Pseudomonas syringae pv. tomato hijacks the Arabidopsis abscisic acid signalling pathway to cause disease

Abscisic acid interacts antagonistically with salicylic acid signaling pathway in rice-Magnaporthe grisea interaction

Plant hormones play pivotal signaling roles in plant-pathogen interactions. Here, we report characterization of an antagonistic interaction of abscisic acid (ABA) with salicylic acid (SA) signaling pathways in the rice-Magnaporthe grisea interaction. Exogenous application of ABA drastically compromised the rice resistance to both compatible and incompatible M. grisea strains, indicating that ABA negatively regulates both basal and resistance gene-mediated blast resistance. ABA markedly suppressed the transcriptional upregulation of WRKY45 and OsNPR1, the two key components of the SA signaling pathway in rice, induced by SA or benzothiadiazole or by blast infection. Overexpression of OsNPR1 or WRKY45 largely negated the enhancement of blast susceptibility by ABA, suggesting that ABA acts upstream of WRKY45 and OsNPR1 in the rice SA pathway. ABA-responsive genes were induced during blast infection in a pattern reciprocal to those of WRKY45 and OsPR1b in the compatible rice-blast interaction but only marginally in the incompatible one. These results suggest that the balance of SA and ABA signaling is an important determinant for the outcome of the rice-M. grisea interaction. ABA was detected in hyphae and conidia of M. grisea as well as in culture media, implying that blast-fungus-derived ABA could play a role in triggering ABA signaling at host infection sites.

Pseudomonas syringae effector protein AvrB perturbs Arabidopsis hormone signaling by activating MAP kinase 4

Pathogenic microbes often modulate phytohormone physiology in the host to their advantage. We previously showed that the Pseudomonas syringae effector protein AvrB perturbs hormone signaling, as exemplified by upregulated expression of jasmonic acid response genes, and enhances plant susceptibility. Here we show that these effects of AvrB require the Arabidopsis mitogen-activated protein kinase MAP kinase 4 (MPK4), HSP90 chaperone components, and the AvrB-interacting protein, RIN4. AvrB interacts with MPK4 and the HSP90 chaperone, and AvrB induces MPK4 activation in a manner promoted by HSP90; RIN4 likely acts downstream of MPK4. These findings link Arabidopsis proteins MPK4, HSP90, and RIN4 into a pathway that P. syringae AvrB activates for the benefit of the bacterium, perturbing hormone signaling and enhancing plant susceptibility.

Involvement of abscisic acid in the coordinated regulation of a stress-inducible hexose transporter (VvHT5) and a cell wall invertase in grapevine in response to biotrophic fungal infection

Biotrophic fungal and oomycete pathogens alter carbohydrate metabolism in infected host tissues. Symptoms such as elevated soluble carbohydrate concentrations and increased invertase activity suggest that a pathogen-induced carbohydrate sink is established. To identify pathogen-induced regulators of carbohydrate sink strength, quantitative real-time polymerase chain reaction was used to measure transcript levels of invertase and hexose transporter genes in biotrophic pathogen-infected grapevine (Vitis vinifera) leaves. The hexose transporter VvHT5 was highly induced in coordination with the cell wall invertase gene VvcwINV by powdery and downy mildew infection. However, similar responses were also observed in response to wounding, suggesting that this is a generalized response to stress. Analysis of the VvHT5 promoter region indicated the presence of multiple abscisic acid (ABA) response elements, suggesting a role for ABA in the transition from source to sink under stress conditions. ABA treatment of grape leaves was found to reproduce the same gene-specific transcriptional changes as observed under biotic and abiotic stress conditions. Furthermore, the key regulatory ABA biosynthetic gene, VvNCED1, was activated under these same stress conditions. VvHT5 promoter::beta-glucuronidase-directed expression in transgenic Arabidopsis (Arabidopsis thaliana) was activated by infection with powdery mildew and by ABA treatment, and the expression was closely associated with vascular tissue adjacent to infected regions. Unlike VvHT1 and VvHT3, which appear to be predominantly involved in hexose transport in developing leaves and berries, VvHT5 appears to have a specific role in enhancing sink strength under stress conditions, and this is controlled through ABA. Our data suggest a central role for ABA in the regulation of VvcwINV and VvHT5 expression during the transition from source to sink in response to infection by biotrophic pathogens.

Deficiencies in jasmonate-mediated plant defense reveal quantitative variation in Botrytis cinerea pathogenesis

Despite the described central role of jasmonate signaling in plant defense against necrotrophic pathogens, the existence of intraspecific variation in pathogen capacity to activate or evade plant jasmonate-mediated defenses is rarely considered. Experimental infection of jasmonate-deficient and jasmonate-insensitive Arabidopsis thaliana with diverse isolates of the necrotrophic fungal pathogen Botrytis cinerea revealed pathogen variation for virulence inhibition by jasmonate-mediated plant defenses and induction of plant defense metabolites. Comparison of the transcriptional effects of infection by two distinct B. cinerea isolates showed only minor differences in transcriptional responses of wild-type plants, but notable isolate-specific transcript differences in jasmonate-insensitive plants. These transcriptional differences suggest B. cinerea activation of plant defenses that require plant jasmonate signaling for activity in response to only one of the two B. cinerea isolates tested. Thus, similar infection phenotypes observed in wild-type plants result from different signaling interactions with the plant that are likely integrated by jasmonate signaling.

While many important elements of plant defense signaling have been identified, the function of these defense signaling pathways may mask additional variation in the plant–pathogen interaction, including both pathogen variation and variation in downstream plant defense responses. Jasmonate plant hormones contribute to both plant development and defense, including plant defense against necrotrophic fungal pathogens such as the grey mold Botrytis cinerea. Ten diverse B. cinerea isolates all showed increased virulence and decreased induction of a plant antimicrobial metabolite in experimental infections of Arabidopsis thaliana lacking functional jasmonate signaling. Yet within this consistent result, B. cinerea isolates varied considerably. Through comparing the transcript profiles of A. thaliana infected with the two most disparate B. cinerea isolates, we found that wild-type plants showed similar transcriptional responses to infection with these two isolates, but the absence of functional jasmonate signaling revealed dramatic differences in plant response, including groups of co-regulated genes that may participate in undescribed plant response networks. Jasmonate signaling appears to integrate plant responses to diverse pathogen inputs, and its absence may reveal novel aspects of plant–pathogen interaction.

MYB96-mediated abscisic acid signals induce pathogen resistance response by promoting salicylic acid biosynthesis in Arabidopsis

*The Arabidopsis MYB96 transcription factor plays a role in abscisic acid (ABA)-mediated drought response. Notably, anthocyanins accumulate in the activation-tagging myb96-1d line, suggesting a role of MYB96 in biotic and abiotic stress responses in plants. Here, we investigate the role of MYB96 in salicylic acid (SA) biosynthesis and plant defense and explore the mechanisms underlying the ABA-SA interaction. *myb96-1d and myb96-1 were subject to pathogen infection assays, and expression of SA biosynthetic and defense genes was examined. myb96-1d was crossed with the NahG transgenic plants to investigate the role of MYB96 in ABA regulation of SA biosynthesis. *Whereas myb96-1d exhibited an enhanced disease resistance, myb96-1 was susceptible to pathogen infection. A subset of pathogenesis-related (PR) genes was up-regulated in myb96-1d. However, PR transcript abundances were reduced in myb96-1d X NahG. Interestingly, a SA biosynthetic gene SALICYLIC ACID INDUCTION DEFICIENT2 (SID2) was up-regulated, and concentrations of SA and SA-beta-glucoside (SAG) were elevated in myb96-1d. In addition, the inductive effects of abiotic stresses on SID2 were reduced in aba3-1. *Our observations indicate that MYB96-mediated ABA signals enhance plant disease resistance by inducing SA biosynthesis. It is envisioned that MYB96 is a molecular link that mediates ABA-SA crosstalks.

