Abstract
Cytokinin (CK) plays an important role for plant development such as regulation of organogenesis, senescence and chloroplast development. We have already identified three genes encoding CK receptors (ZmHK1, ZmHK2, ZmHK3) from maize. Our analysis revealed that the ZmHKs differ in ligand preference, and that the orthologs from maize and Arabidopsis show different ligand specificity (Plant Physiol., 2004, 134: 1654-1661). Here we isolated three new genes encoding CK receptors (ZmHK1a2, ZmHK1b1, ZmHK1b2). ZmHK1 shows over 90% identity with ZmHK1a2 and about 60% with ZmHK1b1 and ZmHK1b2 at amino acid level. Orthologs of the ZmHKs were also found in rice genome, suggesting that they commonly function in monocots. As is the case with ZmHK1, ZmHK1a2 could respond to cis-zeatin at similar extent with trans-zeatin in the Escherichia coli mutant [ΔRcsC, cps::lacZ] assay. These data support our hypothesis that cis-type CK is physiologically active in some plant species including maize.
