Figure 1. Major phloem loading types.

In active apoplasmic loading, the sieve element-companion cell complex (SECCC) is symplasmically isolated. Sucrose produced in the mesophyll cells (MCs) diffuses into phloem parenchyma cells (PPCs), where it is released into the apoplasm by efflux carriers (SWEETs) before being taken up into the SECCC by plasma membrane-localized sucrose transporters. In active symplasmic loading, sucrose can diffuse or convect into the companion cell (CC) through the abundant plasmodesmata (PD). Sucrose in the CC is converted to sugar oligomers, which are hindered from diffusing back into the phloem parenchyma but instead enter the SE through the larger PD at the SE-CC interface. In passive symplasmic loading, sucrose can diffuse or convect along the whole phloem loading pathway from mesophyll cells to SEs following the sugar concentration gradient. Aquaporins facilitate the osmotic uptake of water from the phloem apoplasm into SEs in all loading types. In symplasmic loaders, water also enters the SECCC through PD. SWEET, sugar will eventually be exported transporters.