Figure 12.

Schematic Model of Sugar Supply to Pollen Tubes.

Sucrose delivered into cells of the stigma or transmitting tissue of the style from source tissues is unloaded into the apoplast via SWEET9/SWEET10 or cleaved by intracellular invertases. Sucrose can either be taken up into pollen tubes via SUC transporters or its monosaccharide components are taken up after cleavage by cell wall invertases (cwINV) through PMTs and STPs. Sucrose and monosaccharides derived from extracellular invertase activity contribute to pollen tube growth as signaling molecules, osmotic components or substrates for glycolysis and cell wall synthesis. Release of additional glucose from pistil cells via SWEET1 increases the glucose to fructose ratio in the apoplastic space. After uptake into pollen tubes via STPs glucose activates a signaling pathway through HXK1 localized at mitochondria or in the nucleus. Eventually, this results in the deceleration of pollen tube growth. Activated HXK1 additionally downregulates expression of STP4, STP8, and STP10. In the equimolar presence of glucose and fructose, the glucose signaling pathway is repressed.