Transfers mannose from GDP-mannose to dolichol monophosphate to form dolichol phosphate mannose (Dol-P-Man) which is the mannosyl donor in pathways leading to N-glycosylation, glycosyl phosphatidylinositol membrane anchoring, and O-mannosylation of proteins; catalytic subunit of the dolichol-phosphate mannose (DPM) synthase complex. Plays a role in plant development and physiology, sensitivity to ammonium stress and endoplasmic reticulum stress response. a dolichyl phosphate + GDP-alpha-D-mannose = a dolichyl beta-D-mannosyl phosphate + GDP Protein modification; protein glycosylation. Component of the dolichol-phosphate mannose (DPM) synthase complex composed of DPMS1, DPMS2 and DPMS3; in the complex interacts directly with DPMS3. Anchored to the ER membrane by DPMS2 and DPMS3 of the dolichol-phosphate mannose (DPM) synthase complex. Reduced root growth. Slight reduction of chlorophyll content. Wrinkled seed coat. Hypersensitivity to ammonium. Plants over-expressing DPMS1 show altered stem branch diameter and morphology, perturbation of the vascular bundle arrangements, wrinkled seed coat and constitutive endoplasmic reticulum stress response. Belongs to the glycosyltransferase 2 family. Truncated N-terminus.