A RESPONSE-REGULATOR HOMOLOG 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 p lants. The cloned cDNA contained an open reading frame consisting of 1 57 amino acids with a predicted molecular mass of 16.7 kDa, which poss esses similarity with the response-regulators of bacterial two-compone nt signalling systems. In detached leaves, accumulation of ZmCip1 tran script 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, ZmCip 1 transcript was transiently accumulated exclusively in leaves by supp ly of nitrate or ammonium ions to the roots, whereas the transcript wa s not accumulated in detached leaves by supply of the nitrogen nutrien ts. Both the cytokinin-and nitrate responsive accumulations of ZmCip1 transcript were accompanied by an increase in the immunotitratable pro tein. Isopentenyladenosine and/or its phosphorylated form(s) accumulat ed in roots 2 h after supply of nitrate to plants. These results, take n together, suggest that ZmCip1 is a primary response gene to cytokini ns, and that it involves, at least in part, the nitrogen-signal transd uction mediated by cytokinin in maize.