N-acetylglucosaminyltransferase II (GnTII) is required for stress tolerance in plants

The most abundant N-glycan in plants is the paucimannosidic N-glycan with core β1,2-xylose and α 1,3-fucose residues Man3XylFucGlcNAc2. Here, we report a mechanism in Arabidopsis thaliana that efficiently produces the largest N-glycan in plants. Genetic and biochemical evidence indicates that the addition of the 6-arm β1,2-GlcNAc residue by N-acetylglucosaminyltransferase II (GnTII) is less effective than additions of the core β1,2-xylose and α1,3-fucose residues by XylT, FucTA, and FucTB in Arabidopsis. Furthermore, analysis of gnt2 mutant and 35S:GnTII transgenic plants shows that the addition of the 6-arm non-reducing GlcNAc residue to the common N-glycan acceptor GlcNAcMan3GlcNAc2 inhibits additions of the core β1,2-xylose and α1,3-fucose residues. Our findings indicate that plants limit the rate of the addition of the 6-arm GlcNAc residue to the common N-glycan acceptor as a mechanism to facilitate formation of the prevalent N-glycans with Man3XylFucGlcNAc2 and GlcNAc2Man3XylFucGlcNAc2 structures. To investigate the physiological effects of the addition of the 6-arm GlcNAc residue to the N-glycans in plants, phenotypes of Col-0, gnt2, 35S:GnTII and a triple-knockout mutant with mutated alleles of N-acetyl-beta-D-hexosaminidases (Hex1, Hex2 and Hex3) were analysed in the absence or presence of exogenously supplied tunicamycin (TM) or sodium chloride (NaCl). gnt2, compared with Col-0, 35S:GnTII and the triple mutant, displayed increased sensitivity to TM and NaCl during germination and seedling development, indicating that GnTII is important to confer stress tolerance on plants.