Expression cassettes for the in planta production of H5-S·Tag and S·Protein fusion proteins. The ectodomain of hemagglutinin (H5) was trimerized by c-terminal fusion of trimeric motif (GCN4pII) . While wild-type S·Protein was either expressed alone or multimerized by fusion with different oligomeric motifs, such as trimerization (GCN4-pII, S·Protein-pII) and dimerization (GCN4 wild-type, S·Protein-GCN4) domains , and a mouse tailpiece (TP) element of mouse IgM (S·Protein-TP). Each of recombinant proteins was fused to a c-myc tag to allow for downstream detection by Western blot, a His tag (except for S·Protein-TP) to facilitate their purification by IMAC. The legumine B4 signal peptide and the KDEL motif were used to promote transgene products retention in the endoplasmic reticulum. CaMV35S Pro: cauliflower mosaic virus 35S ubiquitous promoter; CaMV 35S Term: cauliflower mosaic virus 35S terminator; Asterisk: the molecular weight of proteins was calculated for unglycosylated monomers.