SA, JA, ethylene, and disease resistance in plants

Regulation of plant defense responses in Arabidopsis by EDR2, a PH and START domain-containing protein

We have identified an Arabidopsis mutant that displays enhanced disease resistance (edr2) to the biotrophic powdery mildew pathogen Erysiphe cichoracearum. Inhibition of fungal growth on edr2 mutant leaves occurred at a late stage of the infection process and coincided with formation of necrotic lesions approximately 5 days after inoculation. Double-mutant analysis revealed that edr2-mediated resistance is suppressed by mutations that inhibit salicylic acid (SA)-induced defense signaling, including npr1, pad4 and sid2, demonstrating that edr2-mediated disease resistance is dependent on SA. However, edr2 showed normal responses to the bacterial pathogen Pseudomonas syringae pv. tomato strain DC3000. EDR2 appears to be constitutively transcribed in all tissues and organs and encodes a novel protein, consisting of a putative pleckstrin homology (PH) domain and a steroidogenic acute regulatory protein-related lipid-transfer (START) domain, and contains an N-terminal mitochondrial targeting sequence. The PH and START domains are implicated in lipid binding, suggesting that EDR2 may provide a link between lipid signaling and activation of programmed cell death mediated by mitochondria.

Improvement of the fungal biocontrol agent Trichoderma atroviride to enhance both antagonism and induction of plant systemic disease resistance

Biocontrol agents generally do not perform well enough under field conditions to compete with chemical fungicides. We determined whether transgenic strain SJ3-4 of Trichoderma atroviride, which expresses the Aspergillus niger glucose oxidase-encoding gene, goxA, under a homologous chitinase (nag1) promoter had increased capabilities as a fungal biocontrol agent. The transgenic strain differed only slightly from the wild-type in sporulation or the growth rate. goxA expression occurred immediately after contact with the plant pathogen, and the glucose oxidase formed was secreted. SJ3-4 had significantly less N-acetylglucosaminidase and endochitinase activities than its nontransformed parent. Glucose oxidase-containing culture filtrates exhibited threefold-greater inhibition of germination of spores of Botrytis cinerea. The transgenic strain also more quickly overgrew and lysed the plant pathogens Rhizoctonia solani and Pythium ultimum. In planta, SJ3-4 had no detectable improved effect against low inoculum levels of these pathogens. Beans planted in heavily infested soil and treated with conidia of the transgenic Trichoderma strain germinated, but beans treated with wild-type spores did not germinate. SJ3-4 also was more effective in inducing systemic resistance in plants. Beans with SJ3-4 root protection were highly resistant to leaf lesions caused by the foliar pathogen B. cinerea. This work demonstrates that heterologous genes driven by pathogen-inducible promoters can increase the biocontrol and systemic resistance-inducing properties of fungal biocontrol agents, such as Trichoderma spp., and that these microbes can be used as vectors to provide plants with useful molecules (e.g., glucose oxidase) that can increase their resistance to pathogens.

Activation of pepper basic PR-1 gene promoter during defense signaling to pathogen, abiotic and environmental stresses

The basic PR-1 gene, CABPR1, accumulates in pepper leaf tissues during pathogen infection as well as after ethylene treatment. We isolated and functionally characterized the CABPR1 promoter region in tobacco leaves to identify the cis-acting regulatory sequences that are involved in CABPR1 gene expression. Constructs harboring the 5'-serially deleted CABPR1 promoter, which was fused to the beta-glucuronidase (GUS) gene, were evaluated for their promoter activity in the tobacco leaves. The CABPR1 promoter of 1670 bp in size was locally or systemically induced during a compatible interaction with Pseudomonas syringae pv. tabaci. The CABPR1 promoter also was differentially activated by treatment with ethylene, salicylic acid, nitric oxide, high salinity, drought and low temperature. The expression of the pepper transcription factors, CAZFP1 and CARAV1, activated the CABPR1 promoter. Analyses of a series of 5'-deletions of the CABPR1 promoter indicated that novel cis-acting elements essential for induction by pathogen and abiotic elicitors are localized in the region between -1670 bp and -1466 bp upstream from the translation start site. These results suggest that CABPR1 promoter is essential for regulating CABPR1 gene expression in response to pathogen, abiotic and environmental stresses, possibly by transactivating the CAZFP1 and CARAV1 transcription factors.

Genomic cloning and characterization of a Pto-like gene SsPto-2 from Solanum surattense

A Pto-like gene (designated as SsPto-2) was isolated from Solanum surattense by using genomic walker technology which encoded a cytoplasmically localized serine-threonine protein kinase. Analysis of the 2365 bp segment revealed a gene including a 905 bp 5' flanking region, a 924 bp open reading frame (ORF) and a 536 bp 3' flanking region. The deduced amino acid sequence of the SsPto-2 gene shared high homology with other known Ptos. The deduced SsPto-2 protein contained no signal peptide with a calculated molecular weight of 34.61 kDa. The analysis of SsPto-2 promoter region and terminator region was also presented. Semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed that SsPto-2 transcripts were up-regulated by defense-related factors such as gibberellic acid (GA(3)), salicylic acid (SA) and down-regulated by darkness. The cloning of the SsPto-2 gene will allow us to further study its potential role in disease resistance.

Involvement of ethylene in stress-induced expression of the TLC1.1 retrotransposon from Lycopersicon chilense Dun

The TLC1 family is one of the four families of long terminal repeat (LTR) retrotransposons identified in the genome of Lycopersicon chilense. Here, we show that this family of retroelements is transcriptionally active and its expression is induced in response to diverse stress conditions such as wounding, protoplast preparation, and high salt concentrations. Several stress-associated signaling molecules, including ethylene, methyl jasmonate, salicylic acid, and 2,4-dichlorophenoxyacetic acid, are capable of inducing TLC1 family expression in vivo. A representative of this family, named TLC1.1, was isolated from a genomic library from L. chilense. Transient expression assays in leaf protoplasts and stably transformed tobacco (Nicotiana tabacum) plants demonstrate that the U3 domain of the 5'-LTR region of this element can drive stress-induced transcriptional activation of the beta-glucuronidase reporter gene. Two 57-bp tandem repeated sequences are found in this region, including an 8-bp motif, ATTTCAAA, previously identified as an ethylene-responsive element box in the promoter region of ethylene-induced genes. Expression analysis of wild-type LTR and single and double ethylene-responsive element box mutants fused to the beta-glucuronidase gene shows that these elements are required for ethylene-responsive gene expression in protoplasts and transgenic plants. We suggest that ethylene-dependent signaling is the main signaling pathway involved in the regulation of the expression of the TLC1.1 element from L. chilense.

Molecular analysis of the interaction between Olea europaea and the biotrophic fungus Spilocaea oleagina

SUMMARY The mitosporic fungus Spilocaea oleagina is an obligate biotroph of olive (Olea europaea) causing a scab disease associated with leaf fall and substantial losses in production. Using differential display we have identified 162 cDNA fragments corresponding to transcripts that show altered abundance during the defence response of a resistant olive cultivar to S. oleagina. Detailed analyses of 21 selected genes by real-time quantitative RT-PCR revealed different kinetics of induction. Genes involved in signalling, transcriptional control, oxidative stress, biotic and abiotic stress, and several genes with unknown function were found to be induced rapidly after infection. In contrast, genes involved in metabolism and cellular maintenance showed delayed induction. The induction of the selected genes in a susceptible cultivar was delayed and/or reduced during the response to S. oleagina. Interestingly, the basal expression of some genes in the uninfected resistant cultivar was higher than in the susceptible one, suggesting a constitutive activation of defence responses. Expression of these genes in response to salicylic acid, methyl jasmonate, a mixture of both, ethephon, hydrogen peroxide, menadione and wounding was also investigated. The results are discussed in relation to the molecular bases and signalling events involved in this biotrophic interaction.

