Poster: Hormones

Abs # 594: Cytokinin Biosynthesis and Interaction with Auxin

Presenter:	Nordström, Anders , anders.nordstrom@genfys.slu.se
Authors	Nordström, Anders  (A)   Tarkowski, Petr  (B)   Tarkowska, Dana  (B)   Åstot, Crister  (C)   Dolezal, Karel  (B)   Sandberg, Göran  (A)
Affiliations:	(A): Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences (B): Laboratory of Plant Growth Regulators, Department of Experimental Botany, Palackeho University (C): Swedish Defence Research Agency
Web Site:	http://www.upsc.nu

Auxin-cytokinin interactions were investigated in Arabidopsis thaliana and Nicotiana tabacum by the use of in vivo -deuterium labelling and mass spectrometry. 36 hours after induction of cytokinin overproduction in transgenic Arabidopsis thaliana plants, significant lower endogenous IAA biosynthetic rates and pool sizes were detected. In the next set of experiments, the auxin NAA was shown to reduce cytokinin biosynthetic rate in Arabidopsis wt plants within 6 h of incubation. In contrast to the cytokinin-auxin case, this rapid effect indicates a direct effect on transcription or translation of synthase related genes. Furthermore, NAA was found exclusively act on the alternative, iPMP-independent pathway of cytokinin biosynthesis producing ZMP not involving iP-type of cytokinins as intermediates. The de novo biosynthetic activity in shoot tissues was studied in tobacco leaves of different developmental stages. The highest rates of de-novo ZMP biosynthetis were found in very young leaves suggesting a major cytokinin biosynthetic activity in young apical tissues. Cytokinin levels in various organs of tobacco supported the synthesis data. This makes us propose a model, where the iP-type of cytokinins are the major cytokinins synthesized in roots in contrast to the apical tissues where Z-type of cytokinins dominates due to the expression of the alternative iPMP-independent pathway cytokinin biosynthesis on which auxins acts as important regulator.