An analysis of negative and positive components of the gibberellin signaling pathway in <i>Arabidopsis thaliana</i>
Bioactive gibberellins (GAs) are phytohormones that regulate developmental processes ranging from seed germination to vegetative growth and floral organ formation. Thus, production and perception of these hormones are essential to plant development. In comparison to GA biosynthesis, much of the GA signaling pathway remains to be elucidated. The work described here focuses on filling gaps in our understanding of GA signal transduction in Arabidopsis.
The RGA-like genes RGL1, RGL2, and RGL3 were investigated as good candidates for GA signaling molecules. The RGL proteins share high sequence identity with RGA and GAI, two well-characterized negative components of the GA signaling pathway. We hypothesized that RGL1, RGL2, and RGL3 might regulate processes—such as germination and floral development—which are not dramatically affected by RGA and GAI. A characterization of plants homozygous for insertion alleles of each of the RGL genes indicated significant overlap in the functions of RGA, GAI, and the RGLs. Whereas RGL2 is the major repressor of GA-dependent germination, RGA, RGL1, and RGL2 act synergistically to regulate floral development. RGL3 may play a minor role in germination.
To identify additional components of the GA signaling pathway, an activation tagging screen was performed. Activation tagging involves introducing a series of enhancers randomly into the genome to cause the over-expression of genes adjacent to the insertion sites. For the screen, gal-6 plants—which are semi-dwarfed due to a defect in GA biosynthesis—were transformed. Transformants were screened primarily on the basis of height, and several categories of potentially interesting mutants were recovered.
Of these, one activation-tagged line was late-flowering and tall, with increased apical dominance. This line over-expressed a gene for a putative Myb-like transcription factor. The tagged gene has been named TIA for TALL, INCREASED APICAL DOMINANCE. A fusion of TIA with green fluorescent protein exhibited nuclear localization, as would be expected for a transcription factor. Additionally, quantitative-PCR-based analysis and the characterization of promoter-reporter constructs indicated that TIA is expressed in the plant vasculature throughout development. The pleiotropic effects of over-expressing TIA suggest roles for this novel, putative transcription factor in regulating multiple aspects of development, including flowering-time.