Fig. 1

A model for the cytokinin Two Component System (TCS) phosphorelay signaling. In Arabidopsis thaliana, cytokinins (CKs) are perceived by histidine-kinase receptors (HK) containing two (for CRE1/AHK4) or three (for AHK2 and AHK3) transmembrane domains. CKs interact with the CHASE (Cyclases/Histidine kinases Associated Sensory Extracellular) domain of CHK receptors, inducing an autophosphorylation of a conserved histidine (H) residue. The phosphate is then transferred to a conserved aspartate (D) residue in the phosphoreceiver domain of the HKs, and to a conserved H residue of an Histidine Phosphotransfer protein (HPT-H). HPT-H proteins shuttle between the cytoplasm and the nucleus where they can transfer the phosphate group on a conserved D residue of Type-B Response Regulators (RRBs). This leads to an activation of RRBs, acting as transcription factors thanks to their Myb-like DNA binding (Myb) and transactivation domains, regulating the expression of CK-responsive genes such as Type-A Response Regulators (RRA). RRAs act as negative regulators of the CK signaling likely by competing with RRBs for phosphotransfer activation. In the HPT-N AHP6 variant, an asparagine (N) substitution of the conserved H leads to an inhibitory role on the CK signaling. In addition, CKI1, CKI2/AHK5 and the ethylene receptor ETR1 contain all domains defining an active hybrid Histidine Kinase receptor, and can interact with HPT-H, suggesting a potential function as modulators of cytokinin signaling