Mannosyl transferase (ManT) required for the biosynthesis of mannose-carrying glycosylinositol phosphorylceramides (GIPCs) (PubMed:27895225). Maybe involved in cell-cell adhesion that maintains the integrity of organs by providing mechanical strength and facilitating the movement of metabolites throughout the plant during development (PubMed:16045474). Prevents abscisic acid- (ABA-) mediated effects on development (e.g. cell size, flowering time, senescence). Probably implicated in beta-(1,4)-galactan biosynthesis thus being a cell-wall synthesis-related (CWSR) protein (PubMed:17426055, PubMed:20687615). Protein modification; protein glycosylation. Sphingolipid metabolism. Expressed in leaves, roots, stem, and flowers. Disturbed production of mannose-carrying glycosylinositol phosphorylceramides (GIPCs) in gmt1-3 (PubMed:27895225). Dwarf plants exhibiting a constitutive hypersensitive response characterized by elevated salicylic acid (SA) and hydrogen peroxide H(2)O(2) levels (PubMed:27895225). Reduced cellulose content, but increased lignin accumulation in cell walls (PubMed:27895225). In epc1-1, strong growth reduction, mechanical fragility, defects in vascular formation and enhanced secondary growth in hypocotyl tissues probably due to abnormal cell-cell adhesion properties in hypocotyl, cotyledon and cortical parenchyma tissues (PubMed:16045474). Ectopic callose deposition, increased glucose level, but reduced level of galactose in cell walls (PubMed:16045474). In epc1-2, dwarf, reduced cell size, defective root hair development, delayed flowering, early senescence, and hypersensitivity to abscisic acid (ABA) during germination and root elongation (PubMed:17426055). Reduced levels of beta-(1,4)-galactan (PubMed:17426055). Initially thought to be involved in cell-cell adhesion (PubMed:16045474), this function could not be confirmed (PubMed:17426055). Belongs to the glycosyltransferase 64 family.