Probable glycosyltransferase regulating the assembly and trafficking of cellulose synthase complexes. Homo- and heterodimer with STL2 (PubMed:27277162). Interacts with CESA1, CESA3, CESA4, CESA6, CESA7 and CESA8, but not with GOT1 (PubMed:27277162). Expressed in cells that are expanding or producing secondary cell walls. No visible phenotype, due to the redundancy with STL2. Stl1 and stl2 double mutants are impaired in cellulose production and exhibit a stunted growth. In classical Greek, STELLO mean 'to set in order, arrange, send'. Belongs to the STELLO family.