Free <i>N</i>-glycans occurring in plant extracellular fluid or cytosol interact with an auxin, indole-3-acetic acid: Putative biofunction of free <i>N</i>-glycans in plants

Two types of free <i>N</i>-glycans (FNGs), high mannose type (HMT) and plant complex type (PTC), occur ubiquitously in plants, the former mainly in the cytoplasm and the latter in the vacuole or extracellular fluid. It has been hypothesized that these plant FNGs have auxin-like activity that promotes fruit ripening based on the experimental results of adding FNGs to plant tissues; however, the postulated biological functions have not been proven at this time. In this study, using fluorescence analysis in vitro, we found that Man₃Fuc₁Xyl₁GlcNAc₂ (a PCT-FNG) occurring in plant extracellular fluids, significantly decreased the fluorescence intensity of IAA in a concentration-dependent manner at acidic (extracellular fluid) and neutral pH (cytosol), suggesting that this FNG interacts with IAA. These results suggest a possibility that the interaction of PCT-FNG and IAA may reduce the hydrophobicity of IAA in acidic environments and support the movement of IAA in plant extracellular fluids. The Interactions with IAA bearing the indole ring, appear to be unique to free <i>N</i>-glycans, since they were not for other oligosaccharides such as sucrose, lactose, or chitooligosaccharides. Some other PCT-FNGs and HMT-FNGs found in plants have also been confirmed to interact with IAA, suggesting that the common trimannosyl core structure of FNGs may be a prerequisite for such interactions.