N-glycan metabolism and plant cell differentiation and growth

Free N-glycans are present at micromolar concentrations in plant cells during their differentiation, growth and maturation stages, and might play a role in processes such as seed germination and fruit ripening. The structure of free N-glycans, which are found in hypocotyls and developing seeds and fruit, can be classified into two types: a high-mannose type (HMT) and a plant complex type (CT); the former, in most cases, has only one GlcNAc residue, while the latter has a chitobiose unit. It is thought that the enzyme endo-β-N- acetylglucosaminidase (endo-β-GlcNAc-ase) is involved in the production of HMT sugar chains, whereas the enzyme peptide:N-glycanase (PNGase) is involved in the production of plant CT sugar chains. However, the mechanism and significance of free N-glycan production in plant cells remain obscure. To characterize N-glycan metabolism and the physiological function of free sugar chains, we have investigated the substrate specificities, intracellular distributions, and gene structures of endo-β-GlcNAc-ase, PNGase, and α-mannosidase in various plants. Here, we report our discovery that endo-β-GlcNAc-ase activity begins to increase at a specific stage of tomato ripening, and that the amount of free N-glycans dramatically increases in conjunction with this event. In addition, the structural properties of free N-glycans also change notably as the fruit ripens. This review describes N-glycan metabolism in plant cells, and proposes a role for free sugar chains in the differentiation and growth of plants. The recent finding that plant CT sugar chains are immunoactive is also discussed.