The lesion-mimic mutant cpr22 shows alterations in abscisic acid signaling and abscisic acid insensitivity in a salicylic acid-dependent manner

A number of Arabidopsis (Arabidopsis thaliana) lesion-mimic mutants exhibit alterations in both abiotic stress responses and pathogen resistance. One of these mutants, constitutive expresser of PR genes22 (cpr22), which has a mutation in two cyclic nucleotide-gated ion channels, is a typical lesion-mimic mutant exhibiting elevated levels of salicylic acid (SA), spontaneous cell death, constitutive expression of defense-related genes, and enhanced resistance to various pathogens; the majority of its phenotypes are SA dependent. These defense responses in cpr22 are suppressed under high-humidity conditions and enhanced by low humidity. After shifting plants from high to low humidity, the cpr22 mutant, but not the wild type, showed a rapid increase in SA levels followed by an increase in abscisic acid (ABA) levels. Concomitantly, genes for ABA metabolism were up-regulated in the mutant. The expression of a subset of ABA-inducible genes, such as RD29A and KIN1/2, was down-regulated, but that of other genes, like ABI1 and HAB1, was up-regulated in cpr22 after the humidity shift. cpr22 showed reduced responsiveness to ABA not only in abiotic stress responses but also in germination and stomatal closure. Double mutant analysis with nahG plants that degrade SA indicated that these alterations in ABA signaling were attributable to elevated SA levels. Furthermore, cpr22 displayed suppressed drought responses by long-term drought stress. Taken together, these results suggest an effect of SA on ABA signaling/abiotic stress responses during the activation of defense responses in cpr22.

Abscisic acid-induced resistance against the brown spot pathogen Cochliobolus miyabeanus in rice involves MAP kinase-mediated repression of ethylene signaling

The plant hormone abscisic acid (ABA) is involved in an array of plant processes, including the regulation of gene expression during adaptive responses to various environmental cues. Apart from its well-established role in abiotic stress adaptation, emerging evidence indicates that ABA is also prominently involved in the regulation and integration of pathogen defense responses. Here, we demonstrate that exogenously administered ABA enhances basal resistance of rice (Oryza sativa) against the brown spot-causing ascomycete Cochliobolus miyabeanus. Microscopic analysis of early infection events in control and ABA-treated plants revealed that this ABA-inducible resistance (ABA-IR) is based on restriction of fungal progression in the mesophyll. We also show that ABA-IR does not rely on boosted expression of salicylic acid-, jasmonic acid -, or callose-dependent resistance mechanisms but, instead, requires a functional Galpha-protein. In addition, several lines of evidence are presented suggesting that ABA steers its positive effect on brown spot resistance through antagonistic cross talk with the ethylene (ET) response pathway. Exogenous ethephon application enhances susceptibility, whereas genetic disruption of ET signaling renders plants less vulnerable to C. miyabeanus attack, thereby inducing a level of resistance similar to that observed on ABA-treated wild-type plants. Moreover, ABA treatment alleviates C. miyabeanus-induced activation of the ET reporter gene EBP89, while derepression of pathogen-triggered EBP89 transcription via RNA interference-mediated knockdown of OsMPK5, an ABA-primed mitogen-activated protein kinase gene, compromises ABA-IR. Collectively, these data favor a model whereby exogenous ABA enhances resistance against C. miyabeanus at least in part by suppressing pathogen-induced ET action in an OsMPK5-dependent manner.

Characterization of a wheat HSP70 gene and its expression in response to stripe rust infection and abiotic stresses

Members of the family of 70-kD heat shock proteins (HSP70 s) play various stress-protective roles in plants. In this study, a wheat HSP70 gene was isolated from a suppression subtractive hybridization (SSH) cDNA library of wheat leaves infected by Puccinia striiformis f. sp. tritici. The gene, that was designated as TaHSC70, was predicted to encode a protein of 690 amino acids, with a molecular mass of 73.54 KDa and a pI of 5.01. Further analysis revealed the presence of a conserved signature that is characteristic for HSP70s and phylogenetic analysis demonstrated that TaHSC70 is a homolog of chloroplast HSP70s. TaHSC70 mRNA was present in leaves of both green and etiolated wheat seedlings and in stems and roots. The transcript level in roots was approximately threefold less than in leaves but light-dark treatment did not charge TaHSC70 expression. Following heat shock of wheat seedlings at 40°C, TaHSC70 expression increased in leaves of etiolated seedlings but remained stable at the same level in green seedlings. In addition, TaHSC70 was differentially expressed during an incompatible and compatible interaction with wheat-stripe rust, and there was a transient increase in expression upon treatment with methyl jasmonate (MeJA) treatment. Salicylic acid (SA), ethylene (ET) and abscisic acid (ABA) treatments had no influence on TaHSC70 expression. These results suggest that TaHSC70 plays a role in stress-related responses, and in defense responses elicited by infection with stripe rust fungus and does so via a JA-dependent signal transduction pathway.

A plant natriuretic peptide-like molecule of the pathogen Xanthomonas axonopodis pv. citri causes rapid changes in the proteome of its citrus host

BACKGROUND

Plant natriuretic peptides (PNPs) belong to a novel class of peptidic signaling molecules that share some structural similarity to the N-terminal domain of expansins and affect physiological processes such as water and ion homeostasis at nano-molar concentrations. The citrus pathogen Xanthomonas axonopodis pv. citri possesses a PNP-like peptide (XacPNP) uniquely present in this bacteria. Previously we observed that the expression of XacPNP is induced upon infection and that lesions produced in leaves infected with a XacPNP deletion mutant were more necrotic and lead to earlier bacterial cell death, suggesting that the plant-like bacterial PNP enables the plant pathogen to modify host responses in order to create conditions favorable to its own survival.

RESULTS

Here we measured chlorophyll fluorescence parameters and water potential of citrus leaves infiltrated with recombinant purified XacPNP and demonstrate that the peptide improves the physiological conditions of the tissue. Importantly, the proteomic analysis revealed that these responses are mirrored by rapid changes in the host proteome that include the up-regulation of Rubisco activase, ATP synthase CF1 alpha subunit, maturase K, and alpha- and beta-tubulin.

CONCLUSIONS

We demonstrate that XacPNP induces changes in host photosynthesis at the level of protein expression and in photosynthetic efficiency in particular. Our findings suggest that the biotrophic pathogen can use the plant-like hormone to modulate the host cellular environment and in particular host metabolism and that such modulations weaken host defence.

Research on plant abiotic stress responses in the post-genome era: past, present and future

Understanding abiotic stress responses in plants is an important and challenging topic in plant research. Physiological and molecular biological analyses have allowed us to draw a picture of abiotic stress responses in various plants, and determination of the Arabidopsis genome sequence has had a great impact on this research field. The availability of the complete genome sequence has facilitated access to essential information for all genes, e.g. gene products and their function, transcript levels, putative cis-regulatory elements, and alternative splicing patterns. These data have been obtained from comprehensive transcriptome analyses and studies using full-length cDNA collections and T-DNA- or transposon-tagged mutant lines, which were also enhanced by genome sequence information. Moreover, studies on novel regulatory mechanisms involving use of small RNA molecules, chromatin modulation and genomic DNA modification have enabled us to recognize that plants have evolved complicated and sophisticated systems in response to complex abiotic stresses. Integrated data obtained with various 'omics' approaches have provided a more comprehensive picture of abiotic stress responses. In addition, research on stress responses in various plant species other than Arabidopsis has increased our knowledge regarding the mechanisms of plant stress tolerance in nature. Based on this progress, improvements in crop stress tolerance have been attempted by means of gene transfer and marker-assisted breeding. In this review, we summarize recent progress in abiotic stress studies, especially in the post-genomic era, and offer new perspectives on research directions for the next decade.

