An Arabidopsis mutant cex1 exhibits constant accumulation of jasmonate-regulated AtVSP, Thi2.1 and PDF1.2

Phytohormones in a universe of regulatory metabolites: lessons from jasmonate

Small-molecule phytohormones exert control over plant growth, development, and stress responses by coordinating the patterns of gene expression within and between cells. Increasing evidence indicates that currently recognized plant hormones are part of a larger group of regulatory metabolites that have acquired signaling properties during the evolution of land plants. This rich assortment of chemical signals reflects the tremendous diversity of plant secondary metabolism, which offers evolutionary solutions to the daunting challenges of sessility and other unique aspects of plant biology. A major gap in our current understanding of plant regulatory metabolites is the lack of insight into the direct targets of these compounds. Here, we illustrate the blurred distinction between classical phytohormones and other bioactive metabolites by highlighting the major scientific advances that transformed the view of jasmonate from an interesting floral scent to a potent transcriptional regulator. Lessons from jasmonate research generally apply to other phytohormones and thus may help provide a broad understanding of regulatory metabolite-protein interactions. In providing a framework that links small-molecule diversity to transcriptional plasticity, we hope to stimulate future research to explore the evolution, functions, and mechanisms of perception of a broad range of plant regulatory metabolites.

Plant Disease Resistance-Related Signaling Pathways: Recent Progress and Future Prospects

Plant-pathogen interactions induce a signal transmission series that stimulates the plant's host defense system against pathogens and this, in turn, leads to disease resistance responses. Plant innate immunity mainly includes two lines of the defense system, called pathogen-associated molecular pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). There is extensive signal exchange and recognition in the process of triggering the plant immune signaling network. Plant messenger signaling molecules, such as calcium ions, reactive oxygen species, and nitric oxide, and plant hormone signaling molecules, such as salicylic acid, jasmonic acid, and ethylene, play key roles in inducing plant defense responses. In addition, heterotrimeric G proteins, the mitogen-activated protein kinase cascade, and non-coding RNAs (ncRNAs) play important roles in regulating disease resistance and the defense signal transduction network. This paper summarizes the status and progress in plant disease resistance and disease resistance signal transduction pathway research in recent years; discusses the complexities of, and interactions among, defense signal pathways; and forecasts future research prospects to provide new ideas for the prevention and control of plant diseases.

CML8 and GAD4 function in (Z)-3-hexenol-mediated defense by regulating γ-aminobutyric acid accumulation in Arabidopsis

(Z)-3-hexenol, a small gaseous molecule, is produced in plants under biotic stress and induces defense responses in neighboring plants. However, little is known about how (Z)-3-hexenol induces plant defense-related signaling. In this study, we uncovered how (Z)-3-hexenol treatment enhances plant resistance to insect attacks by increasing γ-aminobutyric acid (GABA) contents in Arabidopsis leaves. First, (Z)-3-hexenol increases the intracellular content of calcium as secondary messenger in Arabidopsis leaf mesophyll cells. Both intracellular and extracellular calcium stores regulate changes in calcium content. Then, CML8 and GAD4 transmit calcium signaling to affect (Z)-3-hexenol induced GABA content and plant resistance. Herein, CML8 interaction with GAD4 was examined via yeast two-hybrid assays, firefly luciferase complementation imaging, and GST pull-down assays. These results indicate that (Z)-3-hexenol treatment increased the GABA contents in Arabidopsis leaves based on CML8 and GAD4, thus increasing plant resistance to the insect Plutella xylostella. This study revealed the mechanism of activating plant insect defense induced by (Z)-3-hexenol, which guides the study of volatiles as biological pest control.

Analysis of Tissue-Specific Defense Responses to Sclerotinia sclerotiorum in Brassica napus

