Showing papers on "Biotic stress published in 2009"

Fungal endophytes: diversity and functional roles.

Abstract: Summary 1 Summary All plants in natural ecosystems appear to be symbiotic with fungal endophytes. This highly diverse group of fungi can have profound impacts on plant communities through increasing fitness by conferring abiotic and biotic stress tolerance, increasing biomass and decreasing water consumption, or decreasing fitness by altering resource allocation. Despite more than 100 yr of research resulting in thousands of journal articles, the ecological significance of these fungi remains poorly characterized. Historically, two endophytic groups (clavicipitaceous (C) and nonclavicipitaceous (NC)) have been discriminated based on phylogeny and life history traits. Here, we show that NC-endophytes represent three distinct functional groups based on host colonization and transmission, in planta biodiversity and fitness benefits conferred to hosts. Using this framework, we contrast the life histories, interactions with hosts and potential roles in plant ecophysiology of C- and NC-endophytes, and highlight several key questions for future work in endophyte biology.

Plant-rhizobacteria interactions alleviate abiotic stress conditions.

Abstract: Root-colonizing non-pathogenic bacteria can increase plant resistance to biotic and abiotic stress factors. Bacterial inoculates have been applied as biofertilizers and can increase the effectiveness of phytoremediation. Inoculating plants with non-pathogenic bacteria can provide 'bioprotection' against biotic stresses, and some root-colonizing bacteria increase tolerance against abiotic stresses such as drought, salinity and metal toxicity. Systematic identification of bacterial strains providing cross-protection against multiple stressors would be highly valuable for agricultural production in changing environmental conditions. For bacterial cross-protection to be an effective tool, a better understanding of the underlying morphological, physiological and molecular mechanisms of bacterially mediated stress tolerance, and the phenomenon of cross-protection is critical. Beneficial bacteria-mediated plant gene expression studies under non-stress conditions or during pathogenic rhizobacteria-plant interactions are plentiful, but only few molecular studies on beneficial interactions under abiotic stress situations have been reported. Thus, here we attempt an overview of current knowledge on physiological impacts and modes of action of bacterial mitigation of abiotic stress symptoms in plants. Where available, molecular data will be provided to support physiological or morphological observations. We indicate further research avenues to enable better use of cross-protection capacities of root-colonizing non-pathogenic bacteria in agricultural production systems affected by a changing climate.

Roles of Plant Small RNAs in Biotic Stress Responses

Abstract: A multitude of small RNAs (sRNAs, 18–25 nt in length) accumulate in plant tissues. Although heterogeneous in size, sequence, genomic distribution, biogenesis, and action, most of these molecules mediate repressive gene regulation through RNA silencing. Besides their roles in developmental patterning and maintenance of genome integrity, sRNAs are also integral components of plant responses to adverse environmental conditions, including biotic stress. Until recently, antiviral RNA silencing was considered a paradigm of the interactions linking RNA silencing to pathogens: Virus-derived sRNAs silence viral gene expression and, accordingly, viruses produce suppressor proteins that target the silencing mechanism. However, increasing evidence shows that endogenous, rather than pathogen-derived, sRNAs also have broad functions in regulating plant responses to various microbes. In turn, microbes have evolved ways to inhibit, avoid, or usurp cellular silencing pathways, thereby prompting the deployment of counter-c...

Overexpression of the soybean GmERF3 gene, an AP2/ERF type transcription factor for increased tolerances to salt, drought, and diseases in transgenic tobacco

Abstract: A new member of the AP2/ERF transcription factor family, GmERF3, was isolated from soybean. Sequence analysis showed that GmERF3 contained an AP2/ERF domain of 58 amino acids and two putative nuclear localization signal (NLS) domains. It belonged to a group IV protein in the ERF (ethylene response factor) subfamily as typified by a conserved N-terminal motif [MCGGAI(I/L)]. Expression of GmERF3 was induced by treatments with high salinity, drought, abscisic acid (ABA), salicylic acid (SA), jasmonic acid (JA), ethylene (ET), and soybean mosaic virus (SMV), whereas there was no significant GmERF3 mRNA accumulation under cold stress treatment. GmERF3 could bind to the GCC box and DRE/CRT element, and was targeted to the nucleus when transiently expressed in onion epidermal cells. The GmERF3 protein fused to the GAL4 DNA-binding domain to activate transcription of reporter genes in yeast. Ectopic expression of the GmERF3 gene in transgenic tobacco plants induced the expression of some PR genes and enhanced resistance against infection by Ralstonia solanacearum, Alternaria alternata, and tobacco mosaic virus (TMV), and gave tolerance to high salinity and dehydration stresses. Furthermore, overexpression of GmERF3 in transgenic tobacco led to higher levels of free proline and soluble carbohydrates compared to wild-type plants under drought conditions. The overall results suggested that GmERF3 as an AP2/ERF transcription factor may play dual roles in response to biotic and abiotic stresses in plants.

