Apoplastic exosome-like vesicles: a new way of protein secretion in plants?

The presence of apoplastic proteins without predicted signal peptide in the gene sequence suggests the existence of protein secretion independent of the ER/Golgi classical route. In animals, one of the pathways proposed for alternative protein secretion involves the release of exosomes to the extracellular space. Although this pathway has not been dissected in plants some indirect evidence is emerging. We have reported that apoplastic fractions of sunflower seeds contain exosome-like vesicles. Besides, these vesicles are enriched in the lectin Helja, which is immunolocalized in the extracellular space even if it the protein has no predicted signal peptide. Here we show that Helja is not glycosylated and its secretion is insensitive to brefeldin A, two of the major characteristics to discard ER/Golgi-mediated protein transport. Moreover, the levels of Helja in sunflower extracellular vesicles are not affected by brefeldin A treatment. Our results suggest that Helja could be exported through an exosome-mediated pathway and point out that this mechanism may be responsible for the secretion of at least part of the leaderless proteins detected in the extracellular compartment of plants.