Sclerotinia stem rot (SSR) caused by Sclerotinia sclerotiorum (S. sclerotiorum) is the main disease threat of oilseed rape (Brassica napus), resulting in huge economic losses every year. SSR resistance manifests as quantitative disease resistance (QDR), and no gene with complete SSR resistance has been cloned or reported so far. Transcriptome analysis has revealed a large number of defense-related genes and response processes. However, the similarities and differences in the defense responses of different tissues are rarely reported. In this study, we analyzed the similarities and differences of different tissues in response to S. sclerotiorum at 24 h post inoculation (hpi) by using the published transcriptome data for respective leaf and stem inoculation. At 24 hpi, large differences in gene expression exist in leaf and stem, and there are more differentially expressed genes and larger expression differences in leaf. The leaf is more sensitive to S. sclerotiorum and shows a stronger response than stem. Different defense responses appear in the leaf and stem, and the biosynthesis of lignin, callose, lectin, chitinase, PGIP, and PR protein is activated in leaf. In the stem, lipid metabolism-mediated defense responses are obviously enhanced. For the common defense responses in both leaf and stem, the chain reactions resulting from signal transduction and biological process take the primary responsibility. This research will be beneficial to exploit the potential of different tissues in plant defense and find higher resistance levels of genotypic variability in different environments. Our results are significant in the identification of resistance genes and analysis of defense mechanisms.

The intragenic suppressor mutation Leu59Phe compensates for the effect of detrimental mutations in the jasmonate receptor COI1

The phytohormones jasmonates (JAs) control plant development, growth, and defense against insects and pathogens. The Arabidopsis JA receptor Coronatine Insensitive 1 (COI1) interacts with ARABIDOPSIS SKP-LIKE1 (ASK1)/ASK2 to form the SCF^COI1 E3 ligase and mediate JA responses. Here, we performed a genetic suppressor screen using the leaky coi1-2 (COI1^Leu245Phe ) mutant for restored sensitivity to JA, and identified the intragenic suppressor mutation Leu59Phe, which was in the region connecting the F-box and leucine-rich repeats domains of COI1. The L59F substitution not only restores the COI1^L245F function, but also the COI1^Gly434Glu (coi1-22^rsp ) function in JA responses, through recovering their interactions with ASK1 or ASK2 and their protein levels. The L59F change itself could not enhance the interactions between COI1 and ASK1/2, nor affect JA responses. The present study reveals that the Leu59Phe substitution compensates for the effect of some deleterious mutations in the JA receptor COI1.

MdWRKY75e enhances resistance to Alternaria alternata in Malus domestica

The Alternaria alternata apple pathotype adversely affects apple (Malus domestica Borkh.) cultivation. However, the molecular mechanisms underlying enhanced resistance to this pathogen in apple remain poorly understood. We have previously reported that MdWRKY75 expression is upregulated by A. alternata infection in 'Sushuai' apples. In this study, we discovered that overexpression of MdWRKY75e increased the resistance of transgenic apple lines to A. alternata infection, whereas silencing this gene enhanced susceptibility to A. alternata infection. Furthermore, we found that MdWRKY75e directly binds to the MdLAC7 promoter to regulate the biosynthesis of laccase and increase the biosynthesis of lignin during A. alternata infection. Moreover, the thickening of the cell wall enhanced the mechanical defense capabilities of apple. In addition, we found that jasmonic acid remarkably induced MdWRKY75e expression, and its levels in transgenic apple lines were elevated. These results indicate that MdWRKY75e confers resistance to the A. alternata apple pathotype mainly via the jasmonic acid pathway and that pathogenesis-related genes and antioxidant-related enzyme activity are involved in the disease resistance of MdWRKY75e transgenic plants. In conclusion, our findings provide insights into the importance of MdWRKY75e for resistance to A. alternata infection in apples.

Arabidopsis Flowers Unlocked the Mechanism of Jasmonate Signaling

The Arabidopsis male-sterile phenotype has been a wonderful model for jasmonate action in plants. It has allowed us to identify transcription factors that control gene expression during stamen and pollen maturation and provided for the discovery of the JAZ repressor proteins and the mechanism of jasmonate signaling. More recently, it has revealed intriguing details of the spatial localization of jasmonate synthesis and perception in stamen tissues. The extensive and thoughtful application of protein–protein interaction assays to identify JAZ-interacting partners has led to a much richer appreciation of the mechanisms by which jasmonate integrates with the actions of other hormones to regulate plant growth and physiological responses. This integration is strikingly evident in stamen and pollen development in Arabidopsis, which requires the actions of many hormones. Just as importantly, it is now evident that jasmonate has very different actions during flower development and reproduction in other plant species. This integration and diversity of action indicates that many exciting discoveries remain to be made in this area of jasmonate hormone signaling and response.