Arabidopsis auxin mutants are compromised in systemic acquired resistance and exhibit aberrant accumulation of various indolic compounds

Systemic acquired resistance is a widespread phenomenon in the plant kingdom that confers heightened and often enduring immunity to a range of diverse pathogens. Systemic immunity develops through activation of plant disease resistance protein signaling networks following local infection with an incompatible pathogen. The accumulation of the phytohormone salicylic acid in systemically responding tissues occurs within days after a local immunizing infection and is essential for systemic resistance. However, our knowledge of the signaling components underpinning signal perception and the establishment of systemic immunity are rudimentary. Previously, we showed that an early and transient increase in jasmonic acid in distal responding tissues was central to effective establishment of systemic immunity. Based upon predicted transcriptional networks induced in naive Arabidopsis (Arabidopsis thaliana) leaves following avirulent Pseudomonas syringae challenge, we show that a variety of auxin mutants compromise the establishment of systemic immunity. Linking together transcriptional and targeted metabolite studies, our data provide compelling evidence for a role of indole-derived compounds, but not auxin itself, in the establishment and maintenance of systemic immunity.

Agroinfiltration reduces ABA levels and suppresses Pseudomonas syringae-elicited salicylic acid production in Nicotiana tabacum

BACKGROUND

Agrobacterium tumefaciens strain GV3101 (pMP90) is widely used in transient gene expression assays, including assays to study pathogen effectors and plant disease resistance mechanisms. However, inoculation of A. tumefaciens GV3101 into Nicotiana tabacum (tobacco) leaves prior to infiltration with pathogenic and non-host strains of Pseudomonas syringae results in suppression of macroscopic symptoms when compared with leaves pre-treated with a buffer control.

METHODOLOGY/FINDINGS

To gain further insight into the mechanistic basis of symptom suppression by A. tumefaciens we examined the effect of pre-treatment with A. tumefaciens on the growth of P. syringae, the production of the plant signalling molecules salicylic acid (SA) and abscisic acid (ABA), and the presence of callose deposits. Pre-treatment with A. tumefaciens reduced ABA levels, P. syringae multiplication and P. syringae-elicited SA and ABA production, but promoted increased callose deposition. However, pre-treatment with A. tumefaciens did not suppress necrosis or SA production in leaves inoculated with the elicitor HrpZ.

CONCLUSIONS/SIGNIFICANCE

Collectively, these results show that inoculation of N. tabacum leaves with A. tumefaciens alters plant hormone levels and plant defence responses to P. syringae, and demonstrate that researchers should consider the impact of A. tumefaciens on plant signal transduction when using A. tumefaciens-mediated transient expression assays to investigate ABA-regulated processes or pathogenicity and plant defence mechanisms.

Identification of an Arabidopsis Nodulin-related protein in heat stress

We identified a Nodulin-related protein 1 (NRP1) encoded by At2g03440, which was previously reported to be RPS2 interacting protein in yeast-two-hybrid assay. Northern blotting showed that AtNRP1 expression was suppressed by heat stress (42 degrees C) and induced by low temperature (4 degrees C) treatment. Strong GUS staining was observed in the sites of meristematic tissues of pAtNRP1:: GUS transgenic plants, such as shoot apex and root tips, young leaf veins, stamens and stigmas of flowers, and abscission layers of young siliques. To study AtNRP1 biological functions, we have characterized both loss-of-function T-DNA insertion and transgenic overexpression plants for AtNRP1 in Arabidopsis. The T-DNA insertion mutants displayed no obvious difference as compared to wild-type Arabidopsis under heat stress, but the significant enhanced susceptibility to heat stress was revealed in two independent AtNRP1-overexpressing transgenic lines. Further study found that the decreased thermotolerance in AtNRP1-overexpressing lines accompanied significantly decreased accumulation of ABA after heat treatment, which was probably due to AtNRP1 playing a role in negative-feedback regulation of the ABA synthesis pathway. These results support the viewpoint that the application of ABA inhibits nodulation and nodulin-related gene expression and threaten adverse ambient temperature can impact the nodulin-related gene expression.

Advances in experimental methods for the elucidation of Pseudomonas syringae effector function with a focus on AvrPtoB

Pseudomonas syringae infects a wide range of plant species through the use of a type III secretion system. The effector proteins injected into the plant cell through this molecular syringe serve as promoters of disease by subverting the plant immune response to the benefit of the bacteria in the intercellular space. The targets and activities of a subset of effectors have been elucidated recently. In this article, we focus on the experimental approaches that have proved most successful in probing the molecular basis of effectors, ranging from loss-of-function to gain-of-function analyses utilizing several techniques for effector delivery into plants. In particular, we highlight how these diverse approaches have been applied to the study of one effector--AvrPtoB--a multifunctional protein with the ability to suppress both effector-triggered immunity and pathogen (or microbe)-associated molecular pattern-triggered immunity. Taken together, advances in this field illustrate the need for multiple experimental approaches when elucidating the function of a single effector.

From bacterial avirulence genes to effector functions via the hrp delivery system: an overview of 25 years of progress in our understanding of plant innate immunity

Cloning the first avirulence (avr) gene has led not only to a deeper understanding of gene-for-gene interactions in plant disease, but also to fundamental insights into the suppression of basal defences against microbial attack. This article (focusing on Pseudomonas syringae) charts the development of ideas and research progress over the 25 years following the breakthrough achieved by Staskawicz and coworkers. Advances in gene cloning technology underpinned the identification of both avr and hrp genes, the latter being required for the activation of the defensive hypersensitive reaction (HR) and pathogenicity. The delivery of Avr proteins through the type III secretion machinery encoded by hrp gene clusters was demonstrated, and the activity of the proteins inside plant cells as elicitors of the HR was confirmed. Key roles for avr genes in pathogenic fitness have now been established. The rebranding of Avr proteins as effectors, proteins that suppress the HR and cell wall-based defences, has led to the ongoing search for their targets, and is generating new insights into the co-ordination of plant resistance against diverse microbes. Bioinformatics-led analysis of effector gene distribution in genomes has provided a remarkable view of the interchange of effectors and also their functional domains, as the arms race of attack and defence drives the evolution of microbial pathogenicity. The application of our accrued knowledge for the development of disease control strategies is considered.

Fungal endophytes-induced abscisic acid is required for flavonoid accumulation in suspension cells of Ginkgo biloba

Treatment of suspension cells of Ginkgo biloba with fungal endophytes resulted in accumulation of flavonoids, increased abscisic acid (ABA) production and activation of phenylalanine ammonia-lyase (PAL). Fluridone, an inhibitor of ABA biosynthesis, was effective in inhibiting fungal endophytes-induced ABA biosynthesis, increase of PAL activity and flavonoids accumulation. Moreover, exogenous application of ABA enhanced PAL activity and increased accumulation of flavonoids in G. biloba cells with or without fungal endophytes elicitor. These finding suggest a causal relationship between ABA release and both PAL activity and flavonoid accumulation under fungal endophytes treatment and that ABA is involved in fungal endophytes-induced flavonoids accumulation in this plant.

