1. Supplemental figure 1. The structures of the wild-type (left) and aberrant Man5GlcNAc2 N-glycan (right) is shown.

2. Supplemental figure 2. ELISA assay of total amount of complex type N-glycans in wild-type and alg3-2 seeds. Proteins extracted from wild-type, alg3-2 and cgl mutant seeds and probed in an ELISA assay with a polyclonal anti-HRP antibody that specifically recognizes complex type N-glycans. Error bars represent SD.

3. Supplemental figure 3. Example of LC-QTOF-MS mass spectrum of trypsin and PNGase F treated MBP10 from alg3-2. The non- glycosylated (m/z 595.28, 2+) and deglycosylated (m/z 595.77, 2+) peptide of MBP10 elute at different retention times, 17.14 and 17.84 minutes respectively, as indicated by the arrows. The mass increase of 1 Da is the result of the PNGase F treatment, resulting in a N to D conversion of the glycosylated peptide. In this way the ratio between non-glycosylated and (de)glycosylated MBP10 can be deduced from the corresponding peak intensities in the mass spectrum.

4. Supplemental figure 4. Reactivity of purified MBP10 antibodies against MBP antigen. Protein A purified MBP10 antibodies from total seed protein fractions from wild-type and alg3-2 were tested against the maltose binding protein antigen in an ELISA assay. Error bars represent SD from duplo experiments.

5. Supplemental table 1. Relative amounts of N-glycans in wild-type and alg3-2 seeds. +ManI: treated with α(1-2)- mannosidase %: specific peak area divided by total peak areas in matrix-assisted laser-desorption ionization- time-of-flight (MALDI-TOF) mass spectra. n.d.: not detectable

6. Supplemental table 2. Relative abundance of the different MBP10 dimers (H0, H1 or H2) in wild-type and alg3-2 seeds (based on peak intensities). The calculated relative abundance based on concentrations of unglycosylated and glycosylated heavy chains (figure 6) is given between brackets.