The aromatic cytokinins

After the discovery of kinetin (Miller et al. 1956, J. Am. Chem. Soc. 78: 1345-1350) there was a flurry of syntheses that led to the finding of 6-benzylaminopurine (BA), an active and easily obtainable cytokinin. Much research into cytokinin physiology was subsequently done with this substance. Further, the isolation and unequivocal identification of natural BA and the high biological activity of its meta-hydroxylated analogues stimulated the search for other natural aromatic cytokinins. Screening was accomplished by ELISA of HPLC fractions using antisera against ortho-and meta-hydroxybenzyladenosine. Subsequent isolation and decisive identification by mass spectrometry led to discovery of a brood spectrum of endogenous plant growth substances structurally similar to a highly active compound, meta-topolin (6-[3-hydroxybenzylamino]purine), and to its less active analogue, ortho-topolin (6-[2-hydroxybenzylamino]purine). The structures of such aromatic cytokinins suggest considerably different biosynthetic pathways from that of zeatin and related isoprenoid cytokinins. From a physiological viewpoint, aromatic cytokinin metabolism can be classified under four main headings analogous to isoprenoid cytokinins: interconversion, hydroxylation, conjugation, and oxidative degradation. This review attempts to put into context what is known about 9-alkyl-BAs and compares their metabolism in regard to the practical use of cytokinins in agriculture and biotechnology. The recently discovered unusual specificity of additionally C2,N9-disubstituted aromatic cytokinins toward cell cycle kinases, suggests that these cytokinin-derived growth regulators may selectively inhibit certain steps of the cell cycle. The functional overlap of the aromatic cytokinins with those of their isoprenoid counterparts and cytokinin inhibitors, in relation to growth and developmental processes in plants, has yet to be determined.