A process of producing a recombinant glycoprotein in a plant, in cells of a plant, or in plant cells, comprising expressing in said plant, in cells of said plant or in said plant cells a first nucleic acid sequence encoding a polypeptide having an N-glycosylation site and co-expressing a second nucleic acid sequence encoding a heterologous single-subunit oligosaccharyltransferase (OTase), wherein the first nucleic acid sequence is expressed from a viral vector and the second nucleic acid sequence is expressed from a non-viral vector, and wherein the single-subunit OTase transfers the oligosaccharide Glc3Man9GlcNAc2 from dolichol to an asparagine residue in the consensus sequence Asn-X-Ser or Asn-X-Thr, where X is any amino acid except proline, of a nascent protein in eukaryotic cells, and wherein the nucleic acid sequence encoding the single-subunit OTase is or comprises i. a nucleic acid sequence encoding a protein having an amino acid sequence having at least 80%, preferably at least 90%, sequence identity to the entire amino acid sequence of any one of SEQ ID NO: 2; or ii. a nucleic acid sequence encoding a protein having an amino acid sequence having at least 90%, preferably at least 95%, sequence similarity to the entire amino acid sequence of any one of SEQ ID NO: 2; or iii. a nucleic acid sequence encoding a protein having an amino acid sequence of a length of at least 80 %, preferably at least 90 %, more preferably at least 95% of the number of amino acid residues of any one of SEQ ID NO: 2, and a sequence identity of at least 90% to SEQ ID NO: 2 over such length; or iv. a nucleic acid sequence encoding a protein having an amino acid sequence having from 1 to 30 amino acid additions, substitutions or deletions compared to the amino acid sequence of any one of SEQ ID NO: 2; or v. wherein the single-subunit OTase is the Leishmania STT3A protein, STT3B protein, STT3C protein, STT3D protein or combinations thereof. wherein the first nucleic acid sequence is expressed from a viral vector and the second nucleic acid sequence is expressed from a non-viral vector, and wherein the single-subunit OTase transfers the oligosaccharide Glc3Man9GlcNAc2 from dolichol to an asparagine residue in the consensus sequence Asn-X-Ser or Asn-X-Thr, where X is any amino acid except proline, of a nascent protein in eukaryotic cells, and wherein the nucleic acid sequence encoding the single-subunit OTase is or comprises i. a nucleic acid sequence encoding a protein having an amino acid sequence having at least 80%, preferably at least 90%, sequence identity to the entire amino acid sequence of any one of SEQ ID NO: 2; or ii. a nucleic acid sequence encoding a protein having an amino acid sequence having at least 90%, preferably at least 95%, sequence similarity to the entire amino acid sequence of any one of SEQ ID NO: 2; or iii. a nucleic acid sequence encoding a protein having an amino acid sequence of a length of at least 80 %, preferably at least 90 %, more preferably at least 95% of the number of amino acid residues of any one of SEQ ID NO: 2, and a sequence identity of at least 90% to SEQ ID NO: 2 over such length; or iv. a nucleic acid sequence encoding a protein having an amino acid sequence having from 1 to 30 amino acid additions, substitutions or deletions compared to the amino acid sequence of any one of SEQ ID NO: 2; or v. wherein the single-subunit OTase is the Leishmania STT3A protein, STT3B protein, STT3C protein, STT3D protein or combinations thereof. i. a nucleic acid sequence encoding a protein having an amino acid sequence having at least 80%, preferably at least 90%, sequence identity to the entire amino acid sequence of any one of SEQ ID NO: 2; or ii. a nucleic acid sequence encoding a protein having an amino acid sequence having at least 90%, preferably at least 95%, sequence similarity to the entire amino acid sequence of any one of SEQ ID NO: 2; or iii. a nucleic acid sequence encoding a protein having an amino acid sequence of a length of at least 80 %, preferably at least 90 %, more preferably at least 95% of the number of amino acid residues of any one of SEQ ID NO: 2, and a sequence identity of at least 90% to SEQ ID NO: 2 over such length; or iv. a nucleic acid sequence encoding a protein having an amino acid sequence having from 1 to 30 amino acid additions, substitutions or deletions compared to the amino acid sequence of any one of SEQ ID NO: 2; or v. wherein the single-subunit OTase is the Leishmania STT3A protein, STT3B protein, STT3C protein, STT3D protein or combinations thereof.