JASMONATE-INSENSITIVE1 encodes a MYC transcription factor essential to discriminate between different jasmonate-regulated defense responses in Arabidopsis

Indirect suppression of photosynthesis on individual leaves by arthropod herbivory

BACKGROUND

Herbivory reduces leaf area, disrupts the function of leaves, and ultimately alters yield and productivity. Herbivore damage to foliage typically is assessed in the field by measuring the amount of leaf tissue removed and disrupted. This approach assumes the remaining tissues are unaltered, and plant photosynthesis and water balance function normally. However, recent application of thermal and fluorescent imaging technologies revealed that alterations to photosynthesis and transpiration propagate into remaining undamaged leaf tissue.

SCOPE AND CONCLUSIONS

This review briefly examines the indirect effects of herbivory on photosynthesis, measured by gas exchange or chlorophyll fluorescence, and identifies four mechanisms contributing to the indirect suppression of photosynthesis in remaining leaf tissues: severed vasculature, altered sink demand, defence-induced autotoxicity, and defence-induced down-regulation of photosynthesis. We review the chlorophyll fluorescence and thermal imaging techniques used to gather layers of spatial data and discuss methods for compiling these layers to achieve greater insight into mechanisms contributing to the indirect suppression of photosynthesis. We also elaborate on a few herbivore-induced gene-regulating mechanisms which modulate photosynthesis and discuss the difficult nature of measuring spatial heterogeneity when combining fluorescence imaging and gas exchange technology. Although few studies have characterized herbivore-induced indirect effects on photosynthesis at the leaf level, an emerging literature suggests that the loss of photosynthetic capacity following herbivory may be greater than direct loss of photosynthetic tissues. Depending on the damage guild, ignoring the indirect suppression of photosynthesis by arthropods and other organisms may lead to an underestimate of their physiological and ecological impacts.

MS/MS spectral tag-based annotation of non-targeted profile of plant secondary metabolites

The MS/MS spectral tag (MS2T) library-based peak annotation procedure was developed for informative non-targeted metabolic profiling analysis using LC-MS. An MS2T library of Arabidopsis metabolites was created from a set of MS/MS spectra acquired using the automatic data acquisition function of the mass spectrometer. By using this library, we obtained structural information for the detected peaks in the metabolic profile data without performing additional MS/MS analysis; this was achieved by searching for the corresponding MS2T accession in the library. In the case of metabolic profile data for Arabidopsis tissues containing more than 1000 peaks, approximately 50% of the peaks were tagged by MS2Ts, and 90 peaks were identified or tentatively annotated with metabolite information by searching the metabolite databases and manually interpreting the MS2Ts. A comparison of metabolic profiles among the Arabidopsis tissues revealed that many unknown metabolites accumulated in a tissue-specific manner, some of which were deduced to be unusual Arabidopsis metabolites based on the MS2T data. Candidate genes responsible for these biosyntheses could be predicted by projecting the results to the transcriptome data. The method was also used for metabolic phenotyping of a subset of Ds transposon-inserted lines of Arabidopsis, resulting in clarification of the functions of reported genes involved in glycosylation of flavonoids. Thus, non-targeted metabolic profiling analysis using MS2T annotation methods could prove to be useful for investigating novel functions of secondary metabolites in 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.

The alpha-subunit of the heterotrimeric G-protein affects jasmonate responses in Arabidopsis thaliana

Heterotrimeric G-proteins have been implicated in having a role in many plant signalling pathways. To understand further the role of G-proteins, a preliminary experiment was performed to assess the impact of the G alpha subunit loss-of-function mutation gpa1-1 on the Arabidopsis transcriptome. The analysis indicated that the G alpha subunit may play a role in response to jasmonic acid (JA). Consistent with this, G alpha mutants showed a reduced response to JA in inhibition of chlorophyll accumulation and root growth, whilst G alpha gain-of-function plants overexpressing G alpha showed the opposite phenotype. The levels of JA and related compounds were unaffected in the gpa1-1 mutant, as was autoregulation of the Allene Oxide Synthase (AOS) gene that encodes a key enzyme for JA biosynthesis. In contrast, further analyses using G alpha loss- and gain-of-function Arabidopsis lines indicated that G alpha positively modulates the expression of the Vegetative Storage Protein (VSP) gene. This indicates that the G alpha subunit regulates a subset of JA-regulated genes defining a branch point in this signalling pathway in Arabidopsis. Further analysis of the impact of G alpha loss of function upon the JA-regulated transcriptome using Arabidopsis full genome arrays indicated that up to 29% of genes that are >2-fold regulated by JA in the wild type are misregulated in the G alpha mutant. This supports the observation that a significant proportion of, but not all, JA-regulated gene expression is mediated by G alpha.

A critical role for the TIFY motif in repression of jasmonate signaling by a stabilized splice variant of the JASMONATE ZIM-domain protein JAZ10 in Arabidopsis

JASMONATE ZIM-domain (JAZ) proteins act as repressors of jasmonate (JA) signaling. Perception of bioactive JAs by the F-box protein CORONATINE INSENSITIVE1 (COI1) causes degradation of JAZs via the ubiquitin-proteasome pathway, which in turn activates the expression of genes involved in plant growth, development, and defense. JAZ proteins contain two highly conserved sequence regions: the Jas domain that interacts with COI1 to destabilize the repressor and the ZIM domain of unknown function. Here, we show that the conserved TIFY motif (TIFF/YXG) within the ZIM domain mediates homo- and heteromeric interactions between most Arabidopsis thaliana JAZs. We have also identified an alternatively spliced form (JAZ10.4) of JAZ10 that lacks the Jas domain and, as a consequence, is highly resistant to JA-induced degradation. Strong JA-insensitive phenotypes conferred by overexpression of JAZ10.4 were suppressed by mutations in the TIFY motif that block JAZ10.4-JAZ interactions. We conclude that JAZ10.4 functions to attenuate signal output in the presence of JA and further suggest that the dominant-negative action of this splice variant involves protein-protein interaction through the ZIM/TIFY domain. The ability of JAZ10.4 to interact with MYC2 is consistent with a model in which a JAZ10.4-containing protein complex directly represses the activity of transcription factors that promote expression of JA response genes.

Jasmonates act with salicylic acid to confer basal thermotolerance in Arabidopsis thaliana

* The cpr5-1 Arabidopsis thaliana mutant exhibits constitutive activation of salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) signalling pathways and displays enhanced tolerance of heat stress (HS). * cpr5-1 crossed with jar1-1 (a JA-amino acid synthetase) was compromised in basal thermotolerance, as were the mutants opr3 (mutated in OPDA reductase3) and coi1-1 (affected in an E3 ubiquitin ligase F-box; a key JA-signalling component). In addition, heating wild-type Arabidopsis led to the accumulation of a range of jasmonates: JA, 12-oxophytodienoic acid (OPDA) and a JA-isoleucine (JA-Ile) conjugate. Exogenous application of methyl jasmonate protected wild-type Arabidopsis from HS. * Ethylene was rapidly produced during HS, with levels being modulated by both JA and SA. By contrast, the ethylene mutant ein2-1 conferred greater thermotolerance. * These data suggest that JA acts with SA, conferring basal thermotolerance while ET may act to promote cell death.

