Identification and characterization of cytokinin-signalling gene families in rice

Abstract

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.