Sucrose synthesis in dormant tubers of Helianthus tuberosus.

MARTÍNEZ-NOËL G; BATTAGLIA M; PONTIS H

Baltimore, USA

Congreso; ASPB; 1999

ASPB

Fructans are composed of linear or branched chains of fructosyl units linked to sucrose. In tubers of Jerusalem artichoke (Helianthus tuberosus) they accumulate in late summer and early autumn reaching a degree of polymerization of about 35 fructosyl units. During tuber dormancy there occurs an hydrolysis and depolymerization of fructans with the result that at the end of the winter there are only polymers of up to DP 6. During polymer hydrolysis fructose is produced, however, a very particular characteristic of dormant tubers of J. artichoke is that no free fructose is found, and moreover the total amount of fructose present in the tuber is constant throughout the dormancy. It has been hypothesized that these two facts may be explained if the fructose produced by hydrolysis is used for synthesis of sucrose that acts as an acceptor for the transfer of fructosyl residues catalyzed by the enzyme fructan-fructan fructosyl transferase. We have measured the activities of sucrose synthase (SS) and also sucrose-phosphate synthase (SPS) hitherto not described in J. Artichoke tubers. Even more, we have been able to measure the activity of two isoforms of SPS that have a very different reactivity towards glucose-6-P and Pi. SS is 10 times more active than SPS. These results, however, could not tell which of two sucrose enzymes is responsible for sucrose synthesis during dormancy.