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 subce llular level via immunogold electron microscopy in the phloem-unloading zon e of developing apple fruit. The enzyme (immunogold particles) was found to reside predominantly in the cell walls of the sieve element/ companion cel l (SE/CC) complex, phloem parenchyma cells and other parenchyma cells. Ther e was almost no gold particle found in cytoplasm and vacuole. This distribu tion pattern remained unchanged throughout the growing season, but the enzy me numbers varied. The density of immunogold particles increased during fru it development. The immunoblotting of soluble and insoluble acid invertases provided a supporting proof for the assays of immunolocalization. The bioc hemical analysis showed a predominantly cell-wall-distributed activity of a cid invertase that corresponds essentially with its amount distribution, Th e ultrastructural observations showed that there were numerous plasmodesmat a between the parenchyma cells, but almost no plasmodesmium between the SE/ CC complex and its surrounding parenchyma cells, practically resulting in t he symplasmic isolation of the SE/CC complex. It is therefore suggested tha t the unloading pathway of sucrose from the SE/CC complex may be predominan tly apoplasmic in the developing apple fruit, and that the unloaded sucrose may be hydrolysed by the functional acid invertase localized in the cell w all before it is loaded in sink cells.