Susan Dunford, University of Cincinnati

Transport Sugar May Be Hydrolyzed in the Sink Apoplast

In symplastic phloem unloading, transport sugars such as sucrose move through the plasmodesmata to the sink cells. In the sink cells, sucrose can be metabolized in the cytosol or the vacuole before being stored or entering metabolic pathways associated with growth of the tissue. When phloem unloading is apoplastic, however, there is an additional opportunity for metabolic change. The transport sugar can be partly metabolized in the apoplast, or it can cross the apoplast unchanged (Web Figure 11.8.A). For example, sucrose can be hydrolyzed into glucose and fructose in the apoplast by invertase, and glucose and/or fructose would then enter the sink cells. The fact that many monosaccharide transporters have been localized mainly in sink tissues supports the possible existence of this pathway.

Web Figure 11.8.A   The possible fates of sucrose unloaded apoplastically in sink tissues. (1) Sucrose that enters the apoplast can be split into glucose and fructose by a wall invertase before entering a cell from a sink tissue, or (2) sucrose can be taken up into the cell unaltered. (3) Once in the symplast of the cell from the sink tissue, sucrose can be split into glucose and fructose by a cytoplasmic invertase, or (4) sucrose can enter the vacuole unaltered. (5) Once in the vacuole, sucrose can be split into glucose and fructose by a vacuolar invertase, or it can remain unaltered.

A study with potato plants has shown that apoplastic unloading predominates in elongating stolons (Viola et al. 2001). Stolons are underground lateral shoots that grow from the main stem of the potato plant, characterized by elongated internodes and hooked apical tips, that form tubers at their apices in response to environmental signals. When tuberization started in stolons, phloem unloading shifted from apoplastic to symplastic transport. Histochemical analysis of potato lines transformed with the promoter of an apoplastic invertase gene (invGE) linked to a reporter gene showed invertase activity in the elongating stolon, associated with apoplastic unloading (Web Figure 11.8.B). In the developing tuber, apoplastic loading and invertase activity was observed in a small apical region, which was the apical area of the stolon progressively engulfed by the swelling subapical regions during tuberization. Most of the tuber showed symplastic unloading and lacked expression of the invertase gene.

Web Figure 11.8.B   Expression of an apoplastic invertase (invGE) revealed by GUS staining. (A) GUS staining is restricted to the apical hook region of an elongating stolon (arrow). (B) Developing tuber showing GUS staining associated with the apical bud region (arrow). Bar in (A) = 1 mm; bar in (B) = 500 μm. (From Viola et al. 2001.)

Energy Requirements for Unloading in Developing Seeds and Storage Organs

Developing seeds have proven to be a most interesting system in which to study unloading processes. In legumes such as soybean, the embryo can be removed from the seed coat. In this way, unloading from the seed coat into the apoplast can be studied without the influence of the embryo, and uptake into the embryo can also be investigated separately. Studies with legumes have shown that both entry of sucrose into the apoplast and uptake into the embryo are mediated by transporters and are active. In cereals like wheat, only uptake into the embryo is active; the loss of sucrose (sucrose efflux) from the maternal tissues is passive (down the concentration gradient), because the subsequent active step keeps the sucrose concentration in the apoplast low. In corn, the cell wall invertase helps maintain a low apoplastic sucrose concentration by splitting the disaccharide into monosaccharides. In general, sugar–proton symport mechanisms appear to function in the uptake of sugars from the apoplast, as in sucrose uptake into the soybean embryo.

Storage organs often accumulate sugars to high concentrations, for example, in sugar beet taproot and sugarcane stem. This sugar accumulation requires active membrane transport, since energy is required to move sugars into storage compartments against a concentration gradient.

Sugar transport into the vacuoles of storage cells such as those of sugar beet is thought to be accomplished by a sucrose–proton antiport (see textbook Chapter 6). In this case, a vacuolar H⁺-ATPase pumps protons into the vacuole; the antiport carrier then moves sucrose into the vacuole in exchange for protons, which exit the vacuole down their electrochemical-potential gradient (see textbook Figure 6.11).