Cytokinins metabolism

Certain substituted urea compounds, such as phenylurea or thidiazuron. Fig. (1), are very effective in the replacement of adenine-based cytokinins in promoting callus growth and other bioassays [27,28]. Molecular modeling revealed that the energetically optimal conformation of active urea derivatives have similar geometry as the isoprenoid side chain, so they can bind to the active sites of cytokinin metabolic enzymes and/or activate cytokinin receptors [29]. Thus, these compounds are likely to enhance their cytokinin effect by simultaneous activation of the receptor and inhibition of some of the cytokinin deactivating enzymes [11,30]. 

Ideally, the study of cytokinin metabolism should bring to the fore an association of analytical and biochemical techniques with state-of-the-art cytological approaches, especially with in situ immunolocalisation of cytokinins and molecules reacting with them, as well as of enzymes of cytokinin metabolism. In the future, this approach should facilitate the elucidation of biochemical and physiological processes not only under static conditions (as is the case for most current studies), but dynamically as a real-time biomolecular interaction analysis. 

Vlasakova V, Brezinova A, Hohk J. Study of cytokinin metabolism using HPLC with radioisotope detection. J Pharm Biomed Anal 1998 17 39-44. 

An unsettling feature of the system is the instability of its characteristics other than cytokinin-dependency. For example, the time taken following transfer to cytokinin-free medium (at a fixed cell density) before mitotic arrest is liable to shift between experiments. A simple explanation for the shifts might be the spontaneous occurrence in the culture of variant types with altered cytokinin metabolism. The spontaneous occurrence of an unstable line with enhanced activity of zeatin side chain cleavage has been described [32]. It seems that the renowned phenotypic instability of callus tissue [33] is accentuated by suspension culture, in which each small clump of cells is a separate entity with its own selective advantages in the fierce competition to contribute cells to the inoculum for the next batch. 

Unlike some other cytokinin metabolic enzymes with affinity for cytokinins and other purines, the three Phaseolus enzymes described here appear to recognize only trans-Z (and (diH)Z in the case of the O-xylosyltransferase). Thus, these enzymes are capable of distinguishing between the trans- and cw-isomers of Z as well as the free base and ribonucleoside forms. The fact that similar metabolic conversions of cis-Z do not occur underscores the importance of the enzymes in regulating cytokinin activity in relation to plant growth, which also involves only the trans-isomer of Z. Moreover, the inability of the enzymes to recognize structures other than the free base supports the central role of cytokinin free base as opposed to the ribonucleoside and ribonucleotide forms. 

The differing levels of cytokinins in tobacco leaves of varying age could be due to one or a combination of the following factors (1) differential translocation of xylem cytokinins (2) differential metabolism of xylem cytokinins (3) differential retention of xylem cytokinins and (4) differential biosynthesis of cytokinins in situ in the leaves. These four factors were assessed. By RIA, the principal cytokinins in tobacco xylem exudate were identified as Z, (diH)Z, [9R]Z and (diH)[9R]Z. The distribution and metabolism of the two ribosides in the tobacco shoot were determined after supply via the xylem. The major metabolites of [9R]Z in tobacco leaf laminae were adenine, adenosine and AMP, while the principal metabolite of (diH)[9R]Z was the 7-glucoside of (diH)Z. However, expanded pre-senescent and early senescent laminae did not differ in their cytokinin metabolism, while small expanding laminae showed a higher rate of metabolism. There was no differential distribution of the ribosides to these laminae, and no differential retention in leaves of differing maturity. 

Many studies of cytokinin metabolism have revealed that externally applied cytokinins are broken down to common purinyl compounds. In the case of cytokinins possessing a double bond in the side chain the initial products of breakdown are N amino purines. There has been little research into the breakdown of cytokinins which lack a double bond. The N side chains of such compounds are more resistant to cleavage and the initial products of breakdown appear to be 6-0X0 purines. 

So far the Sinapis story can be interpreted as follows The shoot apical meristem of 2-month-old plants is presumably cytokinin-limited. Exposure to LD causes the production, in the leaves, of a signal which is then transported to the root system. There it alters the course of cytokinin metabolism and/or release. The increase in cytokinin levels in the transpiration stream (xylem) causes an increase in leaf cytokinin levels by 16 h. Some of the leaf cytokinins are then re-exported in the phloem sap to the apical meristem, where they cause a mitotic activation at 26-30 h. Since the cytokinin level in root exudate is altered as early as h 9 (Fig. 2), the initial leaf-to-root signal is apparently produced and transported extremely rapidly, i.e. within the first hour of the photo-extension period of the LD. The nature of this signal is unknown, but bark-ringing experiments indicate that it moves in living tissues (Lejeune, unpublished). 

Mok DWS, Mok MC (2001) Cytokinin metabolism and action. Ann Rev Plant Physiol Plant Mol Biol 52 89-118... 

Fig. 225 Consensus effects of Spd and Spm on cytokinin metabolism and signaling cascade. Description is same as in legends to Fig. 22.1. AHK2 histidine kinase receptor 2, AHK3 histidine kinase receptor 3, AHK4 histidine kinase receptor 4, AHPs histidine-containing phosphotransfer protein, RRs response regulators, CK cytokinin, CKX cytokinin dehydrogenase, CRFs cytokinin response msp.) factors...

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