Jasmonate response locus JAR1 and several related Arabidopsis genes encode enzymes of the firefly luciferase superfamily that show activity on jasmonic, salicylic, and indole-3-acetic acids in an assay for adenylation

COS1: an Arabidopsis coronatine insensitive1 suppressor essential for regulation of jasmonate-mediated plant defense and senescence

The Arabidopsis thaliana CORONATINE INSENSITIVE1 (COI1) gene encodes an F-box protein to assemble SCF(COI1) complexes essential for response to jasmonates (JAs), which are a family of plant signaling molecules required for many essential functions, including plant defense and reproduction. To better understand the molecular basis of JA action, we screened for suppressors of coi1 and isolated a coi1 suppressor1 (cos1) mutant. The cos1 mutation restores the coi1-related phenotypes, including defects in JA sensitivity, senescence, and plant defense responses. The COS1 gene was cloned through a map-based approach and found to encode lumazine synthase, a key component in the riboflavin pathway that is essential for diverse yet critical cellular processes. We demonstrated a novel function for the riboflavin pathway that acts downstream of COI1 in the JA signaling pathway and is required for suppression of the COI1-mediated root growth, senescence, and plant defense.

Metabolic fate of jasmonates in tobacco bright yellow-2 cells

Jasmonic acid and methyl jasmonate play an essential role in plant defense responses and pollen development. Their levels are temporarily and spatially controlled in plant tissue. However, whereas jasmonate biosynthesis is well studied, metabolic pathways downstream of jasmonic acid are less understood. We studied the uptake and metabolism of jasmonic acid and methyl jasmonate in tobacco (Nicotiana tabacum) Bright Yellow-2 suspension culture. We found that upon uptake, jasmonic acid was metabolized to its Glc and gentiobiose esters, and hydroxylation at C-11 or C-12 occurred. Free hydroxylated jasmonates were the preferential fraction of the culture medium. Upon hydrolysis of methyl jasmonate to jasmonic acid, a similar set of conversions occurs. In contrast to jasmonic acid, none of its derivatives interfere with the G2/M transition in synchronized tobacco Bright Yellow-2 cells.

The tobacco bZIP transcription factor BZI-1 binds the GH3 promoter in vivo and modulates auxin-induced transcription

Summary In order to establish the biological function of the tobacco basic leucine zipper (bZIP) transcription factor BZI-1 in hormone signalling, we have analysed transgenic plants which were altered with respect to the protein level or the activation potential of BZI-1. Overexpression of a dominant-negative derivative of BZI-1, lacking the N-terminal activation domain, resulted in plants displaying reduced internode size, enhanced lateral shoot formation and small, curly leaves. The response to auxin monitored with reference to root organogenesis, epinastic leaf curvature and transcription of the auxin-induced GH3 gene was reduced. In vitro, BZI-1 specifically binds to ACGT elements (ACEs) present in the GH3 promoter. In vivo, binding to the GH3 promoter was confirmed by chromatin immunoprecipitation (ChIP). Overexpression of BZI-1 in transgenic plants did not lead to a significant activation of the GH3 target gene. In contrast, plants expressing a VP16 (Herpes simplex virion protein 16)-BZI-1 fusion protein showed enhanced auxin-induced GH3 transcription. However, VP16-BZI-1 is insufficient to trigger GH3 expression independently of the auxin stimulus. Whereas auxin responsiveness has been shown to be mediated by ARF (auxin response factor) transcription factors, we discuss a function of BZI-1 assisting in fine-tuning of auxin-induced transcription.

