Receptor properties and features of cytokinin signaling

Abstract

Cytokinins belong to one of the most important and well-known classes of plant hormones. Discovered over half a century ago, cytokinins have retained the attention of researchers due to the variety of the effects they have on the growth and development of vegetable organisms, their participation in a plant adaptation to external conditions, and the potential to be used in biotechnology, agriculture, medicine and even cosmetics. The molecular mechanism by which cytokinins function remained unknown for a long time. Things started to change only in the 21(st)century, after the discovery of the receptors for these phytohormones. It appeared that plants found ways to adapt a two-component signal transduction system borrowed from prokaryotic organisms for cytokinin signalling. This review covers the recent advances in research of the molecular basis for the perception and transduction of the cytokinin signal. Emphasis is placed on cytokinin receptors, their domain and three-dimensional structures, subcellular localization, signalling activity, effect of mutations, ligand-binding properties, and phylogeny.

Keywords: cytokinins; receptors; sensor histidine kinases; signal transduction; two-component systems.

Fig. 1

Structures of typical cytokinins. The most favoured conformations of cytokinins are shown in the upper line; their chemical structures are shown in the lower line.

Fig. 2

Domain structure of cytokinin receptor (exemplified by CRE1/AHK4 from Arabidopsis). Protein domains: TM – transmembrane; LB – ligand-binding (CHASE); HK – histidine kinase; Ac – acceptor; Ck – cytokinins; H – conserved histidine; D – conserved aspartate; N and C denote the N- and C-termini of the protein. The rightwards arrows indicate the sites for phosphorylation and the transfer of high-energy phosphates (~P).

Fig. 3

Scheme of cytokinin signal transduction based on the principle of multistep His-Asp-His-Asp phosphorelay. The phosphorylation of nuclear transcription factors (ARR-B-type response regulators) leads to their activation and subsequent alteration of the primary response gene transcription.

Fig. 4

A model for long-distance cytokinin action. The arrow in the middle denotes the translocation of cytokinins (Ck) of trans -zeatin type from the root to the shoot via xylem. The lateral arrows denote the translocation of cytokinins of the isopentenyladenine type from the shoot to the root via phloem.

Fig. 5

Three dimensional structure of the CHASE domain of the Arabidopsis cytokinin receptor CRE1/AHK4. The general view (A) and structure of the binding site with a trans -zeatin molecule (B). Cytokinin molecule is shown in spacefill representation, cystine bridge is shown as the ball-and-stick model. The arrow indicates the location of the bound cytokinin.

Fig. 6

Phylogenetic analysis of cytokinin receptors. The sequence alignment was performed using the ClustalW program. The phylogenetic tree was built using the MEGA 5.05 software; the bootstrap analysis includes 1,000 replicates. Bootstrap supports for the individual branches are given as a percentage based on 1,000 bootstrap trials.