OsJAZ1 Attenuates Drought Resistance by Regulating JA and ABA Signaling in Rice

Jasmonates (JAs) and abscisic acid (ABA) are phytohormones known play important roles in plant response and adaptation to various abiotic stresses including salinity, drought, wounding, and cold. JAZ (JASMONATE ZIM-domain) proteins have been reported to play negative roles in JA signaling. However, direct evidence is still lacking that JAZ proteins regulate drought resistance. In this study, OsJAZ1 was investigated for its role in drought resistance in rice. Expression of OsJAZ1 was strongly responsive to JA treatment, and it was slightly responsive to ABA, salicylic acid, and abiotic stresses including drought, salinity, and cold. The OsJAZ1-overexpression rice plants were more sensitive to drought stress treatment than the wild-type (WT) rice Zhonghua 11 (ZH11) at both the seedling and reproductive stages, while the jaz1 T-DNA insertion mutant plants showed increased drought tolerance compared to the WT plants. The OsJAZ1-overexpression plants were hyposensitive to MeJA and ABA, whereas the jaz1 mutant plants were hypersensitive to MeJA and ABA. In addition, there were significant differences in shoot and root length between the OsJAZ1 transgenic and WT plants under the MeJA and ABA treatments. A subcellular localization assay indicated that OsJAZ1 was localized in both the nucleus and cytoplasm. Transcriptome profiling analysis by RNA-seq revealed that the expression levels of many genes in the ABA and JA signaling pathways exhibited significant differences between the OsJAZ1-overexpression plants and WT ZH11 under drought stress treatment. Quantitative real-time PCR confirmed the expression profiles of some of the differentially expressed genes, including OsNCED4, OsLEA3, RAB21, OsbHLH006, OsbHLH148, OsDREB1A, OsDREB1B, SNAC1, and OsCCD1. These results together suggest that OsJAZ1 plays a role in regulating the drought resistance of rice partially via the ABA and JA pathways.

Expression of Vitis amurensis NAC26 in Arabidopsis enhances drought tolerance by modulating jasmonic acid synthesis

The growth and fruit quality of grapevines are widely affected by abnormal climatic conditions such as water deficits, but many of the precise mechanisms by which grapevines respond to drought stress are still largely unknown. Here, we report that VaNAC26, a member of the NAC transcription factor family, was upregulated dramatically during cold, drought and salinity treatments in Vitis amurensis, a cold and drought-hardy wild Vitis species. Heterologous overexpression of VaNAC26 enhanced drought and salt tolerance in transgenic Arabidopsis. Higher activities of antioxidant enzymes and lower concentrations of H2O2 and O2 (-) were found in VaNAC26-OE lines than in wild type plants under drought stress. These results indicated that scavenging by reactive oxygen species (ROS) was enhanced by VaNAC26 in transgenic lines. Microarray-based transcriptome analysis revealed that genes related to jasmonic acid (JA) synthesis and signaling were upregulated in VaNAC26-OE lines under both normal and drought conditions. VaNAC26 showed a specific binding ability on the NAC recognition sequence (NACRS) motif, which broadly exists in the promoter regions of upregulated genes in transgenic lines. Endogenous JA content significantly increased in the VaNAC26-OE lines 2 and 3. Our data suggest that VaNAC26 responds to abiotic stresses and may enhance drought tolerance by transcriptional regulation of JA synthesis in Arabidopsis.

Molecular basis for jasmonate and ethylene signal interactions in Arabidopsis

Jasmonate (JA) and ethylene (ET) are considered to be two essential plant hormones helping plants to tolerate infections by necrotrophic fungi. Phenotypic observations and marker gene expression analysis suggest that JA and ET act synergistically and interdependently in these defence responses. However, JA and ET also interact in an antagonistic way. JA represses ET-induced apical hook formation, while ET inhibits JA-controlled wounding responses. Although these physiological observations have been reported for more than a decade, only recently have the underlying molecular mechanisms been uncovered. Here, I review the recent advances in the understanding of these two hormone interactions and further discuss the biological significance of these apparently opposite interactions between these two hormones in orchestrating plant growth and development.

