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.