CLONING OF SUCROSE - SUCROSE 1-FRUCTOSYLTRANSFERASE FROM ONION AND SYNTHESIS OF STRUCTURALLY DEFINED FRUCTAN MOLECULES FROM SUCROSE

Sucrose (Suc):Suc 1-fructosyltransferase (1-SST) is the key enzyme in plant fructan biosynthesis, since it catalyzes die novo fructan synthe sis from Sue. We have cloned 1-SST from onion (Allium cepa) by screeni ng a cDNA library using acid invertase from tulip (Tulipa gesneriana) as a probe. Expression assays in tobacco (Nicotiana plumbaginifolia) p rotoplasts showed the formation of 1-kestose from Sue. In addition, an onion acid invertase clone was isolated from the same cDNA library. P rotein extracts of tobacco protoplasts transformed with this clone sho wed extensive Suchydrolyzing activity. Conditions that induced fructan accumulation in onion leaves also induced 1-SST mRNA accumulation, wh ereas the acid invertase mRNA level decreased. Structurally different fructan molecules could be produced from Suc by a combined incubation of protein extract of protoplasts transformed with 1-SST and protein e xtract of protoplasts transformed with either the onion fructan:fructa n 6G-fructosyltransferase or the barley Suc:fructan 6-fructosyltransfe rase.