Phloem loading is the process by which sugars are loaded into the phloem of source leaves and then subsequently transported to sink organs via bulk flow driven by hydrostatic pressure. Three loading mechanisms are described: passive, polymer trap, and apoplastic loading. In passive loading, sucrose diffuses from mesophyll through plasmodesmata into the phloem. The two energized loading mechanisms are the polymer trap and apoplastic loading. In the polymer trap, sucrose moves into intermediary cells and is synthesized into oligosaccharides that become "trapped." In apoplastic loading, sucrose is transported into the apoplast by SWEETs, and subsequently taken up by SUTs in a proton-sucrose symport mechanism, concentrating sucrose in companion cells. Herbaceous species tend to use active loading, while woody species tend to use passive loading. Confirming either passive or energized loading is not without ambiguity. Cotton was investigated as a model because its phloem loading mechanism is ambiguous. Cotton was expected to use passive loading. However, experiments showed that active sucrose accumulation occurs in leaves through GhSUT1-L2, suggesting plasmodesmata are not always a reliable indicator of passive loading and passive loading should not be assumed for woody plants. Genetic manipulation of carbohydrate transport could prove helpful for improving productivity and challenging the passive loading hypothesis. To test this, constitutive and phloem-specific AtSUC2 expression in poplar was used to (1) test the conservation of AtSUC2 expression and (2) test for apoplastic phloem loading. Poplar expressing AtSUC2 were expected to show conserved expression and apoplastic loading. Poplar expressing AtSUC2 shared a conserved vascular-specific pattern with Arabidopsis but did not load from the apoplast. These results suggest that there is conservation of companion cell identity between poplar and Arabidopsis, passive loading is the loading mechanism in poplar.

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