The secreted antifungal protein thionin 2.4 in Arabidopsis thaliana suppresses the toxicity of a fungal fruit body lectin from Fusarium graminearum

Plants possess active defense systems and can protect themselves from pathogenic invasion by secretion of a variety of small antimicrobial or antifungal proteins such as thionins. The antibacterial and antifungal properties of thionins are derived from their ability to induce open pore formation on cell membranes of phytopathogens, resulting in release of potassium and calcium ions from the cell. Wheat thionin also accumulates in the cell walls of Fusarium-inoculated plants, suggesting that it may have a role in blocking pathogen infection at the plant cell walls. Here we developed an anti-thionin 2.4 (Thi2.4) antibody and used it to show that Thi2.4 is localized in the cell walls of Arabidopsis and cell membranes of F. graminearum, when flowers are inoculated with F. graminearum. The Thi2.4 protein had an antifungal effect on F. graminearum. Next, we purified the Thi2.4 protein, conjugated it with glutathione-S-transferase (GST) and coupled the proteins to an NHS-activated column. Total protein from F. graminearum was applied to GST-Thi2.4 or Thi2.4-binding columns, and the fungal fruit body lectin (FFBL) of F. graminearum was identified as a Thi2.4-interacting protein. This interaction was confirmed by a yeast two-hybrid analysis. To investigate the biological function of FFBL, we infiltrated the lectin into Arabidopsis leaves and observed that it induced cell death in the leaves. Application of FFBL at the same time as inoculation with F. graminearum significantly enhanced the virulence of the pathogen. By contrast, FFBL-induced host cell death was effectively suppressed in transgenic plants that overexpressed Thi2.4. We found that a 15 kD Thi2.4 protein was specifically expressed in flowers and flower buds and suggest that it acts not only as an antifungal peptide, but also as a suppressor of the FFBL toxicity. Secreted thionin proteins are involved in this dual defense mechanism against pathogen invasion at the plant-pathogen interface.

ASK1 physically interacts with COI1 and is required for male fertility in Arabidopsis

Jasmonates are a new class of plant hormones that play important roles in plant development and plant defense. The COI1 gene was previously shown to be required for jasmonate-regulated plant fertility and defense. We demonstrated for the first time that COI1 interacts with the Arabidopsis SKP1-LIKE1 (ASK1) to form a complex that is required for jasmonate action in planta. Functional analysis by antisense strategy showed that ASK1 is involved in male fertility.

The Arabidopsis P450 protein CYP82C2 modulates jasmonate-induced root growth inhibition, defense gene expression and indole glucosinolate biosynthesis

Jasmonic acid (JA) is a fatty acid-derived signaling molecule that regulates a broad range of plant defense responses against herbivores and some microbial pathogens. Molecular genetic studies have established that JA also performs a critical role in several aspects of plant development. Here, we describe the characterization of the Arabidopsis mutant jasmonic acid-hypersensitive1-1 (jah1-1), which is defective in several aspects of JA responses. Although the mutant exhibits increased sensitivity to JA in root growth inhibition, it shows decreased expression of JA-inducible defense genes and reduced resistance to the necrotrophic fungus Botrytis cinerea . Gene cloning studies indicate that these defects are caused by a mutation in the cytochrome P450 protein CYP82C2. We provide evidence showing that the compromised resistance of the jah1-1 mutant to B . cinerea is accompanied by decreased expression of JA-induced defense genes and reduced accumulation of JA-induced indole glucosinolates (IGs). Conversely, the enhanced resistance to B. cinerea in CYP82C2-overexpressing plants is accompanied by increased expression of JA-induced defense genes and elevated levels of JA-induced IGs. We demonstrate that CYP82C2 affects JA-induced accumulation of the IG biosynthetic precursor tryptophan (Trp), but not the JA-induced IAA or pathogen-induced camalexin. Together, our results support a hypothesis that CYP82C2 may act in the metabolism of Trp-derived secondary metabolites under conditions in which JA levels are elevated. The jah1-1 mutant should thus be important in future studies toward understanding the mechanisms underlying the complexity of JA-mediated differential responses, which are important for plants to adapt their growth to the ever-changing environments.

The power of mutants for investigating jasmonate biosynthesis and signaling

Mutant analysis includes approaches that range from traditional screening of mutant populations (forward genetics), to identifying mutations in known genes (reverse genetics), to examining the effects of site-specific mutations that encode modified proteins. All these methodologies have been applied to study jasmonate synthesis and signaling, and their use has led to important discoveries. The fad3 fad7 fad8 mutant of Arabidopsis, and other mutants defective in jasmonate synthesis, revealed the roles of jasmonate in flower development and plant defense against necrotrophic fungal pathogens. The coi1 mutant identified the F-box protein that is now known to be the receptor for jasmonoyl-isoleucine, the active form of jasmonate hormone. Investigations of how JASMONATE-ZIM DOMAIN (JAZ) proteins bind to COI1 and facilitate jasmonate perception have relied on the jai3 mutant, on JAZDeltaJas constructs, and on site-specific mutations in the Jas and ZIM domains of these proteins.