The importance of cytosolic glutamine synthetase in nitrogen assimilation and recycling - eScholarship

Abstract: Review The Importance of Cytosolic Glutamine Synthetase in Nitrogen Assimilation and Recycling Stephanie M. Bernard 1 and Dimah Z. Habash 2 Earth Sciences Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720, USA; Plant Science Department, Centre for Crop Genetic Improvement, Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK Author for correspondence: Stephanie M. Bernard Tel: +1 510 486 6125 Email: SMBernard@lbl.gov Summary Glutamine synthetase assimilates ammonium into amino acids, thus it is a key enzyme for nitrogen metabolism. The cytosolic isoenzymes of glutamine synthetase assimilate ammonium derived from primary nitrogen uptake and from various internal nitrogen recycling pathways. In this way, cytosolic glutamine synthetase is crucial for the remobilization of protein-derived nitrogen. Cytosolic glutamine synthetase is encoded by a small family of genes that are well conserved across plant species. Members of the cytosolic glutamine synthetase gene family are regulated in response to plant nitrogen status, as well as to environmental cues, such as nitrogen availability and biotic/abiotic stresses. The complex regulation of cytosolic glutamine synthetase at the transcriptional to post-translational levels is key to the establishment of a specific physiological role for each isoenzyme. The diverse physiological roles of cytosolic glutamine synthetase isoenzymes are important in relation to current agricultural and ecological issues. Abbreviations: ATP, adenosine triphosphate; GDH, glutamate dehydrogenase; GOGAT, glutamate synthase; GS, glutamine synthetase; PAL, phenylalanine ammonia lyase; QTL, quantitative trait locus. Key words: abiotic stress, biotic stress, glutamine synthetase (GS), nitrogen metabolism, quantitative trait locus (QTL), regulation, remobilization, seed. Introduction Nitrogen, a key element for plant growth and reproduction, is an essential building block of nucleic acids and proteins. Plants can store nitrogen in large amounts within enzymes involved in carbon fixation, such as leaf Rubisco (Stitt & Schulze, 1994). However, nitrogen is also often a limiting nutrient in many natural environments, and different plant species have evolved specific strategies to acquire nitrogen from their environments and assimilate it into organic compounds. Glutamine synthetase (GS, E.C. 6.3.1.2) plays a major role in fixing ammonium (NH +4 ) to form the amino acid glutamine. The cytosolic isoform of GS is particularly important for assimilating ammonium from different sources, for both primary nitrogen assimilation and recycling. Recent studies have led to a better understanding of the specific roles of this isoform of GS in flowering plants and conifers. Here, we focus on the current knowledge of cytosolic GS and its central role in nitrogen assimilation and recycling. Nitrogen is the most abundant element in the atmosphere. In some systems, biological nitrogen fixation of atmospheric N 2 contributes to much of the primary nitrogen source available to plants, mostly through symbiosis with diazotrophic bacteria (Oldroyd & Downie, 2008). Symbiosis

Ectopic Expression of VvMybPA2 Promotes Proanthocyanidin Biosynthesis in Grapevine and Suggests Additional Targets in the Pathway

Abstract: Grapevine (Vitis vinifera) proanthocyanidins contribute to plant defense mechanisms against biotic stress and also play a critical role in organoleptic properties of wine. In grapevine berry, these compounds are mainly accumulated in exocarps and seeds in the very early stages of development. A previous study has already identified VvMybPA1 as the first transcription factor involved in the regulation of the proanthocyanidin pathway during seed development in grapevine. A novel Myb factor, VvMybPA2, which is described in this study, is in contrast mainly expressed in the exocarp of young berries and in the leaves. This transcription factor shows very high protein sequence homology with other plant Myb factors, which regulate flavonoid biosynthesis. Ectopic expression of either VvMybPA1 or VvMybPA2 in grapevine hairy roots induced qualitative and quantitative changes of the proanthocyanidin profiles. High-throughput transcriptomic analyses of transformed grapevine organs identified a large set of putative targets of the VvMybPA1 and VvMybPA2 transcription factors. Both genes significantly activated enzymes of the flavonoid pathway, including anthocyanidin reductase and leucoanthocyanidin reductase 1, the specific terminal steps in the biosynthesis of epicatechin and catechin, respectively, but not leucoanthocyanidin reductase 2. The functional annotation of the genes whose expression was modified revealed putative new actors of the proanthocyanidin pathway, such as glucosyltransferases and transporters.

