Regulation of cytokinin content in plant cells

Abstract

Cytokinin levels in plant cells are dependent on cytokinin biosynthesis and/or uptake from extracellular sources, metabolic interconversions, inactivation and degradation. Cytokinin conversion to compounds differing in polarity seems to be decisive for their entrapment within the cell and intracellular compartmentation, which affects their metabolic stability. Increase in cytokinin levels, resulting either from their uptake or intracellular biosynthesis, may promote further autoinductive accumulation of cytokinins which may function in the induction of cytokinin-initiated physiological processes. Accumulated cytokinins are capable of inducing cytokinin oxidase which consequently decreases cytokinin levels. This seems to be the mechanism of re-estabhshment and maintenance of cytokinin homeostasis required for further development of physiological events induced by transient cytokinin accumulation. Auxin may influence cytokinin levels by down regulation of cytokinin biosynthesis and/or by promotion of cytokinin degradation. A model of the regulation of cytokinin levels in plant cells based on these phenomena is presented and its physiological role(s) is discussed. n ro uc ion ticular physiological processes is really govemed by changes in levels of endogenous cytokinins. It is obvious The discovery of cytokinins and investigation of their that both application of exogenous cytokinins and maroles in plants originated from testing the effects of ex-nipulated expression of the ipt gene in transgenic plant ogenously applied substances promoting cell division in cells represent useful, if very cmde tools for probing the a number of plant systems. Exogenous application is as-actual role of cytokinins in plants, sociated with many well known limitations arising from Mechanisms of metabolic regulation of plant hordifferences in uptake (Laloue et al. 1981, Auer et al. mone levels should meet two basic requirements essen-1992), metabolism (Kaminek 1992, McGaw and Burch tial for hormonal control of plant development: (1) the 1995) and compartmentation (Jameson 1994) of applied ability to respond to hormonal or other signals and gensubstances in plant cells which complicate the interpre-erate fast and significant changes in the concentration of tation of results. Therefore, it has been encouraging to a particular hormone or its concentration ratio with releam that most of the biological effects described for ex-spect to other plant hormone(s) in particular cells; and ogenously applied cytokinins are also induced by ele-(2) the capability to maintain hormonal homeostasis at vated levels of endogenous cytokinins in transgenic certain stage(s) of cell development. Rapid changes in plant cells expressing the cytokinin biosynthetic ipt gene hormonal levels function in the initiation of major develcoding for the enzyme A^-isopentenyl pyrophosphate:5'-opmental processes (e.g. organ formation) while hor-AMP A'-isopentenyl transferase (see Klee and Romano monal homeostasis is required for further development 1994). These findings indica...

Phytohormone metabolism in human cells: Cytokinins are taken up and interconverted in HeLa cell culture

“…Numbers represent inferred enzymes as follows: 1. adenosine kinase (EC 2.7.1.20); 2. purine nucleotide phosphorylase (EC 2.4.2.1); 3. adenine phophoribosyltransferase (EC 2.4.2.1). The remaining arrows demonstrate alternative conversions that can occur via specific enzymes in other CK producing organisms, such as Arabidopsis . Bolded text refers to the CK forms that were dominant and the proposed enzyme activity that we observed in this study.…”

“…Of the naturally occurring CKs in plants, the isoprenoid‐type CKs, N 6 ‐isopentenyladenine (iP), trans ‐zeatin ( t Z), cis ‐zeatin ( c Z), and dihydrozeatin (DZ), are the most common . Much less has been documented about aromatic‐type CKs, such as N 6 ‐benzyladenine (BA) . The majority of naturally occurring CKs exist in distinctive structural derivatives or forms such as: free bases (FBs), ribosides (Rs), and nucleotides, or conjugates with glucose, xylose, or amino acid residues .…”

Cytokinin‐induced upregulation of cytokinin oxidase activity in tobacco includes changes in enzyme glycosylation and secretion

“…The high proportion of iP7G and iPRMP compared to iP and iPR in Tc‐treated cells probably resulted from their partial or complete resistance to degradation by CKX. It has been suggested that the induction of CKX activity by both exogenous and endogenous cytokinins contribute to the re‐establishment and maintenance of cytokinin homeostasis in plant cells (Kamínek et al. 1997, Zažímalová et al.…”

“…In spite of the impressive progress made in CKX research, little is known about the regulation of CKX activity in plant tissues and cells. It is apparent that the CKX in plant cells is subjected to multiple regulatory mechanisms, most of them depending directly on the concentration and/or compartmentation of cytokinins in the cell (reviewed in Armstrong 1994, Jones and Schreiber 1997, Kamínek et al. 1997).…”

Deuteriumin vivo labelling of cytokinins inArabidopsis thaliana analysed by capillary liquid chromatography/frit-fast atom bombardment mass spectrometry

“…3 The active cytokinin pool could be regulated via modulation of biosynthesis, transport, metabolic interconversion or catabolism. 4 Cytokinin metabolism has been investigated in a range of species. 5,6 N-Glucosyl conjugation and oxidative cleavage of the isoprenoid side-chain have been identified as the major mechanisms of inactivation.…”