Gene expression profiling of ozone-treated Arabidopsis abi1td insertional mutant: protein phosphatase 2C ABI1 modulates biosynthesis ratio of ABA and ethylene

We report on the characterization of the interaction between reactive oxygen species signalling and abscisic acid (ABA)-mediated gene network in ozone (O(3)) stress response. To identify the stress-related signalling pathways and possible cross-talk controlled by an ABA-negative regulator, the protein phosphatase 2C abscisic acid insensitive1 (ABI1), we performed a genome-wide transcription profiling of O(3)-treated wild-type and ABI1 knockout (abi1td) plants. In addition, to better understand ABA signalling and the interactions between stress response pathways, we performed a microarray analysis of drought-treated plants. Functional categorization of the identified genes showed that ABI1 is involved in the modulation of several cellular processes including metabolism, transport, development, information pathways and variant splicing. Comparisons with available transcriptome data sets revealed the extent of ABI1 involvement in both ABA-dependent and ABA-independent gene expression. Furthermore, in O(3) stress the ABA hypersensitivity of abi1td resulted in a significant reduction of the ABA level, ethylene (ET) over-production and O(3) tolerance. Moreover, the physical interaction of ABI1 with ACC synthase2 and ACC synthase6 was shown. We provide a model explaining how ABI1 can regulate both ABA and ET biosynthesis. Altogether, our findings indicate that ABI1 plays the role of a general signal transducer linking ABA and ET biosynthesis as well as signalling pathways to O(3) stress tolerance.

The Arabidopsis ATAF1, a NAC transcription factor, is a negative regulator of defense responses against necrotrophic fungal and bacterial pathogens

Transcription factors of the NAC family are known to be involved in various growth or developmental processes and in regulation of response to environmental stresses. In the present study, we report that Arabidopsis ATAF1 is a negative regulator of defense responses against both necrotrophic fungal and bacterial pathogens. Expression of ATAF1 was downregulated after infection with Botrytis cinerea or Pseudomonas syringae pv. tomato or after treatment with salicylic acid (SA), jasmonic acid, and 1-amino cyclopropane-1-carboxylic acid (the precursor of ethylene biosynthesis). Transgenic plants that overexpress the ATAF1 gene (ATAF1-OE) showed increased susceptibility while expression of an ATAF1 chimeric repressor construct (ATAF1-SRDX) exhibited enhanced resistance to P. syringae pv. tomato DC3000, B. cinerea, and Alternaria brassicicola. The ataf1 mutant plants showed no significant resistance against the pathogens tested. After inoculation with B. cinerea or P. syringae pv. tomato DC3000, expressions of defense-related genes PR-1, PR-5. and PDF1.2 were upregulated in the ATAF1-SRDX plants but attenuated or unchanged in the ATAF1-OE plants. In ATAF1-OE plants, SA-induced expression of pathogenesis-related genes and disease resistance against P. syringae pv. tomato DC3000 was partially suppressed. Increased levels of reactive oxygen species (i.e., H(2)O(2) and superoxide anion) accumulated only in the ATAF1-OE but not in the ATAF1-SRDX plants after Botrytis spp. infection. Our studies provide direct genetic evidence for the role of ATAF1 as a negative regulator of defense response against different type of pathogens.

Disease-specific expression of host genes during downy mildew infection of Arabidopsis

Here, we report on the identification of Arabidopsis genes that are induced during compatible but not during incompatible interactions with the downy mildew pathogen Hyaloperonospora arabidopsidis. This set of so-called compatible specific (CS) genes contrasts with the large group of defense-associated genes that is differentially expressed during both compatible and incompatible interactions. From the 17 identified CS genes, 6 belong to the ethylene response factor (ERF) family of transcription factor genes, suggesting that these ERF have a role during compatibility. The majority of CS genes are differentially regulated in response to various forms of abiotic stress. In silico analysis of the CS genes revealed an over-representation of dehydration-responsive element/C-repeat binding factor (DREB1A/CBF3) binding sites and EveningElement motifs in their promoter regions. The CS-ERF are closely related to the CBF transcription factors and could potentially bind the DREB1A/CBF3 promoter elements in the CS genes. Transcript levels of CS genes peak at 2 to 3 days postinoculation, when pathogen growth is highest, and decline at later stages of infection. The induction of several CS genes was found to be isolate specific. This suggests that the identified CS genes could be the direct or indirect targets of downy mildew effector proteins that promote disease susceptibility.

Episodic Abiotic Stress as a Potential Contributing Factor to Onset and Severity of Disease Caused by Phytophthora ramorum in Rhododendron and Viburnum

Phytophthora ramorum attacks many forest and nursery species, primarily causing trunk or stem cankers, foliar blight, and dieback, and in some species root infection has been demonstrated. However, the abiotic and edaphic factors that influence infection and disease development are unresolved. Root infection by P. ramorum and the potential for mild abiotic stress in disease predisposition was examined with Rhododendron hybrid Cunningham's White and Viburnum tinus cv. Spring Bouquet. To impose water stress in a uniform and synchronous manner, osmotic stress induced with 0.2 M NaCl was selected. Roots were exposed to NaCl for 16 to 24 h in modified hydroponic culture or standard potting soil, removed from the NaCl, and then inoculated with zoospores. In the hydroponic regime, disease symptoms developed in Rhododendron and V. tinus plants within 1 week after inoculation of salt-stressed roots, whereas symptom development was delayed in nonstressed, inoculated plants. Microscopic examination of roots from both species revealed that their apices were covered with sporangia of P. ramorum. On potted Rhododendron plants inoculated by applying zoospores directly to the soil, stem lesions developed rapidly in salt-stressed plants, with death of the plant occurring within 4 weeks after inoculation. Nonstressed plants survived for 6 to 8 weeks before succumbing to disease, and symptom development in these plants was delayed by 1 to 2 weeks relative to the inoculated, salt-stressed plants. A postinfection episode of salt stress to inoculated roots in the hydroponic regime resulted in significantly faster development of stem lesions in Rhododendron relative to nonstressed, inoculated plants.

R/BHC: fast Bayesian hierarchical clustering for microarray data

Background

Although the use of clustering methods has rapidly become one of the standard computational approaches in the literature of microarray gene expression data analysis, little attention has been paid to uncertainty in the results obtained.

Results

We present an R/Bioconductor port of a fast novel algorithm for Bayesian agglomerative hierarchical clustering and demonstrate its use in clustering gene expression microarray data. The method performs bottom-up hierarchical clustering, using a Dirichlet Process (infinite mixture) to model uncertainty in the data and Bayesian model selection to decide at each step which clusters to merge.

Conclusion

Biologically plausible results are presented from a well studied data set: expression profiles of A. thaliana subjected to a variety of biotic and abiotic stresses. Our method avoids several limitations of traditional methods, for example how many clusters there should be and how to choose a principled distance metric.