Evolutionary History and Stress Regulation of Plant Receptor-Like Kinase/Pelle Genes

Abstract: Receptor-Like Kinase (RLK)/Pelle genes play roles ranging from growth regulation to defense response, and the dramatic expansion of this family has been postulated to be crucial for plant-specific adaptations. Despite this, little is known about the history of or the factors that contributed to the dramatic expansion of this gene family. In this study, we show that expansion coincided with the establishment of land plants and that RLK/Pelle subfamilies were established early in land plant evolution. The RLK/Pelle family expanded at a significantly higher rate than other kinases, due in large part to expansion of a few subfamilies by tandem duplication. Interestingly, these subfamilies tend to have members with known roles in defense response, suggesting that their rapid expansion was likely a consequence of adaptation to fast-evolving pathogens. Arabidopsis (Arabidopsis thaliana) expression data support the importance of RLK/Pelles in biotic stress response. We found that hundreds of RLK/Pelles are up-regulated by biotic stress. Furthermore, stress responsiveness is correlated with the degree of tandem duplication in RLK/Pelle subfamilies. Our findings suggest a link between stress response and tandem duplication and provide an explanation for why a large proportion of the RLK/Pelle gene family is found in tandem repeats. In addition, our findings provide a useful framework for potentially predicting RLK/Pelle stress functions based on knowledge of expansion pattern and duplication mechanism. Finally, we propose that the detection of highly variable molecular patterns associated with specific pathogens/parasites is the main reason for the up-regulation of hundreds of RLK/Pelles under biotic stress.

Biotic and abiotic stress down-regulate miR398 expression in Arabidopsis

Abstract: MicroRNA398 targets two Cu/Zn superoxide dismutases (CSD1 and CSD2) in higher plants. Previous investigations revealed both decreased miR398 expression during high Cu2+ or paraquat stress and increased expression under low Cu2+ or high sucrose in the growth medium. Here, we show that additional abiotic stresses such as ozone and salinity also affect miR398 levels. Ozone fumigation decreased miR398 levels that were gradually restored to normal levels after relieved from the stress. Furthermore, miR398 levels decreased in Arabidopsis leaves infiltrated with avirulent strains of Pseudomonas syringae pv. tomato, Pst DC3000 (avrRpm1 or avrRpt2) but not the virulent strain Pst DC3000. To our knowledge, miR398 is the first miRNA shown to be down-regulated in response to biotic stress (P. syringae). CSD1, but not CSD2, mRNA levels were negatively correlated with miR398 levels during ozone, salinity and biotic stress, suggesting that CSD2 regulation is not strictly under miR398 control during diverse stresses. Overall, this study further establishes a link between oxidative stress and miR398 in Arabidopsis.

Dual function of Arabidopsis ATAF1 in abiotic and biotic stress responses.

Abstract: NAC family genes encode plant-specific transcription factors involved in diverse biological processes. In this study, the Arabidopsis NAC gene ATAF1 was found to be induced by drought, high-salinity, abscisic acid (ABA), methyl jasmonate, mechanical wounding, and Botrytis cinerea infection. Significant induction of ATAF1 was found in an ABA-deficient mutant aba2 subjected to drought or high salinity, revealing an ABA-independent mechanism of expression. Arabidopsis ATAF1-overexpression lines displayed many altered phenotypes, including dwarfism and short primary roots. Furthermore, in vivo experiments indicate that ATAF1 is a bona fide regulator modulating plant responses to many abiotic stresses and necrotrophic-pathogen infection. Overexpression of ATAF1 in Arabidopsis increased plant sensitivity to ABA, salt, and oxidative stresses. Especially, ATAF1 overexpression plants, but not mutant lines, showed remarkably enhanced plant tolerance to drought. Additionally, ATAF1 overexpression enhanced plant susceptibility to the necrotrophic pathogen B. cinerea, but did not alter disease symptoms caused by avirulent or virulent strains of P. syringae pv tomato DC3000. Transgenic plants overexpressing ATAF1 were hypersensitive to oxidative stress, suggesting that reactive oxygen intermediates may be related to ATAF1-mediated signaling in response to both pathogen and abiotic stresses.

Abscisic Acid Has a Key Role in Modulating Diverse Plant-Pathogen Interactions

Abstract: We isolated an activation-tagged Arabidopsis (Arabidopsis thaliana) line, constitutive disease susceptibility2-1D (cds2-1D), that showed enhanced bacterial growth when challenged with various Pseudomonas syringae strains. Systemic acquired resistance and systemic PATHOGENESIS-RELATED GENE1 induction were also compromised in cds2-1D. The T-DNA insertion adjacent to NINE-CIS-EPOXYCAROTENOID DIOXYGENASE5 (NCED5), one of six genes encoding the abscisic acid (ABA) biosynthetic enzyme NCED, caused a massive increase in transcript level and enhanced ABA levels >2-fold. Overexpression of NCED genes recreated the enhanced disease susceptibility phenotype. NCED2, NCED3, and NCED5 were induced, and ABA accumulated strongly following compatible P. syringae infection. The ABA biosynthetic mutant aba3-1 showed reduced susceptibility to virulent P. syringae, and ABA, whether through exogenous application or endogenous accumulation in response to mild water stress, resulted in increased bacterial growth following challenge with virulent P. syringae, indicating that ABA suppresses resistance to P. syringae. Likewise ABA accumulation also compromised resistance to the biotrophic oomycete Hyaloperonospora arabidopsis, whereas resistance to the fungus Alternaria brassicicola was enhanced in cds2-1D plants and compromised in aba3-1 plants, indicating that ABA promotes resistance to this necrotroph. Comparison of the accumulation of salicylic acid and jasmonic acid in the wild type, cds2-1D, and aba3-1 plants challenged with P. syringae showed that ABA promotes jasmonic acid accumulation and exhibits a complex antagonistic relationship with salicylic acid. Our findings provide genetic evidence that the abiotic stress signal ABA also has profound roles in modulating diverse plant-pathogen interactions mediated at least in part by cross talk with the jasmonic acid and salicylic acid biotic stress signal pathways.

