A Subfamily of Putative Cytokinin Receptors Is Revealed by an Analysis of the Evolution of the Two-Component Signaling System of Plants    

Abstract

The two-component signaling system-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 (His) kinase receptor, and the signal is transduced by a multistep phosphorelay system of His phosphotransfer proteins and different classes of response regulators (RRs). To shed light on the origin and evolution of the two-component signaling system 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 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. His phosphotransfer proteins of Charophyceae seemed to be more closely related to land plants than to other groups of green algae. Farther 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, of a different evolutionary origin than bona fide RRs.

Members of a recently discovered subfamily of cytokinin receptors display differences and similarities to their classical counterparts

“…9 A cytokinin binding assay confirmed the ability of these 2 receptors to bind cytokinin and a bacterial complementation assay confirmed a dose-dependent translation of the cytokinin signal into a cellular response. 9 In order to fulfill its biological role a protein needs to localize to the appropriate subcellular compartment.…”

“…9 A cytokinin binding assay confirmed the ability of these 2 receptors to bind cytokinin and a bacterial complementation assay confirmed a dose-dependent translation of the cytokinin signal into a cellular response. 9 In order to fulfill its biological role a protein needs to localize to the appropriate subcellular compartment. To investigate to which cellular compartment the 2 members of the newly identified subfamily localize, vector constructs encoding N-terminal GFP fusion proteins of MpCHK1, and PpCHK4, respectively, under the control of the 35S promoter were transiently transformed into Nicotiana benthamiana leaves and fluorescence was analyzed in epidermal cells.…”

“…[4][5][6][7][8] Recently, we have conducted an in-depth analysis of putative cytokinin signaling components comprising more than 40 species ranging from cyanobacteria to angiosperms. 9 This analysis identified a subfamily of cytokinin receptors not described until now. Sequence analysis revealed that while the domain architecture of the members of this subfamily is identical to those of the classical cytokinin receptors, the CHASE domains of the 2 subfamilies are rather diverse.…”

“…In this context it is surprising that residues which have been shown to be important for cytokinin binding in the classical cytokinin receptors 10,11 display a much lower level of conservation among members of the new subfamily (6.5% identity as compared to 50% in the classical receptors). 9 However, despite these differences in key positions of the sequence of the CHASE domain, members from both subfamilies have been shown to bind trans-zeatin, the most active cytokinin in the binding assay.…”

“…[15][16][17] In summary, the additional data presented here corroborate the previously published analyses and thus further emphasize the role of the members of the newly described subfamily as functional cytokinin receptors. 9 However, their biological role is currently unclear. Since members from both subfamilies are encoded in the genome of P. patens, more in-depth information, such as the temporal and spatial expression patterns during plant development and in response to environmental stimuli or their ligand-binding preferences, will provide further insights into possible differences of their in planta functions.…”