Overexpression of a rice NPR1 homolog leads to constitutive activation of defense response and hypersensitivity to light

Arabidopsis NPR1/NIM1 is a key regulator of systemic acquired resistance (SAR), which confers lasting broad-spectrum resistance. Previous reports indicate that rice has a disease-resistance pathway similar to the Arabidopsis SAR pathway. Here we report the isolation and characterization of a rice NPR1 homologue (NH1). Transgenic rice plants overexpressing NH1 (NH1ox) acquire high levels of resistance to Xanthomonas oryzae pv. oryzae. The resistance phenotype is heritable and correlates with the presence of the transgene and reduced bacterial growth. Northern analysis shows that NH1ox rice spontaneously activates defense genes, contrasting with NPR1-overexpressing Arabidopsis, where defense genes are not activated until induction. Wild-type NH1, but not a point mutant corresponding to npr1-1, interacts strongly with the rice transcription factor rTGA2.2 in yeast two-hybrid. Greenhouse-grown NH1ox plants develop lesion-mimic spots on leaves at preflowering stage although no other developmental effects are observed. However, when grown in growth chambers (GCs) under low light, NH1ox plants are dwarfed, indicating elevated sensitivity to light. The GC-grown NH1ox plants show much higher salicylic acid (SA) levels than the wild type, whereas greenhouse-grown NH1ox plants contain lower SA. These results indicate that NH1 may be involved in the regulation of SA in response to environmental changes.

The H2O2-regulated Ep5C gene encodes a peroxidase required for bacterial speck susceptibility in tomato

Bacterial speck caused by the pathogen Pseudomonas syringae pv. tomato (P. s. tomato) is a devastating disease of tomato plants. Here we show that inhibition of Ep5C gene expression, which encodes a secreted cationic peroxidase, is sufficient to confer resistance against P. s. tomato. The inhibition of Ep5C protein accumulation in antisense tomato plants established resistance that was not accompanied by the pre-activation of known defense pathways. Therefore, Ep5C inhibition represents a novel form of disease resistance based on a loss-of-gene function in the plant required for successful infection by a compatible bacterial pathogen. Ep5C expression is rapidly induced by H2O2, a reactive oxygen intermediate normally generated during the course of a plant-pathogen interaction. This was corroborated by monitoring the expression of an Ep5C-GUS gene in transgenic Arabidopsis plants. Collectively, these results identify a signaling pathway that uses early signals generated during the oxidative burst, such as H2O2, for the selective activation of host factors required for mounting a compatible interaction. Thus, Ep5C provides a new resource for developing bacterial speck disease-resistant varieties.

Phytohormones mediate volatile emissions during the interaction of compatible and incompatible pathogens: the role of ethylene in Pseudomonas syringae infected tobacco

Interactions between the phytohormones ethylene, salicylic acid (SA), and jasmonic acid (JA) are thought to regulate the specificity of induced plant defenses against microbial pathogens and herbivores. However, the nature of these interactions leading to induced plant volatile emissions during pathogen infection is unclear. We previously demonstrated that a complex volatile blend including (E)-beta-ocimene, methyl salicylate (MeSA), and numerous sesquiterpenes was released by tobacco plants, Nicotiana tabacum K326, infected with an avirulent/incompatible strain of Pseudomonas syringae pv. tomato (Pst DC3000). In contrast, a volatile blend, mainly consisting of MeSA and two unidentified sesquiterpenes, was released by plants infected with P. syringae pv. tabaci (Pstb) in a virulent/compatible interaction. In this study, we examined the interaction of multiple pathogen stresses, phytohormone signaling, and induced volatile emissions in tobacco. Combined pathogen infection involved the inoculation of one leaf with Pst DC 3000 and of a second leaf, from the same plant, with Pstb. Combined infection reduced emissions of ocimene and MeSA compared to plants infected with Pst DC 3000 alone, but with no significant changes in total sesquiterpene emissions. In the compatible interaction, Pstb elicited a large ethylene burst with a peak emission occurring 3 days after inoculation. In contrast, the incompatible interaction involving Pst DC3000 displayed no such ethylene induction. Pstb-induced ethylene production was not significantly altered by Pst DC3000 in the combined infection. We postulated that Pstb-induced ethylene production may play a regulatory role in altering the typical volatile emission in tobacco in response to Pst DC3000 infection. To clarify the role of ethylene, we dynamically applied ethylene to the headspace of tobacco plants following infection with Pst DC3000. Consistent with Pstb-induced ethylene, exogenous ethylene reduced both ocimene and MeSA emissions, and selectively altered the ratios and amounts of induced sesquiterpene emissions. Our findings suggest that ethylene can regulate the magnitude and blend of induced volatile emissions during pathogen infection.

A constitutive PR-1::luciferase expression screen identifies Arabidopsis mutants with differential disease resistance to both biotrophic and necrotrophic pathogens

SUMMARY A complex signal transduction network involving salicylic acid, jasmonic acid and ethylene underlies disease resistance in Arabidopsis. To understand this defence signalling network further, we identified mutants that expressed the marker gene PR-1::luciferase in the absence of pathogen infection. These cir mutants all display constitutive expression of a suite of defence-related genes but exhibit different disease resistance profiles to two biotrophic pathogens, Pseudomonas syringae pv. tomato and Peronospora parasitica NOCO2, and the necrotrophic pathogen Botrytis cinerea. We further characterized cir3, which displays enhanced resistance only to the necrotrophic pathogen. Cir3-mediated resistance to B. cinerea is dependent on accumulated salicylic acid and a functional EIN2 protein.

Induction of phytohormones and differential gene expression in citrus flowers infected by the fungus Colletotrichum acutatum

Colletotrichum acutatum infects citrus petals and induces premature fruit drop and the formation of persistent calyces. The accumulation of hormones and other growth regulators, and differential gene expression in affected flowers and young fruit, was examined following fungal infection. Ethylene evolution increased threefold and indole-3-acetic acid (IAA) accumulation was as much as 140 times. Abscisic acid (ABA) levels showed no significant response. After infection, both trans- and cis-12-oxo-phytodienoic acid increased 8- to 10-fold. No significant difference of transjasmonic acid (JA) was observed in citrus flower petals or pistils. However, a fivefold increase of cis-JA was detected. The amount of salicylic acid (SA) was elevated twofold in affected petals, but not in pistils. Northern blot analyses revealed that the genes encoding ACC oxidase or ACC synthase, and 12-oxo-phytodienoic acid (12-oxo-PDA) reductase, were highly expressed in affected flowers. The genes encoding auxin-related proteins also were upregulated. Application of 2-(4-chlorophenoxy)-2-methyl-propionic acid (clofibrate; a putative auxin inhibitor), 2,3,5-triiodobenzolic acid (an auxin transport inhibitor), or SA after inoculation significantly decreased the accumulation of the gene transcripts of auxin-responsive, GH3-like protein and 12-oxo-PDA reductase, but resulted in higher percentages of young fruit retention. The results indicate that imbalance of IAA, ethylene, and JA in C. acutatum-infected flowers may be involved in symptom development and young fruit drop.

Phosphorylation of 1-aminocyclopropane-1-carboxylic acid synthase by MPK6, a stress-responsive mitogen-activated protein kinase, induces ethylene biosynthesis in Arabidopsis

Mitogen-activated protein kinases (MAPKs) are implicated in regulating plant growth, development, and response to the environment. However, the underlying mechanisms are unknown because of the lack of information about their substrates. Using a conditional gain-of-function transgenic system, we demonstrated that the activation of SIPK, a tobacco (Nicotiana tabacum) stress-responsive MAPK, induces the biosynthesis of ethylene. Here, we report that MPK6, the Arabidopsis thaliana ortholog of tobacco SIPK, is required for ethylene induction in this transgenic system. Furthermore, we found that selected isoforms of 1-aminocyclopropane-1-carboxylic acid synthase (ACS), the rate-limiting enzyme of ethylene biosynthesis, are substrates of MPK6. Phosphorylation of ACS2 and ACS6 by MPK6 leads to the accumulation of ACS protein and, thus, elevated levels of cellular ACS activity and ethylene production. Expression of ACS6(DDD), a gain-of-function ACS6 mutant that mimics the phosphorylated form of ACS6, confers constitutive ethylene production and ethylene-induced phenotypes. Increasing numbers of stress stimuli have been shown to activate Arabidopsis MPK6 or its orthologs in other plant species. The identification of the first plant MAPK substrate in this report reveals one mechanism by which MPK6/SIPK regulates plant stress responses. Equally important, this study uncovers a signaling pathway that modulates the biosynthesis of ethylene, an important plant hormone, in plants under stress.