Cytoplasmic alkalization precedes reactive oxygen species production during methyl jasmonate- and abscisic acid-induced stomatal closure

Signaling events during abscisic acid (ABA) or methyl jasmonate (MJ)-induced stomatal closure were examined in Arabidopsis wild type, ABA-insensitive (ost1-2), and MJ-insensitive mutants (jar1-1) in order to examine a crosstalk between ABA and MJ signal transduction. Some of the experiments were performed on epidermal strips of Pisum sativum. Stomata of jar1-1 mutant plants are insensitive to MJ but are able to close in response to ABA. However, their sensitivity to ABA is less than that of wild-type plants. Reciprocally, the stomata of ost1-2 are insensitive to ABA but are able to close in response to MJ to a lesser extent compared to wild-type plants. Both MJ and ABA promote H(2)O(2) production in wild-type guard cells, while exogenous application of diphenylene iodonium (DPI) chloride, an inhibitor of NAD(P)H oxidases, results in the suppression of ABA- and MJ-induced stomatal closure. ABA elevates H(2)O(2) production in wild-type and jar1-1 guard cells but not in ost1-2, whereas MJ induces H(2)O(2) production in both wild-type and ost1-2 guard cells, but not in jar1-1. MJ-induced stomatal closing is suppressed in the NAD(P)H oxidase double mutant atrbohD/F and in the outward potassium channel mutant gork1. Furthermore, MJ induces alkalization in guard cell cytosol, and MJ-induced stomatal closing is inhibited by butyrate. Analyses of the kinetics of cytosolic pH changes and reactive oxygen species (ROS) production show that the alkalization of cytoplasm precedes ROS production during the stomatal response to both ABA and MJ. Our results further indicate that JAR1, as OST1, functions upstream of ROS produced by NAD(P)H oxidases and that the cytoplasmic alkalization precedes ROS production during MJ or ABA signal transduction in guard cells.

Metabolic reprogramming in plant innate immunity: the contributions of phenylpropanoid and oxylipin pathways

In their environment, plants interact with a multitude of living organisms and have to cope with a large variety of aggressions of biotic or abiotic origin. To survive, plants have acquired, during evolution, complex mechanisms to detect their aggressors and defend themselves. Receptors and signaling pathways that are involved in such interactions with the environment are just beginning to be uncovered. What has been known for several decades is the extraordinary variety of chemical compounds the plants are capable to synthesize, and many of these products are implicated in defense responses. The number of natural products occurring in plants may be estimated in the range of hundreds of thousands, but only a fraction have been fully characterized. Despite the great importance of these metabolites for plant and also for human health, our knowledge about their biosynthetic pathways and functions is still fragmentary. Recent progress has been made particularly for phenylpropanoid and oxylipin metabolism, which are emphasized in this review. Both pathways are involved in plant resistance at several levels: by providing building units of physical barriers against pathogen invasion, by synthesizing an array of antibiotic compounds, and by producing signals implicated in the mounting of plant resistance.

Methyl jasmonate and cis-jasmone do not dispose of the herbivore-induced jasmonate burst in Nicotiana attenuata

The oxylipin pathway mediates wound- and herbivore-induced defense reactions in Nicotiana attenuata as evidenced by a transient jasmonic acid (JA)-burst that precedes these defense responses. The fate of this induced JA-burst remains unknown. Two derivatives of JA, its methylester, methyl jasmonate (MeJA) and cis-jasmone (cisJ), are thought to be a means of disposing of JA through volatilization at the plant surface. In N. attenuata, the headspace quantities of these compounds did not change over 3 days, although levels of MeJA and cisJ increased 100- and 70-fold, respectively, in surface extracts of attacked leaves after feeding of Manduca sexta larvae or application of larval regurgitant to mechanical wounds. Inhibition of the wound-induced increase in JA with indole-3-acetic acid (IAA) revealed an association between the JA accumulation and subsequent increases in MeJA and cisJ. Induced systemic increases of MeJA were not of local origin and therefore do not contribute to the inactivation of the JA-burst in the wounded leaf. The total amount of MeJA and cisJ produced could only account for 9% of the JA-burst elicited by herbivore attack and therefore their production do not represent major disposal pathways of JA in N. attenuata.

ydk1-D, an auxin-responsive GH3 mutant that is involved in hypocotyl and root elongation

To study the GH3 gene family of Arabidopsis, we investigated a flanking sequence database of Arabidopsis activation-tagged lines. We found a dwarf mutant, named yadokari 1-D (ydk1-D), that had a T-DNA insertion proximal to a GH3 gene. ydk1-D is dominant and has a short hypocotyl not only in light but also in darkness. Moreover, ydk1-D has a short primary root, a reduced lateral root number, and reduced apical dominance. A GH3 gene, named YDK1, was upregulated in ydk1-D, and YDK1 transgenic plants showed the ydk1-D phenotype. YDK1 gene expression was induced by exogenously applied auxin and regulated by auxin-response factor (ARF)7. In addition, YDK1 gene expression was downregulated by blue and far-red (FR) lights. Strong promoter activity of YDK1 was observed in roots and flowers. These results suggest that YDK1 may function as a negative component in auxin signaling by regulating auxin activity.

