Abstract

Cytokinins (CKs) control many plant developmental processes and responses to environmental cues. Although the CK signaling is well understood, we are only beginning to decipher its evolution. Here, we investigated the CK perception apparatus in early-divergent plant species such as bryophyte Physcomitrium patens, lycophyte Selaginella moellendorffii, and gymnosperm Picea abies. Of the eight CHASE-domain containing histidine kinases (CHKs) examined, two CHKs, PpCHK3 and PpCHK4, did not bind CKs. All other CHK receptors showed high-affinity CK binding (KD of nM range), with a strong preference for isopentenyladenine over other CK nucleobases in the moss and for trans-zeatin over cis-zeatin in the gymnosperm. The pH dependences of CK binding for these six CHKs showed a wide range, which may indicate different subcellular localization of these receptors at either the plasma- or endoplasmic reticulum membrane. Thus, the properties of the whole CK perception apparatuses in early-divergent lineages were demonstrated. Data show that during land plant evolution there was a diversification of the ligand specificity of various CHKs, in particular, the rise in preference for trans-zeatin over cis-zeatin, which indicates a steadily increasing specialization of receptors to various CKs. Finally, this distinct preference of individual receptors to different CK versions culminated in vascular plants, especially angiosperms.

Highlights

  • Hormonal regulation of biological processes is an integral part of the plant growth, development and adaptation to the environment

  • containing histidine kinases (CHKs) receptors typically consist of three basic modules

  • The N-terminal extracytosolic sensory module is located on one side of the membrane, while catalytic and receiver modules are located in the cytosol, on the opposite side of the same membrane [14,26,58,59]

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Summary

Introduction

Hormonal regulation of biological processes is an integral part of the plant growth, development and adaptation to the environment. An evolutionary advanced version of a two-component system (TCS) is responsible for the CK signaling This system consists of a transmembrane (TM) receptor—a sensory hybrid histidine kinase (HK), phosphotransmitters (HPts)—small mobile proteins, and response regulators (RRs)—which can be further subdivided into the RR-B, transcription factors, and the RR-A, negative regulators [3,23,24,25,26]. A high-energy phosphate is transferred via conserved protein domains alternately along the histidine-aspartate-histidine-aspartate chain termed Multistep Phosphorelay (MSP), from the TM receptor via HPts to the nuclear type-B response regulator (RR-B). Receiving this phosphorylation signal, RR-Bs are activated and bind to promoters of the primary response genes, changing (mainly activating) their transcription. They are capable of functionally replacing each other, they are not identical in their biochemical properties and physiological roles in the plant [1,2,3,4,5,6,8,27,28,29,30,31,32]

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