Downstream divergence of the ethylene signaling pathway for harpin-stimulated Arabidopsis growth and insect defense

Ethylene (ET) signal transduction may regulate plant growth and defense, depending on which components are recruited into the pathway in response to different stimuli. We report here that the ET pathway controls both insect resistance (IR) and plant growth enhancement (PGE) in Arabidopsis (Arabidopsis thaliana) plants responding to harpin, a protein produced by a plant pathogenic bacterium. PGE may result from spraying plant tops with harpin or by soaking seeds in harpin solution; the latter especially enhances root growth. Plants treated similarly develop resistance to the green peach aphid (Myzus persicae). The salicylic acid pathway, although activated by harpin, does not lead to PGE and IR. By contrast, PGE and IR are induced in both wild-type plants and genotypes that have defects in salicylic acid signaling. In response to harpin, levels of jasmonic acid (JA) decrease, and the COI1 gene, which is indispensable for JA signal transduction, is not expressed in wild-type plants. However, PGE and IR are stimulated in the JA-resistant mutant jar1-1. In the wild type, PGE and IR develop coincidently with increases in ET levels and the expression of several genes essential for ET signaling. The ET receptor gene ETR1 is required because both phenotypes are arrested in the etr1-1 mutant. Consistently, inhibition of ET perception nullifies the induction of both PGE and IR. The signal transducer EIN2 is required for IR, and EIN5 is required for PGE because IR and PGE are impaired correspondingly in the ein2-1 and ein5-1 mutants. Therefore, harpin activates ET signaling while conscribing EIN2 and EIN5 to confer IR and PGE, respectively.

Expression of a salt-induced protein (SALT) in suspension-cultured cells and leaves of rice following exposure to fungal elicitor and phytohormones

Phytohormones are essential signal compounds in the regulation of stress-related and defense-related genes. However, there is no clear evidence for any effect of these signal molecules and biotic elicitors on the regulation of the SALT gene in suspension-cultured rice cells. We characterized the expression of a SALT gene following treatment with fungal elicitor, phytohormones, cycloheximide, and inhibitors of protein kinase/phosphatases. SALT expression was up-regulated following treatment with a fungal elicitor, jasmonic acid (JA), abscisic acid (ABA), and NaCl. However, salicylic acid (SA) alone or in combination with one of the other elicitors not only strongly inhibited SALT gene expression but also exhibited an antagonistic effect in suspension cells and leaves. Cycloheximide inhibited SALT accumulation in suspension cells and in leaves, but the inhibitors of protein kinase/phosphatase did not. Immunolocalization revealed that SALT protein was present in xylem parenchyma cells of vascular bundles in the major and minor leaf veins.

Ornithine decarboxylase gene (CaODC1) is specifically induced during TMV-mediated but salicylate-independent resistant response in hot pepper

A gene encoding putative ornithine decarboxylase (ODC) has been isolated by differential screening of a cDNA library from the resistant hot pepper (Capsicum annuum L.) inoculated with avirulent tobacco mosaic virus (TMV) pathotype P0. In hot pepper plants, transcripts of the CaODC1 (C. annuum ODC1) gene started to accumulate at 24 h post-inoculation of TMV-P0 and the signal was spread systemically. The transcript level of CaODC1 was increased rapidly in a hot pepper resistant to Xanthomonas campestris pv. vesicatoria (Xcv) but not in a susceptible hot pepper after inoculation. These results suggest possible role(s) for CaODC1 in plant defense against a broad range of pathogens including viruses and bacteria.

The use of vapor phase extraction in metabolic profiling of phytohormones and other metabolites

Through complex networks of signaling interactions, phytohormones regulate growth, development, reproduction and responses to biotic and abiotic stress. Comprehensive metabolomic approaches, seeking to quantify changes in vast numbers of plant metabolites, may ultimately clarify these complex signaling interactions and consequently explain pleiotropic effects on plant metabolism. Synergistic and antagonistic phytohormone signaling interactions, referred to as crosstalk, are often considered at the level of transduction without proper consideration of synthesis or accumulation of phytohormones because of the limitation and difficulty in quantifying numerous signals. Significant progress has recently been made in the expansion of metabolic profiling and analysis of multiple phytohormones [Birkemeyer et al. (J. Chromatogr. A, 2003, 993, 89); Chiwocha et al. (Plant J., 2003, 35, 405); Müller et al. (Planta, 2002, 216, 44); Schmelz et al. (Proc. Natl Acad. Sci. USA, 2003, 100, 10552)]. We recently presented a novel metabolic profiling approach to the analysis of acidic phytohormones and other metabolites based on a simplistic preparation scheme and analysis by chemical ionization-gas chromatography/mass spectrometry. We now provide a detailed description of this vapor phase extraction technique and use pathogen infection of Arabidopsis with Pseudomonas syringae DC3000 to illustrate metabolic changes in salicylic acid, cinnamic acid, jasmonic acid, indole-3-acetic acid, abscisic acid, unsaturated C(18) fatty acids, 12-oxo-phytodienoic acid, and phytotoxin coronatine. Directions for further method expansion are provided and include issues of recovery, derivatization, range of accessible analytes, optimization, reproducibility and future directions.

Regulatory mechanisms during the plant – arbuscular mycorrhizal fungus interaction

Abundant data are available on some aspects of the arbuscular mycorrhizal symbiosis, for example, plant nutrition, but because of difficulties immanent to arbuscular mycorrhizal fungi, such as the inability to culture them axenically, the relatively long time it takes to achieve root colonization, and the simultaneous presence of different morphologic stages of the fungus in the root, less information is accumulated on other aspects such as the regulation of mycorrhization. Regulatory processes in the plant – arbuscular mycorrhizal fungus interaction start before root colonization by the fungus and even before a direct physical contact between the host and the fungal symbiont. Some of the signals exchanged are still a matter of debate and will be discussed further on. After the penetration of the root by the fungus, depending on the developmental stage of the arbuscular mycorrhizal association (e.g., early or mature), a range of plant responses is activated. The possible function of several plant responses in the regulation of mycorrhization is discussed.Key words: arbuscular mycorrhiza, Glomales, autoregulation, flavonoid, recognition, root exudates.

Tomato stress-responsive factor TSRF1 interacts with ethylene responsive element GCC box and regulates pathogen resistance to Ralstonia solanacearum

Ethylene responsive factors (ERFs) are important in regulating plant pathogen resistance, abiotic stress tolerance and plant development. Recent studies have greatly enlarged the ERF protein family and revealed more important roles of ERFs in plants. Here, we report our finding of a tomato ERF protein TSRF1, which is transcriptionally up-regulated by ethylene, salicylic acid, or Ralstonia solanacearum strain BJ1057 infection. Biochemical analysis indicates that TSRF1 specifically interacts in vitro with the GCC box, an element present in the promoters of many pathogenesis-related (PR) genes. Further investigation evidences that TSRF1 activates in vivo the expression of reporter beta-glucuronidase gene controlled by GCC box. More importantly, overexpressing TSRF1 in tobacco and tomato constitutively activates the expression of PR genes, and subsequently enhancing transgenic plant resistance to the bacterial wilt caused by Ralstonia solanacearum strain BJ1057. Therefore our investigation not only extends the functions of ERF proteins in plant resistance to R. solanacearum, but also provides further clues to understanding the mechanism of host regulatory proteins in response to the infection of pathogens.

