Jasmonic acid signaling modulates ozone-induced hypersensitive cell death

Heat stress results in loss of chloroplast Cu/Zn superoxide dismutase and increased damage to Photosystem II in combined drought-heat stressed Lotus japonicus

Drought and heat stress have been studied extensively in plants, but most reports involve analysis of response to only one of these stresses. Studies in which both stresses were studied in combination have less commonly been reported. We report the combined effect of drought and heat stress on Photosystem II (PSII) of Lotus japonicus cv. Gifu plants. Photochemistry of PSII was not affected by drought or heat stress alone, but the two stresses together decreased PSII activity as determined by fluorescence emission. Heat stress alone resulted in degradation of D1 and CP47 proteins, and D2 protein was also degraded by combined drought-heat stress. None of these proteins were degraded by drought stress alone. Drought alone induced accumulation of hydrogen peroxide but the drought-heat combination led to an increase in superoxide levels and a decrease in hydrogen peroxide levels. Furthermore, combined drought-heat stress was correlated with an increase in oxidative damage as determined by increased levels of thiobarbituric acid reactive substances. Heat also induced degradation of chloroplast Cu/Zn superoxide dismutase (SOD: EC 1.15.1.1) as shown by reduced protein levels and isozyme-specific SOD activity. Loss of Cu/Zn SOD and induction of catalase (CAT: EC 1.11.1.6) activity would explain the altered balance between hydrogen peroxide and superoxide in response to drought vs combined drought-heat stress. Degradation of PSII could thus be caused by the loss of components of chloroplast antioxidant defence systems and subsequent decreased function of PSII. A possible explanation for energy dissipation by L. japonicus under stress conditions is discussed.

Ectopic expression of the rice lumazine synthase gene contributes to defense responses in transgenic tobacco

Lumazine synthase (LS) catalyzes the penultimate reaction in the multistep riboflavin biosynthesis pathway, which is involved in plant defenses. Plant defenses are often subject to synergistic effects of jasmonic acid and ethylene whereas LS is a regulator of jasmonic acid signal transduction. However, little is known about whether the enzyme contributes to defense responses. To study the role of LS in plant pathogen defenses, we generated transgenic tobacco expressing the rice (Oryza sativa) LS gene, OsLS. OsLS was cloned and found to have strong identity with its homologues in higher plants and less homology to microbial orthologues. The OsLS protein localized to chloroplasts in three OsLS-expressing transgenic tobacco (LSETT) lines characterized as enhanced in growth and defense. Compared with control plants, LSETT had higher content of both riboflavin and the cofactors flavin mononucleotide and flavin adenine dinucleotide. In LSETT, jasmonic acid and ethylene were elevated, the expression of defense-related genes was induced, levels of resistance to pathogens were enhanced, and resistance was effective to viral, bacterial, and oomycete pathogens. Extents of OsLS expression correlated with increases in flavin, jasmonic acid, and ethylene content, and correlated with increases in resistance levels, suggesting a role for OsLS in defense responses.

Natural variation in ozone sensitivity among Arabidopsis thaliana accessions and its relation to stomatal conductance

Genetic variation between naturally occurring populations provides a unique source to unravel the complex mechanisms of stress tolerance. Here, we have analysed O(3) sensitivity of 93 natural Arabidopsis thaliana accessions together with five O(3)-sensitive mutants to acute O(3) exposure. The variation in O(3) sensitivity among the natural accessions was much higher than among the O(3)-sensitive mutants and corresponding wild types. A subset of nine accessions with major variation in their O(3) responses was studied in more detail. Among the traits assayed, stomatal conductance (g(st)) was an important factor determining O(3) sensitivity of the selected accessions. The most O(3)-sensitive accession, Cvi-0, had constitutively high g(st), leading to high initial O(3) uptake rate and dose received during the first 30 min of exposure. Analyzing O(3)-induced changes in stress hormone concentrations indicated that jasmonate (JA) concentration was also positively correlated with leaf damage. Quantitative trait loci (QTL) mapping in a Col-0 x Cvi-0 recombinant inbred line (RIL) population identified three QTLs for O(3) sensitivity, and one for high water loss of Cvi-0. The major O(3) QTL mapped to the same position as the water loss QTL further supporting the role of stomata in regulating O(3) entry and damage.

Functional characterization of Arabidopsis thaliana WRKY39 in heat stress

Arabidopsis thaliana WRKY39, a transcription factor that is induced by heat stress, is a member of the group II WRKY proteins and responds to both abiotic and biotic stress. Heat-treated seeds and plants of WRKY39 knock-down mutants had increased susceptibility to heat stress, showing reduced germination, decreased survival, and elevated electrolyte leakage compared with wild-type plants. In contrast, WRKY39 over-expressing plants exhibited enhanced thermotolerance compared with wild-type plants. RT-PCR and qRT-PCR analysis of wrky39 mutants and WRKY39 over-expressing plants identified putative genes regulated by WRKY39. Consistent with a role for WRKY39 in heat tolerance, the expression levels of salicylic acid (SA)-regulated PR1 and SA-related MBF1c genes were downregulated in wrky39 mutants. In contrast, over-expression of WRKY39 increased the expression of PR1 and MBF1c. The WRKY39 transcript was induced in response to treatment with SA or methyljasmonate. Analysis of heat stress-induced WRKY39 in defense signaling mutants, including coi1, ein2, and sid2, further indicated that WRKY39 was positively co-regulated by the SA and jasmonate (JA) signaling pathways. Together, these findings reveal that heat stress-induced WRKY39 positively regulates the cooperation between the SA- and JA-activated signaling pathways that mediate responses to heat stress.

No interaction between methyl jasmonate and ozone in Pima cotton: growth and allocation respond independently to both

Ozone (O3) is damaging to plants, inducing signalling pathways involving antagonism between jasmonates and ethylene. These pathways mediate O3 responses, particularly to acute exposure, and their manipulation protected several species against acute and chronic O3. We use chronic daily exposure of up to 163 ppb O3, and twice weekly application of up to 320 microg plant(-1) methyl jasmonate (MeJA) to test two hypothesizes: 1) a low rate of MeJA does not affect growth but increases O3 sensitivity; 2) a high rate inhibits growth but reduces O3 sensitivity. Both hypotheses were rejected. Growth declined with increases in both MeJA and O3. MeJA at 40 microg plant(-1) caused no direct effect, and at 160 microg plant(-1) reduced growth similarly at all O3. Neither rate altered O3 sensitivity. These additive responses are not consistent with protection by MeJA in this system. They may reflect inter-specific differences in signalling, since O3 concentrations used here exceeded some reported acute exposures. Alternatively, parallel responses to O3 and MeJA may suggest that O3-induced jasmonates play a developmental role in chronic response but no protective role in the absence of lesions characteristic of acute exposure. MeJA appears useful as a probe of these mechanisms.

