Functional characterization of the two-component signaling system of Arabidopsis

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The objective of the proposal is to study the molecular mechanisms used by the two-component signaling system (TCS) of Arabidopsis to convert signal information into biological processes. Among higher eukaryotes, the TCS is unique to plants. The Arabidopsis genome encodes eight hybrid sensor kinases (AHKs), five histidine phosphotransfer proteins (AHPs) and 22 response regulators of two different types (A-type and B-type ARRs). The TCS perceives and transmits cytokinin signals and has a major role in regulating root and shoot growth, seed development and senescence. In the previous funding period we cloned most of the TCS genes, established a protein-protein interaction map for the TCS, identified interactions with other Arabidopsis proteins, constructed a dominant-negative TCS transcription factor, identified a set of primary target genes and carried out a detailed study of cytokinin receptor mutants. These results set the framework for the current proposal in which we follow two major lines of research. One aim is to understand the functional relevance of selected interactions of TCS proteins, in particular those of the cytokinin receptors and B-type ARRs. A second aim is to study transcriptional regulation by B-type ARRs. This includes functional studies of their three subclasses, identification of their DNA target sequences and the functional analysis of part of the transcriptional cascade they initiate. The study of the TCS functions presents an exemplary analysis of a signaling network and thus addresses an important topic of the AFGN program.