Small extracellular vesicles released from germinated kiwi pollen (pollensomes) present characteristics similar to mammalian exosomes and carry a plant homolog of ALIX

Abstract

Introduction: In the last decade, it has been discovered that allergen-bearing extracellular nanovesicles, termed "pollensomes", are released by pollen during germination. These extracellular vesicles (EVs) may play an important role in pollen-pistil interaction during fertilization, stabilizing the secreted bioactive molecules and allowing long-distance signaling. However, the molecular composition and the biological role of these EVs are still unclear. The present study had two main aims: (I) to clarify whether pollen germination is needed to release pollensomes, or if they can be secreted also in high humidity conditions; and (II) to investigate the molecular features of pollensomes following the most recent guidelines for EVs isolation and identification.

Methods: To do so, pollensomes were isolated from hydrated and germinated kiwi (Actinidia chinensis Planch.) pollen, and characterized using imaging techniques, immunoblotting, and proteomics.

Results: These analyses revealed that only germinated kiwi pollen released detectable concentrations of nanoparticles compatible with small EVs for shape and protein content. Moreover, a plant homolog of ALIX, which is a well-recognized and accepted marker of small EVs and exosomes in mammals, was found in pollensomes.

Discussion: The presence of this protein, along with other proteins involved in endocytosis, is consistent with the hypothesis that pollensomes could comprehend a prominent subpopulation of plant exosome-like vesicles.

Keywords: ALIX; EVs; exosomes; nanovesicles; pollen tube; pollensomes; pollination; signaling.