Jasmonate passes muster: a receptor and targets for the defense hormone

The oxylipin jasmonate (JA) regulates many aspects of growth, development, and environmental responses in plants, particularly defense responses against herbivores and necrotrophic pathogens. Mutants of Arabidopsis helped researchers define the biochemical pathway for synthesis of jasmonoyl-isoleucine (JA-Ile), the active form of JA hormone, and demonstrated that JA is required for plant survival of insect and pathogen attacks and for plant fertility. Transcriptional profiling led to the discovery of the JASMONATE ZIM-DOMAIN (JAZ) proteins, which are repressors of JA signaling. JA-Ile relieves repression by promoting binding of the JAZ proteins to the F-box protein CORONATINE INSENSITIVE1 (COI1) and their subsequent degradation by the ubiquitination/26S-proteasome pathway. Although we now have a much better understanding of the molecular mechanism of JA action, many questions remain. Experimental answers to these questions will expand our knowledge of oxylipin signaling in plants and animals and will also provide new tools for efforts to improve crop protection and reproductive performance.

Priming of plant innate immunity by rhizobacteria and beta-aminobutyric acid: differences and similarities in regulation

Pseudomonas fluorescens WCS417r bacteria and beta-aminobutyric acid can induce disease resistance in Arabidopsis, which is based on priming of defence. In this study, we examined the differences and similarities of WCS417r- and beta-aminobutyric acid-induced priming. Both WCS417r and beta-aminobutyric acid prime for enhanced deposition of callose-rich papillae after infection by the oomycete Hyaloperonospora arabidopsis. This priming is regulated by convergent pathways, which depend on phosphoinositide- and ABA-dependent signalling components. Conversely, induced resistance by WCS417r and beta-aminobutyric acid against the bacterial pathogen Pseudomonas syringae are controlled by distinct NPR1-dependent signalling pathways. As WCS417r and beta-aminobutyric acid prime jasmonate- and salicylate-inducible genes, respectively, we subsequently investigated the role of transcription factors. A quantitative PCR-based genome-wide screen for putative WCS417r- and beta-aminobutyric acid-responsive transcription factor genes revealed distinct sets of priming-responsive genes. Transcriptional analysis of a selection of these genes showed that they can serve as specific markers for priming. Promoter analysis of WRKY genes identified a putative cis-element that is strongly over-represented in promoters of 21 NPR1-dependent, beta-aminobutyric acid-inducible WRKY genes. Our study shows that priming of defence is regulated by different pathways, depending on the inducing agent and the challenging pathogen. Furthermore, we demonstrated that priming is associated with the enhanced expression of transcription factors.

Molecular mechanism for jasmonate-induction of anthocyanin accumulation in Arabidopsis

Anthocyanins are important plant pigments that fulfil many physiological and ecological functions. Anthocyanin biosynthesis is controlled by numerous regulatory factors at the transcriptional level. Jasmonates (JAs) has been shown to induce anthocyanin accumulation in several plant species, however, the molecular mechanism for JA-regulated anthocyanin accumulation remains unknown. In this study, genetic, molecular, and physiological approaches were used to reveal the molecular basis of JA-regulated pigmentation in Arabidopsis. It was found that the F-box protein COI1 was required for JA-specific induced expression of the 'late' anthocyanin biosynthetic genes DFR, LDOX, and UF3GT. It is further demonstrated that COI1 was essential for JA-induction of transcription factors PAP1, PAP2, and GL3. It is speculated that COI1 regulates the expression of the transcription factors, including PAP1, PAP2, and GL3, which mediates the 'late' anthocyanin biosynthetic genes DFR, LDOX, and UF3GT, thereby modulating JA-induced anthocyanin biosynthesis in Arabidopsis.

Elicitor-induced transcription factors for metabolic reprogramming of secondary metabolism in Medicago truncatula

BACKGROUND

Exposure of Medicago truncatula cell suspension cultures to pathogen or wound signals leads to accumulation of various classes of flavonoid and/or triterpene defense molecules, orchestrated via a complex signalling network in which transcription factors (TFs) are essential components.

RESULTS

In this study, we analyzed TFs responding to yeast elicitor (YE) or methyl jasmonate (MJ). From 502 differentially expressed TFs, WRKY and AP2/EREBP gene families were over-represented among YE-induced genes whereas Basic Helix-Loop-Helix (bHLH) family members were more over-represented among the MJ-induced genes. Jasmonate ZIM-domain (JAZ) transcriptional regulators were highly induced by MJ treatment. To investigate potential involvement of WRKY TFs in signalling, we expressed four Medicago WRKY genes in tobacco. Levels of soluble and wall bound phenolic compounds and lignin were increased in all cases. WRKY W109669 also induced tobacco endo-1,3-beta-glucanase (NtPR2) and enhanced the systemic defense response to tobacco mosaic virus in transgenic tobacco plants.

CONCLUSION

These results confirm that Medicago WRKY TFs have broad roles in orchestrating metabolic responses to biotic stress, and that they also represent potentially valuable reagents for engineering metabolic changes that impact pathogen resistance.

The chromatin remodeler SPLAYED regulates specific stress signaling pathways

Organisms are continuously exposed to a myriad of environmental stresses. Central to an organism's survival is the ability to mount a robust transcriptional response to the imposed stress. An emerging mechanism of transcriptional control involves dynamic changes in chromatin structure. Alterations in chromatin structure are brought about by a number of different mechanisms, including chromatin modifications, which covalently modify histone proteins; incorporation of histone variants; and chromatin remodeling, which utilizes ATP hydrolysis to alter histone-DNA contacts. While considerable insight into the mechanisms of chromatin remodeling has been gained, the biological role of chromatin remodeling complexes beyond their function as regulators of cellular differentiation and development has remained poorly understood. Here, we provide genetic, biochemical, and biological evidence for the critical role of chromatin remodeling in mediating plant defense against specific biotic stresses. We found that the Arabidopsis SWI/SNF class chromatin remodeling ATPase SPLAYED (SYD) is required for the expression of selected genes downstream of the jasmonate (JA) and ethylene (ET) signaling pathways. SYD is also directly recruited to the promoters of several of these genes. Furthermore, we show that SYD is required for resistance against the necrotrophic pathogen Botrytis cinerea but not the biotrophic pathogen Pseudomonas syringae. These findings demonstrate not only that chromatin remodeling is required for selective pathogen resistance, but also that chromatin remodelers such as SYD can regulate specific pathways within biotic stress signaling networks.

Transcriptional regulation by an NAC (NAM-ATAF1,2-CUC2) transcription factor attenuates ABA signalling for efficient basal defence towards Blumeria graminis f. sp. hordei in Arabidopsis

ATAF1 is a member of a largely uncharacterized plant-specific gene family encoding NAC transcription factors, and is induced in response to various abiotic and biotic stimuli in Arabidopsis thaliana. Previously, we showed that a mutant allele of ATAF1 compromises penetration resistance in Arabidopsis with respect to the non-host biotrophic pathogen Blumeria graminis f. sp. hordei (Bgh). In this study, we have used genome-wide transcript profiling to characterize signalling perturbations in ataf1 plants following Bgh inoculation. Comparative transcriptomic analyses identified an over-representation of abscisic acid (ABA)-responsive genes, including the ABA biosynthesis gene AAO3, which is significantly induced in ataf1 plants compared to wild-type plants following inoculation with Bgh. Additionally, we show that Bgh inoculation results in decreased endogenous ABA levels in an ATAF1-dependent manner, and that the ABA biosynthetic mutant aao3 showed increased penetration resistance to Bgh compared to wild-type plants. Furthermore, we show that ataf1 plants show ABA-hyposensitive phenotypes during seedling development and germination. Our data support a negative correlation between ABA levels and penetration resistance, and identify ATAF1 as a new stimuli-dependent attenuator of ABA signalling for the mediation of efficient penetration resistance in Arabidopsis upon Bgh attack.