Abscisic Acid Has a Key Role in Modulating Diverse Plant-Pathogen Interactions 1(C)(W)(OA)

Abstract: We isolated an activation-tagged Arabidopsis (Arabidopsis thaliana) line, constitutive disease susceptibility2-1D (cds2-1D), that showed enhanced bacterial growth when challenged with various Pseudomonas syringae strains. Systemic acquired resistance and systemic PATHOGENESIS-RELATED GENE1 induction were also compromised in cds2-1D. The T-DNA insertion adjacent to NINE-CIS-EPOXYCAROTENOID DIOXYGENASE5 (NCED5), one of six genes encoding the abscisic acid (ABA) biosynthetic enzyme NCED, caused a massive increase in transcript level and enhanced ABA levels .2-fold. Overexpression of NCED genes recreated the enhanced disease susceptibility phenotype. NCED2, NCED3, and NCED5 were induced, and ABA accumulated strongly following compatible P. syringae infection. The ABA biosynthetic mutant aba3-1 showed reduced susceptibility to virulent P. syringae, and ABA, whether through exogenous application or endogenous accumulation in response to mild water stress, resulted in increased bacterial growth following challenge with virulent P. syringae, indicating that ABA suppresses resistance to P. syringae. Likewise ABA accumulation also compromised resistance to the biotrophic oomycete Hyaloperonospora arabidopsis, whereas resistance to the fungus Alternaria brassicicola was enhanced in cds2-1D plants and compromised in aba3-1 plants, indicating that ABA promotes resistance to this necrotroph. Comparison of the accumulation of salicylic acid and jasmonic acid in the wild type, cds2-1D, and aba3-1 plants challenged with P. syringae showed that ABA promotes jasmonic acid accumulation and exhibits a complex antagonistic relationship with salicylic acid. Our findings provide genetic evidence that the abiotic stress signal ABA also has profound roles in modulating diverse plantpathogen interactions mediated at least in part by cross talk with the jasmonic acid and salicylic acid biotic stress signal pathways.

Salt stress-induced cell death in the unicellular green alga Micrasterias denticulata.

Abstract: Programmed cell death (PCD) is a key element in normal plant growth and development which may also be induced by various abiotic and biotic stress factors including salt stress. In the present study, morphological, biochemical, and physiological responses of the theoretically immortal unicellular freshwater green alga Micrasterias denticulata were examined after salt (200 mM NaCl or 200 mM KCl) and osmotic stress induced by iso-osmotic sorbitol. KCl caused morphological changes such as cytoplasmic vacuolization, extreme deformation of mitochondria, and ultrastructural changes of Golgi and ER. However, prolonged salt stress (24 h) led to the degradation of organelles by autophagy, a special form of PCD, both in NaCl- and KCl-treated cells. This was indicated by the enclosure of organelles by ER-derived double membranes. DNA of NaCl- and KCl-stressed cells but not of sorbitol-treated cells showed a ladder-like pattern on agarose gel, which means that the ionic rather than the osmotic component of salt stress leads to the activation of the responsible endonuclease. DNA laddering during salt stress could be abrogated by addition of Zn 2+ . Neither cytochrome c release from mitochondria nor increase in caspase-3-like activity occurred after salt stress. Reactive oxygen species could be detected within 5 min after the onset of salt and osmotic stress. Respiration, photosynthetic activity, and pigment composition indicated an active metabolism which supports programmed rather than necrotic cell death in Micrasterias after salt stress.

Probing the mechanisms of silicon-mediated pathogen resistance.

Abstract: Silicon is the second most abundant mineral element in soil, it has important role in alleviating various environmental stresses and enhancing plant resistance against pathogen, but the exact mechanism by which Si mediates pathogen resistance remains unclear. One of the resistance mechanisms is related to silicon deposition in leaf that acts as a physical barrier to hinder pathogen penetration. But more evidence show that silicon can induce defense responses that are functionally similar to systemic acquired resistance, Si-treated plants can significantly increase antioxidant enzyme activities and the production of antifungal compounds such as phenolic metabolism product, phytoalexins and pathogenesis-related proteins etc. Molecular and biochemical detections show that Si can activate the expression of defense-related genes and may play important role in the transduction of plant stress signal such as salicylic acid, jasmonic acid and ethylene.