Ethylene response factor 1 mediates Arabidopsis resistance to the soilborne fungus Fusarium oxysporum

Ethylene response factor 1 (ERF1) is a transcriptional factor from Arabidopsis thaliana that regulates plant resistance to the necrotrophic fungi Botrytis cinerea and Plectosphaerella cucumerina and whose overexpression enhances resistance to these fungi. Here, we show that ERF1 also mediates Arabidopsis resistance to the soilborne fungi Fusarium oxysporum sp. conglutinans and F. oxysporum f. sp. lycopersici, because its constitutive expression in Arabidopsis confers enhanced resistance to these pathogens. Expression of ERF1 was upregulated after inoculation with F. oxysporum f. sp. conglutinans, and this response was blocked in ein2-5 and coi1-1 mutants, impaired in the ethylene (ET) and jasmonic acid (JA) signal pathways, respectively, which further indicates that ERF1 is a downstream component of ET and JA defense responses. The signal transduction network controlling resistance to F. oxysporum fungi was explored using signaling-defective mutants in ET (ein2-5), JA (jar1-1), and salicylic acid (SA) (NahG, sid2-1, eds5-1, npr1-1, pad4-1, eds1-1, and pad2-1) transduction pathways. This analysis revealed that Arabidopsis resistance to F. oxysporum requires the ET, JA, and SA signaling pathways and the NPR1 gene, although it is independent of the PAD4 and EDS1 functions.

Enhanced resistance to Cucumber mosaic virus in the Arabidopsis thaliana ssi2 mutant is mediated via an SA-independent mechanism

The Arabidopsis thaliana SSI2 gene encodes a plastid-localized stearoyl-ACP desaturase. The recessive ssi2 mutant allele confers constitutive accumulation of the pathogenesis-related-1 (PR-1) gene transcript and salicylic acid (SA), and enhanced resistance to bacterial and oomycete pathogens. In addition, the ssi2 mutant is a dwarf and spontaneously develops lesions containing dead cells. Here, we show that the ssi2 mutant also confers enhanced resistance to Cucumber mosaic virus (CMV). Compared with the wild-type plant, viral multiplication and systemic spread were diminished in the ssi2 mutant plant. However, unlike the ssi2-conferred resistance to bacterial and oomycete pathogens, the ssi2-conferred enhanced resistance to CMV was retained in the SA-deficient ssi2 nahG plant. In addition, SA application was not effective in limiting CMV multiplication and systemic spread in the CMV-susceptible wild-type plant. The acd1, acd2, and cpr5 mutants which, like the ssi2 mutant, accumulate elevated SA levels, constitutively express the PR-1 gene, spontaneously develop lesions containing dead cells, and are dwarfs, are, however, fully susceptible to CMV. Our results suggest that dwarfing, cell death, and constitutive activation of SA signaling are not important for the ssi2-conferred enhanced resistance to CMV. However, the sfd1 and sfd4 mutations, which affect lipid metabolism, suppress the ssi2-conferred enhanced resistance to CMV, thus implicating a lipid or lipids in the ssi2-conferred resistance to CMV. Interestingly, the ssi2-conferred resistance to CMV was compromised in the ssi2 eds5 plant, suggesting the involvement of an SA-independent, EDS5-dependent mechanism in the ssi2-conferred resistance to CMV.

Ectopic overexpression of tomato JERF3 in tobacco activates downstream gene expression and enhances salt tolerance

The ethylene, jasmonic acid and osmotic signaling pathways respond to environmental stimuli and in order to understand how plants adapt to biotic and abiotic stresses it is important to understand how these pathways interact each other. In this paper, we report a novel ERF protein--jasmonate and ethylene-responsive factor 3 (JERF3)--that unites these pathways. JERF3, which functions as an in vivo transcription activator in yeast, binds to the GCC box, an element responsive to ethylene/JA signaling, as well as to DRE, a dehydration-responsive element that responds to dehydration, high salt and low-temperature. Expression of JERF3 in tomato is mainly induced by ethylene, JA, cold, salt or ABA. Constitutive expression of JERF3 in transgenic tobacco significantly activated expression of pathogenesis-related genes that contained the GCC box, resulting in enhanced tolerance to salt. These results indicate that JERF3 functions as a linker in ethylene- and osmotic stress-signaling pathways.

Consistency of Nicotiana attenuata's herbivore- and jasmonate-induced transcriptional responses in the allotetraploid species Nicotiana quadrivalvis and Nicotiana clevelandii

We examined the consistency of the native diploid Nicotiana attenuata (Na)'s herbivore-induced transcriptional changes in the two allotetraploid natives, Nicotiana clevelandii (Nc) and Nicotiana quadrivalvis (Nq), which are thought to be derived from hybridizations with an ancestral Na. An analysis of nuclear-encoded chloroplast-expressed Gln synthetase gene (ncpGS) sequences found strong similarity between Nc and Na and between N. trigonophylla and the two allopolyploids. All species were elicited with methyl jasmonate (MeJA), or were wounded and treated with either water, Manduca sexta oral secretions and regurgitant (R), or the two most abundant fatty acid amino acid conjugates (F) in R to simulate herbivory. The induced transcriptional responses in all three species were compared with a cDNA microarray enriched in Na genes. Na had the fastest transcriptional responses followed by Nc and then Nq. Na's R- and F-elicited responses were more similar to those from Nq, while the MeJA- or wound-elicited responses were more consistent in Nc. Treatment of wounds with the full cocktail of elicitors found in R elicits more complex responses than does treatment with F. The species differ in their elicited JA responses, and these differences are mirrored in the expression of oxylipin genes (LOX, HPL, AOS, and alpha-DOX) and downstream JA-elicited genes (TD). Elicitation decreases the expression of growth-related genes in all three species. We propose that this is a valuable system to examine the modification of complex, polygenic, adaptive responses during allopolyploid speciation.

Arabidopsis DND2, a second cyclic nucleotide-gated ion channel gene for which mutation causes the "defense, no death" phenotype

A previous mutant screen identified Arabidopsis dnd1 and dnd2 "defense, no death" mutants, which exhibit loss of hypersensitive response (HR) cell death without loss of gene-for-gene resistance. The dnd1 phenotype is caused by mutation of the gene encoding cyclic nucleotide-gated (CNG) ion channel AtCNGC2. This study characterizes dnd2 plants. Even in the presence of high titers of Pseudomonas syringae expressing avrRpt2, most leaf mesophyll cells in the dnd2 mutant exhibited no HR. These plants retained strong RPS2-, RPM1-, or RPS4-mediated restriction of P. syringae pathogen growth. Mutant dnd2 plants also exhibited enhanced broad-spectrum resistance against virulent P. syringae and constitutively elevated levels of salicylic acid, and pathogenesis-related (PR) gene expression. Unlike the wild type, dnd2 plants responding to virulent and avirulent P. syringae exhibited elevated expression of both salicylate-dependent PR-1 and jasmonate and ethylene-dependent PDF1.2. Introduction of nahG+ (salicylate hydroxylase) into the dnd2 background, which removes salicylic acid and causes other defense alterations, eliminated constitutive disease resistance and PR gene expression but only weakly impacted the HR- phenotype. Map-based cloning revealed that dnd2 phenotypes are caused by mutation of a second CNG ion channel gene, AtCNGC4. Hence, loss of either of two functionally nonredundant CNG ion channels can cause dnd phenotypes. The dnd mutants provide a unique genetic background for dissection of defense signaling.

The role of the jasmonate response in plant susceptibility to diverse pathogens with a range of lifestyles

Plants defend themselves against attack from insects and pathogens with various resistance strategies. The jasmonate and salicylate signaling pathways are two induced responses that protect plants against these attackers. Knowledge of the range of organisms that are affected by each response is important for understanding how plants coordinate their defenses against multiple attackers and the generality of effect of different resistance mechanisms. The jasmonate response is known to protect plants against a wide range of insect herbivores; in this study, we examined the role of the jasmonate response in susceptibility to eight pathogens with diverse lifestyles in the laboratory and field. Recent biochemical models suggest that the lifestyle of the pathogen (necrotroph versus biotroph) should predict whether the jasmonate response will be involved in resistance. We tested this by examining the susceptibility of wild-type (cv Castlemart with no known genes for resistance to the pathogens used) and jasmonate-deficient mutant tomato (Lycopersicon esculentum) plants (def1) and by employing rescue treatments of the mutant. Plant susceptibility to five of the eight pathogens we examined was reduced by the jasmonate response, including two bacteria (Pseudomonas syringae and Xanthomonas campestris), two fungi (Verticillium dahliae and Fusarium oxysporum f. sp. lycopersici), and an oomycete (Phytophthora infestans). Susceptibility to three fungi was unaffected (Cladosporium fulvum, Oidium neolycopersici, and Septoria lycopersici). Our results indicate that the jasmonate response reduces damage by a wide range of pathogens from different lifestyles, a result that contrasts with the emerging picture of diseases on Arabidopsis. Thus, the generality of jasmonate-based resistance of tomato challenges the view that ecologically distinct plant parasites are resisted via different mechanisms.

