RESUMEN
The highly specific recognition of a natural cytokinin, trans-zeatin, by cytokinin-binding protein (CBP) of 67 kDa from barley leaves was detected with an assay developed on the basis of cytokinin competition in ELISA with anti-idiotype antibodies (raised against antibodies to zeatin) for complex formation with CBP. Monoclonal antibodies (mAbs) raised against 70 kDa CBP from etiolated maize seedlings cross-reacted with barley 67 kDa CBP and prevented barley CBP and trans-zeatin induced activation of transcription elongation directed by RNA polymerase I associated with barley chromatin. One mAb (Z-6) had an agonistic effect. Maize CBP replaced barley CBP in activation of RNA synthesis with cytokinin in the barley transcription system. Hence, a new family of cytokinin receptors with common functions and immunodeterminants including maize and barley CBPs was found.