Identification of cell-wall stress as a hexose-dependent and osmosensitive regulator of plant responses.

Abstract: Development, abiotic and biotic stress each affect the physical architecture and chemical composition of the plant cell wall, making maintenance of cell-wall integrity an important component of many plant processes. Cellulose biosynthesis inhibition (CBI) was employed to impair the functional integrity of the cell wall, and the plant's response to this specific stress was characterized in an Arabidopsis seedling model system. CBI caused changes in the expression of genes involved in mechanoperception, the response to microbial challenge, and lignin and cell-wall polysaccharide biosynthesis. Following CBI, activation of a UDP-D-xylose 4-epimerase gene correlated with increases in arabinose and uronic acid content in seedling cell walls. Activation of pathogen response genes, lignin deposition and lesion formation were dependent on externally supplied sugars and were suppressed by osmotic support. Lignin deposition in the root elongation zone caused by CBI was reduced in atrbohd (NADPH oxidase) mutant seedlings but increased in jasmonic acid resistant1 (jar1-1) mutant seedlings. Phytohormone measurements showed that CBI-induced increases in jasmonic (JA) and salicylic acids were dependent on sugar availability and prevented by osmotic support. We show that CBI activates responses commonly attributed to both abiotic and microbial challenges. Glucose/sucrose and turgor pressure are critical components in maintenance of cell-wall integrity and the regulation of induced responses, including JA biosynthesis. Lignin deposition induced by CBI is regulated by JAR1-1 and NADPH oxidase-dependent signalling processes. Our results identify components of the mechanism that mediates the response to impairment of cell-wall integrity in Arabidopsis thaliana.

Upgrading Root Physiology for Stress Tolerance by Ectomycorrhizas: Insights from Metabolite and Transcriptional Profiling into Reprogramming for Stress Anticipation

Abstract: Ectomycorrhizas (EMs) alleviate stress tolerance of host plants, but the underlying molecular mechanisms are unknown. To elucidate the basis of EM-induced physiological changes and their involvement in stress adaptation, we investigated metabolic and transcriptional profiles in EM and non-EM roots of gray poplar (Populus × canescens) in the presence and absence of osmotic stress imposed by excess salinity. Colonization with the ectomycorrhizal fungus Paxillus involutus increased root cell volumes, a response associated with carbohydrate accumulation. The stress-related hormones abscisic acid and salicylic acid were increased, whereas jasmonic acid and auxin were decreased in EM compared with non-EM roots. Auxin-responsive reporter plants showed that auxin decreased in the vascular system. The phytohormone changes in EMs are in contrast to those in arbuscular mycorrhizas, suggesting that EMs and arbuscular mycorrhizas recruit different signaling pathways to influence plant stress responses. Transcriptome analyses on a whole genome poplar microarray revealed activation of genes related to abiotic and biotic stress responses as well as of genes involved in vesicle trafficking and suppression of auxin-related pathways. Comparative transcriptome analysis indicated EM-related genes whose transcript abundances were independent of salt stress and a set of salt stress-related genes that were common to EM non-salt-stressed and non-EM salt-stressed plants. Salt-exposed EM roots showed stronger accumulation of myoinositol, abscisic acid, and salicylic acid and higher K+-to-Na+ ratio than stressed non-EM roots. In conclusion, EMs activated stress-related genes and signaling pathways, apparently leading to priming of pathways conferring abiotic stress tolerance.

Functional analysis of an Arabidopsis transcription factor WRKY25 in heat stress.

Abstract: The WRKY family is one of the major groups of plant-specific transcriptional regulators. Arabidopsisthaliana WRKY25, which is induced by heat stress, is one of the group I WRKY proteins and responds to both abiotic and biotic stress. This study has examined the regulatory role of WRKY25 using wrky25 mutant and over-expressing WRKY25 transgenic A. thaliana. After 45°C for different time periods, wrky25 null mutants showed a moderate increase in thermosensitivity with decreased germination, reduced hypocotyl and root growth, and enhanced conductivity compared to those of wide-type, while WRKY25 over-expressed transgenic seeds exhibited enhanced thermotolerance. Northern blot analysis of wrky25 mutants and WRKY25 over-expressing plants identified putative genes regulated by WRKY25. In consistence with the implication of WRKY25 in heat tolerance, the expression level of six heat-inducible genes and two oxidative stress-responsive genes was more or less down-regulated in wrky25 mutants during heat stress. Among them, heat shock proteinHsp101, heat shock transcription factor HsfB2a, and cytosolic ascrobate peroxidase APX1 were reduced more obviously than other detected genes. Meanwhile, over-expression of WRKY25 increased the expression of HsfA2, HsfB1, HsfB2a, and Hsp101 slightly or moderately. Together, these findings reveal that WRKY25 plays a partial role in thermotolerance.