Proteomics study of COI1-regulated proteins in Arabidopsis flower

Jasmonates (JAs) are a new class of plant hormone that regulate expression of diverse genes to mediate various plant responses. The Arabidopsis F-box protein COI1 is required for plant defense and male fertility in JA signal pathway. To further investigate the regulatory role of COI1 in male fertility, we compared the proteomics profiles of Arabidopsis wild type (WT) flowers with coi1-1 mutant male-sterile flowers using two-dimensional difference gel electrophoresis coupled with matrix-assisted laser desoption/ionization-time-of-flight mass spectrometry. Sixteen proteins with potential function in specific biological processes such as metabolism processes and defense/stress responses were differentially expressed in WT and coi1-1 mutant flowers. Verification on a phi class glutathione transferase AtGSTF9, one out of these 16 identified proteins, revealed that the expression of AtGSTF9 was severely downregulated in flowers of coi1-1 mutant compared with that of WT. Further function analyses of these genes would provide new insights into the molecular basis of COI1-regulated male fertility.

Jasmonate and phytochrome A signaling in Arabidopsis wound and shade responses are integrated through JAZ1 stability

Jasmonate (JA) activates plant defense, promotes pollen maturation, and suppresses plant growth. An emerging theme in JA biology is its involvement in light responses; here, we examine the interdependence of the JA- and light-signaling pathways in Arabidopsis thaliana. We demonstrate that mutants deficient in JA biosynthesis and signaling are deficient in a subset of high irradiance responses in far-red (FR) light. These mutants display exaggerated shade responses to low, but not high, R/FR ratio light, suggesting a role for JA in phytochrome A (phyA) signaling. Additionally, we demonstrate that the FR light-induced expression of transcription factor genes is dependent on CORONATINE INSENSITIVE1 (COI1), a central component of JA signaling, and is suppressed by JA. phyA mutants had reduced JA-regulated growth inhibition and VSP expression and increased content of cis-(+)-12-oxophytodienoic acid, an intermediate in JA biosynthesis. Significantly, COI1-mediated degradation of JASMONATE ZIM DOMAIN1-beta-glucuronidase (JAZ1-GUS) in response to mechanical wounding and JA treatment required phyA, and ectopic expression of JAZ1-GUS resulted in exaggerated shade responses. Together, these results indicate that JA and phyA signaling are integrated through degradation of the JAZ1 protein, and both are required for plant responses to light and stress.

Gibberellin and jasmonate crosstalk during stamen development

Gibberellin (GA) and jasmonate (JA) are two types of phytohormones that play important roles during stamen development. For example, Arabidopsis plants deficient in either of GA or JA develop short stamens. An apparent question to ask is whether GA action and JA action during stamen filament development are independent of each other or are in a hierarchy. Recent studies showed that GA modulates the expression of genes essential for JA biosynthesis to promote JA production and high levels of JA will induce the expression of three MYB genes MYB21, MYB24 and MYB57. These three MYB genes are crucial factors for the normal development of stamen filament in Arabidopsis.

Silencing OsHI-LOX makes rice more susceptible to chewing herbivores, but enhances resistance to a phloem feeder

The jasmonic acid (JA) pathway plays a central role in plant defense responses against insects. Some phloem-feeding insects also induce the salicylic acid (SA) pathway, thereby suppressing the plant's JA response. These phenomena have been well studied in dicotyledonous plants, but little is known about them in monocotyledons. We cloned a chloroplast-localized type 2 13-lipoxygenase gene of rice, OsHI-LOX, whose transcripts were up-regulated in response to feeding by the rice striped stem borer (SSB) Chilo suppressalis and the rice brown planthopper (BPH) Niaparvata lugens, as well as by mechanical wounding and treatment with JA. Antisense expression of OsHI-LOX (as-lox) reduced SSB- or BPH-induced JA and trypsin protease inhibitor (TrypPI) levels, improved the larval performance of SBB as well as that of the rice leaf folder (LF) Cnaphalocrocis medinalis, and increased the damage caused by SSB and LF larvae. In contrast, BPH, a phloem-feeding herbivore, showed a preference for settling and ovipositing on WT plants, on which they consumed more and survived better than on as-lox plants. The enhanced resistance of as-lox plants to BPH infestation correlated with higher levels of BPH-induced H(2)O(2) and SA, as well as with increased hypersensitive response-like cell death. These results imply that OsHI-LOX is involved in herbivore-induced JA biosynthesis, and plays contrasting roles in controlling rice resistance to chewing and phloem-feeding herbivores. The observation that suppression of JA activity results in increased resistance to an insect indicates that revision of the generalized plant defense models in monocotyledons is required, and may help develop novel strategies to protect rice against insect pests.

Top hits in contemporary JAZ: an update on jasmonate signaling

The phytohormone jasmonate (JA) regulates a wide range of growth, developmental, and defense-related processes during the plant life cycle. Identification of the JAZ family of proteins that repress JA responses has facilitated rapid progress in understanding how this lipid-derived hormone controls gene expression. Recent analysis of JAZ proteins has provided insight into the nature of the JA receptor, the chemical specificity of signal perception, and cross-talk between JA and other hormone response pathways. Functional diversification of JAZ proteins by alternative splicing, together with the ability of JAZ proteins to homo- and heterodimerize, provide mechanisms to enhance combinatorial diversity and versatility in gene regulation by JA.

Reactive electrophilic oxylipins: pattern recognition and signalling

Oxidized lipids in plants comprise a variety of reactive electrophiles that contain an alpha,beta-unsaturated carbonyl group. While some of these compounds are formed enzymatically, many of them are formed by non-enzymatic pathways. In addition to their chemical reactivity/toxicity low levels of these compounds are also biologically active. Despite their structural diversity and biosynthetic origin, common biological activities such as induction of defense genes, activation of detoxification responses and growth inhibition have been documented. However, reactive electrophilic oxylipins are poorly defined as a class of compounds but have at least two properties in common, i.e., lipophilicity and thiol-reactivity. Thiol-reactivity is a property of reactive oxylipins (RES) shared by reactive oxygen and nitrogen species (ROS and RNS) and enables these agents to modify proteins in vivo. Thiol-modification is assumed to represent a key mechanism involved in signal transduction. A metaanalysis of proteomic studies reveals that RES oxylipins, ROS and RNS apparently chemically modify a similar set of highly sensitive proteins, virtually all of which are targets for thioredoxins. Moreover, most of these proteins are redox-regulated, i.e., posttranslational thiol-modification alters the activity or function of these proteins. On the transcriptome level, effects of RES oxylipins and ROS on gene induction substantially overlap but are clearly different. Besides electrophilicity other structural properties such as target affinity apparently determine target selectivity and biological activity. In this context, different signalling mechanisms and signal transduction components identified in plants and non-plant organisms as well as putative functions of RES oxylipins are discussed.

