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....

High mobility group box 1 (HMGB1) protein plays multiple roles in transcription, replication, and cellular differentiation. HMGB1 is also secreted by activated monocytes and macrophages and passively released by necrotic or damaged cells, stimulating inflammation. HMGB1 is a novel antigen of antineutrophil cytoplasmic antibodies (ANCA) observed in the sera of patients with ulcerative colitis and autoimmune hepatitis, suggesting that HMGB1 is secreted from neutrophils to the extracellular milieu. However, the actual distribution of HMGB1 in the cytoplasm of neutrophils and the mechanisms responsible for it are obscure. Here we show that HMGB1 in neutrophils is post-translationally mono-methylated at Lys 42 . The methylation alters the conformation of HMGB1 and weakens its DNA binding activity, causing it to become largely distributed in the cytoplasm by passive diffusion out of the nucleus. Thus, post-translational methylation of HMGB1 causes its cytoplasmic localization in neutrophils. This novel pathway explains the distribution of nuclear HMGB1 to the cytoplasm and is important for understanding how neutrophils release HMGB1 to the extracellular milieu. High mobility group box 1 (HMGB1)2 protein is one of the most abundant nonhistone chromosomal proteins in eukaryotic organisms. The primary sequences of HMGB1 in various higher organisms, from birds to mammals, show more than 90% homology with each other (1). The protein has multiple roles in transcription, replication, and cellular differentiation (2, 3). HMGB1 interacts with several transcription factors, thereby allowing them to perform their cellular roles. The phenotype of Hmgb1 knock-out mice confirmed the functional importance of HMGB1 as a regulator of transcription: they die shortly after birth and show a defect in the transcriptional control exerted by the glucocorticoid receptor (4). The subcellular distribution of the protein is tissue-specific: HMGB1 is located in both the nuclei and the cytoplasm of different tissues, such as lymphoid tissue, testis, neurons, and hepatocytes (5). Wang et al. (6) identified HMGB1 as a late mediator of endotoxin lethality in mice and showed that monocytes and macrophages stimulated by lipopolysaccharide (LPS), tumor necrosis factor (TNF) or interleukin-1 (IL-1) secrete HMGB1 in a delayed response. Patients with sepsis show an increased serum level of HMGB1, which is correlated with the severity of infection (7). Moreover, HMGB1 in monocytes and macrophages is extensively acetylated upon activation by LPS, causing localization of the protein to the cytosol (8). Cytosolic HMGB1 is then concentrated into secretory lysosomes and secreted when the cells receive an appropriate second signal (9). The recent discovery of extracellular HMGB1 as a proinflammatory mediator has been supported by a number of studies. In addition, HMGB1 is passively released from the nucleus to the extracellular milieu by cells that die as a result of necrosis or damage (10).Our previous studies showed that HMGB1 and HMGB2 are novel antigens of a...

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.