Gentiobiose: a novel oligosaccharin in ripening tomato fruit

Two neutral disaccharides, gentiobiose [beta- D-Glc p-(1-->6)- D-Glc] and nigerose [alpha- D-Glc p-(1-->3)- D-Glc], were detected in tomato ( Lycopersicon esculentum Mill.) pericarp and locule. Gentiobiose was present in the locule of green fruit and ripe fruit at 0.88 and 5.8 micro mol (kg fresh weight)(-1), respectively. When vacuum-infiltrated into green tomato fruit, exogenous gentiobiose (50 or 200 micro g per fruit) hastened the initiation of ripening (as judged by colour change) by 1-3 days relative to fruit that were infiltrated with glucose or isomaltose. Nigerose plus gentiobiose was particularly effective, but nigerose alone had no significant effect. The endogenous disaccharides were found to be present in the apoplastic fluid of the fruit, compatible with a proposed intercellular signalling role. The origin and metabolic fate of the disaccharides were investigated. Phenolic esters of these disaccharides were not detectable in tomato fruit and it is therefore unlikely that the free disaccharides were formed from a pool of such esters. An alternative possible biosynthetic origin, via transglycosylation, is discussed. When [(14)C]gentiobiose was vacuum-infiltrated into unripe or ripe fruit, the disaccharide remained intact for at least 1 h but was largely degraded within 24 h. The results suggest that gentiobiose is a new, naturally occurring oligosaccharin with a rapid turnover rate.