Accumulation Pattern of IgG Antibodies and Fab Fragments in Transgenic Arabidopsis thaliana Plants

For the further optimization of antibody expression in plants,it is essential to determine the final accumulation sites ofplant-made antibodies. Previously, we have shown that, uponsecretion, IgG antibodies and F_ab fragments can be detectedin the intercellular spaces of leaf mesophyil cells of transgenicArabidopsis thaliana plants. However, immunofluorescence microscopyshowed that this is probably not their final accumulation site.In leaves, IgG and F_abfragments accumulate also at the interiorside of the epidermal cell layers and in xylem vessels. Theseaccumulation sites correspond with the leaf regions where waterof the transpiration stream is entering a space impermeableto the proteins or where water is evaporating. In roots, plant-madeF_ab fragments accumulate in intercellular spaces of cortex cells,in the cytoplasm of pericycle and, to a lesser extent, endodermiscells, and in cells of the vascular cylinder. In other words,antibody accumulation occurs at the sites where water passeson its radial pathway towards and within the vascular bundle.Taken together, our results suggest that, upon secretion ofplant-made antibodies or F_ab fragments, a large proportion ofthese proteins are transported in the apoplast of A. thaliana,possibly by the water flow in the transpiration stream. ⁴Corresponding author. Fax 32-9-2645349; e-mail: anpic{at}gengenp.rug.ac.be