Direct evidence of the symplastic pathway in the trap of the bladderwort Utricularia gibba L

To capture prey, Utricularia gibba, an aquatic angiosperm carnivorous plant, is equipped with specialized bladders containing bifid/quadrifid glands for nutrient absorption. Several studies have focused on the nutrient absorption and subsequent transportation in the bladderworts; more specifically, the apoplastic pathway has been demonstrated as employing radioactive uranium salts. Nevertheless, the symplastic pathway has not been unambiguously demonstrated. Herein, we initially used food dyes as tracers to monitor the absorption processes by light microscope. We then confirmed the observed symplastic pathway using another vital tracer carboxyfluorescein diacetate (CFDA). The absorption and transportation of the CFDA inside the traps were observed using epifluorescence microscope and confocal laser scanning microscope. Our data clearly suggested that the tracers were transported through different tissues in the following order: terminal cells, pedestal cells, basal cells, surrounding epidermal cells, nearby leaves, and, finally, stems. The process was found to be light-sensitive, suggesting that it is energy-dependent. The uptake of the fluorescent dye was observed within seconds, while that of food dyes required 2 to 3 h. Thus, CFDA provides better resolution, while the food dyes afford prolonged tracing procedures. Taken together, the findings lead us to conclude that the symplastic pathway is an important transportation process that has never been shown previously in Utricu-laria.