Effect of tunicamycin on metabolism of unconjugated N-glycans in relation to regulation of tomato fruit ripening.

The unconjugated N-glycans (UNGs) Man3(Xyl)GlcNAc(Fuc)GlcNAc and Man5GlcNAc influence tomato (Lycopersicon esculentum cv 'Rutgers') fruit ripening at nanomolar concentrations. In addition, we have shown a delaying effect on ripening of tomato pericarp discs by 10 ng per g fresh weight Man5GlcNAc, one of 10 UNGs purified from mature green tomato pericarp. In the present study, oligomannosidic UNGs from tomato fruit exhibited activity similar to Man5GlcNAc. However, complex UNGs isolated from tomato pericarp did not show the same inhibitory effect as Man3(Xyl)GlcNAc(Fuc)GlcNAc, a complex N-glycan not detected in tomato. The metabolism of UNGs was also studied by metabolic labeling with 14C-glucosamine, revealing that the amount of oligomannosidic and complex type UNGs may change inversely during ripening. Both 14C-labelled UNGs disappeared after treatment with tunicamycin, an inhibitor of N-glycosylation. There was a correlation between the appearance of UNGs and recovery of ripening after tunicamycin treatment. Furthermore, application of 10 nM Man5GlcNAc with 40 microM tunicamycin prevented the delaying effect of tunicamycin. Our results indicate that endogenous unconjugated Man5GlcNAc may have a vital role in triggering and modulating tomato fruit ripening.