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JXB Advance Access published online on October 20, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm236
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© The Author [2007]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

RESEARCH PAPER

Involvement of hormones and KNOXI genes in early Arabidopsis seedling development

Premysl Soucek1,2, Petr Klíma1, Alena Reková1,2 and Bretislav Brzobohaty1,2,*

1Institute of Biophysics, Academy of Sciences of the Czech Republic V.V.i., Královopolská 135, CZ-61265 Brno, Czech Republic
2Department of Molecular Biology and Radiobiology, Faculty of Agronomy, Mendel University of Agriculture and Forestry, Zemedelská 1, CZ-61300 Brno, Czech Republic

* To whom correspondence should be addressed. E-mail: brzoboha{at}ibp.cz

Plant hormones control plant development by modulating the expression of regulatory genes, including homeobox-containing KNOXI genes. However, much remains to be elucidated about the interactions involved. Therefore, hormonal regulation of KNOXI gene expression was investigated using hormone applications and an inducible transgenic ipt expression system to increase endogenous cytokinin (CK) levels. Treatments with auxin, abscisic acid (ABA), cytokinins, ethylene, and gibberellin (GA) did not result in ectopic expression of the BP (BREVIPEDICELLUS) gene. However, BP expression was strongly reduced by ABA, increased by auxin treatment (correlating with the initiation of lateral root meristems, which strongly express BP), and did not significantly respond to short-term treatments with the other hormones in whole seedlings. Following short-term ipt activation, organ-specific differential regulation of KNOXI gene expression was observed. While several KNOXI genes were transiently up-regulated to low levels, STM was selectively repressed (especially at low light) in hypocotyls. In cotyledons, activation of CK-responsive genes preceded ipt induction, suggesting that CKs are transported more rapidly than the inducing agent (dexamethasone). Long-term increases in CK levels induced raised levels of several KNOXI transcripts in hypocotyls, correlating with the radial expansion of vascular tissues, the main domains of KNOXI gene expression, suggesting that CKs had little effect on KNOXI promoter activity. No alterations in hormone sensitivity were observed in a bp null mutant. Constitutive BP overexpression caused reductions in the length and number of lateral roots, while the primary root remained unaffected. The transgenic seedlings displayed weak, but significant, alterations in sensitivity to ABA, CK, and 1-amino-cyclopropane-1-carboxylic acid.

Key words: Arabidopsis thaliana, KNOX genes, plant hormones, pOp/LhGR system, regulation of gene expression

Received 18 May 2007; Revised 22 August 2007 Accepted 30 August 2007


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