Protein phase separation in plant membrane biology: more than just a compartmentalization strategy.

The formation of biomolecular condensates through phase separation is an important strategy to compartmentalize cellular functions. While it is now well established that condensates exist throughout eukaryotic cells, how condensates assemble and function on lipid membranes is only beginning to be understood. In this perspective, we highlight work from plant, animal, and yeast model systems showing that condensates assemble on many endomembrane surfaces to carry out diverse functions. In vesicle trafficking, condensation has reported roles in the formation of endocytic vesicles and autophagosomes, and in the inactivation of secretory COPII vesicles. We briefly discuss how membranes and membrane lipids regulate the formation and function of membrane-associated condensates. This includes how membranes act as surfaces for condensate assembly, with lipids mediating the nucleation of condensates during endocytosis and other processes. Additionally, membrane-condensate interactions give rise to the biophysical property of 'wetting', which has functional importance in shaping autophagosomal and vacuolar membranes. We also speculate on the existence of membrane-associated condensates during cell polarity in plants, and discuss how condensation may help to establish functional plasma membrane domains. Lastly, we provide advice on relevant in vitro and in vivo approaches and techniques to study membrane-associated phase separation.