Susan Dunford, University of Cincinnati, Cincinnati, OH, USA
Transport Sugar May Be Hydrolyzed in the Sink Apoplast
In symplasmic 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 12.8.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.
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 symplasmic 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 12.8.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 symplasmic unloading and lacked expression of the invertase gene.
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 a 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 8). 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 8.13).
Viola, R., Roberts, A. G., Haupt S., Gazzania S., Hancock, R. D., Marmiroli, N., Machray, G. C., and Oparka, K. J. (2001) Tuberization in potato involves a switch from apoplastic to symplastic phloem unloading. The Plant Cell 13: 385–398.