Cytokinins are structurally diverse and biologically versatile. The chemistry and physiology of cytokinin have been studied extensively, but the regulation of cytokinin biosynthesis, metabolism, and signal transduction is still largely undefined. Recent advances in cloning metabolic genes and identifying putative receptors portend more rapid progress based on molecular techniques. This review centers on cytokinin metabolism with connecting discussions on biosynthesis and signal transduction. Important findings are summarized with emphasis on metabolic enzymes and genes. Based on the information generated to date, implications and future research directions are presented.

Three mechanisms of 2n pollen formation observed in diploid hybrids between Solatium tuberosum Group Phureja (2x) and haploids of S. tuberosum Group Tuberosum (4x) are termed parallel spindles (ps), premature cytokinesis 1 (pc 1) and premature cytokinesis 2 (pc 2). A II spindles are parallel to each other in many cells of clones producing 2n pollen by ps. A single cleavage furrow follows and forms a dyad which generates two 2n microspores. The other two mechanisms, pc 1 and pc 2, produce 2n pollen by omission of the second meiotic division. Premature cytokinesis 1 (pc 1) involves asynchronized movement of bivalents to and from the M I plate and falling apart of chromatids at T I. A cleavage furrow follows the first meiotic division, no second division occurs and products of meiosis are dyads. Premature cytokinesis 2 (pc 2) exhibits no abnormalities at the first meiotic division. However, cytokinesis occurs prematurely at P II, no second division occurs, and only dyads are formed. Genetic data suggested that these three mechanisms may be controlled by single loci, and are inherited independently as simple recessives.

trans-Zeatin is a major and ubiquitous cytokinin in higher plants. cis-Zeatin has traditionally been viewed as an adjunct with low activity and rare occurrence. Recent reports of cis-zeatin and its derivatives as the predominant cytokinin components in some plant tissues may call for a different perspective on cis-isomers. The existence of a maize (Zea mays) gene (cisZOG1) encoding an O-glucosyltransferase specific to cis-zeatin (R.C. Martin, M.C. Mok, J.E. Habben, D.W.S. Proc Natl Acad Sci USA 98: 5922-5926) lends further support to this view. Results described here include the isolation of a second maize cisZOG gene, differential expression of cisZOG1 and cisZOG2, and identification of substantial amounts of cis-isomers in maize tissues. The open reading frame of cisZOG2 has 98.3% identity to cisZOG1 at the nucleotide level and 97.8% at the amino acid level. The upstream regions contain common and unique segments. The recombinant enzymes have similar properties, K m values of 46 and 96 m, respectively, for cis-zeatin and a pH optimum of 7.5. Other cytokinins, including N 6 -(⌬ 2 -isopentenyl)adenine, trans-zeatin, benzyladenine, kinetin, and thidiazuron inhibited the reaction. Expression of cisZOG1 was high in maize roots and kernels, whereas cisZOG2 expression was high in roots but low in kernels. cis-Zeatin, cis-zeatin riboside, and their O-glucosides were detected in all maize tissues, with immature kernels containing very high levels of the O-glucoside of cis-zeatin riboside. The results are a clear indication that O-glucosylation of cis-zeatin is a natural metabolic process in maize. Whether cis-zeatin serves as a precursor to the active trans-isomer or has any other unique function remains to be demonstrated.Cytokinins are plant hormones regulating cell division and a range of developmental events such as bud formation, leaf expansion, senescence, seed germination, and chloroplast formation (Mok, 1994). trans-Zeatin is a major and ubiquitous cytokinin in higher plants. Earlier cytokinin analyses detected ciszeatin and its derivatives in trace amounts in some plants, but due to their low activity (Schmitz et al., 1972), cis-isomers were viewed as adjunct to transisomers. Recent analyses, however, showed that the cis-isomers can be the dominant cytokinins at particular stages of development in plants such as chickpea (Cicer arietinum) and lupine (Lupinus albus; Emery et al., 1998Emery et al., , 2000. Moreover, the presence of cis-isomers was associated with male sterility in Mercurialis spp. flowers (Louis et al., 1990;Durand and Durand, 1994). These are indications that cis-isomers may have unique physiological functions. The ability to regulate the levels of cis-zeatin is evidenced by the maize (Zea mays) cisZOG1 gene, encoding an O-glucosyltransferase with specificity to cis-zeatin (Martin et al., 2001).O-Glucosylation is a major step in the metabolism of trans-zeatin ). The resulting O-glucosides seem to serve as storage compounds and are resistant to degradation by cytokinin oxidases (Armstrong, 1994...

Zeatin is the most active and ubiquitous of the naturally occurring cytokinins. The O-glucoside of zeatin, found in all plants examined, is considered to be important in cytokinin transport, storage, and protection against cytokinin

Genetic data obtained from seven test-crosses (159 progeny) substantiated an earlier hypothesis that all three mechanisms of diplandroid formation (parallel spindles, ps; premature cytokinesis-), pe-l; and premature cytokinesis-2, pe-2) are controlled by simple recessive mutations. Results of tests for allelism suggested that pe-l and pc-2 are not allelic. The expression of ps, pe-l and pc-2 at the tetraploid level was observed when the tetraploid is nulliplex at the corresponding locus.