Characterisation of an Arabidopsis-Leptosphaeria maculans pathosystem: resistance partially requires camalexin biosynthesis and is independent of salicylic acid, ethylene and jasmonic acid signalling

Out of 168 Arabidopsis accessions screened with isolates of Leptosphaeria maculans, one (An-1) showed clear disease symptoms. In order to identify additional components involved in containment of L. maculans in Arabidopsis, a screen for L. maculans-susceptible (lms) mutants was performed. Eleven lms mutants were isolated, which displayed differential susceptibility responses to L. maculans. lms1 was crossed with Columbia (Col-0) and Ws-0, and mapping data for both populations showed the highest linkage to a region on chromosome 2. Reduced levels of PR-1 and PDF1.2 expression were found in lms1 compared to wild-type plants 48 h after pathogen inoculation. In contrast, the lms1 mutant displayed upregulation of either marker gene upon chemical treatment, possibly as an effect of an altered ethylene (ET) response. To assess the contribution of different defence pathways, genotypes implicated in salicylic acid (SA) signalling plants expressing the bacterial salicylate hydroxylase (nahG) gene, non-expressor of PR1 (npr1)-1 and phytoalexin-deficient (pad4-1), jasmonic acid (JA) signalling (coronatine insensitive (coi)1-16, enhanced disease susceptibility (eds)8-1 and jasmonic acid resistant (jar)1-1) and ET signalling (eds4-1, ethylene insensitive (ein)2, ein3-1 and ethylene resistant (etr)1-1) were screened. All the genotypes screened were as resistant as wild-type plants, demonstrating the dispensability of the pathways in L. maculans resistance. When mutants implicated in cell death responses were assayed, responsive to antagonist 1 (ran1)-1 exhibited a weak susceptible phenotype, whereas accelerated cell death (acd)1-20 showed a rapid lesion development. Camalexin is only partially responsible for L. maculans containment in Arabidopsis, as pad3-1 and enhanced susceptibility to Alternaria (esa)1 clearly showed a susceptible response while wild-type levels of camalexin were present in An-1 and lms1. The data presented point to the existence of multiple defence mechanisms controlling the containment of L. maculans in Arabidopsis.

Isolation of an ozone-sensitive and jasmonate-semi-insensitive Arabidopsis mutant (oji1)

A novel ozone-sensitive mutant was isolated from Arabidopsis T-DNA tagging lines. This mutant revealed severe foliar injury and higher ethylene emission than the wild type under ozone exposure. The ozone-induced injury and ethylene emission were suppressed by pretreatment with aminoethoxyvinyl glycine, an inhibitor of ethylene biosynthesis, both in this mutant and wild-type plants. Pretreatment with methyl-jasmonate (MeJA) at 10 micro M, however, suppressed the ozone-induced ethylene emission and foliar injury only in the wild-type plants. This mutant was less sensitive to jasmonate than the wild type, estimated by the MeJA-induced inhibition of root elongation and ozone-induced expression of AtVSP1, a jasmonate-inducible gene. Thus, this mutant was named oji1 (ozone-sensitive and jasmonate-insensitive 1). These results suggest that the ozone sensitivity of oji1 is caused by the increase in ozone-induced emission of ethylene as a result of low sensitivity to jasmonate, which plays defensive roles under stress conditions.

