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Topic 10.8

Sugars in the Phloem

Susan Dunford, University of Cincinnati

Transport Sugar May Be Hydrolyzed in the 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 10.9.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 10.9.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. (Click image to enlarge.)

A recent study with potato plants has shown that apoplastic unloading predominated 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 10.9.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 10.9.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.)