Transformation strategies for stable expression of complex hetero‐multimeric proteins like secretory immunoglobulin A in plants

Plant expression systems have proven to be exceptional in producing high‐value complex polymeric proteins such as secretory IgAs (SIgAs). However, polymeric protein production requires the expression of multiple genes, which can be transformed as single or multiple T‐DNA units to generate stable transgenic plant lines. Here, we evaluated four strategies to stably transform multiple genes and to obtain high expression of all components. Using the in‐seed expression of a simplified secretory IgA (sSIgA) as a reference molecule, we conclude that it is better to spread the genes over two T‐DNAs than to contain them in a single T‐DNA, because of the presence of homologous recombination events and gene silencing. These T‐DNAs can be cotransformed to obtain transgenic plants in one transformation step. However, if time permits, more transformants with high production levels of the polymeric protein can be obtained either by sequential transformation or by in‐parallel transformation followed by crossing of transformants independently selected for excellent expression of the genes in each T‐DNA.