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

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

Interactive effects of jasmonic acid, salicylic acid, and gibberellin on induction of trichomes in Arabidopsis

Leaf trichomes protect plants from attack by insect herbivores and are often induced following damage. Hormonal regulation of this plant induction response has not been previously studied. In a series of experiments, we addressed the effects of artificial damage, jasmonic acid, salicylic acid, and gibberellin on induction of trichomes in Arabidopsis. Artificial damage and jasmonic acid caused significant increases in trichome production of leaves. The jar1-1 mutant exhibited normal trichome induction following treatment with jasmonic acid, suggesting that adenylation of jasmonic acid is not necessary. Salicylic acid had a negative effect on trichome production and consistently reduced the effect of jasmonic acid, suggesting negative cross-talk between the jasmonate and salicylate-dependent defense pathways. Interestingly, the effect of salicylic acid persisted in the nim1-1 mutant, suggesting that the Npr1/Nim1 gene is not downstream of salicylic acid in the negative regulation of trichome production. Last, we found that gibberellin and jasmonic acid had a synergistic effect on the induction of trichomes, suggesting important interactions between these two compounds.

The Arabidopsis mutant iop1 exhibits induced over-expression of the plant defensin gene PDF1.2 and enhanced pathogen resistance

SUMMARY Jasmonate and ethylene are concomitantly involved in the induction of the Arabidopsis plant defensin gene PDF1.2. To define genes in the signal transduction pathway leading to the induction of PDF1.2, we screened for mutants with induced over-expression of a beta-glucuronidase reporter, under the control of the PDF1.2 promoter. One mutant, iop1 (induced over-expressor of PDF1.2) produced small plants that showed induced over-expression of the pathogenesis-related genes PR-3, PR-4 and PR-1,2 (PDF1.2), combined with a down-regulated induction of PR-1 upon pathogen inoculation. The iop1 mutant showed enhanced resistance to a number of necrotrophic pathogens.

Interactive effects of jasmonic acid, salicylic acid, and gibberellin on induction of trichomes in Arabidopsis

Leaf trichomes protect plants from attack by insect herbivores and are often induced following damage. Hormonal regulation of this plant induction response has not been previously studied. In a series of experiments, we addressed the effects of artificial damage, jasmonic acid, salicylic acid, and gibberellin on induction of trichomes in Arabidopsis. Artificial damage and jasmonic acid caused significant increases in trichome production of leaves. The jar1-1 mutant exhibited normal trichome induction following treatment with jasmonic acid, suggesting that adenylation of jasmonic acid is not necessary. Salicylic acid had a negative effect on trichome production and consistently reduced the effect of jasmonic acid, suggesting negative cross-talk between the jasmonate and salicylate-dependent defense pathways. Interestingly, the effect of salicylic acid persisted in the nim1-1 mutant, suggesting that the Npr1/Nim1 gene is not downstream of salicylic acid in the negative regulation of trichome production. Last, we found that gibberellin and jasmonic acid had a synergistic effect on the induction of trichomes, suggesting important interactions between these two compounds.

DFL2, a new member of the Arabidopsis GH3 gene family, is involved in red light-specific hypocotyl elongation

A new GH3-related gene, designated DFL2, causes a short hypocotyl phenotype when overexpressed under red and blue light and a long hypocotyl when antisensed under red light conditions. Higher expression of this gene was observed in continuous white, blue and far-red light but the expression level was low in red light and darkness. DFL2 gene expression was induced transiently with red light pulse treatment. DFL2 transgenic plants exhibited a normal root phenotype including primary root elongation and lateral root formation, although primary root elongation was inhibited in antisense transgenic plants only under red light. The adult phenotypes of sense and antisense transgenic plants were not different from that of wild type. DFL2 promoter activity was observed in the hypocotyl. Our results suggest that DFL2 is located downstream of red light signal transduction and determines the degree of hypocotyl elongation.

Regulation of jasmonate-mediated plant responses in arabidopsis

Jasmonates (JAs) are signalling molecules that play a key role in the regulation of metabolic processes, reproduction, and defence against pathogens and insects. JAs regulate responses that are both local and systemic, and which are affected by outputs from signalling pathways regulated by ethylene, salicylic acid and auxin. This is a review of recent advances in our understanding of the regulation of JA perception in Arabidopsis thaliana, the different signalling functions of biologically active JAs, the post-translational control of JA responses leading to substantial transcriptional reprogramming, and the influence of other signalling pathways of systemic JA responses.