Wound-induced endogenous jasmonates stunt plant growth by inhibiting mitosis

When plants are repeatedly injured their growth is stunted and the size of organs such as leaves is greatly reduced. The basis of this effect is not well-understood however, even though it reduces yield of crops injured by herbivory, and produces dramatic effects exemplified in ornamental bonsai plants. We have investigated the genetic and physiological basis of this “bonsai effect” by repeatedly wounding leaves of the model plant Arabidopsis. This treatment stunted growth by 50% and increased the endogenous content of jasmonate (JA), a growth inhibitor, by seven-fold. Significantly, repeated wounding did not stunt the growth of the leaves of mutants unable to synthesise JA, or unable to respond to JA including coi1, jai3, myc2, but not jar1. The stunted growth did not result from reduced cell size, but resulted instead from reduced cell number, and was associated with reduced expression of CycB1;2. Wounding caused systemic disappearance of constitutively expressed JAZ1::GUS. Wounding also activates plant immunity. We show that a gene, 12-oxo-phytodienoate reductase, which catalyses a step in JA biosynthesis, and which we confirm is not required for defence, is however required for wound-induced stunting. Our data suggest that intermediates in the JA biosynthetic pathway activate defence, but a primary function of wound-induced JA is to stunt growth through the suppression of mitosis.

Systemic resistance in Arabidopsis conferred by the mycorrhizal fungus Piriformospora indica requires jasmonic acid signaling and the cytoplasmic function of NPR1

We analyzed the requirement of specific defense pathways for powdery mildew (Golovinomyces orontii) resistance induced by the basidiomycete Piriformospora indica in Arabidopsis. Piriformospora indica root colonization reduced G. orontii conidia in wild-type (Col-0), npr1-3 (nonexpressor of PR genes 1-3) and NahG plants, but not in the npr1-1 null mutant. Therefore, cytoplasmic but not nuclear localization of NPR1 is required for P. indica-induced resistance. Two jasmonate signaling mutants were non-responsive to P. indica, and jasmonic acid-responsive vegetative storage protein expression was primed and thus elevated in response to powdery mildew, suggesting that P. indica confers resistance reminiscent of induced systemic resistance (ISR).

Pseudomonas syringae elicits emission of the terpenoid (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene in Arabidopsis leaves via jasmonate signaling and expression of the terpene synthase TPS4

Volatile, low-molecular weight terpenoids have been implicated in plant defenses, but their direct role in resistance against microbial pathogens is not clearly defined. We have examined a possible role of terpenoid metabolism in the induced defense of Arabidopsis thaliana plants against leaf infection with the bacterial pathogen Pseudomonas syringae. Inoculation of plants with virulent or avirulent P. syringae strains induces the emission of the terpenoids (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT), beta-ionone and alpha-farnesene. While the most abundant volatile, the C16-homoterpene TMTT, is produced relatively early in compatible and incompatible interactions, emission of both beta-ionone and alpha-farnesene only increases in later stages of the compatible interaction. Pathogen-induced synthesis of TMTT is controlled through jasmonic acid (JA)-dependent signaling but is independent of a functional salicylic acid (SA) pathway. We have identified Arabidopsis T-DNA insertion lines with defects in the terpene synthase gene TPS4, which is expressed in response to P. syringae inoculation. The tps4 knockout mutant completely lacks induced emission of TMTT but is capable of beta-ionone and alpha-farnesene production, demonstrating that TPS4 is specifically involved in TMTT formation. The tps4 plants display at least wild type-like resistance against P. syringae, indicating that TMTT per se does not protect against the bacterial pathogen in Arabidopsis leaves. Similarly, the ability to mount SA-dependent defenses and systemic acquired resistance (SAR) is barely affected in tps4, which excludes a signaling function of TMTT during SAR. Besides P. syringae challenge, intoxication of Arabidopsis leaves with copper sulfate, a treatment that strongly activates JA biosynthesis, triggers production of TMTT, beta-ionone, and alpha-farnesene. Taken together, our data suggest that induced TMTT production in Arabidopsis is a by-product of activated JA signaling, rather than an effective defense response that contributes to resistance against P. syringae.

The genetic network controlling the Arabidopsis transcriptional response to Pseudomonas syringae pv. maculicola: roles of major regulators and the phytotoxin coronatine

Expression profiling of wild-type plants and mutants with defects in key components of the defense signaling network was used to model the Arabidopsis network 24 h after infection by Pseudomonas syringae pv. maculicola ES4326. Results using the Affymetrix ATH1 array revealed that expression levels of most pathogen-responsive genes were affected by mutations in coi1, ein2, npr1, pad4, or sid2. These five mutations defined a small number of different expression patterns displayed by the majority of pathogen-responsive genes. P. syringae pv. tomato strain DC3000 elicited a much weaker salicylic acid (SA) response than ES4326. Additional mutants were profiled using a custom array. Profiles of pbs3 and ndr1 revealed major effects of these mutations and allowed PBS3 and NDR1 to be placed between the EDS1/PAD4 node and the SA synthesis node in the defense network. Comparison of coi1, dde2, and jar1 profiles showed that many genes were affected by coi1 but very few were affected by dde2 or jar1. Profiles of coi1 plants infected with ES4326 were very similar to those of wild-type plants infected with bacteria unable to produce the phytotoxin coronatine, indicating that, essentially, all COI1-dependent gene expression changes in this system are caused by coronatine.

Natural variation in responsiveness of Arabidopsis thaliana to methyl jasmonate is developmentally regulated

A number of Arabidopsis thaliana (L.) Heynh ecotypes were assayed for their responses to methyl jasmonate in order to determine any natural variation in response to this volatile signal. We observed that the regulation of methyl jasmonate-induced expression of the vegetative storage proteins VSP1 and VSP2 is linked to the developmental stage of the plants. In two ecotypes investigated further, Gr-3 and Col-0, it was observed that the VSP1/2 genes became non-responsive to methyl jasmonate stimulation as the plants progressed to bolt formation and flowering. However, the onset of when this transcriptional inactivation occurred differed between the two ecotypes, with Col-0 displaying still high levels of transcript at the onset of flowering whereas Gr-3 showed no induction of VSP1/2 transcription at the same developmental stage. To our knowledge, this is the first time that such a pattern of regulation has been described for a methyl jasmonate-regulated gene. Moreover, in an F(2) population of a cross between these two ecotypes, the trait for 'VSP1/2 methyl jasmonate non-responsiveness' segregated among individuals, indicating the feasibility of mapping the genetic components of this response.

