Sucrose Synthase Activity and Expression during Development and Ripening in Bananas

Banana (Musa acuminata cv. Nanicão, AAA) sucrose synthase (SS, E.C. 2.4.1.13) is a 100kD subunit tetramer found in all stages from development to senescence in two forms, with different pI (4.59 and 4.77). The fruit SS isoforms are similar to those from leaves and roots, as shown by immunoblotting. Their activity is high during the period of starch synthesis but after harvesting and through ripening it drops to very low levels, accompanying starch degradation. The high activity is maintained by expression of SS genes, that are switched off after harvesting and moreover at the onset of ripening. It is concluded that 55 seems to be important for starch formation in bananas, but not for sucrose synthesis, and that gene expression regulates the activity level in the fruit.