Figure 1.

Comparative summary of the eccrine- and merocrine-based models of nectar synthesis and secretion. In the eccrine-based model, starch accumulates in the nectary tissue (1) and is subsequently degraded (2) to provide precursors for the biosynthesis of sucrose (3). Export of sucrose through the plasma membrane into the apoplasm (4) is facilitated by transmembrane transporters, such as the uniporter SWEET9 (Lin et al., 2014) or possibly by H⁺-sucrose antiporters (Vassilyev, 2010). Finally, sucrose in the apoplasm is hydrolyzed (5) by CELL WALL INVERTASE4 (Ruhlmann et al., 2010). In the merocrine-based model, processes 1–3 are synonymous to the eccrine-based model. Sucrose and other nectar metabolites are packaged into endoplasmic reticulum (ER) and/or Golgi-derived vesicle (4), which fuse with the plasma membrane (5) to release the vesicle content into the apoplasm.