The ABC transporter BcatrB from Botrytis cinerea exports camalexin and is a virulence factor on Arabidopsis thaliana

Abstract: Arabidopsis thaliana is known to produce the phytoalexin camalexin in response to abiotic and biotic stress. Here we studied the mechanisms of tolerance to camalexin in the fungus Botrytis cinerea, a necrotrophic pathogen of A. thaliana. Exposure of B. cinerea to camalexin induces expression of BcatrB, an ABC transporter that functions in the efflux of fungitoxic compounds. B. cinerea inoculated on wild-type A. thaliana plants yields smaller lesions than on camalexin-deficient A. thaliana mutants. A B. cinerea strain lacking functional BcatrB is more sensitive to camalexin in vitro and less virulent on wild-type plants, but is still fully virulent on camalexin-deficient mutants. Pre-treatment of A. thaliana with UV-C leads to increased camalexin accumulation and substantial resistance to B. cinerea. UV-C-induced resistance was not seen in the camalexin-deficient mutants cyp79B2/B3, cyp71A13, pad3 or pad2, and was strongly reduced in ups1. Here we demonstrate that an ABC transporter is a virulence factor that increases tolerance of the pathogen towards a phytoalexin, and the complete restoration of virulence on host plants lacking this phytoalexin.

Is the maintenance of homeostatic mitochondrial signaling during stress a physiological role for alternative oxidase

Abstract: All plants maintain a non-energy-conserving pathway of mitochondrial electron transport referred to as alternative oxidase (AOX) respiration. Here, we briefly review some of the most prevailing themes for the metabolic and physiological roles of this respiratory pathway. Many of these themes relate to the potential of AOX to provide metabolic homeostasis in response to fluctuating cellular conditions, such as is often seen during stress. We then review reverse genetic experiments that have been used to test these hypotheses. To date, such experiments have been limited to just two dicot species and have only targeted one member (a stress-induced member) of the AOX multigene family. Nonetheless, the experiments to date strongly reinforce the idea that AOX respiration is of particular importance during abiotic and biotic stress. Finally, we propose that another core role of AOX may be to modulate the strength of a stress-signaling pathway from the mitochondrion that controls cellular responses to stress. In this way, AOX could be acting to provide a degree of signaling homeostasis from the mitochondrion. This hypothesis may provide explanation for some of the disparate results seen in reverse genetic experiments regarding the impact of AOX on the reactive oxygen network and oxidative damage.

DEAR1, a transcriptional repressor of DREB protein that mediates plant defense and freezing stress responses in Arabidopsis

Abstract: Plants have evolved intricate mechanisms to respond and adapt to a wide variety of biotic and abiotic stresses in their environment. The Arabidopsis DEAR1 (DREB and EAR motif protein 1; At3g50260) gene encodes a protein containing significant homology to the DREB1/CBF (dehydration-responsive element binding protein 1/C-repeat binding factor) domain and the EAR (ethylene response factor-associated amphiphilic repression) motif. We show here that DEAR1 mRNA accumulates in response to both pathogen infection and cold treatment. Transgenic Arabidopsis overexpressing DEAR1 (DEAR1ox) showed a dwarf phenotype and lesion-like cell death, together with constitutive expression of PR genes and accumulation of salicylic acid. DEAR1ox also showed more limited P. syringae pathogen growth compared to wild-type, consistent with an activated defense phenotype. In addition, transient expression experiments revealed that the DEAR1 protein represses DRE/CRT (dehydration-responsive element/C-repeat)-dependent transcription, which is regulated by low temperature. Furthermore, the induction of DREB1/CBF family genes by cold treatment was suppressed in DEAR1ox, leading to a reduction in freezing tolerance. These results suggest that DEAR1 has an upstream regulatory role in mediating crosstalk between signaling pathways for biotic and abiotic stress responses.

Disruption of Poly(ADP-ribosyl)ation Mechanisms Alters Responses of Arabidopsis to Biotic Stress

Abstract: Poly(ADP-ribosyl)ation is a posttranslational protein modification in which ADP-ribose (ADP-Rib) units derived from NAD+ are attached to proteins by poly(ADP-Rib) polymerase (PARP) enzymes. ADP-Rib groups are removed from these polymer chains by the enzyme poly(ADP-Rib) glycohydrolase (PARG). In animals, poly(ADP-ribosyl)ation is associated with DNA damage responses and programmed cell death. Previously, we hypothesized a role for poly(ADP-ribosyl)ation in plant defense responses when we detected defense-associated expression of the poly(ADP-ribosyl)ation-related genes PARG2 and NUDT7 and observed altered callose deposition in the presence of a chemical PARP inhibitor. The role of poly(ADP-ribosyl)ation in plant defenses was more extensively investigated in this study, using Arabidopsis (Arabidopsis thaliana). Pharmacological inhibition of PARP using 3-aminobenzamide perturbs certain innate immune responses to microbe-associated molecular patterns (flg22 and elf18), including callose deposition, lignin deposition, pigment accumulation, and phenylalanine ammonia lyase activity, but does not disrupt other responses, such as the initial oxidative burst and expression of some early defense-associated genes. Mutant parg1 seedlings exhibit exaggerated seedling growth inhibition and pigment accumulation in response to elf18 and are hypersensitive to the DNA-damaging agent mitomycin C. Both parg1 and parg2 knockout plants show accelerated onset of disease symptoms when infected with Botrytis cinerea. Cellular levels of ADP-Rib polymer increase after infection with avirulent Pseudomonas syringae pv tomato DC3000 avrRpt2+, and pathogen-dependent changes in the poly(ADP-ribosyl)ation of discrete proteins were also observed. We conclude that poly(ADP-ribosyl)ation is a functional component in plant responses to biotic stress.

