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JXB Advance Access originally published online on September 12, 2005
Journal of Experimental Botany 2005 56(421):2877-2883; doi:10.1093/jxb/eri282
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© The Author [2005]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.

RESEARCH PAPER

Diurnal variation of cytokinin, auxin and abscisic acid levels in tobacco leaves

Marie Nováková1,2, Václav Motyka1, Petre I. Dobrev1, Jirí Malbeck1, Alena Gaudinová1 and Radomíra Vanková1,*

1Institute of Experimental Botany AS CR, Rozvojova 135, 165 02 Prague 6, Czech Republic
2Department of Biochemistry, Faculty of Science, Charles University, Hlavova 2030/8, 128 43 Prague 2, Czech Republic

* To whom correspondence should be addressed. Fax: +420 220 390 446. E-mail: vankova{at}ueb.cas.cz

As many processes are regulated by both light and plant hormones, evaluation of diurnal variations of their levels may contribute to the elucidation of the complex network of light and hormone signal transduction pathways. Diurnal variation of cytokinin, auxin, and abscisic acid levels was tested in tobacco leaves (Nicotiana tabacum L. cv. Wisconsin 38) grown under a 16/8 h photoperiod. The main peak of physiologically active cytokinins (cytokinin bases and ribosides) was found after 9 h of light, i.e. 1 h after the middle of the light period. This peak coincided with the major auxin peak and was closely followed by a minor peak of abscisic acid. Free abscisic acid started to increase at the light/dark transition and reached its maximum 3 h after dark initiation. The content of total cytokinins (mainly N-glucosides, followed by cis-zeatin derivatives and nucleotides) exhibited the main peak after 9 h of light and the minor peak after the transition to darkness. The main, midday peak of active cytokinins was preceded by a period of minimal metabolic conversion of tritiated trans-zeatin (less than 30%). The major cytokinin-degrading enzyme, cytokinin oxidase/dehydrogenase (EC 1.5.99.12), exhibited maximal activity after the dark/light transition and during the diminishing of the midday cytokinin peak. The former peak might be connected with the elimination of the long-distance cytokinin signal. These cytokinin oxidase/dehydrogenase peaks were accompanied by increased activity of ß-glucosidase (EC 3.2.1.21), which might be involved in the hydrolysis of cytokinin O-glucosides and/or in fine-tuning of active cytokinin levels at their midday peak. The achieved data indicate that cytokinin metabolism is tightly regulated by the circadian clock.

Key words: Abscisic acid, auxin, cytokinin, cytokinin oxidase/dehydrogenase, diurnal rhythm, ß-glucosidase, tobacco


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