SOLUBLE ACID INVERTASE DETERMINES THE HEXOSE-TO-SUCROSE RATIO IN COLD-STORED POTATO-TUBERS

Cold storage of potato (Solanum tuberosum L.) tubers is known to cause accumulation of reducing sugars. Hexose accumulation has been shown t o be cultivar-dependent and proposed to be the result of sucrose hydro lysis via invertase. To study whether hexose accumulation is indeed re lated to the amount of invertase activities, two different approaches were used: (i) neutral and acidic invertase activities as well as solu ble sugars were measured in cold-stored tubers of 24 potato cultivars differing in the cold-induced accumulation of reducing sugars and (ii) antisense potato plants with reduced soluble acid invertase activitie s were created and the soluble sugar accumulation in cold-stored tuber s was studied. The cold-induced hexose accumulation in tubers from the different potato cultivars varied strongly (up to eightfold). Large d ifferences were also detected with respect to soluble acid (50-fold) a nd neutral (5-fold) invertase activities among the different cultivars . Although there was almost no correlation between the total amount of invertase activity and the accumulation of reducing sugars there was a striking correlation between the hexose/sucrose ratio and the extrac table soluble invertase activitiy. To exclude the possibility that oth er cultivar-specific features could account for the obtained results, the antisense approach was used to decrease the amount of soluble acid invertase activity in a uniform genetic background. To this end the c DNA of a cold-inducible soluble acid invertase (EMBL nucleic-acid data base accession no. X70368) was cloned from the cultivar Desiree, and t ransgenic potato plants were created expressing this cDNA in the antis ense orientation under control of the constitutive 35S cauliflower mos aic virus promotor. Analysis of the harvested and cold-stored tubers s howed that inhibition of the soluble acid invertase activity leads to a decreased hexose and an increased sucrose content compared with cont rols. As was already found for the different potato cultivars the hexo se/sucrose ratio decreased with decreasing invertase activities but th e total amount of soluble sugars did not significantly change. From th ese data we conclude that invertases do not control the total amount o f soluble sugars in cold-stored potato tubers but are involved in the regulation of the ratio of hexose to sucrose.