Topic 10.9

Energy Requirements for Unloading in Developing Seeds and Storage Organs

Susan Dunford, University of Cincinnati

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).