Jasminum polyanthum Franch. as a natural source of (-)-methyl jasmonate: an alternative to the use of the synthetic standard

INTRODUCTION

Methyl jasmonate (MJ) contains two chiral centres at C-3 and C-7 in its chemical structure, which implies that it can exist in four possible stereoisomeric forms, namely (+)-MJ, (-)-MJ, (+)-epiMJ and (-)-epiMJ. The absolute configuration of the two side chains of MJ affects the biological activity associated with this compound.

OBJECTIVE

To isolate pure (-)-MJ from a natural source, Jasminum polyanthum Franch., with the intention of increasing the knowledge about its biological properties, including its effect on the biosynthesis of plant metabolites.

METHODOLOGY

The method used was based on steam distillation extraction (SDE) as an extraction technique followed by high-performance liquid chromatography (HPLC) as a purification procedure. The HPLC flow-rate as well as the number of fractions accumulated were optimised to achieve the concentration and purity required.

RESULTS

The employment of 0.3 mL/min as HPLC flow-rate and the accumulation of three HPLC fractions allowed the required enantiomeric purity (95%) and concentration (0.36 mg/L in each HPLC fraction) to efficiently obtain (-)-methyl jasmonate from Jasminum polyanthum Franch. to be achieved.

CONCLUSION

The approach proposed may enable the properties and effect of pure (-)-MJ on plant responses to be studied. The use of a natural source to obtain (-)-MJ is presented as an alternative to the enantioselective synthesis and enantiomeric resolution from the standard racaemic mixture.

A previously unknown zinc finger protein, DST, regulates drought and salt tolerance in rice via stomatal aperture control

Abiotic stresses, such as drought and salinity, lead to crop growth damage and a decrease in crop yields. Stomata control CO(2) uptake and optimize water use efficiency, thereby playing crucial roles in abiotic stress tolerance. Hydrogen peroxide (H(2)O(2)) is an important signal molecule that induces stomatal closure. However, the molecular pathway that regulates the H(2)O(2) level in guard cells remains largely unknown. Here, we clone and characterize DST (drought and salt tolerance)-a previously unknown zinc finger transcription factor that negatively regulates stomatal closure by direct modulation of genes related to H(2)O(2) homeostasis-and identify a novel pathway for the signal transduction of DST-mediated H(2)O(2)-induced stomatal closure. Loss of DST function increases stomatal closure and reduces stomatal density, consequently resulting in enhanced drought and salt tolerance in rice. These findings provide an interesting insight into the mechanism of stomata-regulated abiotic stress tolerance, and an important genetic engineering approach for improving abiotic stress tolerance in crops.

Ozone responsive genes in Medicago truncatula: analysis by suppression subtraction hybridization

Acute ozone is a model abiotic elicitor of oxidative stress in plants. In order to identify genes that are important for conferring ozone resistance or sensitivity we used two accessions of Medicago truncatula with contrasting responses to this oxidant. We used suppression subtraction hybridization (SSH) to identify genes differentially expressed in ozone-sensitive Jemalong and ozone-resistant JE154 following exposure to 300 nLL(-1) of ozone for 6h. Following differential screening of more than 2500 clones from four subtraction libraries, more than 800 clones were selected for sequencing. Sequence analysis of these clones identified 239 unique contigs. Fifteen novel genes of unknown functions were identified. A majority of the ozone responsive genes identified in this study were present in the Medicago truncatula EST collections. Genes induced in JE154 were associated with adaptive responses to stress, while in Jemalong, the gene ontologies for oxidative stress, cell growth, and translation were enriched. A meta-analysis of ozone responsive genes using the Genvestigator program indicated enrichment of ABA and auxin responsive genes in JE154, while cytokinin response genes were induced in Jemalong. In resistant JE154, down regulation of photosynthesis-related genes and up regulation of genes responding to low nitrate leads us to speculate that lowering carbon-nitrogen balance may be an important resource allocation strategy for overcoming oxidative stress. Temporal profiles of select genes using real-time PCR analysis showed that most of the genes in Jemalong were induced at the later time points and is consistent with our earlier microarray studies. Inability to mount an early active transcriptional reprogramming in Jemalong may be the cause for an inefficient defense response that in turn leads to severe oxidative stress and culminates in cell death.

Plant oxylipins: role of jasmonic acid during programmed cell death, defence and leaf senescence

Plants are continuously challenged by a variety of abiotic and biotic cues. To deter feeding insects, nematodes and fungal and bacterial pathogens, plants have evolved a plethora of defence strategies. A central player in many of these defence responses is jasmonic acid. It is the aim of this minireview to summarize recent findings that highlight the role of jasmonic acid during programmed cell death, plant defence and leaf senescence.

NbHB1, Nicotiana benthamiana homeobox 1, is a jasmonic acid-dependent positive regulator of pathogen-induced plant cell death

Induction of cell death is an important component of plant defense against pathogens. There have been many reports on the role of phytohormones in pathogen-induced cell death, but jasmonic acid (JA) has not been implicated as a regulator of the response. Here, we report the function of NbHB1, Nicotiana benthamiana homeobox1, in pathogen-induced cell death in connection with JA signaling. Involvement of NbHB1 in cell death was analysed by gain- and loss-of-function studies using Agrobacterium-mediated transient overexpression and virus-induced gene silencing, respectively. Expression of NbHB1 following pathogen inoculations and various treatments was monitored by reverse transcription polymerase chain reaction. Transcript levels of NbHB1 were upregulated by infection with virulent and avirulent bacterial pathogens. Ectopic expression of NbHB1 accelerated cell death following treatment with darkness, methyl jasmonate, or pathogen inoculation. Conversely, when NbHB1 was silenced, pathogen-induced cell death was delayed. NbHB1-induced cell death was also delayed by silencing of NbCOI1, indicating a requirement for JA-mediated signaling. Overexpression of the domain-deleted proteins of NbHB1 revealed that the homeodomain, leucine zipper, and part of the variable N-terminal region were necessary for NbHB1 functionality. These results strongly suggest the role of NbHB1 in pathogen-induced plant cell death via the JA-mediated signaling pathway.

