Cloning, developmental, and tissue-specific expression of Sucrose : Sucrose 1-fructosyl transferase from Taraxacum officinale. Fructan localization in roots

Sucrose:sucrose 1-fructosyl transferase (1-SST) is the key enzyme initiatin g fructan synthesis in Asteraceae. Using reverse transcriptase-PCR, we isol ated the cDNA for 1-SST from Taraxacum officinale. The cDNA-derived amino a cid sequence showed very high homology to other Asteracean 1-SSTs (Cichoriu m intybus 86%, Cynara scolymus 82%, Helianthus tuberosus 80%), but homology to 1-SST from Allium cepa (46%) and Aspergillus foetidus (18%) was much lo wer. Fructan concentrations, 1-SST activities, l-SST protein, and mRNA conc entrations were compared in different organs during vegetative and generati ve development of T. officinale plants. Expression of 1-SST was abundant in young roots but very low in leaves. 1-SST was also expressed at the flower ing stages in roots, stalks, and receptacles. A good correlation was found between northern and western blots showing transcriptional regulation of 1- SST. At the pre-flowering stage, 1-SST mRNA concentrations and 1-SST activi ties were higher in the root phloem than in the xylem, resulting in the hig her fructan concentrations in the phloem. Fructan localization studies indi cated that fructan is preferentially stored in phloem parenchyma cells in t he vicinity of the secondary sieve tube elements. However, inulin-like crys tals occasionally appeared in xylem vessels.