Herbivore-induced plant vaccination. Part II. Array-studies reveal the transience of herbivore-specific transcriptional imprints and a distinct imprint from stress combinations

Summary Microarray technology has given plant biologists the ability to simultaneously monitor changes in the expression of hundreds of genes, and yet, to date, this technology has not been applied to ecological phenomena. In native tobacco (Nicotiana attenuata), prior attack of sap-feeding mirids (Tupiocoris notatus) results in vaccination of the plant against subsequent attacks by chewing hornworms (Manduca sexta). This vaccination is mediated by a combination of direct and indirect defenses and tolerance responses, which act in concert with the attack preferences of a generalist predator. Here, we use microarrays enriched in herbivore-elicited genes with a principal components analysis (PCA) to characterize transcriptional 'imprints' of single, sequential, or simultaneous attacks by these two main herbivores of N. attenuata. The PCA identified distinctly different imprints left by individual attack from the two species after 24 h, but not after 5 days. Moreover, imprints of sequential or simultaneous attacks differed significantly from those of single attack, suggesting the existence of a distinct gene expression program responsive to the combination of biological stressors. A dissection of the transcriptional imprints revealed responses in direct and indirect defense genes that were well correlated with observed increases in defense metabolites. Attack from both herbivores elicits a switch from growth- to defense-related transcriptional processes, and herbivore-specific changes occur largely in primary metabolism and signaling cascades. PCA of these polygenic transcriptional imprints characterizes the ephemeral changes in the transcriptome that occur during the maturation of ecologically relevant phenotypic responses.

Crosstalk and differential response to abiotic and biotic stressors reflected at the transcriptional level of effector genes from secondary metabolism

Plant secondary metabolism significantly contributes to defensive measures against adverse abiotic and biotic cues. To investigate stress-induced, transcriptional alterations of underlying effector gene families, which encode enzymes acting consecutively in secondary metabolism and defense reactions, a DNA array (MetArray) harboring gene-specific probes was established. It comprised complete sets of genes encoding 109 secondary product glycosyltransferases and 63 glutathione-utilizing enzymes along with 62 cytochrome P450 monooxygenases and 26 ABC transporters. Their transcriptome was monitored in different organs of unstressed plants and in shoots in response to herbicides, UV-B radiation, endogenous stress hormones, and pathogen infection. A principal component analysis based on the transcription of these effector gene families defined distinct responses and crosstalk. Methyl jasmonate and ethylene treatments were separated from a group combining reactions towards two sulfonylurea herbicides, salicylate and an avirulent strain of Pseudomonas syringae pv. tomato . The responses to the herbicide bromoxynil and UV-B radiation were distinct from both groups. In addition, these analyses pinpointed individual effector genes indicating their role in these stress responses. A small group of genes was diagnostic in differentiating the response to two herbicide classes used. Interestingly, a subset of genes induced by P. syringae was not responsive to the applied stress hormones. Small groups of comprehensively induced effector genes indicate common defense strategies. Furthermore, homologous members within branches of these effector gene families displayed differential expression patterns either in both organs or during stress responses arguing for their non-redundant functions.

The timing of senescence and response to pathogens is altered in the ascorbate-deficient Arabidopsis mutant vitamin c-1

The ozone-sensitive Arabidopsis mutant vitamin c-1 (vtc1) is deficient in l-ascorbic acid (AsA) due to a mutation in GDP-Man pyrophosphorylase (Conklin et al., 1999), an enzyme involved in the AsA biosynthetic pathway (Smirnoff et al., 2001). In this study, the physiology of this AsA deficiency was initially investigated in response to biotic (virulent pathogens) stress and subsequently with regards to the onset of senescence. Infection with either virulent Pseudomonas syringae or Peronospora parasitica resulted in largely reduced bacterial and hyphal growth in the vtc1 mutant in comparison to the wild type. When vitamin c-2 (vtc2), another AsA-deficient mutant, was challenged with P. parasitica, growth of the fungus was also reduced, indicating that the two AsA-deficient mutants are more resistant to these pathogens. Induction of pathogenesis-related proteins PR-1 and PR-5 is significantly higher in vtc1 than in the wild type when challenged with virulent P. syringae. In addition, the vtc1 mutant exhibits elevated levels of some senescence-associated gene (SAG) transcripts as well as heightened salicylic acid levels. Presumably, therefore, low AsA is causing vtc1 to enter at least some stage(s) of senescence prematurely with an accompanying increase in salicylic acid levels that results in a faster induction of defense responses.

Involvement of cAMP signaling in elicitor-induced phytoalexin accumulation in Cupressus lusitanica cell cultures

•  An increasing body of evidence on plant electrophysiology, biochemistry, and molecular biology shows that cAMP exists in higher plants and plays a role in several physiological processes by affecting potassium (K⁺ ) or calcium (Ca²⁺ ) fluxes. Our study here reports that cAMP is involved in elicitor-induced accumulation of a phytoalexin, β-thujaplicin, in Cupressus lusitanica cell cultures. •  Treatment of C. lusitanica cultured cells with cAMP or its analogues stimulated β-thujaplicin accumulation. Cholera toxin and forskolin, activators of adenylyl cyclase, also stimulated β-thujaplicin accumulation. Enzyme immunoassay showed that after elicitor treatment, cAMP level in the elicited cells quickly increased to about three- to five-fold over the control. Cholera toxin and forskolin also stimulated cAMP accumulation in the absence of elicitor. •  However, K⁺ and Ca²⁺ channel blockers inhibited the β-thujaplicin accumulation induced by cAMP analogues, suggesting that the cAMP-stimulated β-thujaplicin accumulation may involve Ca²⁺ and K⁺ fluxes. Several ionophores mimicked cAMP induction of β-thujaplicin accumulation. •  Cross-talk between cAMP treatment and the ethylene signaling pathway was also observed to work in the cell cultures via Ca²⁺ signaling. The study also indicates an involvement of protein kinase cascades in cAMP signaling processes, leading to both phytoalexin and ethylene production.

Recognition and response in the plant immune system

Molecular communication between plants and potential pathogens determines the ultimate outcome of their interaction. The directed delivery of microbial molecules into and around the host cell, and the subsequent perception of these by the invaded plant tissue (or lack thereof), determines the difference between disease and disease resistance. In theory, any foreign molecule produced by an invading pathogen could act as an elicitor of the broad physiological and transcriptional re-programming indicative of a plant defense response. The diversity of elicitors recognized by plants seems to support this hypothesis. Additionally, these elicitors are often virulence factors from the pathogen recognized by the host. This recognition, though genetically as simple as a ligand-receptor interaction, may require additional host proteins that are the nominal targets of virulence factor action. Transduction of recognition probably requires regulated protein degradation and results in massive changes in cellular homeostasis, including a programmed cell death known as the hypersensitive response that indicates a successful, if perhaps over-zealous, disease resistance response.