Ethylene and salicylic acid control glutathione biosynthesis in ozone-exposed Arabidopsis thaliana

Ozone produces reactive oxygen species and induces the synthesis of phytohormones, including ethylene and salicylic acid. These phytohormones act as signal molecules that enhance cell death in response to ozone exposure. However, some studies have shown that ethylene and salicylic acid can instead decrease the magnitude of ozone-induced cell death. Therefore, we studied the defensive roles of ethylene and salicylic acid against ozone. Unlike the wild-type, Col-0, Arabidopsis mutants deficient in ethylene signaling (ein2) or salicylic acid biosynthesis (sid2) generated high levels of superoxide and exhibited visible leaf injury, indicating that ethylene and salicylic acid can reduce ozone damage. Macroarray analysis suggested that the ethylene and salicylic acid defects influenced glutathione (GSH) metabolism. Increases in the reduced form of GSH occurred in Col-0 6 h after ozone exposure, but little GSH was detected in ein2 and sid2 mutants, suggesting that GSH levels were affected by ethylene or salicylic acid signaling. We performed gene expression analysis by real-time polymerase chain reaction using genes involved in GSH metabolism. Induction of gamma-glutamylcysteine synthetase (GSH1), glutathione synthetase (GSH2), and glutathione reductase 1 (GR1) expression occurred normally in Col-0, but at much lower levels in ein2 and sid2. Enzymatic activities of GSH1 and GSH2 in ein2 and sid2 were significantly lower than in Col-0. Moreover, ozone-induced leaf damage observed in ein2 and sid2 was mitigated by artificial elevation of GSH content. Our results suggest that ethylene and salicylic acid protect against ozone-induced leaf injury by increasing de novo biosynthesis of GSH.

The impact of atmospheric composition on plants: a case study of ozone and poplar

Tropospheric ozone is the main atmospheric pollutant that causes damages to trees. The estimation of the threshold for ozone risk assessment depends on the evaluation of the means that this pollutant impacts the plant and, especially, the foliar organs. The available results show that, before any visible symptom appears, carbon assimilation and the underlying metabolic processes are decreased under chronic ozone exposure. By contrast, the catabolic pathways are enhanced, and contribute to the supply of sufficient reducing power necessary to feed the detoxification processes. Reactive oxygen species delivered during ozone exposure serve as toxic compounds and messengers for the signaling system. In this review, we show that the contribution of genomic tools (transcriptomics, proteomics, and metabolomics) for a better understanding of the mechanistic cellular responses to ozone largely relies on spectrometric measurements.

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.

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.

Hydrogen peroxide is involved in the acclimation of the Mediterranean shrub, Cistus albidus L., to summer drought

This study evaluated the possible role of hydrogen peroxide (H(2)O(2)) in the acclimation of a Mediterranean shrub, Cistus albidus L., to summer drought growing under Mediterranean field conditions. For this purpose, changes in H(2)O(2) concentrations and localization throughout a year were analysed. H(2)O(2) changes in response to environmental conditions in parallel with changes in abscisic acid (ABA) and oxidative stress markers, together with lignin accumulation, xylem and sclerenchyma differentiation, and leaf area were also investigated. During the summer drought, leaf H(2)O(2) concentrations increased 11-fold, reaching values of 10 micromol g(-1) dry weight (DW). This increase occurred mainly in mesophyll cell walls, xylem vessels, and sclerenchyma cells in the differentiation stage. An increase in ABA levels preceded that of H(2)O(2), but both peaked at the same time in conditions of prolonged stress. C. albidus plants tolerated high concentrations of H(2)O(2) because of its localization in the apoplast of mesophyll cells, xylem vessels, and in differentiating sclerenchyma cells. The increase in ABA, and consequently of H(2)O(2), in plants subjected to drought stress might induce a 3.5-fold increase in ascorbic acid (AA), which maintained and even decreased its oxidative status, thus protecting plants from oxidative damage. After recovery from drought following late-summer and autumn rainfall, a decrease in ABA, H(2)O(2), and AA to their basal levels (approximately 60 pmol g(-1) DW, approximately 1 micromol g(-1) DW, and approximately 20 micromol g(-1) DW) was observed.

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.

Aluminum-induced cell wall peroxidase activity and lignin synthesis are differentially regulated by jasmonate and nitric oxide

Cassia tora is an annual legume and cultivated as a traditional medicinal herb for multiple therapies including regulation of blood pressure and blood lipid. Because of naturally occurring acidic soils in southeastern China, this plant species may possess strategies for tolerance to low pH and aluminum toxicity. In the search for the regulatory basis of biochemical response to Al, cell wall-bound peroxidases, including lignin-generated peroxidases and NADH oxidases, were investigated in the root tips of C. tora. Activities of both types of peroxidases significantly increased with Al concentrations. Analysis with native PAGE also demonstrated the strong induction of cell wall peroxidases by Al. The Al-induced increasing activities of peroxidases were closely correlated with lignin accumulation and H 2O 2 production. The biochemical effect of exogenous nitric oxide (NO) and methyl jasmonic acid (MJ) was examined to investigate signal properties and lignin synthesis under Al stress. Application of MJ at 10 microM promoted root sensitivity to Al by activating apoplastic peroxidase activity and accumulating H 2O 2 and lignin, whereas the opposite action was found for NO. The sensitivity of apoplastic peroxidases under Al stress was associated with the cross-talk of MJ and NO signals. The analysis reveals that the activity of lipoxygenase (an enzyme for MJ biosynthesis), with its transcripts increased in Al-exposed roots, was depressed by NO exposure. The effect of MJ on intracellular NO production was also investigated. It is shown that NO staining with 4,5-diaminofluorescein diacetate fluorescence was intensified by Al but was suppressed by MJ. These results suggest that NO and MJ may interplay in signaling the cell wall peroxidase activity and lignin synthesis in the roots exposed to Al.

Gene networks in plant ozone stress response and tolerance

For many plant species ozone stress has become much more severe in the last decade. The accumulating evidence for the significant effects of ozone pollutant on crop and forest yield situate ozone as one of the most important environmental stress factors that limits plant productivity worldwide. Today, transcriptomic approaches seem to give the best coverage of genome level responses. Therefore, microarray serves as an invaluable tool for global gene expression analyses, unravelling new information about gene pathways, in-species and cross-species gene expression comparison, and for the characterization of unknown relationships between genes. In this review we summarize the recent progress in the transcriptomics of ozone to demonstrate the benefits that can be harvested from the application of integrative and systematic analytical approaches to study ozone stress response. We focused our consideration on microarray analyses identifying gene networks responsible for response and tolerance to elevated ozone concentration. From these analyses it is now possible to notice how plant ozone defense responses depend on the interplay between many complex signaling pathways and metabolite signals.