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

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

Transcription factor WRKY70 displays important but no indispensable roles in jasmonate and salicylic acid signaling

The transcription factor WRKY70 was previously reported to be a common component in salicylic acid (SA) and jasmonate (JA) mediated signal pathways in Arabidopsis. Here, we present that the inactivation of the WRKY70 gene in wrky70-1 mutant does not alter the responses of both JA and SA, and that wrky70 mutation is unable to restore the coi1 mutant in JA responses. However, overexpression of WRKY70 reduces JA responses such as expression of JA-induced genes and JA-inhibitory root growth, and activates expression of SA-inducible PR1. These data indicate that the WRKY70 is important but not indispensable for JA and SA signaling, and that other regulators may display the redundant role with WRKY70 in modulation of JA and SA responses in Arabidopsis. Furthermore, we showed that JA inhibits expression of WRKY70 and PR1 by both COI1-dependent and COI1-independent pathways.

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

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

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.

Bestatin, an inhibitor of aminopeptidases, provides a chemical genetics approach to dissect jasmonate signaling in Arabidopsis

Bestatin, a potent inhibitor of some aminopeptidases, was shown previously to be a powerful inducer of wound-response genes in tomato (Lycopersicon esculentum). Here, we present several lines of evidence showing that bestatin specifically activates jasmonic acid (JA) signaling in plants. First, bestatin specifically activates the expression of JA-inducible genes in tomato and Arabidopsis (Arabidopsis thaliana). Second, the induction of JA-responsive genes by bestatin requires the COI1-dependent JA-signaling pathway, but does not depend strictly on JA biosynthesis. Third, microarray analysis using Arabidopsis whole-genome chip demonstrates that the gene expression profile of bestatin-treated plants is similar to that of JA-treated plants. Fourth, bestatin promotes a series of JA-related developmental phenotypes. Taken together, the unique action mode of bestatin in regulating JA-signaled processes leads us to the hypothesis that bestatin exerts its effects through the modulation of some key regulators in JA signaling. We have employed bestatin as an experimental tool to dissect JA signaling through a chemical genetic screening, which yielded a collection of Arabidopsis bestatin-resistant (ber) mutants that are insensitive to the inhibitory effects of bestatin on root elongation. Further characterization efforts demonstrate that some ber mutants are defective in various JA-induced responses, which allowed us to classify the ber mutants into three phenotypic groups: JA-insensitive ber mutants, JA-hypersensitive ber mutants, and mutants insensitive to bestatin but showing normal response to JA. Genetic and phenotypic analyses of the ber mutants with altered JA responses indicate that we have identified several novel loci involved in JA signaling.

Constitutive activation of the jasmonate signaling pathway enhances the production of secondary metabolites in tomato

The phytohormone jasmonic acid (JA) regulates the synthesis of secondary metabolites in a wide range of plant species. Here, we show that exogenous methyl-JA (MeJA) elicits massive accumulation of caffeoylputrescine (CP) in tomato leaves. A mutant (jai1) that is defective in jasmonate perception failed to accumulate CP in flowers and MeJA-treated leaves. Conversely, a transgenic tomato line (called 35S::PS) that exhibits constitutive JA signaling accumulated high levels of leaf CP in the absence of jasmonate treatment. RNA blot analysis showed that genes encoding enzymes in the phenylpropanoid and polyamine pathways for CP biosynthesis are upregulated in MeJA-treated wild-type plants and in untreated 35S::PS plants. These results indicate that CP accumulation in tomato is tightly controlled by the jasmonate signaling pathway, and provide proof-of-concept that the production of some plant secondary metabolites can be enhanced by transgenic manipulation of endogenous JA levels.

