Fig. 1

Cytokinin signaling and repeated polyploidy events during land plant evolution. a Schematic representation of core cytokinin signaling. Cytokinin receptors perceive cytokinins, autophosphorylate and transmit the signal via HPTs to RRAs and RRBs. Pseudo-HPTs may compete for phosphotransfer with HPTs. RRAs are induced by cytokinins and function as negative regulators to form feedback regulatory loops. RRBs encode DNA-binding transcription factors that mediate cytokinin-dependent transcriptional activation [24]. b Repeated WGDs and WGTs during land plant evolution and sampling strategy [4, 21, 37, 40, 41, 43, 44, 50, 95, 101, 102]. The figure illustrates the phylogenetic tree topology for land plants [93, 94, 96, 97]. Klebsormidium flaccidum is placed on the basal lineage of current land plants marking the transition from the aquatic to the terrestrial life form [25]. Ancestral polyploidy events in seed plants and angiosperms are indicated by symbols and were inferred from the literature, given in the key. Gray boxes mark the 14 core species chosen for this study of comparative analyses of cytokinin signaling (Table 1). For all depicted species/lineages, genes encoding CHKs were identified and their evolutionary history was reconstructed. Additionally, the evolutionary history of HPTs, RRAs, and RRBs from species labeled with * was reconstructed