Sugar accumulation and activities of enzymes involved in fructan dynamics from seedling to bulb formation in onion (Allium cepa L.)

Onion (Allium cepa L.) produces a fructose-based oligosaccharide (fructan) in the bulb as the major reserve carbohydrate. We previously found varietal differences in fructan quantity in the harvested bulbs; 'Kita-momiji 2000' accumulated a high level of fructans with a high degree of polymerization, while 'Pole Star' had low amounts and accumulated mainly monosaccharides and sucrose. In this study, we examined fructan accumulation and enzymatic activities involved in the fructan metabolic pathway in more detail in both aerial and subterranean tissues that were harvested throughout onion development. Analyses of fructan levels in the two cultivars revealed that levels in fructan accumulation were almost the same in aerial and basal leaves before bulb formation, but fructan levels differed significantly in the late stage after bulb formation; varietal differences in fructan content in the harvested bulb were apparent only during the 2 or 3 weeks before harvest. At an early developmental stage, fructan levels in 'Pole Star' were equivalent to those in 'Kita-momiji 2000', indicating that 'Pole Star' can synthesize highly polymerized fructans. In analyses of enzymatic activities, 'Pole Star' had high activity for synthesizing fructan after bulb formation compared to 'Kita-momiji 2000' and also greater potential for degrading sucrose and fructan before bulb formation, leading to less sucrose accumulation both in aerial and basal leaves. These results suggest that high activities for fructan degradation enzymes in 'Pole Star' cause less fructan accumulation in the bulb than in 'Kita-Momiji 2000' because this cultivar cannot sustain high levels of sucrose, the starting substrate for fructan synthesis.