Cell-surface receptors enable perception of extracellular cytokinins

Abstract

Cytokinins are mobile multifunctional plant hormones with roles in development and stress resilience. Although their Histidine Kinase receptors are substantially localised to the endoplasmic reticulum, cellular sites of cytokinin perception and importance of spatially heterogeneous cytokinin distribution continue to be debated. Here we show that cytokinin perception by plasma membrane receptors is an effective additional path for cytokinin response. Readout from a Two Component Signalling cytokinin-specific reporter (TCSn::GFP) closely matches intracellular cytokinin content in roots, yet we also find cytokinins in extracellular fluid, potentially enabling action at the cell surface. Cytokinins covalently linked to beads that could not pass the plasma membrane increased expression of both TCSn::GFP and Cytokinin Response Factors. Super-resolution microscopy of GFP-labelled receptors and diminished TCSn::GFP response to immobilised cytokinins in cytokinin receptor mutants, further indicate that receptors can function at the cell surface. We argue that dual intracellular and surface locations may augment flexibility of cytokinin responses.

Cytokinin biosynthesis and transport for systemic nitrogen signaling

“…On the other hand, overexpression of Arabidopsis CYTOKININ OXIDASE/DEHYDROGENASE 1 ( CKX1 ), which encodes a cytokinin‐degrading enzyme that localizes to the ER lumen, leads to a severe phenotype of cytokinin deficiency (Niemann et al ., 2018). Recent studies demonstrated that cytokinins are perceived at the cell surface and at ER‐located receptors (Antoniadi et al ., 2020; Kubiasová et al ., 2020). Both perception sites are important to organize cytokinin signaling, but functional differentiation is still unsolved (Romanov et al ., 2018).…”

The Hulks and the Deadpools of the Cytokinin Universe: A Dual Strategy for Cytokinin Production, Translocation, and Signal Transduction

“…Moreover, recent experiments confirmed that functional receptors are localised on the plasma membrane [ 253 , 255 ]. While Antoniadi et al [ 253 ] showed that extracellular CKs are capable of CK receptor activation, Zürcher et al [ 255 ] showed the significance of CK removal from the apoplast for proper development. Indeed, there are several apoplast-localised CKX enzymes (e.g., [ 132 , 256 ]).…”

“…However, if we accept the premise that the pH of the apoplast is changing, that would also be irrelevant. Further, it is AHK3 that responds strongly to pH change [ 13 ] and it is localised to a lesser extent on the plasma membrane [ 247 , 253 ] as compared to AHK4, which does not respond so strongly to the pH change. The poplar homologues of AHK4 showed, in fact, a negative correlation between t Z binding and pH [ 254 ].…”

“…Similarly, the At CKX1 was shown to localise to the ER [ 215 ]. A higher proportion of AHK4 was reported to localise to non-ER membranes though [ 247 , 253 ]. Interestingly, perception of the extracellular CKs was most impaired in plants lacking AHK4, even in a single mutant [ 253 ].…”

Redundant and Non-redundant Functions of the AHK Cytokinin Receptors During Gynoecium Development

“…Cytokinin receptors are located in the endoplasmic reticulum as well at the plasma membrane where they perceive cytokinin molecules, followed by the activation of a signal transduction pathway, which is a phosphorelay system that reminds us of the bacterial two-component signaling systems ( Kakimoto, 1996 ; Hwang and Sheen, 2001 ; Schaller et al, 2011 ; Hwang et al, 2012 ; Kieber and Schaller, 2018 ; Romanov et al, 2018 ; Antoniadi et al, 2020 ; Kubiasová et al, 2020 ). Cytokinin is first sensed by a CHASE domain of a histidine kinase receptor (in Arabidopsis, AHK), followed by autophosphorylation of its receiver domain.…”