LeMPK3 is a mitogen-activated protein kinase with dual specificity induced during tomato defense and wounding responses

Mitogen-activated protein (MAP) kinase cascades are readily activated during the response of plants to avirulent pathogens or to pathogen-derived elicitors. Here we show that the tomato MAP kinase LeMPK3 is specifically induced at the mRNA level during elicitation of the hypersensitive response in resistant plants infected by avirulent strains of the phytopathogenic bacteria Xanthomonas campestris pv. vesicatoria and Pseudomonas syringae pv. tomato, as well as upon treatment with the fungal elicitor ethylene-inducing xylanase. LeMPK3 gene expression was also induced very rapidly by mechanical stress and wounding much earlier than upon pathogen infection, but not in response to the defense-related plant hormones ethylene and jasmonic acid. Moreover, in resistant tomato plants infected by X. campestris pv. vesicatoria, transcript accumulation was followed by an increase in LeMPK3 kinase activity. Biochemical characterization of a glutathione S-transferase-LeMPK3 fusion protein revealed that the LeMPK3 MAP kinase autophosphorylates in vitro mainly on tyrosine and less so on threonine and serine, whereas it phosphorylates myelin basic protein on serine and threonine. In vitro phosphorylation of a poly-(Glu-Tyr) copolymer by LeMPK3 demonstrated its capability to phosphorylate tyrosine residues on substrates as well. By mutagenesis and phosphoamino acid analysis, Tyr-201 in the kinase activation domain was identified as the main LeMPK3 autophosphorylation site and as critical for kinase activity. Finally, LeMPK3 autophosphorylation showed a preference for Mn(2+) cations and proceeded via an intramolecular mechanism with an estimated K(m) value for ATP of 9.5 microm. These results define LeMPK3 as a MAP kinase with dual specificity and strongly suggest that it represents a convergence point for different signaling pathways inducing the activation of defense responses in tomato.

The diverse roles of ubiquitin and the 26S proteasome in the life of plants

A tightly regulated and highly specific system for the degradation of individual proteins is essential for the survival of all organisms. In eukaryotes, this is achieved by the tagging of proteins with ubiquitin and their subsequent recognition and degradation by the 26S proteasome. In plants, genetic analysis has identified many genes that regulate developmental pathways. Subsequent analysis of these genes has implicated ubiquitin and the 26S proteasome in the control of diverse developmental processes, and indicates that proteolysis is a crucial regulatory step throughout the life cycle of plants.

Pathogenesis-related protein 10 isolated from hot pepper functions as a ribonuclease in an antiviral pathway

A hot pepper (Capsicum annuum) cDNA clone encoding pathogenesis-related protein 10 (CaPR-10) was isolated by differential screening of a cDNA library prepared from pepper leaves inoculated with tobacco mosaic virus pathotype (TMV-P0). CaPR-10 transcripts were induced in the incompatible interaction with TMV-P0 or Xanthomonas campestris pv. vesicatoria (Xcv) but not induced in the compatible interaction. Characterization of enzymatic properties of CaPR-10 indicated that the recombinant protein exhibits a ribonucleolytic activity against TMV RNA, as well as against pepper total RNA, and shows its putative antiviral activity in several conditions. The CaPR-10 protein existed at very low level in leaf tissue but was dramatically induced as soon as plants were inoculated with TMV-P0, and this was correlated with the increase of its ribonucleolytic activity. Immunoblot analysis and pull-down assays using proteins extracted from pepper leaves showed that TMV-P0 inoculation led to the phosphorylation of CaPR-10, a modification that should affect its capacity for RNase function. We present data that the induction and subsequent phosphorylation of CaPR-10 increased its ribonucleolytic activity to cleave invading viral RNAs, and this activity should be important to its antiviral pathway during viral attack in vivo.

Sucrose increases pathogenesis-related PR-2 gene expression in Arabidopsis thaliana through an SA-dependent but NPR1-independent signaling pathway

Pathogenesis-related (PR) protein-coding gene expression was studied in Arabidopsis thaliana grown in liquid medium in the presence of sugars (sucrose or glucose). PR protein transcripts accumulated in the presence of sugar in the medium. A potential effect linked to osmolarity changes induced by sugar addition in the medium was ruled out using osmotica (NaCl or polyethylene glycol). Two major proteins were purified from the culture medium and found to be homologous to A. thaliana PR-2 (acidic form of beta-1, 3-glucanase) and PR-5 (thaumatin-like PR-protein). The expression of the corresponding genes was increased in the presence of sucrose and was detected exclusively in the green parts of the plant. The use of mutants and transgenic plants of A. thaliana indicated that salicylic acid (SA) was involved in the sugar-dependent activation of these PR protein-coding genes. Activation of the PR-2-coding gene was demonstrated not to be hexokinase-dependent and to be linked to a sugar metabolite acting as an internal signal as shown with non-metabolizable sugars, which were inefficient for the induction of the PR-2-coding gene. Moreover, the activation of this gene occurred in the npr1 mutant suggesting that the sugar signal acts either downstream or independently of NPR1.

Tissue-specific and developmentally regulated expression of a cluster of tandemly arrayed cell wall-associated kinase-like kinase genes in Arabidopsis

The Arabidopsis cell wall-associated kinase (WAK) and WAK-like kinase (WAKL) family of receptor-like kinase genes encodes transmembrane proteins with a cytoplasmic serine/threonine kinase domain and an extracellular region containing epidermal growth factor-like repeats. Previous studies have suggested that some WAK members are involved in plant defense and heavy metal responses, whereas others are required for cell elongation and plant development. The WAK/WAKL gene family consists of 26 members in Arabidopsis and can be divided into four groups. Here, we describe the characterization of group 2 members that are composed of a cluster of seven tandemly arrayed WAKL genes. The predicted WAKL proteins are highly similar in their cytoplasmic region but are more divergent in their predicted extracellular ligand-binding region. WAKL7 encodes a truncated WAKL isoform that is predicted to be secreted from the cytoplasm. Ratios of nonsynonymous to synonymous substitutions suggest that the extracellular region is subject to diversifying selection. Comparison of the WAKL and WAK gene clusters suggests that they arose independently. Protein gel-blot and immunolocalization analyses suggest that WAKL6 is associated with the cell wall. Histochemical analyses of WAKL promoters fused with the beta-glucuronidase reporter gene have shown that the expressions of WAKL members are developmentally regulated and tissue specific. Unlike WAK members whose expressions were found predominately in green tissues, WAKL genes are highly expressed in roots and flowers. The expression of WAKL5 and WAKL7 can be induced by wounding stress and by the salicylic acid analog 2,6-dichloroisonicotinic acid in an nonexpressor of pathogenesis-related gene 1-dependent manner, suggesting that they, like some WAK members, are wound inducible and can be defined as pathogenesis-related genes.

Genetic evidence that expression of NahG modifies defence pathways independent of salicylic acid biosynthesis in the Arabidopsis-Pseudomonas syringae pv. tomato interaction

The salicylic acid (SA)-induction deficient (sid) mutants of Arabidopsis, eds5 and sid2 accumulate normal amounts of camalexin after inoculation with Pseudomonas syringae pv. tomato (Pst), while transgenic NahG plants expressing an SA hydroxylase that degrades SA have reduced levels of camalexin and exhibit a higher susceptibility to different pathogens compared to the sid mutants. SID2 encodes an isochorismate synthase necessary for the synthesis of SA. NahG was shown to act epistatically to the sid mutant phenotype regarding accumulation of camalexin after inoculation with Pst in eds5NahG and sid2NahG plants. The effect of the pad4 mutation on the sid mutant phenotype was furthermore tested in eds5pad4 and sid2pad4 double mutants, and it was demonstrated that PAD4 acts epistatically to EDS5 and SID2 regarding the production of camalexin after inoculation with Pst. NahG plants and pad4 mutants were also found to produce less ethylene (ET) after infection with Pst in comparison to the wild type (WT) and sid mutants. Both PAD4 and NahG acted epistatically to SID regarding the Pst-dependent production of ET that was found to be necessary for the accumulation of camalexin. Early production of jasmonic acid (JA) 12 h after inoculation with Pst/avrRpt2 was absent in all plants expressing NahG compared to the other mutants tested here. These genetic studies unravel pleiotropic changes in defence signalling of NahG plants that are unlikely to result from their low SA content. This adds unexpected difficulties in the interpretation of earlier findings based solely on NahG plants.

