Biomedical subjects

Xinnian Dong

Publications and source records attributed to Xinnian Dong.

12 recordsLinked to original sources

PubMed2006

A comprehensive structure-function analysis of Arabidopsis SNI1 defines essential regions and transcriptional repressor activity.

The expression of systemic acquired resistance (SAR) in plants involves the upregulation of many Pathogenesis-Related (PR) genes, which work in concert to confer resistance to a broad spectrum of pathogens. Because SAR is a costly process, SAR-associated transcription must be tightly regulated. Arabidopsis thaliana SNI1 (for Suppressor of NPR1, Inducible) is a negative regulator of SAR required to dampen the basal expression of PR genes. Whole genome transcriptional profiling showed that in the sni1 mutant, Nonexpresser of PR genes (NPR1)-dependent benzothiadiazole S-methylester-responsive genes were specifically derepressed. Interestingly, SNI1 also repressed transcription when expressed in yeast, suggesting that it functions as an active transcriptional repressor through a highly conserved mechanism. Chromatin immunoprecipitation indicated that histone modification may be involved in SNI1-mediated repression. Sequence comparison with orthologs in other plant species and a saturating NAAIRS-scanning mutagenesis of SNI1 identified regions in SNI1 that are required for its activity. The structural similarity of SNI1 to Armadillo repeat proteins implies that SNI1 may form a scaffold for interaction with proteins that modulate transcription.

Amino Acid Sequence↗

PubMed2006

Fitness benefits of systemic acquired resistance during Hyaloperonospora parasitica infection in Arabidopsis thaliana.

We investigated the fitness benefits of systemic acquired resistance (SAR) in Arabidopsis thaliana using a mutational and transformational genetic approach. Genetic lines were designed to differ in the genes determining resistance signaling in a common genetic background. Two mutant lines (cpr1 and cpr5) constitutively activate SAR at different points in SAR signaling, and one mutant line (npr1) has impaired SAR. The transgenic line (NPR1-H) has enhanced resistance when SAR is activated, but SAR is still inducible similarly to wild type. The fitness benefits were also investigated under two nutrient levels to test theories that preventing pathogen damage and realized resistance benefits may be affected by nutrient availability. Under low-nutrient conditions and treatment with the pathogenic oomycete, Hyaloperonospora parasitica, wild type had a higher fitness than the mutant that could not activate SAR, demonstrating that normal inducible SAR is beneficial in these conditions; this result, however, was not found under high-nutrient conditions. The mutants with constitutive SAR all failed to show a fitness benefit in comparison to wild type under a H. parasitica pathogen treatment, suggesting that SAR is induced to prevent an excessive fitness cost.

Arabidopsis↗

PubMed2006

A genomic approach to identify regulatory nodes in the transcriptional network of systemic acquired resistance in plants.

Many biological processes are controlled by intricate networks of transcriptional regulators. With the development of microarray technology, transcriptional changes can be examined at the whole-genome level. However, such analysis often lacks information on the hierarchical relationship between components of a given system. Systemic acquired resistance (SAR) is an inducible plant defense response involving a cascade of transcriptional events induced by salicylic acid through the transcription cofactor NPR1. To identify additional regulatory nodes in the SAR network, we performed microarray analysis on Arabidopsis plants expressing the NPR1-GR (glucocorticoid receptor) fusion protein. Since nuclear translocation of NPR1-GR requires dexamethasone, we were able to control NPR1-dependent transcription and identify direct transcriptional targets of NPR1. We show that NPR1 directly upregulates the expression of eight WRKY transcription factor genes. This large family of 74 transcription factors has been implicated in various defense responses, but no specific WRKY factor has been placed in the SAR network. Identification of NPR1-regulated WRKY factors allowed us to perform in-depth genetic analysis on a small number of WRKY factors and test well-defined phenotypes of single and double mutants associated with NPR1. Among these WRKY factors we found both positive and negative regulators of SAR. This genomics-directed approach unambiguously positioned five WRKY factors in the complex transcriptional regulatory network of SAR. Our work not only discovered new transcription regulatory components in the signaling network of SAR but also demonstrated that functional studies of large gene families have to take into consideration sequence similarity as well as the expression patterns of the candidates.

Arabidopsis↗

PubMed2005

Induction of protein secretory pathway is required for systemic acquired resistance.

