A basic helix-loop-helix 104 (bHLH104) protein functions as a transcriptional repressor for glucose and abscisic acid signaling in Arabidopsis.

Transduction of glucose (Glc) signaling is critical for plant development, metabolism, and stress responses. However, identifying initial Glc sensing and response stimulating mechanisms in plants has been difficult due to dual functions of glucose as energy sources and signaling component. A basic Helix-Loop-Helix 104 (bHLH104) protein is a homolog of bHLH34 previously isolated from Arabidopsis that functions as a transcriptional activator of Glc and abscisic acid (ABA) responses. In this study, we characterized bHLH104 as a transcription factor that binds to the regulatory region of Arabidopsis Plasma membrane Glc-responsive Regulator (AtPGR) gene. The bHLH104 binds to 5'-AANA-3' element of the promoter region of AtPGR in vitro and represses beta-glucuronidase (GUS) activity in AtPGR promoter-GUS transgenic plants. Genetic approaches show that bHLH104 positively regulates Glc and abscisic acid (ABA) response. These results suggest that bHLH104 is involved in Glc- and ABA-mediated signaling pathway. Taken together, these findings provide evidence that bHLH104 is an important transcription regulator in plant-sensitivity to Glc and ABA signaling.

Arabidopsis Basic Helix-Loop-Helix 34 (bHLH34) Is Involved in Glucose Signaling through Binding to a GAGA Cis-Element.

The modulation of glucose (Glc) homeostasis and signaling is crucial for plant growth and development. Nevertheless, the molecular signaling mechanism by which a plant senses a cellular Glc level and coordinates the expression of Glc-responsive genes is still incompletely understood. Previous studies have shown that Arabidopsis thaliana plasma membrane Glc-responsive regulator (AtPGR) is a component of the Glc-responsive pathway. Here, we demonstrated that a transcription factor bHLH34 binds to 5'-GAGA-3' element of the promoter region of AtPGR in vitro, and activates beta-glucuronidase (GUS) activity upon Glc treatment in AtPGR promoter-GUS transgenic plants. Gain- and loss-of-function analyses suggested that the bHLH34 involved in the responses to not only Glc, but also abscisic acid (ABA) and salinity. These results suggest that bHLH34 functions as a transcription factor in the Glc-mediated stress responsive pathway as well as an activator of AtPGR transcription. Furthermore, genetic experiments revealed that in Glc response, the functions of bHLH34 are different from that of a bHLH104, a homolog of bHLH34. Collectively, our findings indicate that bHLH34 is a positive regulator of Glc, and may affect ABA or salinity response, whereas bHLH104 is a negative regulator and epistatic to bHLH34 in the Glc response.

Regulation of Arabidopsis thaliana plasma membrane glucose-responsive regulator (AtPGR) expression by A. thaliana storekeeper-like transcription factor, AtSTKL, modulates glucose response in Arabidopsis.

Biochemical, genetic, physiological, and molecular research in plants has demonstrated a central role of glucose (Glc) in the control of plant growth, metabolism, and development, and has revealed networks that integrate light, stresses, nutrients, and hormone signaling. Previous studies have reported that AtPGR protein as potential candidates for Glc signaling protein. In the present study, we characterized transcription factors that bind to the upstream region of the AtPGR gene isolated using the yeast one-hybrid screening with an Arabidopsis cDNA library. One of the selected genes (AtSTKL) appeared to confer elevated sensitivity to Glc response. Overexpression of AtSTKLs (AtSTKL1 and AtSTKL2) increased the sensitivity to Glc during the post-germination stages. In contrast, atstkl1 and atstkl2 antisense lines displayed reduced sensitivity to high Glc concentration during the early seedling stage. Furthermore, we showed that the two AtSTKLs bind to the 5'-GCCT-3' element of the upstream promoter region of the AtPGR gene in vitro and repress the beta-glucuronidase (GUS) activity in AtPGR promoter-GUS (P999-GUS) transgenic plants. Green fluorescent protein (GFP)-tagged AtSTKLs were localized in the nuclei of transgenic Arabidopsis cells. Collectively, these results suggest that AtSTKL1 and AtSTKL2 function both as repressors of AtPGR transcription and as novel transcription factors in the Glc signaling pathway.

Heterologous expression of the gourd E3 ubiquitin ligase gene LsRZF1 compromises the drought stress tolerance in Arabidopsis thaliana.

