Ethylene and Cytokinin:Partners in Root Growth Regulation

Ethylene and cytokinin,two plant hormones with essential functions throughout the plant's life cycle,contribute to a wide spectrum of physiological,biochemical,and developmental processes.Over the last decades,the molecular mechanisms underlying their biosynthesis and metabolism as well as pathways mediating their perception and signal transduction have been deciphered.Although machineries controlling the activities of these two central hormonal regulators appear robust and autonomous,in the ever-changing environmental conditions that plants face,survival is dependent on their tight communication and mutually balanced activities.A recent work by Zdarska et al.(2019)is trying to address the challenging question regarding the mechanisms underlying the cytokinin and ethylene cross-talk.This study reveals an intriguing,multilayered nature of hormonal interaction,in which the EHYLENE RESISTANT 1(ETR1)receptor upon perceiving an ethylene signal activates a multistep phosphorelay(MSP)cytokinin transduction cascade,integrating ethylene and cytokinin in the control of root apical meristem size.In parallel,activation of canonical ethylene signaling adjusts the responsiveness of the cytokinin pathway by transcriptional upregulation of one of its key signaling components,the ARR10 response regulator(Figure 1).Importantly,unlike the canonical ethylene transduction machinery,ethylene input into the MSP is dependent on histidine kinase activity of ETR1 receptor.

TOLs Function as Ubiquitin Receptors in the Early Steps of the ESCRT Pathway in Higher Plants

Protein abundance and localization at the plasma membrane(PM)shapes plant development and mediates adaptation to changing environmental conditions.It is regulated by ubiquitination,a post-translational modification crucial for the proper sorting of endocytosed PM proteins to the vacuole for subsequent degradation.To understand the significance and the variety of roles played by this reversible modification,the function of ubiquitin receptors,which translate the ubiquitin signature into a cellular response,needs to be elucidated.In this study,we show that TOL(TOM1-like)proteins function in plants as multivalent ubiquitin receptors,governing ubiquitinated cargo delivery to the vacuole via the conserved Endosomal Sorting Complex Required for Transport(ESCRT)pathway.TOL2 and TOL6 interact with components of the ESCRT machinery and bind to K63-linked ubiquitin via two tandemly arranged conserved ubiquitin-binding domains.Mutation of these domains results not only in a loss of ubiquitin binding but also altered localization,abolishing TOL6 ubiquitin receptor activity.Function and localization of TOL6 is itself regulated by ubiquitination,whereby TOL6 ubiquitination potentially modulates degradation of PM-localized cargoes,assisting in the fine-tuning of the delicate interplay between protein recycling and downregulation.Taken together,our findings demonstrate the function and regulation of a ubiquitin receptor that mediates vacuolar degradation of PM proteins in higher plants.