Genetics, Vol. 159, 767-776, October 2001, Copyright © 2001

Gibberellins Are Not Required for Normal Stem Growth in Arabidopsis thaliana in the Absence of GAI and RGA

Kathryn E. Kinga, Thomas Moritzb, and Nicholas P. Harberda
a Department of Molecular Genetics, John Innes Centre, Colney Lane, Norwich, Norfolk NR47UJ, United Kingdom
b Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, S-90183 Umeå, Sweden

Corresponding author: Nicholas P. Harberd, John Innes Centre, Department of Molecular Genetics, Colney Lane, Norwich, Norfolk NR47UJ, United Kingdom., nicholas.harberd{at}bbsrc.ac.uk (E-mail)

Communicating editor: V. L. CHANDLER

The growth of Arabidopsis thaliana is quantitatively regulated by the phytohormone gibberellin (GA) via two closely related nuclear GA-signaling components, GAI and RGA. Here we test the hypothesis that GAI and RGA function as "GA-derepressible repressors" of plant growth. One prediction of this hypothesis is that plants lacking GAI and RGA do not require GA for normal stem growth. Analysis of GA-deficient mutants lacking GAI and RGA confirms this prediction and suggests that in the absence of GAI and RGA, "growth" rather than "no growth" is the default state of plant stems. The function of the GA-signaling system is thus to act as a control system regulating the amount of this growth. We also demonstrate that the GA dose dependency of hypocotyl elongation is altered in mutants lacking GAI and RGA and propose that increments in GAI/RGA repressor function can explain the quantitative nature of GA responses.




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