A Comprehensive Transcriptomic Analysis of the Effect of Silicon on Wheat Plants Under Control and Pathogen Stress Conditions

Abstract: The supply of soluble silicon (Si) to plants has been associated with many benefits that remain poorly explained and often contested. In this work, the effect of Si was studied on wheat plants under both control and pathogen stress (Blumeria graminis f. sp. tritici) conditions by conducting a large transcriptomic analysis (55,000 unigenes) aimed at comparing the differential response of plants under four treatments. The response to the supply of Si on control (uninfected) plants was limited to 47 genes of diverse functions providing little evidence of regulation of a specific metabolic process. Plants reacted to inoculation with B. graminis f. sp. tritici by an upregulation of many genes linked to stress and metabolic processes and a downregulation of genes linked to photosynthesis. Supplying Si to inoculated plants largely prevented disease development, a phenotypic response that translated into a nearly perfect reversal of genes regulated by the effect of B. graminis f. sp. tritici alone. These results suggest that Si plays a limited role on a plant's transcriptome in the absence of stress, even in the case of a high-Si-accumulating monocot such as wheat. On the other hand, the benefits of Si in the form of biotic stress alleviation were remarkably aligned with a counter-response to transcriptomic changes induced by the pathogen B. graminis f. sp. tritici.

Specific and coordinated control of indolic and aliphatic glucosinolate biosynthesis by R2R3-MYB transcription factors in Arabidopsis thaliana

Abstract: Five members of subgroup 12 R2R3-MYB transcription factors, namely MYB51, MYB122, MYB28, MYB29 and MYB76, are novel regulators of glucosinolate biosynthesis in Arabidopsis thaliana. Overexpression of MYB51 and MYB122 led to an increased accumulation of tryptophan-derived indolic glucosinolates whereas MYB28, MYB29 and MYB76 overexpression lines showed an increase in methionine-derived aliphatic glucosinolates. Likewise, disruption of the corresponding genes caused a significant downregulation of indolic and aliphatic glucosinolates, respectively. Expression analysis of promoter-GUS fusions revealed promoter activities at the sites of glucosinolate synthesis and accumulation. Indolic glucosinolate regulators were mainly found in vegetative organs and roots, whereas aliphatic glucosinolate regulators were preferentially expressed in generative organs. Mechanical stimuli such as touch or wounding induced a transient expression of the regulators and overexpression of MYB28 and MYB51 reduced insect performance demonstrating the role of these transcription factors in plant biotic responses. The subgroup 12 R2R3-MYB transcription factors interdependently control the response to biotic challenges. For the regulation of methionine-derived glucosinolates, the coordinated activation of MYB28, MYB76 and MYB29 is required, whereas MYB51, MYB122 and the sixth member of subgroup 12 R2R3-MYB transcription factors, the previously described ATR1/MYB34, are involved in the regulation of tryptophan-derived glucosinolates. Because these two pathways are reciprocally inhibiting each other, a metabolic balance between both biosynthetic pathways can be accomplished in plants exposed to continuous biotic challenges.

Inclusion of biotic stress (consumer pressure) alters predictions from the stress gradient hypothesis

Abstract: The stress gradient hypothesis (SGH) predicts a shift from net negative interactions in benign environments towards net positive in harsh environments in ecological communities. While several studies found support for the SGH, others found evidence against it, leading to a debate on how nature and strength of species interactions change along stress gradients, and to calls for new empirical and theoretical work. In the latest attempt in this journal, it is successfully argued how the SGH should be expanded by considering different life strategies of species (stress tolerance versus competitive ability) and characteristics of abiotic stress (resource versus non-resource based) over wider stress gradients (opposed to low-high contrasts), but the crucial role of biotic stress by consumers is largely ignored in this refinement. We point out that consumers strongly alter the outcome of species interactions in benign and harsh environments, and show how inclusion of consumer-incurred biotic stress alters the predicted outcome of interactions along resource- and non-resource-based stress gradients for stress-tolerant and competitive benefactors and beneficiaries. Synthesis. New studies should include stress gradients consisting of both abiotic and biotic components to disentangle their impacts, and to improve our understanding of how species interactions change along environmental gradients.