Arabidopsis mutants affected in the transcriptional control of allene oxide synthase, the enzyme catalyzing the entrance step in octadecanoid biosynthesis

Allene oxide synthase (AOS) catalyzes the entrance reaction in the biosynthesis of the octadecanoids 12-oxophytodienoic acid (OPDA) and jasmonic acid (JA). The enzyme is feedback-regulated by JA and thus a target of the JA-signalling pathway. A fusion genetic approach was used to isolate mutants in this signalling pathway. Seeds from transgenic Arabidopsis thaliana plants expressing the Escherichia coli uidA gene encoding beta-glucuronidase (GUS) under the control of the AOS promoter were mutagenized with ethylmethane sulfonate and the progeny was screened for individuals exhibiting constitutive expression of uidA in the absence of an added octadecanoid. From 21,000 mutagenized plants, 8 lines showing constitutive AOS expression were obtained. The mutant lines were characterized further and fell into four classes, I to IV. All showed signs of growth inhibition encompassing both shoot and root systems, and accumulated higher than normal levels of OPDA. Mutants belonging to classes I and IV failed to set seeds due to defects in flower development which prevented self-pollination. One mutant, designated cas1, was characterized in more detail and showed, in addition to elevated levels of AOS mRNA, AOS polypeptide, OPDA, and JA, constitutive expression of JA-responsive genes ( VSP2, PDF1.2). The cas1 mutation is recessive and affects a single locus. Using cleaved amplified polymorphic sequences (CAPS) and simple sequence length polymorphisms (SSLP), the mutated gene was mapped to chromosome IV next to the SSLP marker CIW7.

Jasmonates and related oxylipins in plant responses to pathogenesis and herbivory

The vigorous production of oxygenated fatty acids (oxylipins) is a characteristic response to pathogenesis and herbivory, and is often accompanied by the substantial release of small and reactive lipid-fragmentation products. Some oxylipins, most notably those of the jasmonate family, have key roles as potent regulators. Recent advances have been made in understanding oxylipin-regulated signal transduction in response to attack. Much jasmonate signaling takes place via a genetically defined signal network that is linked to the ethylene, auxin, and salicylic acid signal pathways, but a second aspect of jasmonate signaling is emerging. Some jasmonates and several newly discovered cyclopentenone lipids can activate or repress gene expression through the activities of a conserved electrophilic atom group.

Target genes for OBP3, a Dof transcription factor, include novel basic helix-loop-helix domain proteins inducible by salicylic acid

Overexpression of a salicylic-acid (SA)-inducible Arabidopsis DNA binding with one finger (Dof) transcription factor, called OBF-binding protein 3 (OBP3; AtDof3.6), has previously been shown to result in growth defects. In this study, suppressive subtraction hybridization (SSH) was used to isolate genes induced in an OBP3-overexpression line and several putative clones, called OBP3-responsive genes (ORGs), were isolated. The link with the induced expression levels of these genes and OBP3 overexpression was confirmed by analysing additional OBP3-overexpression lines. ORG1 through ORG4 are novel genes, while ORG5 is an extensin gene, AtExt1. While ORG4 has no similarity with other proteins in the database, ORG1 has weak similarity in different regions of the predicted protein with CDC2 and fibrillin. ORG2 and ORG3 share 80% overall identity in their deduced amino acid sequences and contain a basic helix-loop-helix DNA-binding domain, suggesting that ORG2 and ORG3 may be transcription factors. The expression of the ORG1, ORG2 and ORG3 genes was co-regulated under all conditions examined including upregulation by SA and downregulation by jasmonic acid (JA). Fifteen OBP3-silenced lines were generated to further explore the function of OBP3. Although there were no visible phenotypic changes in any of these lines, the expression of ORG1, ORG2 and ORG3 was reduced. Among the ORG genes, ORG1, ORG2 and ORG3 contained the highest number of potential Dof-binding sites in the promoter region, and their expression was significantly increased within 3 h after induction of OBP3 expression using an inducible promoter system, and closely reflected the expression levels of the exogenous OBP3 protein. The results from the gain-of-function and loss-of-function experiments suggest that the ORG1, ORG2 and ORG3 genes are direct target genes of OBP3.

