Abstract
Genes for cytokinin-responsive histidine-protein kinases (ZmHK1, ZmHK2, and ZmHK3a) were isolated from maize (Zea mays L.). Heterologous expression of each of the ZmHKs in Escherichia coli having the ΔrcsC and cps::lacZ genetic background conferred cytokinin-inducibility of lacZ expression on the bacteria. In the recombinant E. coli system, ZmHK1 and ZmHK3a were more sensitive to free-base cytokinins than to the corresponding nucleosides, while ZmHK2 tended to be sensitive to both types. In contrast to a known cytokinin receptor of Arabidopsis thaliana (AHK4/CRE1/WOL), all ZmHKs responded to cis-zeatin (cZ) which is generally believed to be inactive or only weakly active. In cultured maize cells, expression of ZmRR1, a cytokinin-inducible response regulator, was induced by cZ as well as by trans-zeatin. We conclude that maize cytokinin receptors differ in ligand preference, and that cZ is an active cytokinin at least in maize. We will also discuss about the subcellular localizations of ZmHKs.
