Topic 11.10 Partitioning: The Role of Sucrose-Metabolizing Enzymes in Sinks
Acid invertase, which cleaves sucrose into a fructose and a glucose molecule, can be found bound to the cell wall or accumulated in the vacuole (see Web Topic 10.8). Carrot plants (Daucus carota) display altered sucrose partitioning when the cell-wall invertase activity in the roots is decreased due to antisense suppression. Partitioning to the roots is decreased, reducing taproot development. Since these plants also have more leaves than the controls, their dry weight leaf-to-root ratio is greatly increased. Both sucrose and starch levels are increased severalfold in the leaves of the transformed plants. Most of the plants show an opposite change in carbohydrate concentrations in the roots. Since unloading into carrot storage roots is most likely apoplastic, decreasing the formation of glucose and fructose in the cell wall would prevent sugar uptake into the sink storage tissues, causing these profound changes in the partitioning patterns in the plant (Tang et al. 1999).
The reverse experiment is consistent with a role of invertase in sink demand. Expression of a heterologous cell wall invertase under the control of a root-specific promoter in Arabidopsis results in the production of more secondary roots, longer roots, and thus a higher root dry weight. The transgenic plants also developed a higher shoot biomass, presumably due to a greater capacity to acquire nutrients from the soil (von Schweinichen and Buttner 2005).
Genes for invertase and sucrose synthase are often expressed at different times during sink development. In bean pods and corn (maize; Zea mays) kernels, changes in invertase activity are found to precede changes in photosynthate import. These results point to a key role of invertase and sucrose synthase in controlling import patterns, both during the genetic program of sink development and during responses to environmental stresses (see Web Topic 10.8).
The studies above emphasize changes in transport as a result of altered sucrose metablism in sinks of species which mainly transport sucrose. In species which transport other carbohydrates (such as sugar alcohols) in addition to sucrose, changes in source-to-sink ratios also bring about changes in the enzymes which metabolize the sugar alcohol in sink tissues. A greater supply of translocated carbohydrate increases enzyme activity, while restricted supply reduces eznyme activity (Morandi et al. 2008).