plasma membrane at their periphery, just inside the cell wall. However, they do not contain a nucleus at full maturity, and possess only a few cellular organelles (e.g., mitochondria, endoplasmic reticulum). The lack of a nucleus and most other cellular structures means that the cell interior is rather open. This serves to make sieve elements good conduits for long-distance solution flow.

The term sieve in the various names refers to the clusters of pores, or sieve areas, that perforate the common cell walls between adjoining sieve elements and which interconnect these cells. The interconnection is possible because the plasma membrane of each sieve element is extended as a tube through each sieve pore. In sieve cells, the pores are narrow and the structure of the sieve areas is fairly uniform on all walls of the cell. Sieve cells are usually arranged with long, tapering, overlapping ends, and most of the sieve areas are concentrated on these overlapping regions. In sieve-tube members, narrow-pored sieve areas exist, but some walls also possess much larger pores. The areas with larger pores are called sieve plates, and are usually located on the end walls. These end walls tend to be less obliquely oriented than the ends of sieve cells, and in many species can be situated almost perpendicular to the long, side walls. Sieve-tube members are organized end-to-end in columns of cells called sieve tubes, thus forming a long tubular network throughout the plant. Their larger end-wall pores means that the phloem sap can be more readily translocated over long distances.

The cell walls of sieve elements are considered primary walls, as they are composed chiefly of cellulose. The pores of the sieve areas and sieve plates are additionally lined with a substance called callose, which is a polysaccharide consisting of glucose units. The role of callose in the vicinity of pores is to act as a sealing agent in the case of injury to the phloem pathway. When a plant structure is damaged by mechanical stress, such as wind, or by biological attack, such as feeding by an insect, the plant could lose nutrients if these were to "bleed" from the cut end of the sieve elements. This usually does not happen, because following injury callose is rapidly deposited within the wall region of sieve pores. This deposition constricts the interconnecting tube of plasma membrane and thereby blocks the pore. With time, the plant can generate new sieve elements around the cut area to reestablish translocation within that column of phloem cells.

As mentioned earlier, sieve elements do not contain a nucleus in their mature state, yet in some species sieve elements are known to live for decades. How is this possible? Sieve elements are always found to be associated with specialized accessory cells that contain all the components commonly found in living plant cells, including a nucleus. For sieve-tube members, these specialized cells are called companion cells, and the specialized association is referred to as the sieve element-companion cell complex. Companion cells are very densely filled with organelles, and thus they are not structurally suited for the long-distance translocation of nutrients. Functionally, however, companion cells are extremely important, as they are responsible for the coordinated movement of molecules into and out of the sieve-tube members. These molecules include not only substances translocated throughout the plant, but also proteins and nucleic acids that are needed to maintain the life and functions of the sieve-tube member. The movement of these molecules occurs through elaborate channels called plasmodesmata that interconnect companion cells and sieve-tube members. Less specialized plasmodesmata also exist between certain other cell types. Although very important, the movement of molecules through plasmodesmata is poorly understood, and scientists are currently focusing much attention on this area of plant biology.

Accessory cells are also found associated with the sieve cells of gymnosperms, where they are called albuminous cells. The albuminous cells are structurally comparable to and perform a role similar to that of companion cells.

Loading and Unloading of Sugars and the Pressure-Flow Mechanism

With the presence of a continuous, membrane-bound pathway, phloem sap can flow from source to sink regions within the plant. But how do the components of the phloem sap get in to or out of the pathway, and what is the mechanism, or driving force, that moves the solution? As noted earlier, the predominant solute in phloem sap is sugar, and in many species the translocated sugar is sucrose. For these species, sucrose is manufactured primarily in the photosynthetic mesophyll cells of the leaf, from where it must be transported to the minor veins of the phloem system. Sucrose can move to the minor veins using an intracellular pathway, referred to as symplastic movement, or it can diffuse through a path along the cell walls, a process known as apoplastic movement. In either case, sucrose is eventually pumped into sieve elements through an active, energy-requiring process called phloem loading. The amino acids and mineral ions found in phloem sap also are said to be "phloem loaded."