Elevated ozone alters soybean-virus interaction

Increasing concentrations of ozone (O(3)) in the troposphere affect many organisms and their interactions with each other. To analyze the changes in a plant-pathogen interaction, soybean plants were infected with Soybean mosaic virus (SMV) while they were fumigated with O(3). In otherwise natural field conditions, elevated O(3) treatment slowed systemic infection and disease development by inducing a nonspecific resistance against SMV for a period of 3 weeks. During this period, the negative effect of virus infection on light-saturated carbon assimilation rate was prevented by elevated O(3) exposure. To identify the molecular basis of a soybean nonspecific defense response, high-throughput gene expression analysis was performed in a controlled environment. Transcripts of fungal, bacterial, and viral defense-related genes, including PR-1, PR-5, PR-10, and EDS1, as well as genes of the flavonoid biosynthesis pathways (and concentrations of their end products, quercetin and kaempherol derivatives) increased in response to elevated O(3). The drastic changes in soybean basal defense response under altered atmospheric conditions suggest that one of the elements of global change may alter the ecological consequences and, eventually, coevolutionary relationship of plant-pathogen interactions in the future.

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.

Monoclonal antibody-based enzyme linked immunosorbent assay for the analysis of jasmonates in plants

Methyl jasmonate (MeJA) and its free-acid form, jasmonic acid (JA) are naturally occurring plant growth regulators widely distributed in higher plants. In order to improve the sensitivity for the analysis of MeJA at low levels in small amounts of plant samples, a monoclonal antibody (MAb) (designated as MAb 3E(5)D(7)C(4)B(6)) against MeJA was derived from a JA-bovine serum albumin (BSA) conjugate as an immunogen. The antibody belongs to the IgG(1) subclass with a kappa type light chain and has a dissociation constant of approximately 6.07 x 10(-9) M. MAb3E(5)D(7)C(4)B(6) is very specific to MeJA. It was used to develop a direct competitive enzyme-linked immunosorbent assay (dcELISA), conventional and simplified indirect competitive ELISAs (icELISA). JA was derivatized into MeJA for the ELISA analysis. The IC(50) value and detection range for MeJA were, respectively, 34 and 4-257 ng/mL by the conventional icELISA, 21 and 3-226 ng/mL by the simplified icELISA and 5.0 and 0.7-97.0 ng/mL by the dcELISA. The dcELISA was more sensitive than either the conventional or simplified icELISA. The assays were used to measure the content of jasmonates as MeJA in tobacco leaves under drought stress or inoculated with tobacco mosaic virus and tomato leaves inoculated with tomato mosaic virus or Lirioinyza sativae Blanchard as compared with the corresponding healthy leaves. The increased jasmonates content indicated its role in response to the drought stress and pathogens.

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.

Kinetics of salicylate-mediated suppression of jasmonate signaling reveal a role for redox modulation

Cross talk between salicylic acid (SA) and jasmonic acid (JA) signaling pathways plays an important role in the regulation and fine tuning of induced defenses that are activated upon pathogen or insect attack. Pharmacological experiments revealed that transcription of JA-responsive marker genes, such as PDF1.2 and VSP2, is highly sensitive to suppression by SA. This antagonistic effect of SA on JA signaling was also observed when the JA pathway was biologically activated by necrotrophic pathogens or insect herbivores, and when the SA pathway was triggered by a biotrophic pathogen. Furthermore, all 18 Arabidopsis (Arabidopsis thaliana) accessions tested displayed SA-mediated suppression of JA-responsive gene expression, highlighting the potential significance of this phenomenon in induced plant defenses in nature. During plant-attacker interactions, the kinetics of SA and JA signaling are highly dynamic. Mimicking this dynamic response by applying SA and methyl jasmonate (MeJA) at different concentrations and time intervals revealed that PDF1.2 transcription is readily suppressed when the SA response was activated at or after the onset of the JA response, and that this SA-JA antagonism is long lasting. However, when SA was applied more than 30 h prior to the onset of the JA response, the suppressive effect of SA was completely absent. The window of opportunity of SA to suppress MeJA-induced PDF1.2 transcription coincided with a transient increase in glutathione levels. The glutathione biosynthesis inhibitor l-buthionine-sulfoximine strongly reduced PDF1.2 suppression by SA, suggesting that SA-mediated redox modulation plays an important role in the SA-mediated attenuation of the JA signaling pathway.

The role of JAR1 in Jasmonoyl-L: -isoleucine production during Arabidopsis wound response

The Arabidopsis thaliana (L.) Heynh. JASMONATE RESISTANT 1( JAR1) locus is essential for pathogen defense, but its role in wound response has not been investigated. JAR1 encodes an enzyme that conjugates jasmonic acid (JA) to isoleucine, which was recently shown to function directly in CORONATINE INSENSITIVE 1 (COI1)-mediated signal transduction. Leaf wounding rapidly increased the level of JA-Ile by about 60-fold to a peak of 279 pmole/g FW at 40 min after wounding. Conjugates with Leu, Val and Phe remained near basal level or were not detected. Kinetic analysis showed that JAR1 had a K (m) of 0.03 mM for Ile, which was 60-80-fold lower than for Leu, Val and Phe. JA-Ile accumulated mostly near the wound site with a minor increase in unwounded portions of wounded leaves. JAR1 transcript also increased dramatically in wounded tissue, reaching a maximum after about 1 h. In the jar1-1 mutant JA-Ile was only about 10% of the WT level at 40 min after leaf wounding, and reached a maximum of 47 pmole/g FW at 2 h. However, the reduced accumulation of JA-Ile had little or no effect on several jasmonate-dependent wound-induced genes. Wound induction of the VSP2 transcript was only slightly delayed while transcripts for LOX2, PDF1.2, WRKY33, TAT3 and CORI3 were unaffected. These results suggest that the rapid increase in JA-Ile mediated by the JAR1 enzyme plays only a minor role in transcriptional modulation of genes induced by mechanical wounding.

