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....

REPRESSION OF SHOOT GROWTH (RSG) is a tobacco (Nicotiana tabacum) transcriptional activator with a basic Leu zipper domain that regulates endogenous amounts of gibberellins (GAs) by the control of a GA biosynthetic enzyme. The 14-3-3 signaling proteins have been suggested to suppress RSG by sequestering it in the cytoplasm. Here, we show that RSG phosphorylation on Ser-114 is important for 14-3-3 binding. We found that GA levels regulate the intracellular localization of RSG. RSG translocated into the nucleus in response to a reduction in GA levels. GA treatment could reverse this nuclear accumulation. The GA-induced disappearance of RSG–green fluorescent protein from the nucleus did not depend on protein degradation. By contrast, the mutant RSG (S114A) that could not bind to 14-3-3 continued to be localized predominantly in the nucleus after GA application. Analysis of the mRNA levels of GA biosynthetic genes showed that the feedback regulation of the GA 20-oxidase gene was inhibited in transgenic plants expressing a dominant negative form of RSG. Our results suggest that RSG is negatively modulated by GAs by 14-3-3 binding and might be involved in GA homeostasis

Gibberellins (GAs) are essential regulators of plant development, and DELLAs are negative regulators of GA signaling. The mechanism of GA-dependent transcription has been explained by DELLA-mediated titration of transcriptional activators and their release through the degradation of DELLAs in response to GA. However, the effect of GA on genome-wide expression is predominantly repression, suggesting the existence of unknown mechanisms of GA function. In this study, we identified an Arabidopsis thaliana DELLA binding transcription factor, GAI-ASSOCIATED FACTOR1 (GAF1). GAF1 shows high homology to INDETERMINATE DOMAIN1 (IDD1)/ENHYDROUS. GA responsiveness was decreased in the double mutant gaf1 idd1, whereas it was enhanced in a GAF1 overexpressor. GAF1 binds to genes that are subject to GA feedback regulation. Furthermore, we found that GAF1 interacts with the corepressor TOPLESS RELATED (TPR) and that DELLAs and TPR act as coactivators and a corepressor of GAF1, respectively. GA converts the GAF1 complex from transcriptional activator to repressor via the degradation of DELLAs. These results indicate that DELLAs turn on or off two sets of GA-regulated genes via dual functions, namely titration and coactivation, providing a mechanism for the integrative regulation of plant growth and GA homeostasis.

Negative feedback is a fundamental mechanism of organisms to maintain the internal environment within tolerable limits. Gibberellins (GAs) are essential regulators of many aspects of plant development, including seed germination, stem elongation, and flowering. GA biosynthesis is regulated by the feedback mechanism in plants. GA 3-oxidase (GA3ox) catalyzes the final step of the biosynthetic pathway to produce the physiologically active GAs. Here, we found that only the AtGA3ox1 among the AtGA3ox family of Arabidopsis (Arabidopsis thaliana) is under the regulation of GA-negative feedback. We have identified a cis-acting sequence responsible for the GA-negative feedback of AtGA3ox1 using transgenic plants. Furthermore, we have identified an AT-hook protein, AGF1 (for the AT-hook protein of GA feedback regulation), as a DNA-binding protein for the cis-acting sequence of GA-negative feedback. The mutation in the cis-acting sequence abolished both GA-negative feedback and AGF1 binding. In addition, constitutive expression of AGF1 affected GA-negative feedback in Arabidopsis. Our results suggest that AGF1 plays a role in the homeostasis of GAs through binding to the cis-acting sequence of the GA-negative feedback of AtGA3ox1.