FIGURE 7.

Simplified model of CsSWEET7a function during phloem unloading in cucumber receptacle and nectary. The top light-yellow color region indicates the nectary (Nec) tissue, and the bottom light-green color region is the receptacle (Re) tissue. The droplet above the nectary represents the secreted nectar. A similar phloem unloading process is proposed in receptacle and nectary of male and female flowers. Photosynthetic sugars are transported to flower receptacles and nectary tissues via the phloem. As an RFOs-transporting plant, the majority of sugar in the cucumber phloem transport stream [in sieve element (SE)] is stachyose and raffinose, with a small amount of sucrose detected. The stachyose and raffinose could be broken down by α-galactosidase 1 (AGA1) to sucrose, which has three possible routes to phloem parenchyma cell (PPC): (1) sucrose is hydrolyzed by sucrose synthase 4 (SUS4) and/or cytosol invertase 1 (CINV1) to hexoses in companion cell (CC), and the resulting hexoses are exported to apoplasmic space by hexose transporter, e.g., CsSWEET7a or other SWEETs (Clade I/II which transport hexoses), before loading into PPC by hexose transporter; (2) sucrose is exported from CC to apoplasmic space by sucrose transporter, followed by hydrolyzation via CWINV4 to hexoses, and hexoses are taken up to PPC by hexose transporter; (3) sucrose is exported to apoplasmic space by sucrose transporter and is taken up by sucrose transporter to PPC.