Acid invertase is predominantly localized to cell walls of both the practically symplasmically isolated sieve element/companion cell complex and parenchyma cells in developing apple fruits

The acid invertase (beta -fructosidase, EC 3.2.1.26) was localized at subcellular level via immunogold electron microscopy in the phloem-unloading zone of developing apple fruit. The enzyme (immunogold particles) was found to reside predominantly in the cell walls of the sieve element/ companion cell (SE/CC) complex, phloem parenchyma cells and other parenchyma cells. There was almost no gold particle found in cytoplasm and vacuole. This distribution pattern remained unchanged throughout the growing season, but the enzyme numbers varied. The density of immunogold particles increased during fruit development. The immunoblotting of soluble and insoluble acid invertases provided a supporting proof for the assays of immunolocalization. The biochemical analysis showed a predominantly cell-wall-distributed activity of acid invertase that corresponds essentially with its amount distribution, The ultrastructural observations showed that there were numerous plasmodesmata between the parenchyma cells, but almost no plasmodesmium between the SE/CC complex and its surrounding parenchyma cells, practically resulting in the symplasmic isolation of the SE/CC complex. It is therefore suggested that the unloading pathway of sucrose from the SE/CC complex may be predominantly apoplasmic in the developing apple fruit, and that the unloaded sucrose may be hydrolysed by the functional acid invertase localized in the cell wall before it is loaded in sink cells.