The substrate specificity-determining amino acid code of 4-coumarate:CoA ligase

To reveal the structural principles determining substrate specificity of 4-coumarate:CoA ligase (4CL), the crystal structure of the phenylalanine activation domain of gramicidin S synthetase was used as a template for homology modeling. According to our model, 12 amino acid residues lining the Arabidopsis 4CL isoform 2 (At4CL2) substrate binding pocket (SBP) function as a signature motif generally determining 4CL substrate specificity. We used this substrate specificity code to create At4CL2 gain-of-function mutants. By increasing the space within the SBP we generated ferulic- and sinapic acid-activating At4CL2 variants. Increasing the hydrophobicity of the SBP resulted in At4CL2 variants with strongly enhanced conversion of cinnamic acid. These enzyme variants are suitable tools for investigating and influencing metabolic channeling mediated by 4CL. Knowledge of the 4CL specificity code will facilitate the prediction of substrate preference of numerous, still uncharacterized 4CL-like proteins.

Methyl jasmonate as a vital substance in plants

The plant floral scent methyl jasmonate (MeJA) has been identified as a vital cellular regulator that mediates diverse developmental processes and defense responses against biotic and abiotic stresses. The pleiotropic effects of MeJA have raised numerous questions about its regulation for biogenesis and mode of action. Characterization of the gene encoding jasmonic acid carboxyl methyltransferase has provided basic information on the role(s) of this phytohormone in gene-activation control and systemic long-distance signaling. Recent approaches using functional genomics and bioinformatics have identified a whole set of MeJA-responsive genes, and provide insights into how plants use volatile signals to withstand diverse and variable environments.

Arabidopsis contains a large superfamily of acyl-activating enzymes. Phylogenetic and biochemical analysis reveals a new class of acyl-coenzyme a synthetases

Acyl-activating enzymes are a diverse group of proteins that catalyze the activation of many different carboxylic acids, primarily through the formation of a thioester bond. This group of enzymes is found in all living organisms and includes the acyl-coenzyme A synthetases, 4-coumarate:coenzyme A ligases, luciferases, and non-ribosomal peptide synthetases. The members of this superfamily share little overall sequence identity, but do contain a 12-amino acid motif common to all enzymes that activate their acid substrates using ATP via an enzyme-bound adenylate intermediate. Arabidopsis possesses an acyl-activating enzyme superfamily containing 63 different genes. In addition to the genes that had been characterized previously, 14 new cDNA clones were isolated as part of this work. The protein sequences were compared phylogenetically and grouped into seven distinct categories. At least four of these categories are plant specific. The tissue-specific expression profiles of some of the genes of unknown function were analyzed and shown to be complex, with a high degree of overlap. Most of the plant-specific genes represent uncharacterized aspects of carboxylic acid metabolism. One such group contains members whose enzymes activate short- and medium-chain fatty acids. Altogether, the results presented here describe the largest acyl-activating enzyme family present in any organism thus far studied at the genomic level and clearly indicate that carboxylic acid activation metabolism in plants is much more complex than previously thought.

Characterization of the early response of Arabidopsis to Alternaria brassicicola infection using expression profiling