Plant pathogenesis-related (PR) proteins: a focus on PR peptides

The novel classes of plant pathogenesis-related (PR) proteins identified during the last decade also include novel peptide families. This review specifically focuses on these pathogenesis-related peptides, including proteinase inhibitors (PR-6 family), plant defensins (PR-12 family), thionins (PR-13 family) and lipid transfer proteins (PR-14 family). For each family of PR peptides, the general features concerning occurrence, expression and possible functions of their members are described. Next, more specifically the occurrence of each PR peptide family in the model plant Arabidopsis thaliana is discussed. Single-gene studies performed on particular gene members of a PR peptide family are reported. In addition, expression data of yet undescribed gene members of that particular PR peptide family are presented by consultation of publicly available micro-array databases. Finally an update is provided on the potential role of these PR peptides in A. thaliana, with a focus on their possible involvement in plant defense.

Fine-tuning of early events in the jasmonate response

Jasmonates (JAs) control many aspects of plant defense and development, for instance by inhibiting growth and eliciting secondary metabolism. The mechanisms by which JAs regulate these processes are currently under intensive investigation. Examination of transcriptional changes upon methyl jasmonate (MeJA) perception in a fast-growing Arabidopsis thaliana cell suspension culture revealed a quick and direct dual effect of JAs on the plant's cellular processes. Simultaneously, JA-elicited Arabidopsis cells activated phenylpropanoid metabolism and repressed cell cycle progression. Early JA response genes were predominantly implicated in transcriptional regulation and JA biosynthesis. In two parallel screens, we identified both early responsive transcriptional activators (ORA47 and MYC2) and transcriptional repressors (STZ/ZAT10 and AZF2) that putatively regulate the expression of the JA biosynthesis gene LOX3. In this addendum, we provide additional data demonstrating that MYC2, STZ/ZAT10 and AZF2 might also control the expression of JAZ1/TIFY10a that encodes a key repressor in the JA signaling cascade.

SHW1, a common regulator of abscisic acid (ABA) and light signaling pathways

In our recent paper in Plant Physiology, we have reported the identification and functional characterization of a unique regulator, SHW1, a serine-arginine-aspartate rich protein in Arabidopsis seedling development.1 Genetic and molecular analyses have revealed that SHW1 functions in an independent and interdependent manner with COP1, and differentially regulates photomorphogenic growth and light regulated gene expression. Here, we show the involvement of photoreceptors in the function of SHW1. Our results have further revealed that SHW1 is a common regulator of light and ABA signaling pathways. These results along with some data described in Plant Physiology paper have been discussed here in a broader perspective.

Comprehensive transcriptome analysis of phytohormone biosynthesis and signaling genes in microspore/pollen and tapetum of rice

To investigate the involvement of phytohormones during rice microspore/pollen (MS/POL) development, endogenous levels of IAA, gibberellins (GAs), cytokinins (CKs) and abscisic acid (ABA) in the mature anther were analyzed. We also analyzed the global expression profiles of genes related to seven phytohormones, namely auxin, GAs, CKs, brassinosteroids, ethylene, ABA and jasmonic acids, in MS/POL and tapetum (TAP) using a 44K microarray combined with a laser microdissection technique (LM-array analysis). IAA and GA(4) accumulated in a much higher amount in the mature anther compared with the other tissues, while CKs and ABA did not. LM-array analysis revealed that sets of genes required for IAA and GA synthesis were coordinately expressed during the later stages of MS/POL development, suggesting that these genes are responsible for the massive accumulation of IAA and GA(4) in the mature anther. In contrast, genes for GA signaling were preferentially expressed during the early developmental stages of MS/POL and throughout TAP development, while their expression was down-regulated at the later stages of MS/POL development. In the case of auxin signaling genes, such mirror-imaged expression observed in GA synthesis and signaling genes was not observed. IAA receptor genes were mostly expressed during the late stages of MS/POL development, and various sets of AUX/IAA and ARF genes were expressed during the different stages of MS/POL or TAP development. Such cell type-specific expression profiles of phytohormone biosynthesis and signaling genes demonstrate the validity and importance of analyzing the expression of phytohormone-related genes in individual cell types independently of other cells/tissues.

JAZ repressors set the rhythm in jasmonate signaling

Jasmonates (JAs) are essential hormones for plant defense and development. In spite of their importance, the molecular details of their signaling pathways remain largely unknown. A new family of regulators of JA signaling named JAZ, jasmonate ZIM-domain proteins, has recently been described. JAZ proteins repress of JA signaling and are targeted by the E3-ubiquitin ligase SCF(COI1) for proteasome degradation in response to JA. Hormone binding depends on a functional COI1 protein suggesting that COI1 is the JA receptor. MYC2, a positive regulator of JA-dependent responses, has been identified as a target of JAZ repressors. Interestingly, MYC2 and JAZ proteins are involved in a negative regulatory feedback loop, suggesting a model to explain how transcriptional reprogramming is turned on and off in response to JA. The discovery of JAZ repressors provides a new framework to understand JA-signaling pathways from hormonal perception to transcriptional activation.

Role of the Arabidopsis thaliana NAC transcription factors ANAC019 and ANAC055 in regulating jasmonic acid-signaled defense responses

Jasmonic acid (JA) is an important phytohormone that regulates plant defense responses against herbivore attack, pathogen infection and mechanical wounding. In this report, we provided biochemical and genetic evidence to show that the Arabidopsis thaliana NAC family proteins ANAC019 and ANAC055 might function as transcription activators to regulate JA-induced expression of defense genes. The role of the two NAC genes in JA signaling was examined with the anac019 anac055 double mutant and with transgenic plants overexpressing ANAC019 or ANAC055. The anac019 anac055 double mutant plants showed attenuated JA-induced VEGETATIVE STORAGE PROTEIN1 (VSP1) and LIPOXYGENASE2 (LOX2) expression, whereas transgenic plants overexpressing the two NAC genes showed enhanced JA-induced VSP1 and LOX2 expression. That the JA-induced expression of the two NAC genes depends on the function of COI1 and AtMYC2, together with the finding that overexpression of ANAC019 partially rescued the JA-related phenotype of the atmyc2-2 mutant, has led us to a hypothesis that the two NAC proteins act downstream of AtMYC2 to regulate JA-signaled defense responses. Further evidence to substantiate this idea comes from the observation that the response of the anac019 anac055 double mutant to a necrotrophic fungus showed high similarity to that of the atmyc2-2 mutant.

The AtGenExpress hormone and chemical treatment data set: experimental design, data evaluation, model data analysis and data access

We analyzed global gene expression in Arabidopsis in response to various hormones and in related experiments as part of the AtGenExpress project. The experimental agents included seven basic phytohormones (auxin, cytokinin, gibberellin, brassinosteroid, abscisic acid, jasmonate and ethylene) and their inhibitors. In addition, gene expression was investigated in hormone-related mutants and during seed germination and sulfate starvation. Hormone-inducible genes were identified from the hormone response data. The effects of each hormone and the relevance of the gene lists were verified by comparing expression profiles for the hormone treatments and related experiments using Pearson's correlation coefficient. This approach was also used to analyze the relationships among expression profiles for hormone responses and those included in the AtGenExpress stress-response data set. The expected correlations were observed, indicating that this approach is useful to monitor the hormonal status in the stress-related samples. Global interactions among hormones-inducible genes were analyzed in a pairwise fashion, and several known and novel hormone interactions were detected. Genome-wide transcriptional gene-to-gene correlations, analyzed by hierarchical cluster analysis (HCA), indicated that our data set is useful for identification of clusters of co-expressed genes, and to predict the functions of unknown genes, even if a gene's function is not directly related to the experiments included in AtGenExpress. Our data are available online from AtGenExpressJapan; the results of genome-wide HCA are available from PRIMe. The data set presented here will be a versatile resource for future hormone studies, and constitutes a reference for genome-wide gene expression in Arabidopsis.