Manipulation of plant innate immunity and gibberellin as factor of compatibility in the mutualistic association of barley roots with Piriformospora indica

Fungi of the order Sebacinales (Basidiomycota) are involved in a wide spectrum of mutualistic symbioses with various plants, thereby exhibiting unique potential for biocontrol strategies. Piriformospora indica, a model organism of this fungal order, is able to increase the biomass and grain yield of crop plants, and induces local and systemic resistance to fungal diseases and tolerance to abiotic stress. To elucidate the molecular basis for root colonization, we characterized the interaction of P. indica with barley roots by combining global gene expression profiling, metabolic profiling, and genetic studies. At the metabolic level, we show that fungal colonization reduces the availability of free sugars and amino acids to the root tip. At the transcriptional level, consecutive interaction stages covering pre-penetration-associated events and progressing through to root colonization showed differential regulation of signal perception and transduction components, secondary metabolism, and genes associated with membrane transport. Moreover, we observed stage-specific up-regulation of genes involved in phytohormone metabolism, mainly encompassing gibberellin, auxin and abscisic acid, but salicylic acid-associated gene expression was suppressed. The changes in hormone homoeostasis were accompanied with a general suppression of the plant innate immune system. Further genetic studies showed reduced fungal colonization in mutants that are impaired in gibberellin synthesis as well as perception, and implicate gibberellin as a modulator of the root's basal defence. Our data further reveal the complexity of compatibility mechanisms in host-microbe interactions, and identify gibberellin signaling as potential target for successful fungi.

Antagonism between salicylic and abscisic acid reflects early host-pathogen conflict and moulds plant defence responses

The importance of phytohormone balance is increasingly recognized as central to the outcome of plant-pathogen interactions. Recently it has been demonstrated that abscisic acid signalling pathways are utilized by the bacterial phytopathogen Pseudomonas syringae to promote pathogenesis. In this study, we examined the dynamics, inter-relationship and impact of three key acidic phytohormones, salicylic acid, abscisic acid and jasmonic acid, and the bacterial virulence factor, coronatine, during progression of P. syringae infection of Arabidopsis thaliana. We show that levels of SA and ABA, but not JA, appear to play important early roles in determining the outcome of the infection process. SA is required in order to mount a full innate immune responses, while bacterial effectors act rapidly to activate ABA biosynthesis. ABA suppresses inducible innate immune responses by down-regulating SA biosynthesis and SA-mediated defences. Mutant analyses indicated that endogenous ABA levels represent an important reservoir that is necessary for effector suppression of plant-inducible innate defence responses and SA synthesis prior to subsequent pathogen-induced increases in ABA. Enhanced susceptibility due to loss of SA-mediated basal resistance is epistatically dominant over acquired resistance due to ABA deficiency, although ABA also contributes to symptom development. We conclude that pathogen-modulated ABA signalling rapidly antagonizes SA-mediated defences. We predict that hormonal perturbations, either induced or as a result of environmental stress, have a marked impact on pathological outcomes, and we provide a mechanistic basis for understanding priming events in plant defence.

Abscisic acid has a key role in modulating diverse plant-pathogen interactions

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.

Cre-lox univector acceptor vectors for functional screening in protoplasts: analysis of Arabidopsis donor cDNAs encoding ABSCISIC ACID INSENSITIVE1-like protein phosphatases

The 14,200 available full length Arabidopsis thaliana cDNAs in the universal plasmid system (UPS) donor vector pUNI51 should be applied broadly and efficiently to leverage a "functional map-space" of homologous plant genes. We have engineered Cre-lox UPS host acceptor vectors (pCR701- 705) with N-terminal epitope tags in frame with the loxH site and downstream from the maize Ubiquitin promoter for use in transient protoplast expression assays and particle bombardment transformation of monocots. As an example of the utility of these vectors, we recombined them with several Arabidopsis cDNAs encoding Ser/Thr protein phosphatase type 2C (PP2Cs) known from genetic studies or predicted by hierarchical clustering meta-analysis to be involved in ABA and stress responses. Our functional results in Zea mays mesophyll protoplasts on ABA-inducible expression effects on the Late Embryogenesis Abundant promoter ProEm:GUS reporter were consistent with predictions and resulted in identification of novel activities of some PP2Cs. Deployment of these vectors can facilitate functional genomics and proteomics and identification of novel gene activities.

Abscisic acid negatively regulates elicitor-induced synthesis of capsidiol in wild tobacco

In the Solanaceae, biotic and abiotic elicitors induce de novo synthesis of sesquiterpenoid stress metabolites known as phytoalexins. Because plant hormones play critical roles in the induction of defense-responsive genes, we have explored the effect of abscisic acid (ABA) on the synthesis of capsidiol, the major wild tobacco (Nicotiana plumbaginifolia) sesquiterpenoid phytoalexin, using wild-type plants versus nonallelic mutants Npaba2 and Npaba1 that are deficient in ABA synthesis. Npaba2 and Npaba1 mutants exhibited a 2-fold higher synthesis of capsidiol than wild-type plants when elicited with either cellulase or arachidonic acid or when infected by Botrytis cinerea. The same trend was observed for the expression of the capsidiol biosynthetic genes 5-epi-aristolochene synthase and 5-epi-aristolochene hydroxylase. Treatment of wild-type plants with fluridone, an inhibitor of the upstream ABA pathway, recapitulated the behavior of Npaba2 and Npaba1 mutants, while the application of exogenous ABA reversed the enhanced synthesis of capsidiol in Npaba2 and Npaba1 mutants. Concomitant with the production of capsidiol, we observed the induction of ABA 8'-hydroxylase in elicited plants. In wild-type plants, the induction of ABA 8'-hydroxylase coincided with a decrease in ABA content and with the accumulation of ABA catabolic products such as phaseic acid and dihydrophaseic acid, suggesting a negative regulation exerted by ABA on capsidiol synthesis. Collectively, our data indicate that ABA is not required per se for the induction of capsidiol synthesis but is essentially implicated in a stress-response checkpoint to fine-tune the amplification of capsidiol synthesis in challenged plants.

The high light response in Arabidopsis involves ABA signaling between vascular and bundle sheath cells

Previously, it has been shown that Arabidopsis thaliana leaves exposed to high light accumulate hydrogen peroxide (H2O2) in bundle sheath cell (BSC) chloroplasts as part of a retrograde signaling network that induces ASCORBATE PEROXIDASE2 (APX2). Abscisic acid (ABA) signaling has been postulated to be involved in this network. To investigate the proposed role of ABA, a combination of physiological, pharmacological, bioinformatic, and molecular genetic approaches was used. ABA biosynthesis is initiated in vascular parenchyma and activates a signaling network in neighboring BSCs. This signaling network includes the Galpha subunit of the heterotrimeric G protein complex, the OPEN STOMATA1 protein kinase, and extracellular H2O2, which together coordinate with a redox-retrograde signal from BSC chloroplasts to activate APX2 expression. High light-responsive genes expressed in other leaf tissues are subject to a coordination of chloroplast retrograde signaling and transcellular signaling activated by ABA synthesized in vascular cells. ABA is necessary for the successful adjustment of the leaf to repeated episodes of high light. This process involves maintenance of photochemical quenching, which is required for dissipation of excess excitation energy.

Phytohormones in plant root-Piriformospora indica mutualism

Piriformospora indica is a mutualistic root-colonising basidiomycete that tranfers various benefits to colonized host plants including growth promotion, yield increases as well as abiotic and biotic stress tolerance. The fungus is characterized by a broad host spectrum encompassing various monocots and dicots. Our recent microarray-based studies indicate a general plant defense suppression by P. indica and significant changes in the GA biosynthesis pathway. Furthermore, barley plants impaired in GA synthesis and perception showed a significant reduction in mutualistic colonization, which was associated with an elevated expression of defense-related genes. Here, we discuss the importance of plant hormones for compatibility in plant root-P. indica associations. Our data might provide a first explanation for the colonization success of the fungus in a wide range of higher plants.

