Cytokinin Perception Also Starts at the Plasma Membrane

Abstract

The plant hormone cytokinin regulates diverse biological processes, including embryogenesis, cell division and differentiation, root and shoot architecture, vascular development, and formation of shoot and root lateral organs. The endoplasmic reticulum (ER) is considered as the primary cytokinin perception site based on the known subcellular localization of the cytokinin receptors. In the canonical cytokinin signaling pathway, the perception of cytokinin starts with the CHASE domain- containing AHK (Arabidopsis histidine kinase) receptors, which are located primarily in the ER. Upon perception through AHKs, AHPs (Arabidopsis histidine phosphotransferases) act as intermediates to transfer a phosphate from AHKs to the downstream type B ARRs (Arabidopsis response regulators) for initiating a cytokinin-mediated transcriptional response. A previous study, however, suggested that cytokinin signaling could also initiate from the plasma membrane (PM) (Zürcher et al., 2016Zürcher E. Liu J. di Donato M. Geisler M. Müller B. Plant development regulated by cytokinin sinks.Science. 2016; 353: 1027-1030Crossref PubMed Scopus (82) Google Scholar). Two recent reports confirmed the role of PM-localized AHKs in the perception of extracellular cytokinin. Antoniadi et al., 2020Antoniadi I. Novák O. Gelová Z. Johnson A. Plíhal O. Simerský R. Mik V. Vain T. Mateo-Bonmatí E. Karady M. et al.Cell-surface receptors enable perception of extracellular cytokinins.Nat. Commun. 2020; 11: 1-10Crossref PubMed Scopus (14) Google Scholar reported that extracellular cytokinins can bind to cell-surface receptors to elicit signaling. They used sorted protoplasts based on expression of the Two Component Signaling cytokinin-specific reporter (TCSn::GFP) and discovered that extracellular cytokinin can initiate signaling through PM receptors. When AHK receptors were tested for an extracellular cytokinin response, based on TCSn::GFP expression, the extracellular cytokinin response was confirmed to involve three AHK receptors,AHK2, AHK3, and AHK4. In addition, the authors observed a non-ER-localized signal, 25% of AHK3 and 36% of AHK4, most of which are localized at the cell surface. Using an alternative approach, Kubiasová et al., 2020Kubiasová K. Montesinos J.C. Šamajová O. Nisler J. Mik V. Semerádová H. Plíhalová L. Novák O. Marhavý P. Cavallari N. et al.Cytokinin fluoroprobe reveals multiple sites of cytokinin perception at plasma membrane and endoplasmic reticulum.Nat. Commun. 2020; 11: 1-11Crossref PubMed Scopus (17) Google Scholar demonstrated that ER-localized cytokinin receptors can be transported to the PM. The authors developed a cytokinin fluoroprobe called iP-NBD, which can bind to cytokinin receptors and track their subcellular localizations in planta. A colocalization study using the fluoroprobe and GFP-tagged AHK4 suggests that the cytokinin receptor resides in the PM and vesicles in addition to the ER. Furthermore, the treatment of brefeldin-A (BFA), a compound that inhibits ER-Golgi trafficking, resulted in the accumulation of iP-NBD in BFA compartments, suggesting that the vesicular trafficking of cytokinin receptors occurs between the ER and PM. These two recent studies have revised our current understanding about the initiation sites of cytokinin perception, and clearly suggested that extracellular cytokinin can initiate signaling at the PM and cytokinin receptors, localized both in the ER and PM undergo endosomal trafficking between the ER and PM (Figure 1). The author thanks Ioanna Antoniadi (Swedish University of Agricultural Sciences), Colin Turnbull (Imperial College London), and Lukáš Spíchal (Palacký University) for editing the text; Lukáš Spíchal (Palacký University) for suggestion in improving the figure; Amber De Neve (University of Massachusetts Amherst) for reading and editing the manuscript.