Role of auxin-responsive genes in biotic stress responses.

Abstract: Although the phytohormone auxin has been implicated primarily in developmental processes, some recent studies suggest its involvement in stress/defense responses as well. Recently, we identified auxin-responsive genes and reported their comprehensive transcript profiling during various stages of development and abiotic stress responses in crop plant rice. The analysis revealed tissue-specific and overlapping expression profiles of auxin-responsive genes during various stages of reproductive development. In addition, a large number of auxin-responsive genes were also found to be differentially expressed under various abiotic stress conditions. Here, we further analyze the expression profiles of auxin-responsive genes during various biotic stress conditions. Several auxin-responsive genes showed response to biotic stress as well. Our analysis provides evidence for role of auxin in plant defense responses and suggests cross-talk between auxin, abiotic stress and biotic stress signaling pathways.

Strategies for Crop Improvement Against Salinity and Drought Stress: An Overview

Abstract: Abiotic stresses such as salinity, drought, nutrient defi ciency or toxicity, and fl ooding limit crop productivity world-wide. However, this situation becomes more problematic in developing countries, where they cause food insecurity for large populations and poverty, particularly in rural areas. For example, drought stress has affected more than 70 million hectares of rice-growing land world-wide. While salt stress and nutrient stress render more than 100 million hectares of agricultural land uncultivable thereby resulting in low outputs, poor human nutrition and reduced educational and employment opportunities. Thus, abiotic stresses are the major factors of poverty for millions of people. In this scenario, it is widely urged that strategies should be adopted which may be used to get maximum crop stand and economic returns from stressful environments. Major strategies include breeding of new crop varieties, screening and selection of the existing germ-plasm of potential crops, production of genetically modifi ed (GM) crops, exogenous use of osmoprotec-tants etc. In the last century, conventional selection and breeding program proved to be highly effective in improving crops against abiotic stresses. Therefore, breeding for abiotic stress tolerance in crop plants (for food supply) should be given high research priority. However, extent and rate of progress in improving stress tolerance in crops through conventional breeding program is limited. This is due to complex mechanism of abiotic stress tolerance, which is controlled by the expression of several minor genes. Furthermore, techniques employed for selecting tolerant plants are time consumable and consequently expensive. During the last decade, using advanced molecular biology techniques different researchers showed some promising results in understanding molecular mechanisms of abiotic stress tolerance as well as in inducing stress tolerance in some potential crops. These fi ndings emphasized that future research should focus on molecular, physiological and metabolic aspects of stress tolerance to facilitate the development of crops with an inherent capacity to withstand abiotic stresses. This would help stabilize the crop production, and signifi cantly contribute to food and nutritional security in developing countries and semi-arid tropical regions.

Very long chain fatty acid and lipid signaling in the response of plants to pathogens.

Abstract: Recent findings indicate that lipid signaling is essential for plant resistance to pathogens. Besides oxylipins and unsaturated fatty acids known to play important signaling functions during plant-pathogen interactions, the very long chain fatty acid (VLCFA) biosynthesis pathway has been recently associated to plant defense through different aspects. VLCFAs are indeed required for the biosynthesis of the plant cuticle and the generation of sphingolipids. Elucidation of the roles of these lipids in biotic stress responses is the result of the use of genetic approaches together with the identification of the genes/proteins involved in their biosynthesis. This review focuses on recent observations which revealed the complex function of the cuticle and cuticle-derived signals, and the key role of sphingolipids as bioactive molecules involved in signal transduction and cell death regulation during plant-pathogen interactions.

Bax Inhibitor-1, a Conserved Cell Death Suppressor, Is a Key Molecular Switch Downstream from a Variety of Biotic and Abiotic Stress Signals in Plants

Abstract: In Nature plants are constantly challenged by a variety of environmental stresses that could lead to disruptions in cellular homeostasis. Programmed cell death (PCD) is a fundamental cellular process that is often associated with defense responses to pathogens, during development and in response to abiotic stresses in fungi, animals and plants. Although there are many characteristics shared between different types of PCD events, it remains unknown whether a common mechanism drives various types of PCD in eukaryotes. One candidate regulator for such a mechanism is Bax Inhibitor-1 (BI-1), an evolutionary conserved, endoplasmic reticulum (ER)-resident protein that represents an ancient cell death regulator that potentially regulates PCD in all eukaryotes. Recent findings strongly suggested that BI-1 plays an important role in the conserved ER stress response pathway to modulate cell death induction in response to multiple types of cell death signals. As ER stress signaling pathways has been suggested to play important roles not only in the control of ER homeostasis but also in other biological processes such as the response to pathogens and abiotic stress in plants, BI-1 might function to control the convergence point that modulates the level of the “pro-survival and pro-death” signals under multiple stress conditions.