Sucrose synthase activity and expression during development and ripening in bananas

Banana (Musa acuminata cv. Nanicao, AAA) sucrose synthase (SS, E.C. 2.4.1.1 3) is a 100 kD subunit tetramer found in all stages from development to sen escence in two forms, with different pi (4.59 and 4.77). The fruit SS isofo rms are similar to those from leaves and roots, as shown by immunoblotting. Their activity is high during the period of starch synthesis but after har vesting and through ripening it drops to very low levels, accompanying star ch degradation. The high activity is maintained by expression of SS genes, that are switched off after harvesting and moreover at the onset of ripenin g. It is concluded that SS seems to be important for starch formation in ba nanas, but not for sucrose synthesis, and that gene expression regulates th e activity level in the fruit.