A plant natriuretic peptide-like gene in the bacterial pathogen Xanthomonas axonopodis may induce hyper-hydration in the plant host: a hypothesis of molecular mimicry

BACKGROUND: Plant natriuretic peptides (PNPs) are systemically mobile molecules that regulate homeostasis at nanomolar concentrations. PNPs are up-regulated under conditions of osmotic stress and PNP-dependent processes include changes in ion transport and increases of H(2)O uptake into protoplasts and whole tissue. PRESENTATION OF THE HYPOTHESIS: The bacterial citrus pathogen Xanthomonas axonopodis pv. Citri str. 306 contains a gene encoding a PNP-like protein. We hypothesise that this bacterial protein can alter plant cell homeostasis and thus is likely to represent an example of molecular mimicry that enables the pathogen to manipulate plant responses in order to bring about conditions favourable to the pathogen such as the induced plant tissue hyper-hydration seen in the wet edged lesions associated with Xanthomonas axonopodis infection. TESTING THE HYPOTHESIS: We found a Xanthomonas axonopodis PNP-like protein that shares significant sequence similarity and identical domain organisation with PNPs. We also observed a significant excess of conserved residues between the two proteins within the domain previously identified as being sufficient to induce biological activity. Structural modelling predicts identical six stranded double-psi β barrel folds for both proteins thus supporting the hypothesis of similar modes of action. No significant similarity between the Xanthomonas axonopodis protein and other bacterial proteins from GenBank was found. Sequence similarity of the Xanthomonas axonopodis PNP-like protein with the Arabidopsis thaliana PNP (AtPNP-A), shared domain organisation and incongruent phylogeny suggest that the PNP-gene may have been acquired by the bacteria in an ancient lateral gene transfer event. Finally, activity of a recombinant Xanthomonas axonopodis protein in plant tissue and changes in symptoms induced by a Xanthomonas axonopodis mutant with a knocked-out PNP-like gene will be experimental proof of molecular mimicry. IMPLICATION OF THE HYPOTHESIS: If the hypothesis is true, it could at least in part explain why the citrus pathogen Xanthomonas campestris that does not contain a PNP-like gene produces dry corky lesions while the closely related Xanthomonas axonopodis forms lesions with wet edges. It also suggests that genes typically found in the host, horizontally transferred or heterologous, can help to explain aspects of the physiology of the host-pathogen interactions

Drought Stress Causes Specific Changes to the Spliceosome and Stress Granule Components.

The spliceosome processes RNAs from a pre-RNA state to a mature mRNA thereby influencing RNA availability for translation, localization, and turnover. It consists of complex structures containing RNA-binding proteins (RBPs) essential for post-transcriptional gene expression control. Here we investigate the dynamic modifications of spliceosomal RBPs under stress and in particular drought stress. We do so by mRNA interactome capture in Arabidopsis thaliana using label free quantitation. This approach identified 44 proteins associated with the spliceosome and further 32 proteins associated with stress granules. We noted a high enrichment in the motifs RDRR and RSRSRS that are characteristic of RNA interacting proteins. Identification of splicing factors reflect direct and/or indirect stress induced splicing events that have a direct effect on transcriptome and proteome changes under stress. Furthermore, detection of stress granule components is consistent with transcriptional arrest. Identification of drought induced stress granule components is critical in determining common abiotic stress-induced foci that can have biotechnological applications. This study may therefore open ways to modify plant stress responses at a systems level through the modification of key spliceosome components

Gametophytic Selection in Arabidopsis thaliana Supports the Selective Model of Intron Length Reduction

Why do highly expressed genes have small introns? This is an important issue, not least because it provides a testing ground to compare selectionist and neutralist models of genome evolution. Some argue that small introns are selectively favoured to reduce the costs of transcription. Alternatively, large introns might permit complex regulation, not needed for highly expressed genes. This “genome design” hypothesis evokes a regionalized model of control of expression and hence can explain why intron size covaries with intergene distance, a feature also consistent with the hypothesis that highly expressed genes cluster in genomic regions with high deletion rates. As some genes are expressed in the haploid stage and hence subject to especially strong purifying selection, the evolution of genes in Arabidopsis provides a novel testing ground to discriminate between these possibilities. Importantly, controlling for expression level, genes that are expressed in pollen have shorter introns than genes that are expressed in the sporophyte. That genes flanking pollen-expressed genes have average-sized introns and intergene distances argues against regional mutational biases and genomic design. These observations thus support the view that selection for efficiency contributes to the reduction in intron length and provide the first report of a molecular signature of strong gametophytic selection

