The potato protease inhibitor gene, St-Inh, plays roles in the cold-induced sweetening of potato tubers by modulating invertase activity

Reducing sugar accumulation is determined mainly by acid invertase activity in cold-stored potato tubers. The potato Kunitz-type protease inhibitor St-Inh reduces acid invertase activity in vitro, is linked with a quantitative trait locus for sugar content, and is therefore speculated to be involved in the cold-induced sweetening (CIS) of potato tubers. In this study, the expression profile of St-Inh in various organs of potato plants and stored tubers was characterized, and it was found that expression was highest in tubers and was strongly suppressed by low temperatures. This expression pattern was opposite to reducing sugar accumulation in the cold-stored tubers, suggesting a possible involvement of St-Inh in CIS in tubers. Overexpression of St-Inh in tubers resulted in lower acid invertase activities and reducing sugar contents in comparison with wild-type tubers, confirming the role of St-Inh in resistance to CIS. Interestingly, a greater reduction in potato tuber CIS was obtained after overexpression of the tobacco invertase inhibitor NtInvInh2, which belongs to the pectin methylesterase/invertase inhibitor family. The NtInvInh2 transgenic tubers had an even lighter chip color than did the St-Inh transgenic tubers. Both inhibitors are confirmed to be involved in reducing potato CIS, although protease inhibitors of the pectin methylesterase/invertase type may have a stronger capacity to inhibit acid invertase activity than Kunitz-type ones. These results provide novel clues to the mechanism by which potato CIS is regulated.