The role of JAR1 in Jasmonoyl-L: -isoleucine production during Arabidopsis wound response

The Arabidopsis thaliana (L.) Heynh. JASMONATE RESISTANT 1( JAR1) locus is essential for pathogen defense, but its role in wound response has not been investigated. JAR1 encodes an enzyme that conjugates jasmonic acid (JA) to isoleucine, which was recently shown to function directly in CORONATINE INSENSITIVE 1 (COI1)-mediated signal transduction. Leaf wounding rapidly increased the level of JA-Ile by about 60-fold to a peak of 279 pmole/g FW at 40 min after wounding. Conjugates with Leu, Val and Phe remained near basal level or were not detected. Kinetic analysis showed that JAR1 had a K (m) of 0.03 mM for Ile, which was 60-80-fold lower than for Leu, Val and Phe. JA-Ile accumulated mostly near the wound site with a minor increase in unwounded portions of wounded leaves. JAR1 transcript also increased dramatically in wounded tissue, reaching a maximum after about 1 h. In the jar1-1 mutant JA-Ile was only about 10% of the WT level at 40 min after leaf wounding, and reached a maximum of 47 pmole/g FW at 2 h. However, the reduced accumulation of JA-Ile had little or no effect on several jasmonate-dependent wound-induced genes. Wound induction of the VSP2 transcript was only slightly delayed while transcripts for LOX2, PDF1.2, WRKY33, TAT3 and CORI3 were unaffected. These results suggest that the rapid increase in JA-Ile mediated by the JAR1 enzyme plays only a minor role in transcriptional modulation of genes induced by mechanical wounding.

A lesion-mimic syntaxin double mutant in Arabidopsis reveals novel complexity of pathogen defense signaling

The lesion-mimic Arabidopsis mutant, syp121 syp122, constitutively expresses the salicylic acid (SA) signaling pathway and has low penetration resistance to powdery mildew fungi. Genetic analyses of the lesion-mimic phenotype have expanded our understanding of programmed cell death (PCD) in plants. Inactivation of SA signaling genes in syp121 syp122 only partially rescues the lesion-mimic phenotype, indicating that additional defenses contribute to the PCD. Whole genome transcriptome analysis confirmed that SA-induced transcripts, as well as numerous other known pathogen-response transcripts, are up-regulated after inactivation of the syntaxin genes. A suppressor mutant analysis of syp121 syp122 revealed that FMO1, ALD1, and PAD4 are important for lesion development. Mutant alleles of EDS1, NDR1, RAR1, and SGT1b also partially rescued the lesion-mimic phenotype, suggesting that mutating syntaxin genes stimulates TIR-NB-LRR and CC-NB-LRR-type resistances. The syntaxin double knockout potentiated a powdery mildew-induced HR-like response. This required functional PAD4 but not functional SA signaling. However, SA signaling potentiated the PAD4-dependent HR-like response. Analyses of quadruple mutants suggest that EDS5 and SID2 confer separate SA-independent signaling functions, and that FMO1 and ALD1 mediate SA-independent signals that are NPR1-dependent. These studies highlight the contribution of multiple pathways to defense and point to the complexity of their interactions.

Transcriptomic changes induced by acute ozone in resistant and sensitive Medicago truncatula accessions

BACKGROUND

Tropospheric ozone, the most abundant air pollutant is detrimental to plant and animal health including humans. In sensitive plant species even a few hours of exposure to this potent oxidant (200-300 nL. L-1) leads to severe oxidative stress that manifests as visible cell death. In resistant plants usually no visible symptoms are observed on exposure to similar ozone concentrations. Naturally occurring variability to acute ozone in plants provides a valuable resource for examining molecular basis of the differences in responses to ozone. From our earlier study in Medicago truncatula, we have identified cultivar Jemalong is ozone sensitive and PI 464815 (JE154) is an ozone-resistant accession. Analyses of transcriptome changes in ozone-sensitive and resistant accession will provide important clues for understanding the molecular changes governing the plant responses to ozone.

RESULTS

Acute ozone treatment (300 nL L-1 for six hours) led to a reactive oxygen species (ROS) burst in sensitive Jemalong six hours post-fumigation. In resistant JE154 increase in ROS levels was much reduced compared to Jemalong. Based on the results of ROS profiling, time points for microarray analysis were one hour into the ozone treatment, end of treatment and onset of an ozone-induced ROS burst at 12 hours. Replicated temporal transcriptome analysis in these two accessions using 17 K oligonucleotide arrays revealed more than 2000 genes were differentially expressed. Significantly enriched gene ontologies (GOs) were identified using the Cluster Enrichment analysis program. A striking finding was the alacrity of JE154 in altering its gene expression patterns in response to ozone, in stark contrast to delayed transcriptional response of Jemalong. GOs involved in signaling, hormonal pathways, antioxidants and secondary metabolism were altered in both accessions. However, the repertoire of genes responding in each of these categories was different between the two accessions. Real-time PCR analysis confirmed the differential expression patterns of a subset of these genes.

CONCLUSION

This study provided a cogent view of the unique and shared transcriptional responses in an ozone-resistant and sensitive accession that exemplifies the complexity of oxidative signaling in plants. Based on this study, and supporting literature in Arabidopsis we speculate that plants sensitive to acute ozone are impaired in perception of the initial signals generated by the action of this oxidant. This in turn leads to a delayed transcriptional response in the ozone sensitive plants. In resistant plants rapid and sustained activation of several signaling pathways enables the deployment of multiple mechanisms for minimizing the toxicity effect of this reactive molecule.

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.

Foliar CO2 fixation in bean (Phaseolus vulgaris L.) submitted to elevated ozone: distinct changes in Rubisco and PEPc activities in relation to pigment content

Using open-top chambers, the impact of ozone (O(3)) on foliar carboxylases of bean (Phaseolus vulgaris L.) was investigated. From sowing, beans were exposed to non-filtered air (NF) and NF supplied with 40 (+40) and 80 (+80) nL L(-1) O(3). Twenty days after emergence, primary and first trifoliate leaves were sampled. Biochemical characteristics of leaves from +40 were quite similar to those from NF. Conversely, +80 induced distinct biochemical effects in primary and first trifoliate leaves. Regarding primary leaves, +80 reduced ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity by 33% whereas it stimulated phosphoenolpyruvate carboxylase (PEPc) activity by 376%. The reduction in Rubisco activity was accompanied by a decrease in both Rubisco subunit amounts and a consistent oxidative modification of the Rubisco small subunit (SSU). These changes came with a drastic loss in pigmentation. Regarding first trifoliate leaves, +80 stimulated Rubisco activity by 33% while it disturbed neither PEPc activity nor pigmentation. Surprisingly, the enhanced Rubisco activity was associated with a slight decrease in Rubisco protein quantity, which was not coupled with the formation of carbonyl groups in Rubisco-SSU.