In plants, systemic acquired resistance (SAR) is established as a result of NPR1-regulated expression of pathogenesis-related (PR) genes. Using gene expression profiling in Arabidopsis, we found that in addition to controlling the expression of PR genes, NPR1 also directly controls the expression of the protein secretory pathway genes. Up-regulation of these genes is essential for SAR, because mutations in some of them diminished the secretion of PR proteins (for example, PR1), resulting in reduced resistance. We provide evidence that NPR1 coordinately regulates these secretion-related genes through a previously undescribed cis-element. Activation of this cis-element is controlled by a transcription factor that is translocated into the nucleus upon SAR induction.

Arabidopsis↗

PubMed2004

The role of membrane-bound ankyrin-repeat protein ACD6 in programmed cell death and plant defense.

Programmed cell death (PCD) is a common defense response in plants against pathogen infection. The recently cloned ACD6 gene was identified in an Arabidopsis mutant, accelerated cell death 6 (acd6), that undergoes PCD in the absence of a pathogen challenge. ACD6 is a founding member of a large family of genes that encode proteins with a short amino-terminal region, nine ankyrin repeats in the middle, and five putative transmembrane domains in the carboxyl-terminal region. Characterization of the original gain-of-function acd6 mutant and a transferred-DNA knockout mutant acd6-T showed that ACD6 is an activator of the defense pathway against bacterial pathogens and plays a role in PCD through regulation of the defense signal salicylic acid (SA). SA mediates not only downstream pathogenesis-related (PR) genes, which encode proteins with antimicrobial activities, but also ACD6, forming a feedback signal amplification loop.

Ankyrins↗

PubMed2004

NPR1, all things considered.

Recent work has shown that the Arabidopsis NPR1 protein not only plays an essential role in salicylic acid (SA)-mediated systemic acquired resistance and rhizobacterium-triggered induced systemic resistance, but also is involved in crosstalk inhibition of jasmonic acid (JA)-mediated defense responses. Molecular characterization has revealed that activation of NPR1 and certain TGA transcription factors occurs under the reducing conditions that follow an initial oxidative burst after the induction of defense responses. In addition to NPR1 and TGA, the single-stranded DNA-binding transcription factor AtWhy1 and the WRKY70 transcription factor were recently found to be involved in SA-mediated defense and SA-JA crosstalk, respectively.

Arabidopsis↗

PubMed2004

Pathogen-induced systemic DNA rearrangement in plants.

Infection of tobacco plants with tobacco mosaic virus and oilseed rape mosaic virus was shown to induce a threefold increase in homologous DNA recombination in non-infected tissues. Grafting experiments by Igor Kovalchuk et al. demonstrated that this increase was mediated by a systemic recombination signal that traveled ahead of the virus. A similar increase in DNA recombination was also observed in the progeny of the infected plants, indicating that pathogen-induced recombination can lead to heritable adaptations to environmental stresses.

Recombination, Genetic↗

PubMed2004

Fitness costs of mutations affecting the systemic acquired resistance pathway in Arabidopsis thaliana.

This study investigated the fitness effects of four mutations (npr1, cpr1, cpr5, and cpr6) and two transgenic genotypes (NPR1-L and NPR1-H) affecting different points of the systemic acquired resistance (SAR) signaling pathway associated with pathogen defense in Arabidopsis thaliana. The npr1 mutation, which resulted in a failure to express SAR, had no effect on fitness under growth chamber conditions, but decreased fitness in the field. The expression of NPR1 positively correlated with the fitness in the field. Constitutive activation of SAR by cpr1, cpr5, and cpr6 generally decreased fitness in the field and under two nutrient levels in two growth chamber conditions. At low-nutrient levels, fitness differences between wild type and the constitutive mutants were unchanged or reduced (especially in cpr5). The reduced fitness of the constitutive mutants suggests that this pathway is costly, with the precise fitness consequences highly dependent on the environmental context.

Arabidopsis↗

PubMed2003

A gain-of-function mutation in a plant disease resistance gene leads to constitutive activation of downstream signal transduction pathways in suppressor of npr1-1, constitutive 1.