All tested accessions of Arabidopsis are resistant to the fungal pathogen Alternaria brassicicola. Resistance is compromised by pad3 or coi1 mutations, suggesting that it requires the Arabidopsis phytoalexin camalexin and jasmonic acid (JA)-dependent signaling, respectively. This contrasts with most well-studied Arabidopsis pathogens, which are controlled by salicylic acid-dependent responses and do not benefit from absence of camalexin or JA. Here, mutants with defects in camalexin synthesis (pad1, pad2, pad3, and pad5) or in JA signaling (pad1, coi1) were found to be more susceptible than wild type. Mutants with defects in salicylic acid (pad4 and sid2) or ethylene (ein2) signaling remained resistant. Plant responses to A. brassicicola were characterized using expression profiling. Plants showed dramatic gene expression changes within 12 h, persisting at 24 and 36 h. Wild-type and pad3 plants responded similarly, suggesting that pad3 does not have a major effect on signaling. The response of coi1 plants was quite different. Of the 645 genes induced by A. brassicicola in wild-type and pad3 plants, 265 required COI1 for full expression. It is likely that some of the COI1-dependent genes are important for resistance to A. brassicicola. Responses to A. brassicicola were compared with responses to Pseudomonas syringae infection. Despite the fact that these pathogens are limited by different defense responses, approximately 50% of the induced genes were induced in response to both pathogens. Among these, requirements for COI1 were consistent after infection by either pathogen, suggesting that the regulatory effect of COI1 is similar regardless of the initial stimulus.

Biochemical and molecular characterization of a hydroxyjasmonate sulfotransferase from Arabidopsis thaliana

12-Hydroxyjasmonate, also known as tuberonic acid, was first isolated from Solanum tuberosum and was shown to have tuber-inducing properties. It is derived from the ubiquitously occurring jasmonic acid, an important signaling molecule mediating diverse developmental processes and plant defense responses. We report here that the gene AtST2a from Arabidopsis thaliana encodes a hydroxyjasmonate sulfotransferase. The recombinant AtST2a protein was found to exhibit strict specificity for 11- and 12-hydroxyjasmonate with K(m) values of 50 and 10 microm, respectively. Furthermore, 12-hydroxyjasmonate and its sulfonated derivative are shown to be naturally occurring in A. thaliana. The exogenous application of methyljasmonate to A. thaliana plants led to increased levels of both metabolites, whereas treatment with 12-hydroxyjasmonate led to increased level of 12-hydroxyjasmonate sulfate without affecting the endogenous level of jasmonic acid. AtST2a expression was found to be induced following treatment with methyljasmonate and 12-hydroxyjasmonate. In contrast, the expression of the methyljasmonate-responsive gene Thi2.1, a marker gene in plant defense responses, is not induced upon treatment with 12-hydroxyjasmonate indicating the existence of independent signaling pathways responding to jasmonic acid and 12-hydroxyjasmonic acid. Taken together, the results suggest that the hydroxylation and sulfonation reactions might be components of a pathway that inactivates excess jasmonic acid in plants. Alternatively, the function of AtST2a might be to control the biological activity of 12-hydroxyjasmonic acid.

Negative cross-talk between salicylate- and jasmonate-mediated pathways in the Wassilewskija ecotype of Arabidopsis thaliana

Plants often respond to attack by insect herbivores and necrotrophic pathogens with induction of jasmonate-dependent resistance traits, but respond to attack by biotrophic pathogens with induction of salicylate-dependent resistance traits. To assess the degree to which the jasmonate- and salicylate-dependent pathways interact, we compared pathogenesis-related protein activity and bacterial performance in four mutant Arabidopsis thaliana lines relative to their wild-type backgrounds. We found that two salicylate-dependent pathway mutants (cep1, nim1-1) exhibited strong effects on the growth of the generalist biotrophic pathogen, Pseudomonas syringae pv. tomato, whereas two jasmonate-dependent pathway mutants (fad3-2fad7-2fad8, jar1-1) did not. Leaf peroxidase and exochitinase activity were negatively correlated with bacterial growth, whereas leaf polyphenol oxidase activity and trypsin inhibitor concentration were not. Interestingly, leaf total glucosinolate concentration was positively correlated with bacterial growth. In the same experiment, we also found that application of jasmonic acid generally increased leaf peroxidase activity and trypsin inhibitor concentration in the mutant lines. However, the cep1 mutant, shown previously to overexpress salicylic acid, exhibited no detectable biological or chemical responses to jasmonic acid, suggesting that high levels of salicylic acid may have inhibited a plant response. In a second experiment, we compared the effect of jasmonic acid and/or salicylic acid on two ecotypes of A. thaliana. Application of salicylic acid to the Wassilewskija ecotype decreased bacterial growth. However, this effect was not observed when both salicylic acid and jasmonic acid were applied, suggesting that jasmonic acid negated the beneficial effect of salicylic acid. Collectively, our results confirm that the salicylate-dependent pathway is more important than the jasmonate-dependent pathway in determining growth of P. syringae pv. tomato in A. thaliana, and suggest important negative interactions between these two major defensive pathways in the Wassilewskija ecotype. In contrast, the Columbia ecotype exhibited little evidence of negative interactions between the two pathways, suggesting intraspecific variability in how these pathways interact in A. thaliana.

