Modulation of abscisic acid signaling via endosomal TOL proteins

The green ESCRTs: Newly defined roles for ESCRT proteins in plants

Endocytosis and endosomal trafficking of plasma membrane proteins for degradation regulate cellular homeostasis and development. As part of these processes, ubiquitinated plasma membrane proteins (cargo) are recognized, clustered, and sorted into intraluminal vesicles of multivesicular endosomes by endosomal sorting complexes required for transport (ESCRT) proteins. At endosomes, ESCRT proteins recognize ubiquitinated cargo and mediate the deformation of the endosomal membrane in a negative geometry, away from the cytosol. ESCRTs are organized in five major complexes that are sequentially recruited to the endosomal membrane where they mediate its vesiculation and cargo sequestration. ESCRTs also participate in other membrane remodeling events and are widely conserved across organisms, both eukaryotes and prokaryotes. Plants contain both conserved and unique ESCRT components and show a general trend toward gene family expansion. Plant endosomes show a wide range of membrane budding patterns with potential implications in cargo sequestration efficiency, plant development, and hormone signaling. Understanding the diversification and specialization of plant ESCRT proteins can provide valuable insights in the mechanisms of ESCRT-mediated membrane bending. In this review, we discuss the endosomal function of ESCRT proteins, their unique features in plants, and the potential connections to the modes of plant endosomal vesiculation.

Recent Advances in Ubiquitin Signals Regulating Plant Membrane Trafficking

Ubiquitination is a reversible post-translational modification involving the attachment of ubiquitin, a 76-amino acid protein conserved among eukaryotes. The protein 'ubiquitin' was named after it was found to be ubiquitously expressed in cells. Ubiquitination was first identified as a post-translational modification that mediates energy-consuming protein degradation by the proteasome. After half a century, the manifold functions of ubiquitin are widely recognized to play key roles in diverse molecular pathways and physiological processes. Compared to humans, the number of enzymes related to ubiquitination is almost twice as high in plant species, such as Arabidopsis and rice, suggesting that this modification plays a critical role in many aspects of plant physiology including development and environmental stress responses. Here, we summarize and discuss recent knowledge of ubiquitination focusing on the regulation of membrane trafficking in plants. Ubiquitination of plasma membrane-localized proteins often leads to endocytosis and vacuolar targeting. In addition to cargo proteins, ubiquitination of membrane trafficking regulators regulates the morphodynamics of the endomembrane system. Thus, throughout this review, we focus on the physiological responses regulated by ubiquitination and their underlying mechanisms to clarify what is already known and what would be interesting to investigate in the future.