Plants have evolved sophisticated defense mechanisms against pathogen infections, during which resistance (R) genes play central roles in recognizing pathogens and initiating defense cascades. Most of the cloned R genes share two common domains: the central domain, which encodes a nucleotide binding adaptor shared by APAF-1, certain R proteins, and CED-4 (NB-ARC), plus a C-terminal region that encodes Leu-rich repeats (LRR). In Arabidopsis, a dominant mutant, suppressor of npr1-1, constitutive 1 (snc1), was identified previously that constitutively expresses pathogenesis-related (PR) genes and resistance against both Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2. The snc1 mutation was mapped to the RPP4 cluster. In snc1, one of the TIR-NB-LRR-type R genes contains a point mutation that results in a single amino acid change from Glu to Lys in the region between NB-ARC and LRR. Deletions of this R gene in snc1 reverted the plants to wild-type morphology and completely abolished constitutive PR gene expression and disease resistance. The constitutive activation of the defense responses was not the result of the overexpression of the R gene, because its expression level was not altered in snc1. Our data suggest that the point mutation in snc1 renders the R gene constitutively active without interaction with pathogens. To analyze signal transduction pathways downstream of snc1, epistasis analyses between snc1 and pad4-1 or eds5-3 were performed. Although the resistance signaling in snc1 was fully dependent on PAD4, it was only partially affected by blocking salicylic acid (SA) synthesis, suggesting that snc1 activates both SA-dependent and SA-independent resistance pathways.

Amino Acid Sequence↗

PubMed2003

Inducers of plant systemic acquired resistance regulate NPR1 function through redox changes.

NPR1 is an essential regulator of plant systemic acquired resistance (SAR), which confers immunity to a broad-spectrum of pathogens. SAR induction results in accumulation of the signal molecule salicylic acid (SA), which induces defense gene expression via activation of NPR1. We found that in an uninduced state, NPR1 is present as an oligomer formed through intermolecular disulfide bonds. Upon SAR induction, a biphasic change in cellular reduction potential occurs, resulting in reduction of NPR1 to a monomeric form. Monomeric NPR1 accumulates in the nucleus and activates gene expression. Inhibition of NPR1 reduction prevents defense gene expression, whereas mutation of Cys82 or Cys216 in NPR1 leads to constitutive monomerization, nuclear localization of the mutant proteins, and defense gene expression. These data provide a missing link between accumulation of SA and activation of NPR1 in the SAR signaling pathway.

Arabidopsis↗

PubMed2003

NPR1 modulates cross-talk between salicylate- and jasmonate-dependent defense pathways through a novel function in the cytosol.

Plant defenses against pathogens and insects are regulated differentially by cross-communicating signal transduction pathways in which salicylic acid (SA) and jasmonic acid (JA) play key roles. In this study, we investigated the molecular mechanism of the antagonistic effect of SA on JA signaling. Arabidopsis plants unable to accumulate SA produced 25-fold higher levels of JA and showed enhanced expression of the JA-responsive genes LOX2, PDF1.2, and VSP in response to infection by Pseudomonas syringae pv tomato DC3000, indicating that in wild-type plants, pathogen-induced SA accumulation is associated with the suppression of JA signaling. Analysis of the Arabidopsis mutant npr1, which is impaired in SA signal transduction, revealed that the antagonistic effect of SA on JA signaling requires the regulatory protein NPR1. Nuclear localization of NPR1, which is essential for SA-mediated defense gene expression, is not required for the suppression of JA signaling, indicating that cross-talk between SA and JA is modulated through a novel function of NPR1 in the cytosol.

Arabidopsis Proteins↗

PubMed2002

In vivo interaction between NPR1 and transcription factor TGA2 leads to salicylic acid-mediated gene activation in Arabidopsis.

The Arabidopsis NPR1 protein is a key regulator of salicylic acid (SA)-mediated gene expression in systemic acquired resistance. Based on yeast two-hybrid analysis, NPR1 has been suggested to interact with members of the TGA family of transcription factors, including TGA2 (AHBP-1b). However, genetic evidence demonstrating that the NPR1-TGA interaction occurs in planta is still lacking, and the role of this interaction in SA-mediated gene activation has yet to be determined. In this study, we expressed a truncated form of TGA2 in Arabidopsis and found that the resulting transgenic lines displayed phenotypes similar to those of npr1 mutants. This dominant-negative effect of the TGA2 mutant shows that TGA2 and NPR1 interact in planta. We also present biochemical evidence indicating that this interaction is specific and enhanced by SA treatment. Moreover, using a chimera reporter system, we found that a chimeric TGA2GAL4 transcription factor activated a UAS(GAL)::GUS reporter gene in response to SA and that this activation was abolished in the npr1 mutant. NPR1 is required for the DNA binding activity of the transcription factor. These genetic data clearly demonstrate that TGA2 is a SA-responsive and NPR1-dependent transcription activator.

Arabidopsis↗