Dissecting the phytochrome A-dependent signaling network in higher plants

Plants monitor their ambient light environment using a network of photoreceptors. In Arabidopsis, phytochrome A (phyA) is the primary photoreceptor responsible for perceiving and mediating various responses to far-red light. Several breakthroughs in understanding the signaling network mediating phyA-activated responses have been made in recent years. Here, we highlight several key advances: the demonstration that light regulates nuclear translocation of phyA and its associated kinase activity; the revelation of a transcriptional cascade controlling phyA-regulated gene expression; the detection of a direct interaction between phyA and a transcription factor; and the identification and characterization of many phyA-specific signaling intermediates, some of them suggesting the involvement of the ubiquitin-proteasome pathway.

Phytochrome-hormonal signalling networks

Through time, plants have evolved an extraordinary ability to interpret environmental cues. One of the most reliable of these cues is light, and plants are particularly adept at sensing and translating environmental light signals. The phytochrome family of photoreceptors monitor cues such as daylength or vegetative shade and adjust development to reflect change in these parameters. Indeed, it is their ability to coordinate these complex developmental changes that underpins the remarkable success of plants. Evidence is mounting that hormones control many of these light-mediated changes. Therefore, if we are to understand how light manipulates development we need to explore the interplay between light and hormonal signalling. Toward this goal, this review highlights the known convergence points of the phytochrome and the hormonal networks and explores their interactions. Contents Summary 449 I. Introduction 449 II. The phytochrome protein 450 III. Bacteriophytochromes 450 IV. IBacteriophytochrome signalling 450 V. Plant phytochrome signalling 451 VI. Ethylene perception and signalling 451 VII. Cytokinin perception and signalling 452 VIII. Brassinosteroid perception and signalling 453 IX. Gibberellin signalling 455 X. Auxin signalling 456 XI. Proteolysis in light and hormonal signalling 458 XII. Conclusion 459 Acknowledgements 459 References 459.

An Arabidopsis mutant defective in jasmonate response is allelic to the auxin-signaling mutant axr1

A screen for Arabidopsis mutants that were insensitive to methyl jasmonate (MeJA) in an assay for seedling root growth yielded only alleles of previously isolated mutants jar1 and coi1, with one exception. Mapping of the locus and morphological characterization of the new mutant suggested it might be allelic to axr1, which had not previously been reported to show resistance to MeJA. The F(1) from a cross of the new mutant with axr1-3 did not show complementation, confirming that these are the same genes. The new allele is called axr1-24. In addition to MeJA and indole-3-acetic acid (IAA), axr1-24 had decreased sensitivity to 1-aminocyclopropane-1-carboxylic acid, 6-benzylamino-purine, epi-brassinolide, and abscisic acid. Both axr1-24 and the previously characterized axr1-3 allele were shown to be susceptible to the opportunistic pathogen Pythium irregulare, a trait found in other jasmonate response mutants, including jar1-1. The double mutant jar1-1/axr1-3 was more resistant to inhibition of root growth by MeJA and was more susceptible to P. irregulare infection than either single mutant, suggesting these genes might act in independent response pathways. In contrast, resistance to IAA in the double mutant was not different from axr1-3. Northern-blot analysis showed that IAA induced the jasmonate-responsive lipoxygenase 2, AOS, and AtVSP gene transcripts and induction was strongly impaired in axr1-3. However, transcript induction by MeJA was only minimally affected in axr1-3. This study demonstrates that in addition to auxin signaling, the AXR1 locus is involved in MeJA response, providing a mechanistic link between jasmonate and auxin-signaling pathways.