Protein ubiquitination is one of the major regulatory processes used by eukaryotic cells. The ubiquitin E3 ligase acts as a main determinant of substrate specificity. However, the precise roles of E3 ligase in plants to drought stress are poorly understood. In this study, a gourd family (Lagenaria siceraria) ortholog of Arabidopsis thaliana RING Zinc Finger 1 (AtRZF1) gene, designated LsRZF1, was identified and characterized. LsRZF1 was reduced by abscisic acid (ABA), osmotic stress, and drought conditions. Compared to wild type, transgenic Arabidopsis plants ectopic expressing LsRZF1 were hypersensitive to ABA and osmotic stress during early seedling development, indicating that LsRZF1 negatively regulates drought-mediated control of early seedling development. Moreover, the ectopic expression of the LsRZF1 gene was very influential in drought sensitive parameters including proline content, water loss, and the expression of dehydration stress-related genes. Furthermore, ubiquitin E3 ligase activity and genetic data indicate that AtRZF1 and LsRZF1 function in similar pathway to control proline metabolism in Arabidopsis under drought condition. Together, these results suggest that the E3 ligase LsRZF1 is an important regulator of water deficit stress during early seedling development.

The atrzf1 mutation of the novel RING-type E3 ubiquitin ligase increases proline contents and enhances drought tolerance in Arabidopsis.

The covalent attachment of ubiquitin to proteins plays a fundamental role in the regulation of cellular function through biological events involving abiotic or biotic stress responses, immune responses, and apoptosis. Here, we characterize the biological function of the Arabidopsis thaliana RING Zinc Finger 1 (AtRZF1) in dehydration response. AtRZF1 was significantly reduced by drought stress. The atrzf1 mutant was less sensitive to osmotic stress than the wild-type during early seedling development, whereas transgenic plants overexpressing AtRZF1 were hypersensitive, indicating that AtRZF1 negatively regulates drought-mediated control of early seedling development. Moreover, the ectopic expression of the AtRZF1 gene was very significantly influential in drought sensitive parameters including proline content, water loss, membrane ion leakage and the expression of dehydration stress-related genes. AtRZF1 is a functional E3 ubiquitin ligase, and its conserved C3H2C3-type RING domain is likely important for the biological function of AtRZF1 in drought response. Together, these results suggest that the E3 ligase AtRZF1 is an important regulator of water deficit stress during early seedling development.

Overexpression of L-type lectin-like protein kinase 1 confers pathogen resistance and regulates salinity response in Arabidopsis thaliana.

Plant receptor-like protein kinases are thought to be involved in various cellular processes mediated by signal transduction pathways. There are about 45 lectin receptor kinases in Arabidopsis, but only a few have been studied. Here, we investigated the effect of the disruption and overexpression of a plasma membrane-localized L-type lectin-like protein kinase 1, AtLPK1 (At4g02410), on plant responses to abiotic and biotic stress. Expression of AtLPK1 was strongly induced by abscisic acid, methyl jasmonate, salicylic acid and stress treatments. Overexpression of AtLPK1 in Arabidopsis resulted in enhanced seed germination and cotyledon greening under high salinity condition, while antisense transgenic lines were more sensitive to salt stress. Activity of three abiotic stress responsive genes, RD29A, RD29B and COR15A, was elevated in AtLPK1-overexpressing plants than that in wild type (WT) plants with salt treatment, whereas the transcript level of these genes in antisense plants decreased compared with WT. Furthermore, AtLPK1-overexpressing plants displayed increased resistance to infection by Botrytis cinerea and exhibited stronger expression of a group of defense-related genes than did WT. The data implicates AtLPK1 plays essential roles at both abiotic and biotic stress response in Arabidopsis thaliana.

Molecular and physiological characterization of the Arabidopsis thaliana Oxidation-related Zinc Finger 2, a plasma membrane protein involved in ABA and salt stress response through the ABI2-mediated signaling pathway.

CCCH-type zinc finger proteins are important for developmental and environmental responses. However, the precise roles of these proteins in plant stress tolerance are poorly understood. Arabidopsis thaliana Oxidation-related Zinc Finger 2 (AtOZF2) (At4g29190) is an AtOZF1 homolog previously isolated from Arabidopsis, which confers oxidative stress tolerance on plants. The AtOZF2 protein is localized in the plasma membrane, as is AtOZF1. Disruption expression of AtOZF2 led to reduced root length and leaf size. AtOZF2 was implicated to be involved in the ABA and salinity responses. atozf2 antisense lines were more sensitive to ABA and salt stress during the seed germination and cotyledon greening processes. In contrast, AtOZF2-overexpressing plants were more insensitive to ABA and salt stress than the wild type. Interestingly, in the presence of ABA and salt stress, the transcript level of ABA insensitive 2 (ABI2), but not that of ABI1, in AtOZF2-overexpressing plants was lower than that in the wild type, whereas the expression of ABI2 in atozf2 was significantly enhanced. Thus, AtOZF2 is involved in the ABA and salt stress response through the ABI2-mediated signaling pathway. Taken together, these findings provide compelling evidence that AtOZF2 is an important regulator for plant tolerance to abiotic stress.

The R-R-type MYB-like transcription factor, AtMYBL, is involved in promoting leaf senescence and modulates an abiotic stress response in Arabidopsis.