The targeting of plant cellular systems by injected type III effector proteins

The battle between phytopathogenic bacteria and their plant hosts has revealed a diverse suite of strategies and mechanisms employed by the pathogen or the host to gain the higher ground. Pathogens continually evolve tactics to acquire host resources and dampen host defences. Hosts must evolve surveillance and defence systems that are sensitive enough to rapidly respond to a diverse range of pathogens, while reducing costly and damaging inappropriate misexpression. The primary virulence mechanism employed by many bacteria is the type III secretion system, which secretes and translocates effector proteins directly into the cells of their plant hosts. Effectors have diverse enzymatic functions and can target specific components of plant systems. While these effectors should favour bacterial fitness, the host may be able to thwart infection by recognizing the activity or presence of these foreign molecules and initiating retaliatory immune measures. We review the diverse host cellular systems exploited by bacterial effectors, with particular focus on plant proteins directly targeted by effectors. Effector-host interactions reveal different stages of the battle between pathogen and host, as well as the diverse molecular strategies employed by bacterial pathogens to hijack eukaryotic cellular systems.

Fusarium graminearum exploits ethylene signalling to colonize dicotyledonous and monocotyledonous plants

Ethylene signalling affects the resistance of dicotyledonous plant species to diverse pathogens but almost nothing is known about the role of this pathway in monocotyledonous crop species. Fusarium graminearum causes Fusarium head blight (FHB) of cereals, contaminating grain with mycotoxins such as deoxynivalenol (DON). Very little is known about the mechanisms of resistance/susceptibility to this disease. Genetic and chemical genetic studies were used to examine the influence of ethylene (ET) signalling and perception on infection of dicotyledonous (Arabidopsis) and monocotyledonous (wheat and barley) species by F. graminearum. Arabidopsis mutants with reduced ET signalling or perception were more resistant to F. graminearum than wild-type, while mutants with enhanced ET production were more susceptible. These findings were confirmed by chemical genetic studies of Arabidopsis, wheat and barley. Attenuation of expression of EIN2 in wheat, a gene encoding a core component of ethylene signalling, reduced both disease symptoms and DON contamination of grain. Fusarium graminearum appears to exploit ethylene signalling in both monocotyledonous and dicotyledonous species. This demonstration of translation from model to crop species provides a foundation for improving resistance of cereal crops to FHB through identification of allelic variation for components of the ethylene-signalling pathway.

The multifaceted role of ABA in disease resistance

Long known only for its role in abiotic stress tolerance, recent evidence shows that abscisic acid (ABA) also has a prominent role in biotic stress. Although it acts as a negative regulator of disease resistance, ABA can also promote plant defense and is involved in a complicated network of synergistic and antagonistic interactions. Its role in disease resistance depends on the type of pathogen, its specific way of entering the host and, hence, the timing of the defense response and the type of affected plant tissue. Here, we discuss the controversial evidence pointing to either a repression or a promotion of resistance by ABA. Furthermore, we propose a model in which both possibilities are integrated.

Hormone (dis)harmony moulds plant health and disease

Diseased plants often display phenotypes consistent with hormone perturbations. We review recent data that have revealed roles in plant-microbe interactions for cellular components and signaling molecules that previously were associated only with hormone signaling. A better understanding of cross-talk between hormonal and defense signaling pathways should reveal new potential targets for microbial effectors that attenuate host resistance mechanisms.

Fitness and beyond: preparing for the arrival of GM crops with ecologically important novel characters

The seemingly inexorable expansion of global human population size, significant increases in the use of biofuel crops and the growing pressures of multifunctional land-use have intensified the need to improve crop productivity. The widespread cultivation of high-yielding genetically modified (GM) crops could help to address these problems, although in doing so, steps must also be taken to ensure that any gene flow from these crops to wild or weedy recipients does not cause significant ecological harm. It is partly for this reason that new GM cultivars are invariably subjected to strict regulatory evaluation in order to assess the risks that each may pose to the environment. Regulatory bodies vary in their approach to decision-making, although all require access to large quantities of detailed information. Such an exhaustive case-by-case approach has been made tractable by the comparative simplicity of the portfolio of GM crops currently on the market, with four crops and two classes of traits accounting for almost all of the area under cultivation of GM crops. This simplified situation will change shortly, and will seriously complicate and potentially slow the evaluation process. Nowhere will the increased diversity of GM crops cause more difficulty to regulators than in those cases where there is a need to assess whether the transgene(s) will enhance fitness in a non-transgenic relative and thereafter cause ecological harm. Current practice to test this risk hypothesis focuses on attempting to detect increased fitness in the recipient. In this paper we explore the merits and shortcomings of this strategy, and investigate the scope for developing new approaches to streamline decision-making processes for transgenes that could cause unwanted ecological change.

Networking by small-molecule hormones in plant immunity

Plants live in complex environments in which they intimately interact with a broad range of microbial pathogens with different lifestyles and infection strategies. The evolutionary arms race between plants and their attackers provided plants with a highly sophisticated defense system that, like the animal innate immune system, recognizes pathogen molecules and responds by activating specific defenses that are directed against the invader. Recent advances in plant immunity research have provided exciting new insights into the underlying defense signaling network. Diverse small-molecule hormones play pivotal roles in the regulation of this network. Their signaling pathways cross-communicate in an antagonistic or synergistic manner, providing the plant with a powerful capacity to finely regulate its immune response. Pathogens, on the other hand, can manipulate the plant's defense signaling network for their own benefit by affecting phytohormone homeostasis to antagonize the host immune response.

Drought tolerance in Arabidopsis is controlled by the OCP3 disease resistance regulator

Water scarcity and corresponding abiotic drought stress is one of the most important factors limiting plant performance and yield. In addition, plant productivity is severely compromised worldwide by infection with microbial pathogens. Two of the most prominent pathways responsible for drought tolerance and disease resistance to fungal pathogens in Arabidopsis are those controlled by the phytohormones abscisic acid (ABA) and the oxylipin methyl jasmonate (MeJA), respectively. Here, we report on the functional characterization of OCP3, a transcriptional regulator from the homeodomain (HD) family. The Arabidopsis loss-of-function ocp3 mutant exhibits both drought resistance and enhanced disease resistance to necrotrophic fungal pathogens. Double-mutant analysis revealed that these two resistance phenotypes have different genetic requirements. Whereas drought tolerance in ocp3 is ABA-dependent but MeJA-independent, the opposite holds true for the enhanced disease resistance characteristics. These observations lead us to propose a regulatory role of OCP3 in the adaptive responses to these two stresses, functioning as a modulator of independent and specific aspects of the ABA- and MeJA-mediated signal transduction pathways.

The short-rooted phenotype of the brevis radix mutant partly reflects root abscisic acid hypersensitivity

To gain further insight into abscisic acid (ABA) signaling and its role in growth regulation, we have screened for Arabidopsis (Arabidopsis thaliana) mutants hypersensitive to ABA-mediated root growth inhibition. As a result, we have identified a loss-of-function allele of BREVIS RADIX (BRX) in the Columbia background, named brx-2, which shows enhanced response to ABA-mediated inhibition of root growth. BRX encodes a key regulator of cell proliferation and elongation in the root, which has been implicated in the brassinosteroid (BR) pathway as well as in the regulation of auxin-responsive gene expression. Mutants affected in BR signaling that are not impaired in root growth, such as bes1-D, bzr1-D, and bsu1-D, also showed enhanced sensitivity to ABA-mediated inhibition of root growth. Triple loss-of-function mutants affected in PP2Cs, which act as negative regulators of ABA signaling, showed impaired root growth in the absence of exogenous ABA, indicating that disturbed regulation of ABA sensitivity impairs root growth. In agreement with this result, diminishing ABA sensitivity of brx-2 by crossing it with a 35S:HAB1 ABA-insensitive line allowed significantly higher recovery of root growth after brassinolide treatment. Finally, transcriptomic analysis revealed that ABA treatment negatively affects auxin signaling in wild-type and brx-2 roots and that ABA response is globally altered in brx-2. Taken together, our results reveal an interaction between BRs, auxin, and ABA in the control of root growth and indicate that altered sensitivity to ABA is partly responsible for the brx short-root phenotype.

