REPRESSION OF SHOOT GROWTH, a bZIP Transcriptional Activator, Regulates Cell Elongation by Controlling the Level of Gibberellins

Abstract

Cell expansion, a developmental process regulated by both endogenous programs and environmental stimuli, is critically important for plant growth. Here, we report the isolation and characterization of RSG (for repression of shoot growth), a transcriptional activator with a basic leucine zipper (bZIP) domain. To examine the role of RSG in plant development, we generated transgenic tobacco plants expressing a dominant-negative form of RSG, which repressed the activity of full-length RSG. In transgenic plants, this expression severely inhibited stem internode growth, specifically cell elongation. These plants also had less endogenous amounts of the major active gibberellin (GA) in tobacco, GA 1 . Applying GAs restored the dwarf phenotypes of transgenic tobacco plants that expressed the dominant-negative form of RSG. To investigate the function of RSG in the regulation of the endogenous amounts of GAs, we identified a target for RSG. RSG bound and activated the promoter of Arabidopsis GA3 , one of the genes encoding enzymes involved in GA biosynthesis. Moreover, the dominant-negative form of RSG decreased expression of the GA3 homolog in transgenic tobacco plants. Our results show that RSG, a bZIP transcriptional activator, regulates the morphology of plants by controlling the endogenous amounts of GAs. INTRODUCTIONThe growth of multicellular organisms is accomplished by orderly cell division and regulated cell expansion. In plants, the contribution of cell expansion to growth and development is much more important than in most other organisms, the direction and extent of cell expansion determining the final shapes and sizes of all plant organs. In the phase of controlled expansion that generally follows cell division in meristems, the daughter cells may often increase in volume by у 50-fold (Steeves and Sussex, 1989). Phytohormones such as auxins, gibberellins (GAs), and brassinolides induce the elongation of cells along the longitudinal axis, whereas cytokinins and ethylene cause expansion of cells along the transverse axis (Shibaoka, 1994).GAs, which are tetracyclic diterpenoid growth factors, are essential regulators in many aspects of plant development, including stem elongation, seed germination, and flowering. The control of polarized cell expansion is thought to involve cortical microtubules, characteristically found beneath the cell membrane in plant cells (Giddings and Staehelin, 1988;Cyr, 1994). GAs promote the orientation of cortical microtubules perpendicularly to the growing axis of the cell; the corresponding cellulose deposition that results allows the cells to expand only in the growing axis, thereby producing thin, elongated plants (Shibaoka, 1994). GA-deficient Arabidopsis mutants display characteristic phenotypes that include dark green leaves and stunted growth attributable to the inhibition of stem elongation, especially during the cell elongation phase. Investigators using these mutants have isolated several genes that encode gibberellin biosynthetic enzymes (Sun and Kamiya, 1994;Chiang et al....

Arabidopsis thaliana transcription factors bZIP19 and bZIP23 regulate the adaptation to zinc deficiency

“…The ZDRE does not have the typical ACGT core as found in the A-box (TACGTA), C-box (GACGTC) or G-box (CACGTG) (10,16,17) DNA elements, to which plant bZIPs are known to preferentially bind. Although other consensus bZIP binding sites have been reported (18,19), the ZDRE element identified in this study is not among them.…”

The Transcription Factors HvABI5 and HvVP1 Are Required for the Abscisic Acid Induction of Gene Expression in Barley Aleurone Cells

“…In the case of barley, for which mutants of HvABI5 are not available, one way to study the effect of a transcription factor is through the expression of a dominant-negative form of the protein to repress the function of the wild-type gene. This has been particularly suitable for bZIP factors (Unger et al, 1993;Fukazawa et al, 2000;Sprenger-Haussels and Weisshaar, 2000). We constructed a dominant-negative form of HvABI5, and when introduced into aleurones, it efficiently repressed the ABA induction of the ABRC3-GUS reporter construct (Figure 4).…”

“…It has been shown that introducing a dominant-negative form of a bZIP protein can suppress the activity of the endogenous transcription factor in stable and transient expression studies (Fukazawa et al, 2000;Sprenger-Haussels and Weisshaar, 2000). To investigate the role of HvABI5 in the ABA induction of HVA1 , we tried to repress HvABI5 activity using a construct made of the C-terminal region of the protein that contains only the basic and Leu zipper regions lacking the first 279 amino acids.…”

The Effect of α-Amanitin on the Arabidopsis Seed Proteome Highlights the Distinct Roles of Stored and Neosynthesized mRNAs during Germination

“…et al, 2000). Transgenic tobacco plants expressing a dominant-negative form of RSG exhibit reduced germination frequency (Fukazawa et al, 2000), showing that RSG does play a role in seed germination. Recent work also documented the role played by thioredoxins, which are small M r proteins containing a redox-active disulfide group (Yano et al, 2002), in seed germination.…”