Jasmonate Signaling Pathway

Jasmonates in plants are cyclic fatty acid-derived regulators structurally similar to prostaglandins in metazoans. These chemicals mediate many of plants' transcriptional responses to wounding and pathogenesis by acting as potent regulators for the expression of numerous frontline immune response genes, including those for defensins and antifungal proteins. Additionally, the pathway is critical for fertility. Ongoing genetic screens and protein-protein interaction assays are identifying components of the canonical jasmonate signaling pathway. A massive molecular machine, based on two multiprotein complexes, SCF(COI1) and the COP9 signalosome (CNS), plays a central role in jasmonate signaling. This machine functions in vivo as a ubiquitin ligase complex, probably targeting regulatory proteins, some of which are expected to be transcriptional repressors. Some defense-related mediators, notably salicylic acid, antagonize jasmonates in controlling the expression of many genes. In Arabidopsis, NONEXPRESSOR OF PR GENES (NPR1) mediates part of this interaction, with another layer of control provided further downstream by the mitogen-activated protein kinase (MAPK) homolog MPK4. Numerous other interpathway connections influence the jasmonate pathway. Insights from Arabidopsis have shown that an allele of the auxin signaling gene AXR1, for example, reduces the sensitivity of plants to jasmonate. APETALA2 (AP2)-domain transcription factors, such as ETHYLENE RESPONSE FACTOR 1 (ERF1), link the jasmonate pathway to the ethylene signaling pathway. As progress in characterizing several new mutants (some of which are hypersensitive to jasmonic acid) augments our understanding of jasmonate signaling, the Connections Map will be updated to include this new information.

Fine-Tuning Plant Defence Signalling: Salicylate versus Jasmonate

Plant defences against pathogens and herbivorous insects form a comprehensive network of interacting signal transduction pathways. The signalling molecules salicylic acid (SA) and jasmonic acid (JA) play important roles in this network. SA is involved in signalling processes providing systemic acquired resistance (SAR), protecting the plant from further infection after an initial pathogen attack. SAR is long-lasting and provides broad spectrum resistance to biotrophic pathogens that feed on a living host cell. The regulatory protein NPR1 is a central positive regulator of SAR. SA-activated NPR1 localizes to the nucleus where it interacts with TGA transcription factors to induce the expression of a large set of pathogenesis-related proteins that contribute to the enhanced state of resistance. In a distinct signalling process, JA protects the plant from insect infestation and necrotrophic pathogens that kill the host cell before feeding. JA activates the regulatory protein COI1 that is part of the E3 ubiquitin ligase-containing complex SCFCOI1, which is thought to derepress JA-responsive genes involved in plant defence. Both synergistic and antagonistic interactions have been observed between SA- and JA-dependent defences. NPR1 has emerged as a critical modulator of cross-talk between the SA and JA signal and is thought to aid in fine tuning defence responses specific to the encountered attacker. Here we review SA- and JA-dependent signal transduction and summarize our current understanding of the molecular mechanisms of cross-talk between these defences.

Plant gamma-thionins: novel insights on the mechanism of action of a multi-functional class of defense proteins

This review focuses on the first plant defense protein class described in literature, with growth inhibition activity toward pathogens. These peptides were named gamma-thionins or defensins, which are small proteins that can be classified into four main subtypes according to their specific functions. Gamma-thionins are small cationic peptides with different and special abilities. They are able to inhibit digestive enzymes or act against bacteria and/or fungi. Current research in this area focuses particularly these two last targets, being the natural crop plant defenses improved through the use of transgenic technology. Here, we will compare primary and tertiary structures of gamma-thionins and also will analyze their similarities to scorpion toxins and insect defensins. This last comparison offers some hypothesis for gamma-thionins mechanisms of action against certain pathogens. This specific area has benefited from the recent determination of many gamma-thionin structures. Furthermore, we also summarize molecular interactions between plant gamma-thionins and fungi receptors, which include membrane proteins and lipids, shedding some light over pathogen resistance. Researches on gamma-thionins targets could help on plant genetic improvement for production of increased resistance toward pathogens. Thus, positive results recently obtained for transgenic plants and future prospects in the area are also approached. Finally, gamma-thionins activity has also been studied for future drug development, capable of inhibit tumor cell growth in human beings.

