Cytokinin two-component (His-Asp phosphorelay) signaling in Arabidopsis

The cytokinin signal transduction pathway of Arabidopsis thaliana is a eukaryotic multistep His-Asp phosphorelay. Cytokinins bind the extracytosolic CHASE domains of three partially redundant hybrid sensor histidine kinase receptors (AHK2, AHK3 and CRE1/AHK4), located mainly in the endoplasmic reticulum membrane. Ligand binding activates autophosphorylation of a conserved histidine in the transmitter domain; the phosphoryl group passes to an aspartate in the receptor's receiver domain and then to a conserved histidine of the small, nucleocytosolic histidine phosphotransfer proteins (AHP1, AHP2, AHP3, AHP5). AHPs deliver the phosphoryl group to the receiver aspartate of nuclear type-B response regulators (principally ARR1, ARR10 and ARR12), which are Myb-related (GARP) DNA-binding transcription factors; receiver phosphorylation activates them to bind AGAT(T)-containing cis-elements and induce cytokinin primary-response genes. Among these targets are the type-A response regulators (e.g. ARR5), receiver-only proteins that are themselves phosphorylated and stabilized through the relay and attenuate the pathway, closing a negative feedback loop. Without ligand, CRE1/AHK4 can reverse the relay and dephosphorylate AHPs. The phosphorelay core (receptor -> AHP -> type-B ARR) conforms to the generic two-component relay motif; transcriptional output and type-A feedback are this pathway's extensions. Cytokinin biosynthesis, transport and degradation, the cytokinin-independent histidine kinase CKI1, CRFs, and downstream developmental targets are outside the module boundary.

Concrete Arabidopsis realization. The phosphorelay core is declared as an inner bundle (`ck_phosphorelay`) that conforms to MODULE:two_component_relay (sensor kinase -> Hpt -> response regulator); the receptor tier uses the receptor-kinase term GO:0009885 (transmembrane histidine kinase cytokinin receptor activity), a cytokinin-specific realization of the sensor-kinase role (GO:0000155 phosphorelay sensor kinase activity is also annotated to all three receptors), hence status WITH_DEVIATIONS. Members are grounded with verified UniProtKB accessions (AHK2 Q9C5U2, AHK3 Q9C5U1, AHK4 Q9C5U0, AHP1 Q9ZNV9, ARR1 Q940D0, ARR12 P62598, ARR5 Q9SB04). Only the seven reviewed members are listed as participants; redundant paralogs (AHP2, AHP3, AHP5; ARR10 and other subfamily-1 type-B ARRs; ARR3/4/6/7/8/9/15/16/17 type-A ARRs) perform the same roles and are named in prose only. AHP6 (a pseudo-HPt lacking the phospho-accepting histidine) is an inhibitor and outside this boundary. GOA IBA rows place the receptor kinase function at PAINT node PTN000703785 and the AHP phosphotransfer function at PTN002000688; these are recorded here as provenance rather than asserted ancestral nodes. No production GO-CAM in gocams/index.tsv contains these genes, so no gocam_associations are declared.