Jasmonate signaling: a conserved mechanism of hormone sensing

The lipid-derived hormone jasmonate (JA) regulates diverse aspects of plant immunity and development. Among the central components of the JA signaling cascade are the E3 ubiquitin ligase SCFCOI1 and Jasmonate ZIM-domain (JAZ) proteins that repress transcription of JA-responsive genes. Recent studies provide evidence that amino acid-conjugated forms of JA initiate signal transduction upon formation of a coronatine-insensitive1 (COI1)-JA-JAZ ternary complex in which JAZs are ubiquitinated and subsequently degraded. Coronatine, a virulence factor produced by the plant pathogen Pseudomonas syringae, is a potent agonist of this hormone receptor system. Coronatine-induced targeting of JAZs to COI1 obstructs host immune responses to P. syrinage, providing a striking example of how pathogens exploit hormone signaling pathways in the host to promote disease. These findings, together with homology between COI1 and the auxin receptor, TIR1, extend the paradigm of F-box proteins as intracellular sensors of small molecules, and suggest a common evolutionary origin of the auxin and JA response pathways.

Signaling in plant disease resistance and symbiosis

Interactions between plants and microbes result in plant disease and symbiosis. The former causes considerable economic damage in modern agriculture, while the latter has produced great beneficial effects to our agriculture system. Comparison of the two interactions has revealed that a common panel of signaling pathways might participate in the establishment of the equilibrium between plant and microbes or its break-up. Plants appear to detect both pathogenic and symbiotic microbes by a similar set of genes. All symbiotic microbes seem to produce effectors to overcome plant basal defenses and it is speculated that symbiotic effectors have functions similar to pathogenic ones. Signaling molecules, salicylic acid (SA), jasmonic acid (JA) and ethylene (ET), are involved in both plant defense and symbiosis. Switching off signals contributing to deterioration of disease symptom would establish a new equilibrium between plant and pathogenic microbes. This would facilitate the development of strategies for durable disease resistance.

Jasmonate-induced nicotine formation in tobacco is mediated by tobacco COI1 and JAZ genes

Biosynthesis of many plant alkaloids is enhanced by endogenous accumulation and exogenous application of jasmonates, but the general and specific signaling components are not well understood. In Arabidopsis, jasmonate-induced ZIM-domain-containing (JAZ) proteins have recently been found to be critical transcriptional repressors linking CORONATINE INSENSTIVE1 (COI1)-mediated jasmonate perception and jasmonate-regulated transcriptional regulation. Insect herbivory on tobacco leaves activates the jasmonate signaling pathway, leading to up-regulation of nicotine biosynthesis genes in roots. We show here that roots of COI1-silenced tobacco plants are insensitive to growth inhibition by methyl jasmonate, and do not activate nicotine biosynthesis genes after jasmonate treatment or wounding of leaves. Tobacco JAZ proteins appeared to be rapidly degraded after jasmonate treatment, whereas a C-terminally truncated form lacking the conserved Jas motif did not. When the non-degradable JAZ forms were expressed in tobacco hairy roots, jasmonate induction of nicotine biosynthesis was strongly inhibited. Formation of tobacco alkaloids in jasmonate-elicited tobacco BY-2 cells was also effectively suppressed by the COI1 RNAi (RNA interference) construct and by the dominant-negative truncated JAZ constructs. In addition, jasmonate-mediated induction of nicotine biosynthesis genes was diminished by treatment with a proteasome inhibitor MG132. These results indicate that jasmonate-triggered, COI1-mediated degradation of JAZ repressors activates transcriptional regulation of nicotine biosynthesis genes in tobacco roots.

Induction of the Arabidopsis PHO1;H10 gene by 12-oxo-phytodienoic acid but not jasmonic acid via a CORONATINE INSENSITIVE1-dependent pathway

Expression of AtPHO1;H10, a member of the Arabidopsis (Arabidopsis thaliana) PHO1 gene family, is strongly induced following numerous abiotic and biotic stresses, including wounding, dehydration, cold, salt, and pathogen attack. AtPHO1;H10 expression by wounding was localized to the cells in the close vicinity of the wound site. AtPHO1;H10 expression was increased by application of the jasmonic acid (JA) precursor 12-oxo-phytodienoic acid (OPDA), but not by JA or coronatine. Surprisingly, induction of AtPHO1;H10 by OPDA was dependent on the presence of CORONATINE INSENSITIVE1 (COI1). The induction of AtPHO1;H10 expression by wounding and dehydration was dependent on COI1 and was comparable in both the wild type and the OPDA reductase 3-deficient (opr3) mutant. In contrast, induction of AtPHO1;H10 expression by exogenous abscisic acid (ABA) was independent of the presence of either OPDA or COI1, but was strongly decreased in the ABA-insensitive mutant abi1-1. The involvement of the ABA pathway in regulating AtPHO1;H10 was distinct between wounding and dehydration, with induction of AtPHO1;H10 by wounding being comparable to wild type in the ABA-deficient mutant aba1-3 and abi1-1, whereas a strong reduction in AtPHO1;H10 expression occurred in aba1-3 and abi1-1 following dehydration. Together, these results reveal that OPDA can modulate gene expression via COI1 in a manner distinct from JA, and independently from ABA. Furthermore, the implication of the ABA pathway in coregulating AtPHO1;H10 expression is dependent on the abiotic stress applied, being weak under wounding but strong upon dehydration.

Global switches and fine-tuning-ABA modulates plant pathogen defense

Plants are obliged to defend themselves against a wide range of biotic and abiotic stresses. Complex regulatory signaling networks mount an appropriate defense response depending on the type of stress that is perceived. In response to abiotic stresses such as drought, cold, and salinity, the function of abscisic acid (ABA) is well documented: elevation of plant ABA levels and activation of ABA-responsive signaling result in regulation of stomatal aperture and expression of stress-responsive genes. In response to pathogens, the role of ABA is more obscure and is a research topic that has long been overlooked. This article aims to evaluate and review the reported modes of ABA action on pathogen defense and highlight recent advances in deciphering the complex role of ABA in plant-pathogen interactions. The proposed mechanisms responsible for positive or negative effects of ABA on pathogen defense are discussed, as well as the regulation of ABA signaling and in planta ABA concentrations by beneficial and pathogenic microorganisms. In addition, the fast-growing number of reports that characterize antagonistic and synergistic interactions between abiotic and biotic stress responses point to ABA as an essential component in integrating and fine-tuning abiotic and biotic stress-response signaling networks.