Identification of a novel protein with guanylyl cyclase activity in Arabidopsis thaliana

Guanylyl cyclases (GCs) catalyze the formation of the second messenger guanosine 3 ,5 -cyclic monophos- phate (cGMP) from guanosine 5 -triphosphate (GTP). While many cGMP-mediated processes in plants have been reported, no plant molecule with GC activity has been identified. When the Arabidopsis thaliana genome is queried with GC sequences from cyanobacteria, lower and higher eukaryotes no unassigned proteins with sig- nificant similarity are found. However, a motif search of the A. thaliana genome based on conserved and func- tionally assigned amino acids in the catalytic center of annotated GCs returns one candidate that also contains the adjacent glycine-rich domain typical for GCs

A transcriptional analysis of carotenoid, chlorophyll and plastidial isoprenoid biosynthesis genes during development and osmotic stress responses in Arabidopsis thaliana

<p>Abstract</p> <p>Background</p> <p>The carotenoids are pure isoprenoids that are essential components of the photosynthetic apparatus and are coordinately synthesized with chlorophylls in chloroplasts. However, little is known about the mechanisms that regulate carotenoid biosynthesis or the mechanisms that coordinate this synthesis with that of chlorophylls and other plastidial synthesized isoprenoid-derived compounds, including quinones, gibberellic acid and abscisic acid. Here, a comprehensive transcriptional analysis of individual carotenoid and isoprenoid-related biosynthesis pathway genes was performed in order to elucidate the role of transcriptional regulation in the coordinated synthesis of these compounds and to identify regulatory components that may mediate this process in <it>Arabidopsis thaliana</it>.</p> <p>Results</p> <p>A global microarray expression correlation analysis revealed that the phytoene synthase gene, which encodes the first dedicated and rate-limiting enzyme of carotenogenesis, is highly co-expressed with many photosynthesis-related genes including many isoprenoid-related biosynthesis pathway genes. Chemical and mutant analysis revealed that induction of the co-expressed genes following germination was dependent on gibberellic acid and brassinosteroids (BR) but was inhibited by abscisic acid (ABA). Mutant analyses further revealed that expression of many of the genes is suppressed in dark grown plants by Phytochrome Interacting transcription Factors (PIFs) and activated by photoactivated phytochromes, which in turn degrade PIFs and mediate a coordinated induction of the genes. The promoters of <it>PSY </it>and the co-expressed genes were found to contain an enrichment in putative BR-auxin response elements and G-boxes, which bind PIFs, further supporting a role for BRs and PIFs in regulating expression of the genes. In osmotically stressed root tissue, transcription of Calvin cycle, methylerythritol 4-phosphate pathway and carotenoid biosynthesis genes is induced and uncoupled from that of chlorophyll biosynthesis genes in a manner that is consistent with the increased synthesis of carotenoid precursors for ABA biosynthesis. In all tissues examined, induction of β-carotene hydroxylase transcript levels are linked to an increased demand for ABA.</p> <p>Conclusions</p> <p>This analysis provides compelling evidence to suggest that coordinated transcriptional regulation of isoprenoid-related biosynthesis pathway genes plays a major role in coordinating the synthesis of functionally related chloroplast localized isoprenoid-derived compounds.</p

Unraveling Plant Responses to Bacterial Pathogens through Proteomics

Plant pathogenic bacteria cause diseases in important crops and seriously and negatively impact agricultural production. Therefore, an understanding of the mechanisms by which plants resist bacterial infection at the stage of the basal immune response or mount a successful specific R-dependent defense response is crucial since a better understanding of the biochemical and cellular mechanisms underlying these interactions will enable molecular and transgenic approaches to crops with increased biotic resistance. In recent years, proteomics has been used to gain in-depth understanding of many aspects of the host defense against pathogens and has allowed monitoring differences in abundance of proteins as well as posttranscriptional and posttranslational processes, protein activation/inactivation, and turnover. Proteomics also offers a window to study protein trafficking and routes of communication between organelles. Here, we summarize and discuss current progress in proteomics of the basal and specific host defense responses elicited by bacterial pathogens

Dynamic origin of the morphotropic phase boundary - Soft modes and phase instability in 0.68Pb(Mg1/3Nb2/3O3)-0.32PbTiO3

