Modulation of light signalling by the response regulator ARR4 and other two-component signalling elements

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The two-component system (TCS) is a phosphotransfer mechanism that confers the ability to sense and respond to exogenous or endogenous stimuli. In higher plants, TCS usually com-prises three elements: a hybrid histidine kinase, a histidine-containing phosphotransfer do-main protein and a response regulator. In the past years two-component signalling systems have been intensively studied especially in hormone signalling. Recent analyses performed by us in Arabidopsis plants indicate that TCS also functions as a molecular mechanism that integrates endogenous (e.g. phytohormones) and environmental (e.g. light) signals to opti-mally influence plant growth, development and adaptation to environmental conditions. For instance, the response regulator ARR4 is able to stabilize the active Pfr form of phytochrome B and thereby modifies red light dependent photomorphogenesis. In the proposed research we would like to continue our work on the basis of existing results. We will thereby concentrate on the molecular and functional aspects of the interaction of ARR4 with light signal transduction in Arabidopsis. This includes (i) detailed analysis of the physiological properties of arr4 mutants and ARR4 overexpressing lines, (ii) determination of the intracellular location, at which the interaction of ARR4 with phyB functionally occurs and (iii) identification of upstream signals and TCS elements regulating ARR4 activity. For this purpose, we will use state-of-the-art molecular, cell biological, biochemical and physiological tools.