Multiple hormones act sequentially to mediate a susceptible tomato pathogen defense response

Phytohormones regulate plant responses to a wide range of biotic and abiotic stresses. How a limited number of hormones differentially mediate individual stress responses is not understood. We have used one such response, the compatible interaction of tomato (Lycopersicon esculentum) and Xanthomonas campestris pv vesicatoria (Xcv), to examine the interactions of jasmonic acid (JA), ethylene, and salicylic acid (SA). The role of JA was assessed using an antisense allene oxide cyclase transgenic line and the def1 mutant to suppress Xcv-induced biosynthesis of jasmonates. Xcv growth was limited in these lines as was subsequent disease symptom development. No increase in JA was detected before the onset of terminal necrosis. The lack of a detectable increase in JA may indicate that an oxylipin other than JA regulates basal resistance and symptom proliferation. Alternatively, there may be an increase in sensitivity to JA or related compounds following infection. Hormone measurements showed that the oxylipin signal must precede subsequent increases in ethylene and SA accumulation. Tomato thus actively regulates the Xcv-induced disease response via the sequential action of at least three hormones, promoting expansive cell death of its own tissue. This sequential action of jasmonate, ethylene, and SA in disease symptom development is different from the hormone interactions observed in many other plant-pathogen interactions.

Pathogen-responsive expression of a putative ATP-binding cassette transporter gene conferring resistance to the diterpenoid sclareol is regulated by multiple defense signaling pathways in Arabidopsis

The ATP-binding cassette (ABC) transporters are encoded by large gene families in plants. Although these proteins are potentially involved in a number of diverse plant processes, currently, very little is known about their actual functions. In this paper, through a cDNA microarray screening of anonymous cDNA clones from a subtractive library, we identified an Arabidopsis gene (AtPDR12) putatively encoding a member of the pleiotropic drug resistance (PDR) subfamily of ABC transporters. AtPDR12 displayed distinct induction profiles after inoculation of plants with compatible and incompatible fungal pathogens and treatments with salicylic acid, ethylene, or methyl jasmonate. Analysis of AtPDR12 expression in a number of Arabidopsis defense signaling mutants further revealed that salicylic acid accumulation, NPR1 function, and sensitivity to jasmonates and ethylene were all required for pathogen-responsive expression of AtPDR12. Germination assays using seeds from an AtPDR12 insertion line in the presence of sclareol resulted in lower germination rates and much stronger inhibition of root elongation in the AtPDR12 insertion line than in wild-type plants. These results suggest that AtPDR12 may be functionally related to the previously identified ABC transporters SpTUR2 and NpABC1, which transport sclareol. Our data also point to a potential role for terpenoids in the Arabidopsis defensive armory.

A developmental response to pathogen infection in Arabidopsis

We present evidence that susceptible Arabidopsis plants accelerate their reproductive development and alter their shoot architecture in response to three different pathogen species. We infected 2-week-old Arabidopsis seedlings with two bacterial pathogens, Pseudomonas syringae and Xanthomonas campestris, and an oomycete, Peronospora parasitica. Infection with each of the three pathogens reduced time to flowering and the number of aerial branches on the primary inflorescence. In the absence of competition, P. syringae and P. parasitica infection also increased basal branch development. Flowering time and branch responses were affected by the amount of pathogen present. Large amounts of pathogen caused the most dramatic changes in the number of branches on the primary inflorescence, but small amounts of P. syringae caused the fastest flowering and the production of the most basal branches. RPS2 resistance prevented large changes in development when it prevented visible disease symptoms but not at high pathogen doses and when substantial visible hypersensitive response occurred. These experiments indicate that phylogenetically disparate pathogens cause similar changes in the development of susceptible Arabidopsis. We propose that these changes in flowering time and branch architecture constitute a general developmental response to pathogen infection that may affect tolerance of and/or resistance to disease.

A novel transcription factor involved in plant defense endowed with protein phosphatase activity

In plants, expression of a disease-resistance character following perception of a pathogen involves massive deployment of transcription-dependent defenses. Thus, if rapid and effective defense responses have to be achieved, it is crucial that the pathogenic signal is transduced and amplified through pre-existing signaling pathways. Reversible phosphorylation of specific transcription factors, by a concerted action of protein kinases and phosphatases, may represent a mechanism for rapid and flexible regulation of selective gene expression by environmental stimuli. Here we identified a novel DNA-binding protein from tobacco plants, designated DBP1, with protein phosphatase activity, which binds in a sequence-specific manner to a cis- acting element of a defense-related gene and participates in its transcriptional regulation. This finding helps delineate a terminal event in a signaling pathway for the selective activation of early transcription-dependent defense responses in plants, and suggests that stimulus-dependent reversible phosphorylation of regulatory proteins may occur directly in a transcription protein-DNA complex.

The role of proteolysis in R gene mediated defence in plants

SUMMARY Within the last 10 years, numerous R genes have been cloned from natural genetic variation in model as well as crop plants, and these have been classified according to their motifs. Some of the downstream signalling components have also been identified by artificial mutagenesis. Recently, cloning of three of these signalling genes (COI1, RAR1 and SGT1b) from Arabidopsis, barley and tobacco have helped uncover the physiological link between defence signalling and ubiquitin-mediated protein degradation. The physical association of COI1 and SGT1b with the components of ubiquitin-ligase complexes has been shown. In addition, post-transcriptional silencing of some of the subunits of the ubiquitin-ligase complex has led to a loss of resistance, indicating that protein degradation may also act as a regulatory mechanism in plant defence. Over the next few years, we should expect to see more examples of the interplay between the defence response and protein degradation in plants.

Two Brassica napus polygalacturonase inhibitory protein genes are expressed at different levels in response to biotic and abiotic stresses

Plants encode a distinct set of polygalacturonase inhibitory proteins (PGIPs) that function to inhibit polygalacturonase enzymes produced by soft-rot fungal pathogens. We characterized two PGIP-encoding genes ( Bnpgip1 and Bnpgip2) from Brassica napus DH12075 (a double-haploid line derived from a cross between 'Crésor' and 'Westar'). The two proteins exhibit 67.4% identity at the amino acid level and contain 10 imperfect leucine-rich repeats. The pgip genes are present as a small multigene family in B. napus with at least four members. Bnpgip1 and Bnpgip2 are constitutively expressed in roots, stems, flower buds and open flowers. In mature leaf tissue, different levels of induction were observed in response to biotic and abiotic stresses. Bnpgip1 expression was highly responsive to flea beetle feeding and mechanical wounding, weakly responsive to Sclerotinia sclerotiorum infection and exposure to cold but not to dehydration. Conversely, Bnpgip2 expression was strongly induced by S. sclerotiorum infection and to a lesser degree by wounding but not by flea beetle feeding. Application of jasmonic acid to leaves induced both Bnpgip1 and Bnpgip2 gene expression; however, salicylic acid did not activate either gene. Taken together, these results suggest that separate pathways regulate Bnpgip1 and Bnpgip2, and that their roles in plant development or resistance to biotic and abiotic stress differ.

A role for the GCC-box in jasmonate-mediated activation of the PDF1.2 gene of Arabidopsis

The PDF1.2 gene of Arabidopsis encoding a plant defensin is commonly used as a marker for characterization of the jasmonate-dependent defense responses. Here, using PDF1.2 promoter-deletion lines linked to the beta-glucoronidase-reporter gene, we examined putative promoter elements associated with jasmonate-responsive expression of this gene. Using stably transformed plants, we first characterized the extended promoter region that positively regulates basal expression from the PDF1.2 promoter. Second, using promoter deletion constructs including one from which the GCC-box region was deleted, we observed a substantially lower response to jasmonate than lines carrying this motif. In addition, point mutations introduced into the core GCC-box sequence substantially reduced jasmonate responsiveness, whereas addition of a 20-nucleotide-long promoter element carrying the core GCC-box and flanking nucleotides provided jasmonate responsiveness to a 35S minimal promoter. Taken together, these results indicated that the GCC-box plays a key role in conferring jasmonate responsiveness to the PDF1.2 promoter. However, deletion or specific mutations introduced into the core GCC-box did not completely abolish the jasmonate responsiveness of the promoter, suggesting that the other promoter elements lying downstream from the GCC-box region may also contribute to jasmonate responsiveness. In other experiments, we identified a jasmonate- and pathogen-responsive ethylene response factor transcription factor, AtERF2, which when overexpressed in transgenic Arabidopsis plants activated transcription from the PDF1.2, Thi2.1, and PR4 (basic chitinase) genes, all of which contain a GCC-box sequence in their promoters. Our results suggest that in addition to their roles in regulating ethylene-mediated gene expression, ethylene response factors also appear to play important roles in regulating jasmonate-responsive gene expression, possibly via interaction with the GCC-box.

