Sucrose is produced in leaf mesophyll cells via photosynthesis and exported to non-photosynthetic sink tissues through the phloem. The molecular basis of source-to-sink long-distance transport in cereal crop plants is of importance due to its direct influence on grain yield-pollen grains, essential for male fertility, are filled with sugary starch, and rely on long-distance sugar transport from source leaves. Here, we overview sugar partitioning via phloem transport in rice, especially where relevant for male reproductive development. Phloem loading and unloading in source leaves and sink tissues uses a combination of the symplastic, apoplastic, and/or polymer trapping pathways. The symplastic and polymer trapping pathways are passive processes, correlated with source activity and sugar gradients. In contrast, apoplastic phloem loading/unloading involves active processes and several proteins, including SUcrose Transporters (SUTs), Sugars Will Eventually be Exported Transporters (SWEETs), Invertases (INVs), and MonoSaccharide Transporters (MSTs). Numerous transcription factors combine to create a complex network, such as DNA binding with One Finger 11 (DOF11), Carbon Starved Anther (CSA), and CSA2, which regulates sugar metabolism in normal male reproductive development and in response to changes in environmental signals, such as photoperiod.
Keywords: phloem; sink; source; sugar partitioning; sugar signaling.