Cytokinin activity of zeatin allylic phosphate, a natural compound

Zeatin allylic phosphate (ZAP) retarded chlorophyll loss in the barley leaf senescence assay at a concentration 20 times higher than for 6-benzyladeni ne (BA): the effective concentrations for ZAP and BA were 10 muM and 0.5 mu M, respectively. Sodium molybdate, an inhibitor of phosphatases, decreased the ZAP effective concentration to 0.5 muM without affecting leaf senescenc e and trans-zeatin activity in the control. This demonstrates the importanc e of the phosphate group for ZAP activity or its penetration into leaf cell s. ZAP up-regulated the protein kinase activity of the barley leaf chromati n with concentration dependence similar to that of trans-zeatin. Conversely , ZAP was 1000 times less active than trans-zeatin in the competition with anti-idiotype antibodies (raised against antibody to zeatin) for binding wi th a trans-zeatin-binding site of trans-zeatin-binding protein ZBP67 isolat ed from barley leaves. In contrast to trans-zeatin, ZAP did not activate RN A synthesis in the presence of ZBP in the in vitro system containing chroma tin and RNA polymerase I isolated from barley leaves. In summary, data pres ented show that ZAP possesses cytokinin activity as demonstrated by the ret ardation of barley leaf senescence, but molecular target(s) for ZAP in barl ey leaf cells differs, at least partially, from these for trans-zeatin. It seems possible that the cytokinin activity of ZAP results from its hydrolys is while producing zeatin.