Molecular mechanism of enzymatic allene oxide cyclization in plants

Jasmonates, a collective term combining both jasmonic acid (JA) and related derivatives, are ubiquitously distributed in the plant kingdom. They are characterized as lipid-derived signal molecules which mediate a plethora of physiological functions, in particular stress responses, male fertility, and a multitude of developmental processes. In the course of JA biosynthesis, the first oxylipin with signal character, cis-(+)-12-oxo-phytodienoic acid (OPDA), is produced in a cyclization reaction catalyzed by allene oxide cyclase (AOC). This enzyme-catalyzed ring closure is of particular importance, as it warrants the enantiomeric structure at the cyclopentenone ring which in the end results in the only bioactive JA enantiomer, cis-(+)-JA. In this review, we focus on the structural and molecular mechanisms underlying the above mentioned cyclization reaction. In this context, we will discuss the crystal structure of AOC2 of Arabidopsis thaliana with respect to putative binding sites of the instable substrate, 12,13-epoxy-9(Z),11,15(Z)-octadecatrienoic acid (12,13-EOT), as well as possible intermolecular rearrangements during the cyclization reaction.

The role of phytohormone signaling in ozone-induced cell death in plants

Ozone is the main photochemical oxidant that causes leaf damage in many plant species, and can thereby significantly decrease the productivity of crops and forests. When ozone is incorporated into plants, it produces reactive oxygen species (ROS), such as superoxide radicals and hydrogen peroxide. These ROS induce the synthesis of several plant hormones, such as ethylene, salicylic acid, and jasmonic acid. These phytohormones are required for plant growth, development, and defense responses, and regulate the extent of leaf injury in ozone-fumigated plants. Recently, responses to ozone have been studied using genetically modified plants and mutants with altered hormone levels or signaling pathways. These researches have clarified the roles of phytohormones and the complexity of their signaling pathways. The present paper reviews the biosynthesis of the phytohormones ethylene, salicylic acid, and jasmonic acid, their roles in plant responses to ozone, and multiple interactions between these phytohormones in ozone-exposed plants.

The Arabidopsis her1 mutant implicates GABA in E-2-hexenal responsiveness

When wounded or attacked by herbivores or pathogens, plants produce a blend of six-carbon alcohols, aldehydes and esters, known as C6-volatiles. Undamaged plants, when exposed to C6-volatiles, respond by inducing defense-related genes and secondary metabolites, suggesting that C6-volatiles can act as signaling molecules regulating plant defense responses. However, to date, the molecular mechanisms by which plants perceive and respond to these volatiles are unknown. To elucidate such mechanisms, we decided to isolate Arabidopsis thaliana mutants in which responses to C6-volatiles were altered. We observed that treatment of Arabidopsis seedlings with the C6-volatile E-2-hexenal inhibits root elongation. Among C6-volatiles this response is specific to E-2-hexenal, and is not dependent on ethylene, jasmonic and salicylic acid. Using this bioassay, we isolated 18 E-2-hexenal-response (her) mutants that showed sustained root growth after E-2-hexenal treatment. Here, we focused on the molecular characterization of one of these mutants, her1. Microarray and map-based cloning revealed that her1 encodes a gamma-amino butyric acid transaminase (GABA-TP), an enzyme that degrades GABA. As a consequence of the mutation, her1 plants accumulate high GABA levels in all their organs. Based on the observation that E-2-hexenal treatment induces GABA accumulation, and that high GABA levels confer resistance to E-2-hexenal, we propose a role for GABA in mediating E-2-hexenal responses.

The isochorismate pathway is negatively regulated by salicylic acid signaling in O3-exposed Arabidopsis

Ozone (O3), a major photochemical oxidant, causes leaf injury in plants. Plants synthesize salicylic acid (SA), which is reported to greatly affect O3 sensitivity. However, the mechanism of SA biosynthesis under O3 exposure remains unclear. Plants synthesize SA either by a pathway involving phenylalanine as a substrate or another involving isochorismate. To clarify how SA is produced in O3-exposed Arabidopsis, we examined the activities of phenylalanine ammonia lyase (PAL) and isochorismate synthase (ICS), which are components of the phenylalanine and isochorismate pathways, respectively. Exposure of Arabidopsis to O3 enhanced the accumulation of SA and the increase of ICS activity but did not affect PAL activity. In sid2 mutants, which have a defect in ICS1, the level of SA and the activity of ICS did not increase in response to O3 exposure. These results suggest that SA is mainly synthesized from isochorismate in Arabidopsis. Furthermore, the level of ICS1 expression and the activity of ICS during O3 exposure elevated in plants deficient for SA signaling (npr1 and eds5 mutants and NahG transgenics). Treatment of plants with SA also suppressed the enhancement of ICS1 expression by O3. These results suggest that SA synthesis is negatively regulated by SA signaling.

Proteomics-based dissection of stress-responsive pathways in plants

Abiotic stress has an ability to alter the levels of a number of proteins, which may be soluble or structural in nature or which may exist before and after folding in the plant cell. The most crucial function of plant cell is to respond to stress by developing defence mechanisms. This defence is brought about by alteration in the pattern of gene expression. This leads to modulation of certain metabolic and defensive pathways. Owing to gene expression altered under stress, qualitative and quantitative changes in proteins are obvious. These proteins might play a role in signal transduction, antioxidative defence, antifreezing, heat shock, metal binding, antipathogenesis or osmolyte synthesis. A significant part of the literature shows the quantitative and qualitative changes in proteins, mainly employing western analysis, enzymatic kinetics, fraction isolation, one-dimensional SDS-PAGE electrophoresis, etc. Fortunately, recent developments in sensitivity and accuracy for proteome analysis have provided new dimensions to assess the changes in protein types and their expression levels under stress. The novel aim of this review is to do a side-by-side comparison of the proteins that are induced or overexpressed under abiotic stress, examining those from biochemical literature and the ones observed, sequenced and identified using the advanced proteomics and bioinformatic techniques.

A mitochondrial protein homologous to the mammalian peripheral-type benzodiazepine receptor is essential for stress adaptation in plants

The cloning of abiotic stress-inducible genes from the moss Physcomitrella patens led to the identification of the gene PpTSPO1, encoding a protein homologous to the mammalian mitochondrial peripheral-type benzodiazepine receptor and the bacterial tryptophane-rich sensory protein. This class of proteins is involved in the transport of intermediates of the tetrapyrrole biosynthesis pathway. Like the mammalian homologue, the PpTSPO1 protein is localized to mitochondria. The generation of PpTSPO1-targeted moss knock-out lines revealed an essential function of the gene in abiotic stress adaptation. Under stress conditions, the PpTSPO1 null mutants show elevated H(2)O(2) levels, enhanced lipid peroxidation and cell death, indicating an important role of PpTSPO1 in redox homeostasis. We hypothesize that PpTSPO1 acts to direct porphyrin precursors to the mitochondria for heme formation, and is involved in the removal of photoreactive tetrapyrrole intermediates.