Activation of defense response pathways by OGs and Flg22 elicitors in Arabidopsis seedlings

We carried out transcriptional profiling analysis in 10-d-old Arabidopsis thaliana seedlings treated with oligogalacturonides (OGs), oligosaccharides derived from the plant cell wall, or the bacterial flagellin peptide Flg22, general elicitors of the basal defense response in plants. Although detected by different receptors, both OGs and Flg22 trigger a fast and transient response that is both similar and comprehensive, and characterized by activation of early stages of multiple defense signaling pathways, particularly JA-associated processes. However, the response to Flg22 is stronger in both the number of genes differentially expressed and the amplitude of change. The magnitude of induction of individual genes is in both cases dose-dependent, but, even at very high concentrations, OGs do not induce a response that is as comprehensive as that seen with Flg22. While high doses of either microbe-associated molecular pattern (MAMP) elicit a late response that includes activation of senescence processes, SA-dependent secretory pathway genes and PR1 expression are substantially induced only by Flg22. These results suggest a lower threshold for activation of early responses than for sustained or SA-mediated late defenses. Expression patterns of amino-cyclopropane-carboxylate synthase genes also implicate ethylene biosynthesis in regulation of the late innate immune response.

Further characterization of a rice AGL12 group MADS-box gene, OsMADS26

Plant MADS-box genes can be divided into 11 groups. Genetic analysis has revealed that most of them function in flowering-time control, reproductive organ development, and vegetative growth. Here, we elucidated the role of OsMADS26, a member of the AGL12 group. Transcript levels of OsMADS26 were increased in an age-dependent manner in the shoots and roots. Transgenic plants of both rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana) overexpressing this gene manifested phenotypes related to stress responses, such as chlorosis, cell death, pigment accumulation, and defective root/shoot growth. In addition, apical hook development was significantly suppressed in Arabidopsis. Plants transformed with the OsMADS26-GR (glucocorticoid receptor) fusion construct displayed those stress-related phenotypes when treated with dexamethasone. Microarray analyses using this inducible system showed that biosynthesis genes for jasmonate, ethylene, and reactive oxygen species, as well as putative downstream targets involved in the stress-related process, were up-regulated in OsMADS26-overexpressing plants. These results suggest that OsMADS26 induces multiple responses that are related to various stresses.

Repression of the auxin response pathway increases Arabidopsis susceptibility to necrotrophic fungi

In plants, resistance to necrotrophic pathogens depends on the interplay between different hormone systems, such as those regulated by salicylic acid (SA), jasmonic acid (JA), ethylene, and abscisic acid. Repression of auxin signaling by the SA pathway was recently shown to contribute to antibacterial resistance. Here, we demonstrate that Arabidopsis auxin signaling mutants axr1, axr2, and axr6 that have defects in the auxin-stimulated SCF (Skp1-Cullin-F-box) ubiquitination pathway exhibit increased susceptibility to the necrotrophic fungi Plectosphaerella cucumerina and Botrytis cinerea. Also, stabilization of the auxin transcriptional repressor AXR3 that is normally targeted for removal by the SCF-ubiquitin/proteasome machinery occurs upon P. cucumerina infection. Pharmacological inhibition of auxin transport or proteasome function each compromise necrotroph resistance of wild-type plants to a similar extent as in non-treated auxin response mutants. These results suggest that auxin signaling is important for resistance to the necrotrophic fungi P. cucumerina and B. cinerea. SGT1b (one of two Arabidopsis SGT1 genes encoding HSP90/HSC70 co-chaperones) promotes the functions of SCF E3-ubiquitin ligase complexes in auxin and JA responses and resistance conditioned by certain Resistance (R) genes to biotrophic pathogens. We find that sgt1b mutants are as resistant to P. cucumerina as wild-type plants. Conversely, auxin/SCF signaling mutants are uncompromised in RPP4-triggered resistance to the obligate biotrophic oomycete, Hyaloperonospora parasitica. Thus, the predominant action of SGT1b in R gene-conditioned resistance to oomycetes appears to be at a site other than assisting SCF E3-ubiquitin ligases. However, genetic additivity of sgt1b axr1 double mutants in susceptibility to H. parasitica suggests that SCF-mediated ubiquitination contributes to limiting biotrophic pathogen colonization once plant-pathogen compatibility is established.

Functional characterization of a cotton late embryogenesis-abundant D113 gene promoter in transgenic tobacco

Previous studies have shown that mRNA and protein encoded by late embryogenesis-abundant (LEA) gene D113 from Gossypium hirsutum L. accumulate at high levels in mature seeds and also in response to abscisic acid (ABA) in young embryo. In this study, we studied the expression of four promoter 5' deletion constructs (-1383, -974, -578 and -158) of the LEA D113 gene fused to beta-glucuronidase (GUS). GUS activity analysis revealed that the -578 promoter fragment was necessary to direct seed-specific GUS expression in transgenic tobacco plants (Nicotiana tabacum L.). To further investigate the expression pattern of LEA D113 promoter under environmental stresses, 2-week-old transgenic tobacco seedlings were exposed to ABA, dehydration, high salinity and cold treatments. GUS activity in the seedlings was quantified fluorimetrically, and expression was also observed by histochemical staining. An apparent increase in GUS activity was found in plants harboring constructs -1383, -974 and -578 after 24 h of ABA or high-salinity treatments, as well as after 10 days of dehydration. By contrast, only a slight increase was observed in all the three lines after cold treatment. Virtually no change in expression was found in construct -158 in response to dehydration, salinity and cold, but there was a moderate response to ABA, suggesting that the region between -574 and -158 was necessary for dehydration- and salinity-dependent expression, whereas ABA-responsive cis-acting elements might be located in the -158 region of the promoter.

Peroxisome proliferation, wound-activated responses and expression of peroxisome-associated genes are cross-regulated but uncoupled in Arabidopsis thaliana

Plant peroxisomes are multifunctional organelles that show plasticity in number, size, morphology, cellular location and metabolic functions. Many of these changes occur in response to environmental factors and are decisive for the development and defence of the plant. Among them, peroxisomal beta-oxidation-mediated synthesis of jasmonic acid (JA) is a key process in regulating development as well as wound- or pathogen-triggered defence responses. This work seeks for the connection between wound, JA and the proliferation of peroxisomes in Arabidopsis thaliana. The hypolipidemic drug clofibrate (CFB) induced the proliferation of peroxisomes and the expression of the beta-oxidation 3-ketoacyl-CoA thiolase 2 (KAT2) gene, coding for a key enzyme in the biosynthesis of JA, among other wound- and JA-responsive gene transcripts in Arabidopsis leaves. The CFB-activated expression of wound-responsive genes was not dependent on JA synthesis or perception and those responsive to JA required the function of the F-box protein COI1. In turn, wounding neither triggered peroxisome proliferation nor required peroxisome integrity to activate gene expression. Interestingly, cells from JA-treated leaves contained fewer but larger peroxisomes than cells from untreated leaves. The proliferation of peroxisomes, the synthesis of JA and the activation of wound-responsive genes by CFB, although functionally connected, were uncoupled in Arabidopsis.

