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Plant and Cell Physiology, 2002, Vol. 43, No. 8 939-950
© 2002 Oxford University Press

Resistance of Transgenic Tobacco Seedlings Expressing the Agrobacterium tumefaciens C58-6b Gene, to Growth-Inhibitory Levels of Cytokinin is Associated with Elevated IAA Levels and Activation of Phenylpropanoid Metabolism

Ivan Gális1,2, Petr Simek3, Henri A. Van Onckelen4, Yasutaka Kakiuchi1 and Hiroetsu Wabiko1,5

1 Biotechnology Institute, Akita Prefectural University, 2-2 Minami, Ohgata, Akita, 010-0444 Japan
2 Institute of Plant Molecular Biology AS CR, Branisovská 31, 37005 Ceské Budejovice, Czech Republic
3 Institute of Entomology AS CR, Branisovská 31, 37005 Ceské Budejovice, Czech Republic
4 Universitaire Instelling Antwerpen, Universiteitsplein 1, Antwerpen, Belgium

We previously reported that the Agrobacterium tumefaciens C58-6b gene confers resistance to growth-inhibitory levels of exogenously applied N6-benzyladenine (BA, cytokinin) in transgenic tobacco (Nicotiana tabacum) seedlings. Here, we found that intracellular levels of indoleacetic acid (IAA, auxin) increased in transgenics but declined in wild-type seedlings upon BA treatment. Since exogenously supplied 1-naphthalene acetic acid (NAA), a stable synthetic auxin, counteracted the growth inhibition of wild-type seedlings by BA, we suggest that BA-induced growth inhibition in wild-type seedlings occurs, at least in part, as a result of intracellular IAA deficiency. Further HPLC analysis of cell extracts from BA-treated seedlings revealed that a fluorescent compound, later identified as the phenylpropanoid, scopolin, and the major phenolic compound, chlorogenic acid, accumulated earlier in transgenics than in wild-type seedlings. Gene transcripts encoding phenylalanine ammonia-lyase, cinnamate 4-hydroxylase, and 4-coumarate:CoA ligase, which are responsible for the early steps of phenylpropanoid biosynthesis, accumulated earlier and to higher levels in transgenics than in wild-type seedlings as determined by Northern hybridization analysis, thus accounting for the early accumulation of scopolin and chlorogenic acid in transgenics. As some phenolic compounds, including chlorogenic acid and scopoletin (aglycon of scopolin) are suggested to inhibit IAA catabolism, we further propose that C58-6b gene expression protects IAA from degradation by inducing the early phenylpropanoid pathway.

5 Corresponding author: E-mail, wabi@agri.akita-pu.ac.jp; Fax, +81-185-45-2678.


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