Two‐Component Signalling in Plants

Abstract

Two‐component signalling pathways are used by bacteria to perceive and respond to a wide variety of environmental signals. In their simplest form, these are comprised of a histidine kinase receptor, whose activity is regulated by a signal, and a response regulator, whose activity is controlled by Asp phosphorylation mediated by the histidine kinase. Plants use two‐component elements to respond to endogenous and environmental signals. The plant pathways are either an extended version of the two‐component pathway called a phosphorelay or include degenerated elements that no longer function as histidine kinases. These elements are conserved throughout the plant kingdom, although they are best understood in the model Arabidopsis thaliana. The most complete and best understood of these is the signalling pathway for the phytohormone cytokinin, which is perceived by endoplasmic reticulum‐localised histidine kinase receptors and ultimately regulates the phosphorylation of a set of response regulators that mediate the transcriptional response to cytokinin.

Key Concepts:

  • The canonical two‐component system consists of a histidine kinase and response regulator protein.

  • Multiple permutations of the canonical two‐component system exist, including a multistep phosphorelay system.

  • Plants respond to a variety of signals using two‐component signalling systems, including cytokinin, ethylene and light.

  • Two‐component signalling elements are conserved in both dicot and monocot plant species.

  • Cytokinin, a plant hormone, uses a multistep phosphorelay system to transduce its signal from membrane‐bound receptors to the nucleus to mediate transcription of cytokinin response genes.

Keywords: cytokinin; ethylene; two‐component signalling; histidine kinases; response regulators

Figure 1.

The prototypical two‐component system and the cytokinin signalling pathway in Arabidopsis. (a) In the prototypical two‐component system, the sensor His kinase receives a signal, triggering autophosphorylation on a conserved His (H) residue in the transmitter domain. The phosphate group is then transferred to an aspartate (D) residue in the receiver domain of the response regulator protein, which regulates its activity. (b) The cytokinin signalling pathway is an example of a multistep phosphorelay system. The CHASE domain in the His kinase receptor binds to cytokinin, triggering autophosphorylation of a conserved His residue in the transmitter domain, which then transfers the phosphate group to the receiver domain within the receptor. The phosphate is transferred from the receiver domain to the Hpt, which shuttles the phosphate to the receiver domain of either the type B or the type A response regulator proteins, most of which are in the nucleus. (c) The ethylene signalling pathway is an example of a degenerate TCS. Only a subset of the ethylene receptors has a functional transmitter and/or receiver domain. In air, the ethylene receptors activate the CTR1 Ser/Thr protein kinase, which phosphorylates EIN2, thus blocking an activating proteolytic cleavage. In ethylene, CTR1 does not phosphorylate EIn2 and is thus cleaved and the C‐terminal domain then migrates into the nucleus.

Figure 2.

Domain architecture of the response regulator proteins in plants. Plant response regulators fall into four groups: Type A, Type B, Type C and pseudo/clock‐related RR. Typically, response regulator proteins consist of two domains, a receiver domain and an output domain. Type B RRs have a Myb‐like DNA‐binding output domain C‐terminal to the receiver domain. The type A and type C RRs have a short C‐terminal extension and lack an output domain. The pseudo/clock‐related response regulators are split into two groups, containing either a Myb‐like motif or CCT motif in their output domain. The type A, type B and type C response regulators contain a conserved aspartate residue in the receiver domain that is the target of phosphorylation, but the pseudo/clock‐related response regulators lack this Asp residue, and it is often replaced with a glutamate residue.

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Further Reading

Bourret RB and Silversmith RE (2010) Two‐component signal transduction. Current Opinion in Microbiology 13: 113–115.

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Wuichet K, Cantwell BJ and Zhulin IB (2010) Evolution and phyletic distribution of two‐component signal transduction systems. Current Opinion in Microbiology 13: 19–225.

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Burr, Christian A, Sacks, Carly M, and Kieber, Joseph J(Apr 2014) Two‐Component Signalling in Plants. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0025266]