Cell-surface receptors enable perception of extracellular cytokinins

Ioanna Antoniadi,Thomas Vain,Alexander Johnson,Jan Hejátko,Markéta Pernisová,Stéphanie Robert,Václav Mik,Radim Simerský,Zuzana Gelová,Ondřej Novák,Lenka Plačková,Eduardo Mateo-Bonmatí,Jiří Friml,Colin Turnbull,Ondřej Plíhal,Karin Ljung,Korawit Opassathian,Karel Doležal,Michal Karady

doi:10.1038/s41467-020-17700-9

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.

Highlights

Cytokinins are mobile multifunctional plant hormones with roles in development and stress resilience
The evidence we provide here is that root cells with significant Two Component Signalling cytokinin-specific reporter (TCSn)::GFP expression contain nearly threefold more total cytokinin than cells sorted into the GFP− category
When cytokinin catabolism was blocked with the inhibitor INCYDE, TCSn::GFP signal in whole roots was greatly enhanced especially in stele cells, and protoplasts showed higher levels of tZ, but not of the other bioactive cytokinins cZ and isopentenyl adenine (iP)

Summary

Introduction

Cytokinins are mobile multifunctional plant hormones with roles in development and stress resilience. The cytokinin-responsive synthetic promoter fusion TCSn::GFP was developed to reflect global ARR-B transcriptional activity[19] and has facilitated in vivo monitoring of cytokinin responses, leading to new discoveries about cytokinin function[20,21]. It is unclear whether TCSn::GFP signal strength is quantitatively related to cellular cytokinin content and uncertainty remains about which active cytokinin(s) are responsible for different responses in the root tip and other tissues. In addition to the known route of cytokinin signalling via ER receptors, plasma membrane receptors are able to initiate signalling in response to extracellular cytokinins

Methods

Results

Conclusion