What phloem loading accomplishes is to create a very high concentration of solutes within the interior of the sieve elements in a source region. Because the sieve element interior is surrounded by a largely non-permeable plasma membrane, it is able to retain these solutes within the cell. On the other hand, the plasma membrane also contains special channels that make it highly permeable to water molecules and water molecules enter by osmosis. This is critical because the movement of water into sieve elements increases the hydrostatic pressure (i.e., the water pressure) of phloem sap within these cells. The end result is that the interior of the sieve element becomes pressurized with respect to other cells of the source region.

At the sink end of the pathway, an opposite chain of events is occurring. Sugars and other solutes are moved out of the sieve elements through a process called phloem unloading, as these solutes are used by other cell types for growth, metabolism, or storage. In response to this release of solutes water molecules move out of the sieve element, and the result is a localized decrease in the sieve element hydrostatic pressure. The lowered pressure within the sieve elements of the sink region in conjunction with the higher pressure within the sieve elements of the source region creates a gradient of pressure along the length of the interconnected phloem pathway. Because of this pressure gradient, a bulk flow of phloem sap occurs from high to low pressure, or from source to sink tissues. The pressure gradient remains in place, even as flow proceeds, as long as solutes are continuously loaded into and unloaded from the pathway. This translocation process is known as the pressure-flow mechanism.

It should be noted that the larger the gradient in pressure between two points in the pathway, the greater the potential for translocation of phloem sap. Thus, actively photosynthetic tissues have the ability to load more sugars into the pathway, creating higher localized sieve element pressures in these regions. Similarly, an actively growing sink tissue, which is consuming/removing large quantities of sugars and other solutes from the pathway, will create lower localized sieve element pressures in this region, which will help sustain translocation flow to the sink.

Ways to Determine the Chemical Nature of Phloem Sap and the Rate of Translocation

Scientists have been interested in studying the composition of phloem sap for many years because of its importance to plant growth and development. Unfortunately, access to pure phloem sap is difficult for a number of reasons: sieve elements are very narrow cells (approximately 10^-8 meters in diameter), they are embedded within other tissues of the plant, and most plants have a sealing mechanism that prevents the loss of phloem sap upon cutting. Certain techniques do exist, however, that get around these problems. One approach involves the use of aphids, which are insects that feed selectively upon the contents of sieve elements but do not induce a sealing reaction. Scientists allow an aphid to insert its stylet, a long tube-shaped mouth part, into the side of a sieve element within a stem or leaf. The insect is then sacrificed and removed, with its stylet still inserted in the plant tissue, either by using a razor blade or a laser burst. Because the phloem sap is pressurized, phloem sap will flow out the cut end of the stylet for a short period of time and it can be collected for analysis. Standard analytical chemistry techniques are then used to determine carbohydrate and mineral composition of the phloem sap, or more modern techniques of protein chemistry and molecular biology are used to quantify and characterize the protein and nucleic acid composition of the collected solution.

The rate of translocation in different plants, especially in response to various environmental conditions, is also of interest to scientists who study phloem function. Because sugars are the predominant component of the phloem sap, researchers have used radioactively labeled sugars to monitor and quantify phloem translocation. A source leaf, for instance, can be exposed to radioactive carbon dioxide within a sealed glass chamber, allowing it to convert the carbon dioxide to radioactive sugars via the process of photosynthesis. These sugars are phloem loaded and can be monitored as they move throughout the plant using external radiation detectors, or sink regions can be harvested and analyzed for radioactivity following some period of translocation. In either case, rates of translocation can be quantified, and the effect of various physical or biological factors on translocation rate can be determined. These types of studies help scientists determine ways to improve plants, both in terms of yield and nutritional quality.

SEE ALSO LEAVES; PHOTOSYNTHESIS, CARBON FIXATION AND; STEMS; VASCULAR TISSUES.

Michael A. Grusak

Bibliography

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Evans, Lloyd T. Crop Evolution, Adaptation and Yield. Cambridge: Cambridge University Press, 1993.

Lucas, William J., Biao Ding, and Chris van der Schoot. "Plasmodesmata and the Supracellular Nature of Plants." New Phytologist 125 (1993): 435-76.

Zamski, Eli, and Arthur A. Schaffer. Photoassimilate Distribution in Plants and Crops: Source-Sink Relationships. New York: Marcel Dekker, 1996.