Various plant developmental processes involve phytohormones such as cytokinins. Isopentenyltransferase (IPT) reaction is the key rate-limiting step in cytokinin biosynthesis that transfers the isopentenyl (iP) group from dimethylallyl diphosphate to the N6-amino group of adenine. Here, a series of diadenosine polyphosphates (Ap. nA) were screened as possible substrates of IPT, among which diadenosine tetraphosphate, diadenosine pentaphosphate and diadenosine hexaphosphate showed higher affinity than did the authentic substrates ADP and ATP. In addition, formation of mono-isopentenyl Ap. nA and di-isopentenyl Ap. nA was observed. Judging by the existing biosynthetic and hydrolytic systems for Ap. nA in plants, Ap. nA and isopentenyl-Ap. nA may occur in the plant cells, with functional importance.
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