Poster: Hormones

Abs # 611: Molecular characterization of cytokinin-responsive histidine kinases (ZmHKs) in maize

Presenter:	Yonekura-Sakakibara, Keiko , keikoys@psc.riken.go.jp
Authors	Yonekura-Sakakibara, Keiko  (A)   Yamaya, Tomoyuki  (A)   Sakakibara, Hitoshi  (A)
Affiliations:	(A): Plant Science Center, RIKEN (The Institute of Physical and Chemical Research)

In higher plants, the cytokinin signal is mediated via histidyl-aspartyl (His-Asp) phosphorelay system, sensory histidine-kinase (HK), histidine-containing phosphotransmitter (HP) and reponse regulator. In order to understand the cytokinin-mediated signal transduction, we isolated three genes for HK (ZmHK1, ZmHK2 and ZmHK3) from Zea mays L. The deduced amino acid sequences were similar to those of cytokinin-responsive HKs of Arabidopsis thaliana. The expressions of ZmHK1 and ZmHK2 in Escherichia coli having the DrcsC and cps::lacZ genetic background conferred cytokinin-responsive expression of lacZ. In the recombinant E. coli, ZmHK1 is more responsive to free-base type cytokinins such as trans-zeatin, cis-zeatin and isopentenyladenine than to their ribosides. Isopentenyladenine was more effective for ZmHK1 and trans-zeatin was for ZmHK2. Transient expression of fusion products of the ZmHKs with green fluorescent protein in maize protoplasts implied that ZmHK1 was abundant in endoplasmic reticulum. These differences imply that the ZmHKs are functionally differentiated in terms of the ligand specificity and subcellular localization. Single substitution of aspartic acid residue of receiver domain of ZmHK1 to glutamic acid, which is the putative phosphorylation site, enabled it to interact with ZmHPs. This suggests that His-Asp phosphotransfer with physical interaction between the ZmHK and the ZmHPs occurs in the signaling pathway.