Heterotrimeric G proteins-mediated resistance to necrotrophic pathogens includes mechanisms independent of salicylic acid-, jasmonic acid/ethylene- and abscisic acid-mediated defense signaling

Heterotrimeric G proteins are involved in the defense response against necrotrophic fungi in Arabidopsis. In order to elucidate the resistance mechanisms involving heterotrimeric G proteins, we analyzed the effects of the Gβ (subunit deficiency in the mutant agb1-2 on pathogenesis-related gene expression, as well as the genetic interaction between agb1-2 and a number of mutants of established defense pathways. Gβ-mediated signaling suppresses the induction of salicylic acid (SA)-, jasmonic acid (JA)-, ethylene (ET)- and abscisic acid (ABA)-dependent genes during the initial phase of the infection with Fusarium oxysporum (up to 48 h after inoculation). However, at a later phase it enhances JA/ET-dependent genes such as PDF1.2 and PR4. Quantification of the Fusarium wilt symptoms revealed that Gβ- and SA-deficient mutants were more susceptible than wild-type plants, whereas JA- and ET-insensitive and ABA-deficient mutants demonstrated various levels of resistance. Analysis of the double mutants showed that the Gβ-mediated resistance to F. oxysporum and Alternaria brassicicola was mostly independent of all of the previously mentioned pathways. However, the progressive decay of agb1-2 mutants was compensated by coi1-21 and jin1-9 mutations, suggesting that at this stage of F. oxysporum infection Gβ acts upstream of COI1 and ATMYC2 in JA signaling.

Ethylene modulates the role of NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 in cross talk between salicylate and jasmonate signaling

The plant hormones salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) play crucial roles in the signaling network that regulates induced defense responses against biotic stresses. Antagonism between SA and JA operates as a mechanism to fine-tune defenses that are activated in response to multiple attackers. In Arabidopsis (Arabidopsis thaliana), NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) was demonstrated to be required for SA-mediated suppression of JA-dependent defenses. Because ET is known to enhance SA/NPR1-dependent defense responses, we investigated the role of ET in the SA-JA signal interaction. Pharmacological experiments with gaseous ET and the ET precursor 1-aminocyclopropane-1-carboxylic acid showed that ET potentiated SA/NPR1-dependent PATHOGENESIS-RELATED1 transcription, while it rendered the antagonistic effect of SA on methyl jasmonate-induced PDF1.2 and VSP2 expression NPR1 independent. This overriding effect of ET on NPR1 function in SA-JA cross talk was absent in the npr1-1/ein2-1 double mutant, demonstrating that it is mediated via ET signaling. Abiotic and biotic induction of the ET response similarly abolished the NPR1 dependency of the SA-JA signal interaction. Furthermore, JA-dependent resistance against biotic attackers was antagonized by SA in an NPR1-dependent fashion only when the plant-attacker combination did not result in the production of high levels of endogenous ET. Hence, the interaction between ET and NPR1 plays an important modulating role in the fine tuning of the defense signaling network that is activated upon pathogen and insect attack. Our results suggest a model in which ET modulates the NPR1 dependency of SA-JA antagonism, possibly to compensate for enhanced allocation of NPR1 to function in SA-dependent activation of PR genes.

Use of a synthetic salicylic acid analog to investigate the roles of methyl salicylate and its esterases in plant disease resistance

We previously demonstrated that salicylic acid-binding protein 2 (SABP2) of tobacco is an integral component of systemic acquired resistance (SAR). SABP2 is a methyl salicylate (MeSA) esterase that has high affinity for SA, which feedback inhibits its esterase activity. MeSA esterase activity is required in distal, healthy tissue of pathogen-infected plants to hydrolyze MeSA, which functions as a long-distance, phloem-mobile SAR signal; this hydrolysis releases the biologically active defense hormone SA. In this study, we examined the inhibitory interaction of SA with SABP2, and identified a synthetic SA analog, 2,2,2,2'-tetra-f luoroacetophenone (tetraFA) that, like SA, competitively inhibits the activity of SABP2 and targets esterases, which utilize MeSA as a substrate. However, in contrast to SA, tetraFA does not induce downstream defense responses and, therefore, is effective in planta at blocking SAR development in tobacco mosaic virus (TMV)-infected tobacco and Pseudomonas syringae-infected Arabidopsis. These results confirm the importance of SABP2 and MeSA for SAR development in tobacco and establish similar roles for MeSA and the orthologs of SABP2 in Arabidopsis. Moreover, they demonstrate that tetraFA can be used to determine whether MeSA and its corresponding esterase(s) play a role in SAR signaling in other plant species. In planta analyses using tetraFA, in conjunction with leaf detachment assays and MeSA quantification, were used to assess the kinetics with which MeSA is generated in pathogen-infected leaves, transmitted through the phloem, and processed in the distal healthy leaves. In TMV-infected tobacco, these studies revealed that critical amounts of MeSA are generated, transmitted, and processed between 48 and 72 h post primary infection.

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

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.

Role of plant hormones in plant defence responses

Plant hormones play important roles in regulating developmental processes and signaling networks involved in plant responses to a wide range of biotic and abiotic stresses. Significant progress has been made in identifying the key components and understanding the role of salicylic acid (SA), jasmonates (JA) and ethylene (ET) in plant responses to biotic stresses. Recent studies indicate that other hormones such as abscisic acid (ABA), auxin, gibberellic acid (GA), cytokinin (CK), brassinosteroids (BR) and peptide hormones are also implicated in plant defence signaling pathways but their role in plant defence is less well studied. Here, we review recent advances made in understanding the role of these hormones in modulating plant defence responses against various diseases and pests.

The 7B-1 mutation in tomato (Solanum lycopersicum L.) confers a blue light-specific lower sensitivity to coronatine, a toxin produced by Pseudomonas syringae pv. tomato

The spontaneous mutant 7B-1 in tomato (Solanum lycopersicum=Lycopersicon esculentum) is a photoperiod-dependent male-sterile mutant previously reported as resistant to various abiotic stresses specifically under blue light. Since this finding improved the potential of 7B-1's use in breeding programmes, its susceptibility to stress induced by coronatine (COR), the phytotoxine produced by several Pseudomonas syringae strains, was assessed in this study. The 7B-1 mutant was found to be less sensitive than the corresponding wild type (WT) to COR treatment in a blue light-dependent manner. Treatment of WT and 7B-1 plants with COR induced a strong accumulation of salicylic acid (SA), jasmonic acid (JA), and abscisic acid (ABA) in hypocotyls. Interestingly, accumulation of ABA and SA in the 7B-1 mutant was distinctly greater than in WT, especially in blue light. Based on the cross-talk between SA- and JA-signalling pathways, expression analysis of NPR1 and COI1 genes, respectively involved in these pathways, was investigated in COR-stressed plants. The blue light-specific lower sensitivity of 7B-1 plants to COR was found to be associated with blue light-specific overexpression of the NPR1 gene. These data suggest that the SA-dependent NPR1-dependent pathway could be involved in the lower sensitivity of the 7B-1 mutant to COR. The role of anthocyanins and ABA accumulation during the response to COR is also discussed in the present study.

