Is a 67-kD Cytokinin-Binding Protein from Barley and Arabidopsis thaliana Leaves Involved in the Leaf Responses to Phenylurea Derivatives? (A Review)

Abstract

Cytokinin-binding proteins (67-kD CBP) were isolated from barley first leaves of 8–10-day-old plants and Arabidopsis thaliana rosette leaves of 9-week-old plants. A capability of these proteins for cytokinin binding was assessed from trans-zeatin competition with antiidiotypic antibodies (ABa-i) for complex formation with immobilized 67-kD CBP under competitive ELISA conditions. ABa-i were obtained against anti-trans-zeatin antibody. 67-kD CBP from A. thaliana and barley leaves, in combination with trans-zeatin, activated transcriptional elongation in the in vitro system containing chromatin and RNA polymerase I from the same leaves. A phenylurea derivative demonstrating a high cytokinin activity, N-phenyl-N1-(2-chloro-4-pyridyl)urea (4-PU-30) could not displace ABa-i from its complex with 67-kD CBP from either barley or A. thaliana leaves, which indicates the absence of its interaction with CBP. In the presence of 67-kD CBP, 4-PU-30 did not activate transcription in the in vitro system containing chromatin and RNA polymerase I from A. thaliana or barley leaves. This means that, as distinct from trans-zeatin, 4-PU-30 did not use 67-kD CBP for transcription activation. A. thaliana leaf preincubation in the 4-PU-30 solution for 1 h enhanced RNA synthesis in the transcriptional system containing chromatin and RNA polymerase I from these pretreated leaves. Thus, leaves recognized the 4-PU-30 signal using another receptor. 4-PU-30 capacity to retard leaf senescence also supports this supposition. Another highly active phenylurea derivative thidiazuron also enhanced RNA synthesis in leaves and retarded their senescence.