FIGURE 1.

Simplified schematic presentation of sugar metabolism in sink tissue cells. Sucrose may be hydrolyzed in the apoplast by cell-wall invertase (cwINV) to yield glucose and fructose, which can be brought into the cell by a monosaccharide transporter. Alternatively, sucrose can be brought into the sink cell by a sucrose transporter or enter through plasmodesmata. Inside the cell, the sucrose can be stored in the vacuole or hydrolyzed by vacuolar invertase (vINV). In the cytosol, sucrose can be hydrolyzed by cytosolic invertase (cINV) to yield glucose and fructose, or cleaved by SUS to yield fructose and UDP-G. Glucose can be phosphorylated by mitochondria-associated HXK so that it can be used for glycolysis and then respiration, or be brought into the plastids by a plastidic glucose transporter and then phosphorylated by plastidic hexokinase so that it can be fed into plastidic metabolic pathways. Fructose can be phosphorylated by cytosolic FRK (marked in light green) and then used for cytosolic glycolysis or be brought into the plastid by an unknown transporter, phosphorylated by plastidic FRK (marked in light green) and then fed into plastidic metabolic pathways.