High humidity induces abscisic acid 8'-hydroxylase in stomata and vasculature to regulate local and systemic abscisic acid responses in Arabidopsis

Levels of endogenous abscisic acid (ABA) are changed dynamically in response to environmental conditions. The ABA 8'-hydroxylase is a key enzyme in ABA catabolism and is encoded by CYP707A genes. In this study, we examined physiological roles of Arabidopsis (Arabidopsis thaliana) CYP707As in the plant's response to changes in humidity. The cyp707a1 and cyp707a3 mutants displayed lower stomatal conductance under turgid conditions (relative humidity 60%) than the wild type. When wild-type plants were transferred to high-humidity conditions (relative humidity 90%), CYP707A1 and CYP707A3 transcript levels increased, followed by the reduction of ABA levels. The cyp707a3 mutant exhibited high ABA levels even after transferring to high-humidity conditions, whereas, under similar conditions, the cyp707a1 mutant exhibited low ABA levels comparable to the wild type. Analysis of spatial expression patterns by using transgenic plants harboring a promoterbeta-glucuronidase gene indicated that high-humidity-induced expression of CYP707A1 and CYP707A3 occurred primarily in guard cells and vascular tissues, respectively. Furthermore, stomatal closure of the cyp707a1 mutant, but not cyp707a3 mutant, was ABA hypersensitive when epidermal peel was treated with exogenous ABA, suggesting that CYP707A1 is essential for ABA catabolism inside the guard cells. These results implicate that CYP707A3 reduces the amount of mobile ABA in vascular tissues in response to high humidity, whereas CYP707A1 inactivates local ABA pools inside the guard cells. Taken together, ABA catabolism in both vascular tissues and guard cells participates in the systemic ABA action that controls stomatal movement in response to high humidity.

Pseudomonas syringae type III secretion system effectors: repertoires in search of functions

The ability of Pseudomonas syringae to grow and cause diseases in plants is dependent on the injection of multiple effector proteins into plant cells via the type III secretion system (T3SS). Genome-enabled bioinformatic/experimental methods have comprehensively identified the repertoires of effectors and related T3SS substrates for P. syringae pv. tomato DC3000 and three other sequenced strains. The effector repertoires are diverse and internally redundant. Insights into effector functions are being gained through the construction of mutants lacking one or more effector genes, which may be reduced in growth in planta, and through gain-of-function assays for the ability of single effectors to suppress plant innate immune defenses, manipulate hormone signaling, elicit cell death, and/or display biochemical activities on plant protein targets.

Response of sweet orange (Citrus sinensis) to 'Candidatus Liberibacter asiaticus' infection: microscopy and microarray analyses

Citrus greening or huanglongbing (HLB) is a devastating disease of citrus. HLB is associated with the phloem-limited fastidious prokaryotic alpha-proteobacterium 'Candidatus Liberibacter spp.' In this report, we used sweet orange (Citrus sinensis) leaf tissue infected with 'Ca. Liberibacter asiaticus' and compared this with healthy controls. Investigation of the host response was examined with citrus microarray hybridization based on 33,879 expressed sequence tag sequences from several citrus species and hybrids. The microarray analysis indicated that HLB infection significantly affected expression of 624 genes whose encoded proteins were categorized according to function. The categories included genes associated with sugar metabolism, plant defense, phytohormone, and cell wall metabolism, as well as 14 other gene categories. The anatomical analyses indicated that HLB bacterium infection caused phloem disruption, sucrose accumulation, and plugged sieve pores. The up-regulation of three key starch biosynthetic genes including ADP-glucose pyrophosphorylase, starch synthase, granule-bound starch synthase and starch debranching enzyme likely contributed to accumulation of starch in HLB-affected leaves. The HLB-associated phloem blockage resulted from the plugged sieve pores rather than the HLB bacterial aggregates since 'Ca. Liberibacter asiaticus' does not form aggregate in citrus. The up-regulation of pp2 gene is related to callose deposition to plug the sieve pores in HLB-affected plants.

Fatty Acid-derived signals in plant defense

Fatty acids (FAs) consist of long hydrophobic, often unbranched chains of hydrocarbons, with hydrophilic carboxylic acid groups at one end. They are an important source of reserve energy and essential components of membrane lipids in all living organisms. In plants, FA metabolic pathways play significant roles in pathogen defense. Historically, FAs were only assigned passive roles in plant defense such as biosynthetic precursors for cuticular components or the phytohormone jasmonic acid. However, recent discoveries demonstrate more direct roles for FAs and their breakdown products in inducing various modes of plant defenses. Both 16- and 18-carbon FAs participate in defense to modulate basal, effector-triggered, and systemic immunity in plants. Studies of FA metabolic mutants also reveal an active signaling role for the cuticle in plant defense. This review summarizes the current knowledge of the involvement of FAs, FA-derived oxylipins, and enzymes catalyzing FA metabolism in plant defense.

Functional roles of the pepper antimicrobial protein gene, CaAMP1, in abscisic acid signaling, and salt and drought tolerance in Arabidopsis

Biotic signaling molecules including abscisic acid (ABA) are involved in signal transduction pathways that mediate the defense response of plants to environmental stresses. The antimicrobial protein gene CaAMP1, previously isolated from pepper (Capsicum annuum), was strongly induced in pepper leaves exposed to ABA, NaCl, drought, or low temperature. Because transformation is very difficult in pepper, we overexpressed CaAMP1 in Arabidopsis. CaAMP1-overexpressing (OX) transgenic plants exhibited reduced sensitivity to ABA during the seed germination and seedling stages. Overexpression of CaAMP1 conferred enhanced tolerance to high salinity and drought, accompanied by altered expression of the AtRD29A gene, which is correlated with ABA levels and environmental stresses. The transgenic plants were also highly tolerant to osmotic stress caused by high concentrations of mannitol. Together, these results suggest that overexpression of the CaAMP1 transgene modulates salt and drought tolerance in Arabidopsis through ABA-mediated cell signaling.

The Arabidopsis A4 subfamily of lectin receptor kinases negatively regulates abscisic acid response in seed germination

Abscisic acid (ABA) is an important plant hormone for a wide array of growth and developmental processes and stress responses, but the mechanism of ABA signal perception on the plasma membrane remains to be dissected. A previous GeneChip analysis revealed that a member of the A4 subfamily of lectin receptor kinases (LecRKs) of Arabidopsis (Arabidopsis thaliana), At5g01540 (designated LecRKA4.1), is up-regulated in response to a low dose of ABA in the rop10-1 background. Here, we present functional evidence to support its role in ABA response. LecRKA4.1 is expressed in seeds and leaves but not in roots, and the protein is localized to the plasma membrane. A T-DNA knockout mutant, lecrka4.1-1, slightly enhanced ABA inhibition of seed germination. Interestingly, LecRKA4.1 is adjacent to two other members of the A4 subfamily of LecRK genes, At5g01550 (LecRKA4.2) and At5g01560 (LecRKA4.3). We found that loss-of-function mutants of LecRKA4.2 and LecRKA4.3 exhibited similarly weak enhancement of ABA response in seed germination inhibition. Furthermore, LecRKA4.2 suppression by RNA interference in lecrka4.1-1 showed stronger ABA inhibition of seed germination than lecrka4.1-1, while the response to gibberellic acid was not affected in lecrka4.1-1 and lecrka4.1-1; LecRKA4.2 (RNAi) lines. Expression studies, together with network-based analysis, suggest that LecRKA4.1 and LecRKA4.2 regulate some of the ABA-responsive genes. Taken together, our results demonstrate that the A4 subfamily of LecRKs has a redundant function in the negative regulation of ABA response in seed germination.