The cDNA microarray analysis using an Arabidopsis pad3 mutant reveals the expression profiles and classification of genes induced by Alternaria brassicicola attack

The hypersensitive response (HR) was induced in a wild-type Arabidopsis thaliana plant (Columbia) (Col-wt) by inoculation with Alternaria brassicicola that causes the development of small brown necrotic lesions on the leaves. By contrast, pad3-1 mutants challenged with A. brassicicola produced spreading lesions. The cell death in pad3-1 mutants could not inhibit the pathogen growth and development, although both production of H(2)O(2) and localized cell death were similar in Col-wt and pad3-1 plants after the inoculation. The difference between Col-wt and pad3-1 plants is defense responses after the occurrence of cell death. In other words, PAD3 is necessary for defense response to A. brassicicola. Therefore, we examined the changes in the expression patterns of ca. 7,000 genes by cDNA microarray analysis after inoculation with A. brassicicola. The cDNA microarrays were also done to analyze Arabidopsis responses after treatment with signal molecules, reactive oxygen species (ROS)-inducing compounds and UV-C. The results suggested that the pad3-1 mutation altered not only the accumulation of camalexin but also the timing of expression of many defense-related genes in response to the challenge with A. brassicicola. Furthermore, the plants integrate two or more signals that act together for promoting the induction of multiple defense pathways.

Brassinosteroid functions in a broad range of disease resistance in tobacco and rice

Brassinolide (BL), considered to be the most important brassinosteroid (BR) and playing pivotal roles in the hormonal regulation of plant growth and development, was found to induce disease resistance in plants. To study the potentialities of BL activity on stress responding systems, we analyzed its ability to induce disease resistance in tobacco and rice plants. Wild-type tobacco treated with BL exhibited enhanced resistance to the viral pathogen tobacco mosaic virus (TMV), the bacterial pathogen Pseudomonas syringae pv. tabaci (Pst), and the fungal pathogen Oidium sp. The measurement of salicylic acid (SA) in wild-type plants treated with BL and the pathogen infection assays using NahG transgenic plants indicate that BL-induced resistance does not require SA biosynthesis. BL treatment did not induce either acidic or basic pathogenesis-related (PR) gene expression, suggesting that BL-induced resistance is distinct from systemic acquired resistance (SAR) and wound-inducible disease resistance. Analysis using brassinazole 2001, a specific inhibitor for BR biosynthesis, and the measurement of BRs in TMV-infected tobacco leaves indicate that steroid hormone-mediated disease resistance (BDR) plays part in defense response in tobacco. Simultaneous activation of SAR and BDR by SAR inducers and BL, respectively, exhibited additive protective effects against TMV and Pst, indicating that there is no cross-talk between SAR- and BDR-signaling pathway downstream of BL. In addition to the enhanced resistance to a broad range of diseases in tobacco, BL induced resistance in rice to rice blast and bacterial blight diseases caused by Magnaporthe grisea and Xanthomonas oryzae pv. oryzae, respectively. Our data suggest that BDR functions in the innate immunity system of higher plants including dicotyledonous and monocotyledonous species.

Changes in hydrogen peroxide homeostasis trigger an active cell death process in tobacco

In transgenic tobacco plants with reduced catalase activity, high levels of hydrogen peroxide (H2O2) can accumulate under photorespiratory conditions. Such a perturbation in H2O2 homeostasis induced cell death in clusters of palisade parenchyma cells, primarily along the veins. Ultrastructural alterations, such as chromatin condensation and disruption of mitochondrial integrity, took place before cell death. Furthermore, enhanced transcript levels of mitochondrial defense genes accompanied these mitochondrial changes. Pharmacological data indicated that the initiation and execution of cell death require de novo protein synthesis and that the signal transduction pathway leading to cell death involved changes in ion homeostasis, (de)phosphorylation events and an oxidative burst, as observed during hypersensitive responses. This oxidase-dependent oxidative burst is essential for cell death, but it is not required for the accumulation of defense proteins, suggesting a more prominent role for the oxidative burst in abiotic stress-induced cell death.

Susceptible to intolerance--a range of hormonal actions in a susceptible Arabidopsis pathogen response

Ethylene and salicylic acid (SA) are key intermediates in a host's response to pathogens. Previously, we have shown using a tomato compatible interaction that ethylene and SA act sequentially and are essential for disease symptom production. Here, we have examined the relationship between the two signals in the Arabidopsis-Xanthomonas campestris pv. campestris (Xcc) compatible interaction. Preventing SA accumulation by expression of the nahG gene reduced subsequent ethylene production and altered the development of disease symptoms, with plants showing no visible chlorosis. The ethylene insensitive lines, etr1-1 and etr2-1, on the other hand, accumulated SA and exhibited normal but precocious symptom development. Therefore, Arabidopsis, like tomato, was found to exhibit co-operative ethylene and SA action for the production of disease symptoms. However, in Arabidopsis, SA was found to act upstream of ethylene. Jasmonic acid and indole-3-acetic acid levels were also found to increase in response to Xcc. In contrast to ethylene, accumulation of these hormones was not found to be dependent on SA action. These results indicate that the plants response to a virulent pathogen is a composite of multiple signaling pathways.

Characterizing the stress/defense transcriptome of Arabidopsis

BACKGROUND

To understand the gene networks that underlie plant stress and defense responses, it is necessary to identify and characterize the genes that respond both initially and as the physiological response to the stress or pathogen develops. We used PCR-based suppression subtractive hybridization to identify Arabidopsis genes that are differentially expressed in response to ozone, bacterial and oomycete pathogens and the signaling molecules salicylic acid (SA) and jasmonic acid.

RESULTS

We identified a total of 1,058 differentially expressed genes from eight stress cDNA libraries. Digital northern analysis revealed that 55% of the stress-inducible genes are rarely transcribed in unstressed plants and 17% of them were not previously represented in Arabidopsis expressed sequence tag databases. More than two-thirds of the genes in the stress cDNA collection have not been identified in previous studies as stress/defense response genes. Several stress-responsive cis-elements showed a statistically significant over-representation in the promoters of the genes in the stress cDNA collection. These include W- and G-boxes, the SA-inducible element, the abscisic acid response element and the TGA motif.

CONCLUSIONS

The stress cDNA collection comprises a broad repertoire of stress-responsive genes encoding proteins that are involved in both the initial and subsequent stages of the physiological response to abiotic stress and pathogens. This set of stress-, pathogen- and hormone-modulated genes is an important resource for understanding the genetic interactions underlying stress signaling and responses and may contribute to the characterization of the stress transcriptome through the construction of standardized specialized arrays.

trans-Resveratrol and grape disease resistance. A dynamical study by high-resolution laser-based techniques

Two modern laser-based techniques were synchronously applied to study the dynamics of the trans-resveratrol activity in Botrytis cinerea-infected grapes. Direct analysis of trans-resveratrol in both infected and noninfected grapes (Vitis vinifera, Aledo variety) was performed by using an analytical technique incorporating laser desorption coupled with laser resonant ionization and time-of-flight mass spectrometry. On the other hand, one of the most sensitive on-line methods for trace gas detection, laser photoacoustic spectroscopy, was used to investigate the involvement of the plant hormone ethylene (C(2)H(4)) in the B. cinerea grapes interaction and its temporal relationship with the trans-resveratrol content upon infection. The trans-resveratrol content and the ethylene released by noninfected grapes showed an opposite behavior. In this case, a high trans-resveratrol content corresponds to a low ethylene emission. For the B. cinerea-infected grapes, ethylene emission rises up after 48 h when the analogous content of trans-resveratrol started to decrease irreversibly. Moreover, the activity of trans-resveratrol as natural pesticide has been investigated by exogenous application on grapes. A short submerge (5 s) of the grapes in 1.6 x 10(-4) M solution of trans-resveratrol delays the increase of C(2)H(4) emission with about 48 h and produces a decrease of the C(2)H(4) concentration and its emission rate. The treatment has positive effects on fruit conservation during storage; it doubled the normal shelf-life of grapes at room temperature, maintaining their post-harvest quality within 10 d.