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.