We report neutron inelastic scattering on single crystal 0.68Pb(Mg1/3Nb2/3O3)-0.32PbTiO3 (PMN-0.32PT), a relaxor ferroelectric material that lies within the compositional range of the morphotropic phase boundary (MPB). Data were obtained between 100 K and 600 K under zero and non-zero electric field applied along the cubic [001] direction. The lowest energy, zone-center, transverse optic phonon is strongly damped and softens slowly at high temperature; however the square of the soft mode energy begins to increase linearly with temperature as in a conventional ferroelectric, which we term the soft mode "recovery," upon cooling into the tetragonal phase at TC. Our data show that the soft mode in PMN-0.32PT behaves almost identically to that in pure PMN, exhibiting the same temperature dependence and recovery temperature even though PMN exhibits no well-defined structural transition (no TC). The temperature dependence of the soft mode in PMN-0.32PT is also similar to that in PMN-0.60PT; however in PMN-0.60PT the recovery temperature equals TC. These results suggest that the temperature dependence and the energy scale of the soft mode dynamics in PMN-xPT are independent of concentration on the Ti-poor side of the MPB, but scale with TC for Ti-rich compositions. Thus the MPB may be defined in lattice dynamical terms as the concentration where TC first matches the recovery temperature of the soft mode. High-resolution x-ray studies show that the cubic-to-ferroelectric phase boundary shifts to higher temperatures by an abnormal amount within the MPB region in the presence of an electric field. This suggests that an unusual instability exists within the apparently cubic phase at the MPB.Comment: 13 pages, 6 figure

Co-expression and promoter content analyses assign a role in biotic and abiotic stress responses to plant natriuretic peptides

<p>Abstract</p> <p>Background</p> <p>Plant natriuretic peptides (PNPs) are a class of systemically mobile molecules distantly related to expansins. While several physiological responses to PNPs have been reported, their biological role has remained elusive. Here we use a combination of expression correlation analysis, meta-analysis of gene expression profiles in response to specific stimuli and in selected mutants, and promoter content analysis to infer the biological role of the <it>Arabidopsis thaliana </it>PNP, AtPNP-A.</p> <p>Results</p> <p>A gene ontology analysis of <it>AtPNP-A </it>and the 25 most expression correlated genes revealed a significant over representation of genes annotated as part of the systemic acquired resistance (SAR) pathway. Transcription of these genes is strongly induced in response to salicylic acid (SA) and its functional synthetic analogue benzothiadiazole S-methylester (BTH), a number of biotic and abiotic stresses including many SA-mediated SAR-inducing conditions, as well as in the constitutive SAR expressing mutants <it>cpr5 </it>and <it>mpk4 </it>which have elevated SA levels. Furthermore, the expression of <it>AtPNP-A </it>was determined to be significantly correlated with the SAR annotated transcription factor, <it>WRKY 70</it>, and the promoters of <it>AtPNP-A </it>and the correlated genes contain an enrichment in the core WRKY binding W-box <it>cis</it>-elements. In constitutively expressing <it>WRKY 70 </it>lines the expression of <it>AtPNP-A </it>and the correlated genes, including the SAR marker genes, <it>PR-2 </it>and <it>PR-5</it>, were determined to be strongly induced.</p> <p>Conclusion</p> <p>The co-expression analyses, both in wild type and mutants, provides compelling evidence that suggests <it>AtPNP-A </it>may function as a component of plant defence responses and SAR in particular. The presented evidence also suggests that the expression of <it>AtPNP-A </it>is controlled by WRKY transcription factors and WRKY 70 in particular. <it>AtPNP-A </it>shares many characteristics with PR proteins in that its transcription is strongly induced in response to pathogen challenges, it contains an N-terminal signalling peptide and is secreted into the extracellular space and along with PR-1, PR-2 and PR-5 proteins it has been isolated from the Arabidopsis apoplast. Based on these findings we suggest that <it>AtPNP-A </it>could be classified as a newly identified PR protein.</p

Ozone and nitric oxide induce cGMP-dependent and independent transcription of defence genes in tobacco

Here, we analyse the temporal signatures of ozone (O3)-induced hydrogen peroxide (H2O2) and nitric oxide (NO) and the role of the second messenger guanosine 3′,5′-cyclic monophosphate (cGMP) in transcriptional changes of genes diagnostic for biotic and abiotic stress responses. Within 90 min O3 induced H2O2 and NO peaks and we demonstrate that NO donors cause rapid H2O2 accumulation in tobacco (Nicotiana tabacum) leaf. Ozone also causes highly significant, late (> 2 h) and sustained cGMP increases, suggesting that the second messenger may not be required in all early (< 2 h) responses to O3, but is essential and sufficient for the induction of some O3-dependent pathways