Sulfur metabolism in plants: are trees different?

Sulfur metabolite levels and sulfur metabolism have been studied in a significant number of herbaceous and woody plant species. However, only a limited number of datasets are comparable and can be used to identify similarities and differences between these two groups of plants. From these data, it appears that large differences in sulfur metabolite levels, as well as the genetic organization of sulfate assimilation and metabolism do not exist between herbaceous plants and trees. The general response of sulfur metabolism to internal and/or external stimuli, such as oxidative stress, seems to be conserved between the two groups of plants. Thus, it can be expected that, generally, the molecular mechanisms of regulation of sulfur metabolism will also be similar. However, significant differences have been found in fine tuning of the regulation of sulfur metabolism and in developmental regulation of sulfur metabolite levels. It seems that the homeostasis of sulfur metabolism in trees is more robust than in herbaceous plants and a greater change in conditions is necessary to initiate a response in trees. This view is consistent with the requirement for highly flexible defence strategies in woody plant species as a consequence of longevity. In addition, seasonal growth of perennial plants exerts changes in sulfur metabolite levels and regulation that currently are not understood. In this review, similarities and differences in sulfur metabolite levels, sulfur assimilation and its regulation are characterized and future areas of research are identified.

Phytochrome chromophore deficiency leads to overproduction of jasmonic acid and elevated expression of jasmonate-responsive genes in Arabidopsis

An Arabidopsis mutant line named hy1-101 was isolated because it shows stunted root growth on medium containing low concentrations of jasmonic acid (JA). Subsequent investigation indicated that even in the absence of JA, hy1-101 plants exhibit shorter roots and express higher levels of a group of JA-inducible defense genes. Here, we show that the hy1-101 mutant has increased production of JA and its jasmonate-related phenotype is suppressed by the coi1-1 mutation that interrupts JA signaling. Gene cloning and genetic complementation analyses revealed that the hy1-101 mutant contains a mutation in the HY1 gene, which encodes a heme oxygenase essential for phytochrome chromophore biosynthesis. These results support a hypothesis that phytochrome chromophore deficiency leads to overproduction of JA and activates COI1-dependent JA responses. Indeed, we show that, like hy1-101, independent alleles of the phytochrome chromophore-deficient mutants, including hy1-100 and hy2 (CS68), also show elevated JA levels and constant expression of JA-inducible defense genes. We further provide evidence showing that, on the other hand, JA inhibits the expression of a group of light-inducible and photosynthesis-related genes. Together, these data imply that the JA-signaled defense pathway and phytochrome chromophore-mediated light signaling might have antagonistic effects on each other.

Natural variation among Arabidopsis thaliana accessions for transcriptome response to exogenous salicylic acid

Little is known about how gene expression variation within a given species controls phenotypic variation under different treatments or environments. Here, we surveyed the transcriptome response of seven diverse Arabidopsis thaliana accessions in response to two treatments: the presence and absence of exogenously applied salicylic acid (SA), an important signaling molecule in plant defense. A factorial experiment was conducted with three biological replicates per accession with and without applications of SA and sampled at three time points posttreatment. Transcript level data from Affymetrix ATH1 microarrays were analyzed on both per-gene and gene-network levels to detect expression level polymorphisms associated with SA response. Significant variation in transcript levels for response to SA was detected among the accessions, with relatively few genes responding similarly across all accessions and time points. Twenty-five of 54 defined gene networks identified from other microarray studies (pathogen-challenged Columbia [Col-0]) showed a significant response to SA in one or more accessions. A comparison of gene-network relationships in our data to the pathogen-challenged Col-0 data demonstrated a higher-order conservation of linkages between defense response gene networks. Cvi-1 and Mt-0 appeared to have globally different SA responsiveness in comparison to the other five accessions. Expression level polymorphisms for SA response were abundant at both individual gene and gene-network levels in the seven accessions, suggesting that natural variation for SA response is prevalent in Arabidopsis.

Independently silencing two JAR family members impairs levels of trypsin proteinase inhibitors but not nicotine

Jasmonic acid (JA)-amino acid conjugates are important JA metabolites that activate JA responses. However, our understanding of their involvement in herbivore defenses is limited. We identified a new Arabidopsis jasmonate resistant 1 (JAR1) homologue in Nicotiana attenuata (N. attenuata) and named it jasmonate resistant 6 (JAR6). JAR6 clustered closely with Arabidopsis JAR1 and the recently reported jasmonate resistant 4 (JAR4), another JAR1 homologue in N. attenuata, in a phylogenic analysis. The strong elicitation of JAR6 transcripts by wounding and treatment with Manduca sexta (M. sexta) oral secretions (OS), which mimics herbivore attack, suggests it plays a role in herbivore defense. Independently silencing JAR4 or JAR6 by transforming N. attenuata with inverted repeat JAR4 or JAR6 constructs significantly reduced levels of not only JA-Ile plus JA-Leu but also JA-Val in OS-elicited leaves, suggesting JAR4 and JAR6 are functionally redundant and their amino acid substrates are not highly specific to individual amino acids. A new JA conjugate, JA-Gln, whose levels are much higher than those of the other JA conjugates in WT plants, was not affected in JAR4- or JAR6-silenced lines, implying that another JA-conjugating enzyme exists in N. attenuata. Neither JA-ACC, the second most abundant JA conjugate in Arabidopsis seedlings, nor JA-Met or JA-Trp, was detectable in N. attenuata. Levels of trypsin proteinase inhibitors (TPIs) in JAR4- and JAR6-silenced plants were significantly reduced, but nicotine levels were normal. We conclude that both JAR4 and JAR6 conjugate JA to Ile, Val, and Leu, and that both positively regulate TPI activity.

Linking stress with macroscopic and microscopic leaf response in trees: new diagnostic perspectives

Visible symptoms in tree foliage can be used for stress diagnosis once validated with microscopical analyses. This paper reviews and illustrates macroscopical and microscopical markers of stress with a biotic (bacteria, fungi, insects) or abiotic (frost, drought, mineral deficiency, heavy metal pollution in the soil, acidic deposition and ozone) origin helpful for the validation of symptoms in broadleaved and conifer trees. Differentiation of changes in the leaf or needle physiology, through ageing, senescence, accelerated cell senescence, programmed cell death and oxidative stress, provides additional clues raising diagnosis efficiency, especially in combination with information about the target of the stress agent at the tree, leaf/needle, tissue, cell and ultrastructural level. Given the increasing stress in a changing environment, this review discusses how integrated diagnostic approaches lead to better causal analysis to be applied for specific monitoring of stress factors affecting forest ecosystems.