Identification and characterization of cytokinin-signalling gene families in rice

We identified four histidine kinase (HK) genes of a cytokinin receptor family, two histidine-containing phosphotransmitter (HPt) genes, thirteen A-type response regulator (RR) genes and six B-type RR genes in the rice genome. The HK genes (OHK2, OHK3, OHK4 and OHK5 for Oryza sativa HK), the HPt genes (OHP1 and OHP2 for O. sativa HPt) and the B-type RR genes (ORR1, ORR2, ORR3, ORR4 and ORR6 for O. sativa RR) except one (ORR5) showed expression in various organs. ORR5 was expressed in callus and flower. Three A-type RR genes (OsRR4, OsRR9 and OsRR10 for O. sativa RR) showed cytokinin-induced expression, and three (OsRR8, OsRR12 and OsRR13) showed expression in flower. We also identified two other genes named OHK1 and CHARK (CHASE domain Receptor-like serine/threonine Kinase). OHK1 encodes an HK similar to Arabidopsis CKI1, which is involved in female gametophyte development. CHARK encodes a protein with an extracellular cytokinin-perceiving CHASE domain and a cytoplasmic serine/threonine kinase domain which are connected with a single transmembrane domain. The presence of all four gene families and CHARK in the rice genome suggests that a cytokinin signal is transduced by the phosphotransfer mechanism as is the case in Arabidopsis, and that rice may have an additional novel signalling pathway involving serine/threonine phosphorylation.