A new subfamily of putative cytokinin receptors is revealed by an analysis of the evolution of the two-component signaling system of plants.

The two-component signaling system (TCS) - the major signaling pathway of bacteria - is found among higher eukaryotes only in plants where it regulates diverse processes such as the signaling of the phytohormone cytokinin. Cytokinin is perceived by a hybrid-histidine kinase receptor and the signal is transduced by a multi-step phospho-relay system of histidine phosphotransfer proteins (HPT) and different classes of response regulators (RR). To shed light on the origin and evolution of TCS members in plants, we conducted a comprehensive domain-based phylogenetic study across the relevant kingdoms including charophyceae algae, the group of green algae giving rise to land plants. Surprisingly, we identified a novel subfamily of cytokinin receptors with members only from the early diverging land plants Marchantia polymorpha and Physcomitrella patens and then experimentally characterized two members of this subfamily. HTPs of charophyceae seemed to be more closely related to those of land plants than to other groups of green algae. Further down the signaling pathway, the type-B RRs were found across all plant clades, but many members lack either the canonical Asp residue or the DNA-binding domain. In contrast, the type-A RRs seemed to be limited to land plants. Finally, the analysis provided hints that one additional group of RRs, the type-C RRs, might be degenerated receptors and thus evolutionary of a different origin than bona fide response regulators.