Pathogenicity of Pseudomonas syringae pv. tomato on tomato seedlings: phenotypic and gene expression analyses of the virulence function of coronatine

Bacterial speck disease, which is caused by Pseudomonas syringae pv. tomato, is an economically important disease on tomato. In the present study, we show that P. syringae pv. tomato DC3000 is a pathogen of tomato seedlings, an aspect of pathogen biology that has not been previously investigated. This resulted in the development of a virulence assay on tomato seedlings that has several advantages over labor-intensive foliar assays, including a shorter growth and incubation period, ease of inoculation and handling, and rapid generation of larger sample sizes per experiment. The utility of this assay was investigated by exploring the virulence function of coronatine (COR) on tomato seedlings. Using the COR- mutant DB29 and a MAPMAN display of transcript data from TOM1 microarrays, COR-dependent expression of genes involved in secondary metabolism, polyamine biosynthesis, reactive oxygen species homeostasis, and the novel transcription factor SlNAC2 were identified. Furthermore, during pathogenesis, genes involved in photosynthetic light reactions and the Calvin-Benson cycle were strongly repressed by COR. In conclusion, we show that P. syringae pv. tomato infects tomato seedlings and that COR is required for virulence in seedlings. The seedling assay can be used in high-throughput screens for the identification of molecular targets for COR and for the identification of genes involved in pathogenesis.

Arabidopsis defense response against Fusarium oxysporum

The plant fungal pathogen Fusarium oxysporum (Fox) is the causal agent of root rot or wilt diseases in several plant species, including crops such as tomato (Solanum lycopersicum), banana (Musa sapientum) and asparagus (Asparagus officinalis). Colonization of plants by Fox leads to the necrosis of the infected tissues, a subsequent collapse of vascular vessels and decay of the plant. Plant resistance to Fox appears to be monogenic or oligogenic depending on the host. Perception of Fox by plants follows the concept of elicitor-induced immune response, which in turn activates several plant defense signaling pathways. Here, we review the Fox-derived elicitors identified so far and the interaction among the different signaling pathways mediating plant resistance to Fox.

Regulation and function of Arabidopsis JASMONATE ZIM-domain genes in response to wounding and herbivory

Jasmonate (JA) and its amino acid conjugate, jasmonoyl-isoleucine (JA-Ile), play important roles in regulating plant defense responses to insect herbivores. Recent studies indicate that JA-Ile promotes the degradation of JASMONATE ZIM-domain (JAZ) transcriptional repressors through the activity of the E(3) ubiquitin-ligase SCF(COI1). Here, we investigated the regulation and function of JAZ genes during the interaction of Arabidopsis (Arabidopsis thaliana) with the generalist herbivore Spodoptera exigua. Most members of the JAZ gene family were highly expressed in response to S. exigua feeding and mechanical wounding. JAZ transcript levels increased within 5 min of mechanical tissue damage, coincident with a large (approximately 25-fold) rise in JA and JA-Ile levels. Wound-induced expression of JAZ and other CORONATINE-INSENSITIVE1 (COI1)-dependent genes was not impaired in the jar1-1 mutant that is partially deficient in the conversion of JA to JA-Ile. Experiments performed with the protein synthesis inhibitor cycloheximide provided evidence that JAZs, MYC2, and genes encoding several JA biosynthetic enzymes are primary response genes whose expression is derepressed upon COI1-dependent turnover of a labile repressor protein(s). We also show that overexpression of a modified form of JAZ1 (JAZ1Delta3A) that is stable in the presence of JA compromises host resistance to feeding by S. exigua larvae. These findings establish a role for JAZ proteins in the regulation of plant anti-insect defense, and support the hypothesis that JA-Ile and perhaps other JA derivatives activate COI1-dependent wound responses in Arabidopsis. Our results also indicate that the timing of JA-induced transcription in response to wounding is more rapid than previously realized.

The F-box protein ACRE189/ACIF1 regulates cell death and defense responses activated during pathogen recognition in tobacco and tomato

Virus-induced gene silencing identified the Avr9/Cf-9 RAPIDLY ELICITED gene ACRE189 as essential for the Cf-9- and Cf-4-mediated hypersensitive response (HR) in Nicotiana benthamiana. We report a role for ACRE189 in disease resistance in tomato (Solanum lycopersicum) and tobacco (Nicotiana tabacum). ACRE189 (herein renamed Avr9/Cf-9-INDUCED F-BOX1 [ACIF1]) encodes an F-box protein with a Leu-rich-repeat domain. ACIF1 is widely conserved and is closely related to F-box proteins regulating plant hormone signaling. Silencing of tobacco ACIF1 suppressed the HR triggered by various elicitors (Avr9, Avr4, AvrPto, Inf1, and the P50 helicase of Tobacco mosaic virus [TMV]). ACIF1 is recruited to SCF complexes (a class of ubiquitin E3 ligases), and the expression of ACIF1 F-box mutants in tobacco compromises the HR similarly to ACIF1 silencing. ACIF1 affects N gene-mediated responses to TMV infection, including lesion formation and salicylic acid accumulation. Loss of ACIF1 function also reduced confluent cell death induced by Pseudomonas syringae pv tabaci. ACIF1 silencing in Cf9 tomato attenuated the Cf-9-dependent HR but not Cf-9 resistance to Cladosporium fulvum. Resistance conferred by the Cf-9 homolog Cf-9B, however, was compromised in ACIF1-silenced tomato. Analysis of public expression profiling data suggests that Arabidopsis thaliana homologs of ACIF1 (VFBs) regulate defense responses via methyl jasmonate- and abscisic acid-responsive genes. Together, these findings support a role of ACIF1/VFBs in plant defense responses.

A MYB transcription factor regulates very-long-chain fatty acid biosynthesis for activation of the hypersensitive cell death response in Arabidopsis

Plant immune responses to pathogen attack include the hypersensitive response (HR), a form of programmed cell death occurring at invasion sites. We previously reported on Arabidopsis thaliana MYB30, a transcription factor that acts as a positive regulator of a cell death pathway conditioning the HR. Here, we show by microarray analyses of Arabidopsis plants misexpressing MYB30 that the genes encoding the four enzymes forming the acyl-coA elongase complex are putative MYB30 targets. The acyl-coA elongase complex synthesizes very-long-chain fatty acids (VLCFAs), and the accumulation of extracellular VLCFA-derived metabolites (leaf epidermal wax components) was affected in MYB30 knockout mutant and overexpressing lines. In the same lines, a lipid extraction procedure allowing high recovery of sphingolipids revealed changes in VLCFA contents that were amplified in response to inoculation. Finally, the exacerbated HR phenotype of MYB30-overexpressing lines was altered by the loss of function of the acyl-ACP thioesterase FATB, which causes severe defects in the supply of fatty acids for VLCFA biosynthesis. Based on these findings, we propose a model in which MYB30 modulates HR via VLCFAs by themselves, or VLCFA derivatives, as cell death messengers in plants.

