A response-regulator homologue possibly involved in nitrogen signal transduction mediated by cytokinin in maize

A cDNA clone, pZmCip1, encoding a maize (Zea mays) cytokinin-inducible protein 1 was isolated utilizing the differential display technique, and studied using the expression of ZmCip1 in nitrogen-starved maize plants. The cloned cDNA contained an open reading frame consisting of 157 amino acids with a predicted molecular mass of 16.7 kDa, which possesses similarity with the response-regulators of bacterial two-component signalling systems. In detached leaves, accumulation of ZmCip1 transcript by t-zeatin was dose-dependent in a range of 10(-9) M to 10(-7) M, and occurred within 30 min after treatment. The effect of t-zeatin was replaceable by isopentenyl-adenosine or isopentenyl-adenosine-5'-monophosphate. Pretreatment of detached leaves with cycloheximide did not inhibit the accumulation of the transcript. In whole plants, ZmCip1 transcript was transiently accumulated exclusively in leaves by supply of nitrate or ammonium ions to the roots, whereas the transcript was not accumulated in detached leaves by supply of the nitrogen nutrients. Both the cytokinin- and nitrate-responsive accumulations of ZmCip1 transcript were accompanied by an increase in the immunotitratable protein. Isopentenyladenosine and/or its phosphorylated form(s) accumulated in roots 2 h after supply of nitrate to plants. These results, taken together, suggest that ZmCip1 is a primary response gene to cytokinins, and that it involves, at least in part, the nitrogen-signal transduction mediated by cytokinin in maize.