Figure 7.

Model of starch synthesis in pollen grains. A schematic representation is shown for carbohydrate metabolism and transport in the pollen grain. All expressed isoforms during pollen development are indicated in red. Sugar uptake by the pollen occurs only via the apoplast, because developing microspores are symplastically isolated. Suc is either transported via the apoplast (SUC transporters) or metabolized first by cell wall invertases, and the resulting monosaccharides are transported by STP, PMT, and INT transporters. In the cytosol, expressed genes of the different metabolic routes for Suc, monosaccharide, hexose phosphate, and starch metabolism are depicted. Not or lowly expressed but important genes are indicated in green; expressed genes with unknown functions or with unknown substrate specificities are in blue; and the three genes for which we analyzed insertional mutants are highlighted by red boxes. Based on these expression patterns, the most likely pathway of starch biosynthesis within the developing pollen grain is highlighted in yellow. For the full names of enzymes and genes, see Table I.