General detoxification and stress responses are mediated by oxidized lipids through TGA transcription factors in Arabidopsis

12-oxo-phytodienoic acid and several phytoprostanes are cyclopentenone oxylipins that are formed via the enzymatic jasmonate pathway and a nonenzymatic, free radical-catalyzed pathway, respectively. Both types of cyclopentenone oxylipins induce the expression of genes related to detoxification, stress responses, and secondary metabolism, a profile clearly distinct from that of the cyclopentanone jasmonic acid. Microarray analyses revealed that 60% of the induction by phytoprostanes and 30% of the induction by 12-oxo-phytodienoic acid was dependent on the TGA transcription factors TGA2, TGA5, and TGA6. Moreover, treatment with phytoprostanes and 12-oxo-phytodienoic acid inhibited cell division and root growth, a property also shared by jasmonic acid. Besides being potent signals, cyclopentenones and other lipid peroxidation products are reactive electrophiles that can covalently bind to and damage proteins. To this end, we show that at least two of the induced detoxification enzymes efficiently metabolize cyclopentenones in vitro. Accumulation of two of these metabolites was detectable during Pseudomonas infection. The cyclopentenone oxylipin gene induction profile resembles the defense response induced by a variety of lipophilic xenobiotics. Hence, oxidized lipids may activate chemosensory mechanisms of a general broad-spectrum detoxification network involving TGA transcription factors.

JAZing up jasmonate signaling

Recent discoveries show that jasmonate ZIM-domain (JAZ) transcriptional repressors are key regulators of jasmonate hormonal response. Jasmonate promotes interaction between JAZ proteins and the SCF(COI1) ubiquitin ligase, leading to JAZ degradation via the 26S proteasome in Arabidopsis thaliana. Elimination of JAZ repressors then frees the MYC2 transcription factor to stimulate jasmonate-dependent gene expression. Although jasmonic acid and methyl jasmonate were thought to be key regulators of jasmonate responses, they were ineffective in promoting SCF(COI1)-JAZ interaction and it is the isoleucine conjugate of jasmonic acid that acts in this signal transduction pathway. The discovery of JAZ transcriptional regulators greatly advances our understanding of how jasmonate signaling regulates plant growth and response to the environment.

Identification of the OsOPR7 gene encoding 12-oxophytodienoate reductase involved in the biosynthesis of jasmonic acid in rice

Enzyme 12-oxophytodienoate (OPDA) reductase (EC1.3.1.42), which is involved in the biosynthesis of jasmonic acid (JA), catalyses the reduction of 10, 11-double bonds of OPDA to yield 3-oxo-2-(2'-pentenyl)-cyclopentane-1-octanoic acid (OPC-8:0). The rice OsOPR1 gene encodes OPDA reductase (OPR) converting (-)-cis-OPDA preferentially, rather than (+)-cis-OPDA, a natural precursor of JA. Here, we provide evidence that an OPR family gene in rice chromosome 8, designated OsOPR7, encodes the enzyme involved in the JA biosynthesis. Recombinant OsOPR7-His protein efficiently catalysed the reduction of both enantiomers of cis-OPDA, similar to the OPR3 protein in Arabidopsis thaliana (L.) Heynh. The expression of OsOPR7 mRNA was induced and reached maximum levels within 0.5 h of mechanical wounding and drought stress, and the endogenous JA level started to increase in accordance with the increase in OsOPR7 expression. The GFP-OsOPR7 fusion protein was detected exclusively in peroxisomes in onion epidermal cells. Furthermore, complementation analysis using an Arabidopsis opr3 mutant indicated that the OsOPR7 gene, but not OsOPR1, was able to complement the phenotypes of male sterility in the mutant caused by JA deficiency, and that JA production in the opr3 mutant was also restored by the expression of the OsOPR7 gene. We conclude that the OsOPR7 gene encodes the enzyme catalysing the reduction of natural (+)-cis-OPDA for the JA biosynthesis in rice.

Mapping methyl jasmonate-mediated transcriptional reprogramming of metabolism and cell cycle progression in cultured Arabidopsis cells

Jasmonates (JAs) are plant-specific signaling molecules that steer a diverse set of physiological and developmental processes. Pathogen attack and wounding inflicted by herbivores induce the biosynthesis of these hormones, triggering defense responses both locally and systemically. We report on alterations in the transcriptome of a fast-dividing cell culture of the model plant Arabidopsis thaliana after exogenous application of methyl JA (MeJA). Early MeJA response genes encoded the JA biosynthesis pathway proteins and key regulators of MeJA responses, including most JA ZIM domain proteins and MYC2, together with transcriptional regulators with potential, but yet unknown, functions in MeJA signaling. In a second transcriptional wave, MeJA reprogrammed cellular metabolism and cell cycle progression. Up-regulation of the monolignol biosynthesis gene set resulted in an increased production of monolignols and oligolignols, the building blocks of lignin. Simultaneously, MeJA repressed activation of M-phase genes, arresting the cell cycle in G(2). MeJA-responsive transcription factors were screened for their involvement in early signaling events, in particular the regulation of JA biosynthesis. Parallel screens based on yeast one-hybrid and transient transactivation assays identified both positive (MYC2 and the AP2/ERF factor ORA47) and negative (the C2H2 Zn finger proteins STZ/ZAT10 and AZF2) regulators, revealing a complex control of the JA autoregulatory loop and possibly other MeJA-mediated downstream processes.

Identification and characterization of COI1-dependent transcription factor genes involved in JA-mediated response to wounding in Arabidopsis plants

The phytohormone jasmonic acid (JA) is an important signaling molecular involved in many developmental and physiological processes, especially in the response of plants to wounding. In this study, we adopted a new strategy, taking into consideration the microarray data of the CHX treatment, to identify 15 COI1-dependent JA-inducible transcription factors (JCTFs) that have distinct expression patterns in response to wounding. After the analysis on the JCTFs over-expressor plants, we identified four JCTFs, i.e., WRKY18, At1g74930 and At3g53600 in addition to AtMYC2, as the positive regulators in the JA-mediated signaling pathway in response to Arabidopsis wounding.

Transcription factor MYC2 is involved in priming for enhanced defense during rhizobacteria-induced systemic resistance in Arabidopsis thaliana

Upon appropriate stimulation, plants can develop an enhanced capacity to express infection-induced cellular defense responses, a phenomenon known as the primed state. Colonization of the roots of Arabidopsis thaliana by the beneficial rhizobacterial strain Pseudomonas fluorescens WCS417r primes the leaf tissue for enhanced pathogen- and insect-induced expression of jasmonate (JA)-responsive genes, resulting in an induced systemic resistance (ISR) that is effective against different types of pathogens and insect herbivores. Here the molecular mechanism of this rhizobacteria-induced priming response was investigated using a whole-genome transcript profiling approach. Out of the 1879 putative methyl jasmonate (MeJA)-responsive genes, 442 genes displayed a primed expression pattern in ISR-expressing plants. Promoter analysis of ISR-primed, MeJA-responsive genes and ISR-primed, Pseudomonas syringae pv. tomato DC3000 (Pst DC3000)-responsive genes revealed over-representation of the G-box-like motif 5'-CACATG-3'. This motif is a binding site for the transcription factor MYC2, which plays a central role in JA- and abscisic acid-regulated signaling. MYC2 expression was consistently up-regulated in ISR-expressing plants. Moreover, mutants impaired in the JASMONATE-INSENSITIVE1/MYC2 gene (jin1-1 and jin1-2) were unable to mount WCS417r-ISR against Pst DC3000 and the downy mildew pathogen Hyaloperonospora parasitica. Together, these results pinpoint MYC2 as a potential regulator in priming for enhanced JA-responsive gene expression during rhizobacteria-mediated ISR.