N-linked glycosylation is an essential protein modification that helps protein folding, trafficking and translocation in eukaryotic systems. The initial process for N-linked glycosylation shares a common pathway with assembly of a dolichol-linked core oligosaccharide. Here we characterize a new Arabidopsis thaliana mutant lew3 (leaf wilting 3), which has a defect in an alpha-1,2-mannosyltransferase, a homolog of ALG11 in yeast, that transfers mannose to the dolichol-linked core oligosaccharide in the last two steps on the cytosolic face of the ER in N-glycan precursor synthesis. LEW3 is localized to the ER membrane and expressed throughout the plant. Mutation of LEW3 caused low-level accumulation of Man(3)GlcNAc(2) and Man(4)GlcNAc(2) glycans, structures that are seldom detected in wild-type plants. In addition, the lew3 mutant has low levels of normal high-mannose-type glycans, but increased levels of complex-type glycans. The lew3 mutant showed abnormal developmental phenotypes, reduced fertility, impaired cellulose synthesis, abnormal primary cell walls, and xylem collapse due to disturbance of the secondary cell walls. lew3 mutants were more sensitive to osmotic stress and abscisic acid (ABA) treatment. Protein N-glycosylation was reduced and the unfolded protein response was more activated by osmotic stress and ABA treatment in the lew3 mutant than in the wild-type. These results demonstrate that protein N-glycosylation plays crucial roles in plant development and the response to abiotic stresses.