Functional analysis of a putative novel transcription factor Arabidopsis MYB-like protein designated AtMYBL, which contains two predicted DNA-binding domains, was performed. The physiological role of the R-R-type MYB-like transcription factor has not been reported in any plant. Analyses of an AtMYBL promoter-ß-glucuronidase (GUS) construct revealed substantial gene expression in old leaves and induction of GUS activity by ABA and salt stress. AtMYBL-overexpressing plants displayed a markedly enhanced leaf senescence phenotype. Moreover, the ectopic expression of the AtMYBL gene was very significantly influential in senescence parameters including Chl content, membrane ion leakage and the expression of senescence-related genes. Although the seed germination rate was improved under ABA and saline stress conditions in the AtMYBL-overexpressing plants, decreased salt tolerance was evident compared with the wild type and atmybl RNA interference lines during later seedling growth when exposed to long-term salt stress, indicating that AtMYBL protein is able to developmentally regulate stress sensitivity. Furthermore, AtMYBL protein activated the transcription of a reporter gene in yeast. Green fluorescent protein-tagged AtMYBL was localized in the nuclei of transgenic Arabidopsis cells. Taken together, these results suggest that AtMYBL functions in the leaf senescence process, with the abiotic stress response implicated as a putative potential transcription factor.

Physiological characterization of the Arabidopsis thaliana oxidation-related zinc finger 1, a plasma membrane protein involved in oxidative stress.

The CCCH-type zinc finger proteins are a superfamily containing tandem zinc-binding motifs involved in many aspects of plant growth and development. However, the precise role of these proteins involved in plant stress tolerance is poorly understood. This study was to examine the regulatory and functional role of the CCCH-type zinc finger protein, AtOZF1 (At2g19810), under oxidative stress. Interestingly, the AtOZF1 protein was localized in the plasma membrane. The AtOZF1 transcripts were highly induced by treatment with hydrogen peroxide, abscisic acid and salinity. The AtOZF1-overexpressing plants were relatively resistant to oxidative stress than wild-type and T-DNA insertion mutant atozf1. Malondialdehyde, a decomposition product of lipid peroxidation, accumulated in atozf1 mutants more than in wild-type and AtOZF1-overexpressing plants. Furthermore, atozf1 mutants displayed lower activities of catalase and guaiacol peroxidase, higher chlorosis, and down-regulated expression of antioxidant genes under oxidative stress. Taken together, these observations demonstrate that AtOZF1 is required for the tolerance of Arabidopsis to oxidative stress.

Glucosamine causes overproduction of reactive oxygen species, leading to repression of hypocotyl elongation through a hexokinase-mediated mechanism in Arabidopsis.

Glucosamine (GlcN) is a naturally occurring amino-sugar that is synthesized by amidation of fructose-6-phosphate. Although a number of reports have examined the biological effects of GlcN on insulin resistance in mammalian systems, little is known about its effects on plant growth. In this study, we have shown that exogenous GlcN inhibits hypocotyl elongation in Arabidopsis, whereas glucose and its analogs alleviate this inhibitory effect. The hexokinase (HXK)-specific inhibitor mannoheptulose also restored hypocotyl elongation. The gin2-1 mutants with an alteration in AtHXK1 exhibited higher tolerance to GlcN. We also found that GlcN induces a significant increase in the production of reactive oxygen species (ROS). In addition, the GlcN-mediated inhibition of hypocotyl elongation was relieved by reducing agents such as ascorbic acid and glutathione. GlcN treatment resulted in significant induction of expression of GST1, GST2 and GST6, which are marker genes for ROS production. The gin2 mutation also represses the ROS production and the GST2 induction by GlcN treatment. Taken together, these results provide evidence that GlcN induces HXK-mediated induction of oxidative stress, leading to growth repression in Arabidopsis thaliana.

Arabidopsis hot2 encodes an endochitinase-like protein that is essential for tolerance to heat, salt and drought stresses.

The Arabidopsis hot2 mutant was originally identified based on its lack of thermotolerance, but pleiotropic abnormal phenotypes are also exhibited under normal conditions, including semi-dwarfism, ethylene overproduction and aberrant cell shape with incomplete cell walls. Here we present additional characterization of the hot2 mutant, and the map-based cloning of HOT2. Mutants of hot2 had an aberrant tolerance to salt and drought stresses, and accumulated high levels of Na(+) in cells under either normal or NaCl stress conditions. Expression of the stress-inducible COR15A and KIN1 gene in hot2 mutants in response to increased NaCl concentrations was normal. HOT2 encoded a chitinase-like protein (AtCTL1) that has not previously been shown to be involved in tolerance to salt stress. Ten-day-old seedlings of wild-type plants exhibited constitutive expression of the AtCTL1 transcript, the level of which was unaffected by treatment with NaCl, mannitol or mild heat. These observations provide genetic evidence that a chitinase-like protein prevents the overaccumulation of Na(+) ions, thereby contributing to the salt tolerance in Arabidopsis. A possible role for this chitinase-like protein in Arabidopsis tolerance to abiotic stress is discussed.