GmCOI1, a soybean F-box protein gene, shows ability to mediate jasmonate-regulated plant defense and fertility in Arabidopsis

The F-box protein gene COI1 from Arabidopsis plays a fundamental role in response to jasmonates, which regulate plant root growth, pollen fertility, wounding and healing, and defense against pathogens and insects. Null mutations in COI1 were previously found to abolish all the jasmonate responses, and the Arabidopsis coil-1 mutant is male sterile and susceptible to pathogen infection. In this study, we isolated an F-box protein gene from soybean, which shares significant homology with the Arabidopsis COI1 and similarly contains an F-box motif and leucine rich repeats (LRR), here designated GmCOI1 (Glycine max L. (Merr.) COI1). To test whether the sequence homology and structural similarity are indicative of functional conservation, we expressed GmCOI1 in the Arabidopsis coil-1 mutant. The transgenic coil-1 plants with expression of the GmCOI1 gene were found to exhibit normal jasmonate responses, including jasmonate-regulated plant defense and fertility. In addition, the chimerical proteins with swapped domain of the F-box motif or LRR between GmCOI1 and COI1 were shown to functionally complement the coil-1 mutation. Furthermore, GmCOI1 was found to assemble into the Skpl-Cullin-F-box (SCF) complexes, similar to the formation of the Arabidopsis SCF(COO1). These data demonstrate the soybean F-box protein gene GmCOI1 is able to mediate jasmonate-regulated plant defense and fertility in Arabidopsis, which implies a generic jasmonate pathway with conserved signal components in different plant species.

Molecular players regulating the jasmonate signalling network

Many plant developmental and stress responses require the coordinated interaction of the jasmonate and other signalling pathways, such as those for ethylene, salicylic acid and abscisic acid. Recent research in Arabidopsis has uncovered several key players that regulate crosstalk between these signalling pathways and that shed light on the molecular mechanisms modulating this coordinated interaction. Genes that are involved in the regulation of protein stability through the ubiquitin-proteasome pathway (COI1, AXR1 and SGT1b), signalling proteins (MPK4) and transcription factors (AtMYC2, ERF1, NPR1 and WRKY70) form a regulatory network that allows the plant to fine-tune specific responses to different stimuli.

Point mutations in Arabidopsis Cullin1 reveal its essential role in jasmonate response

The SKP1-Cullin/Cdc53-F-box protein ubiquitin ligases (SCF) target many important regulatory proteins for degradation and play vital roles in diverse cellular processes. In Arabidopsis there are 11 Cullin members (AtCUL). AtCUL1 was demonstrated to assemble into SCF complexes containing COI1, an F-box protein required for response to jasmonates (JA) that regulate plant fertility and defense responses. It is not clear whether other Cullins also associate with COI1 to form SCF complexes, thus, it is unknown whether AtCUL1, or another Cullin that assembles into SCF(COI1) (even perhaps two or more functionally redundant Cullins), plays a major role in JA signaling. We present genetic and physiological data to directly demonstrate that AtCUL1 is necessary for normal JA responses. The homozygous AtCUL1 mutants axr6-1 and axr6-2, the heterozygous mutants axr6/AXR6, and transgenic plants expressing mutant AtCUL1 proteins containing a single amino acid substitution from phenylalanine-111 to valine, all exhibit reduced responses to JA. We also demonstrate that ax6 enhances the effect of coi1 on JA responses, implying a genetic interaction between COI1 and AtCUL1 in JA signaling. Furthermore, we show that the point mutations in AtCUL1 affect the assembly of COI1 into SCF, thus attenuating SCF(COI1) formation.

Jasmonate: an oxylipin signal with many roles in plants

Jasmonic acid is an oxylipin signaling molecule derived from linolenic acid. So far, jasmonate (JA) (including the free acid and a number of conjugates) has been shown to regulate or co-regulate a wide range of processes in plants, from responses to biotic and abiotic stresses to the developmental maturation of stamens and pollen in Arabidopsis. This review focuses on discoveries in several of these areas. Most work described is from studies in Arabidopsis. While the results are expected to be broadly applicable to other higher plants, there are cases where related but distinct phenotypes have been observed in other species (e.g., tomato). Investigation of JA action in wound- and insect-defense responses has established that this compound is an essential component of the systemic signal that activates defense genes throughout the plant. It is possible that JA acts indirectly through the production of reactive oxygen species including hydrogen peroxide (H2O2). The availability of Arabidopsis mutants deficient in JA synthesis has been central to the identification of additional roles for JA in defense against microbial pathogens and in reproductive development. Currently, the key issues in JA action are to understand the role of the skip/cullin/F-box ubiquitination complex, SCF(COI1), and to identify additional protein components that act in the early steps of JA signaling.

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.

Jasmonates and octadecanoids: signals in plant stress responses and development

Plants are sessile organisms. Consequently they have to adapt constantly to fluctuations in the environment. Some of these changes involve essential factors such as nutrients, light, and water. Plants have evolved independent systems to sense nutrients such as phosphate and nitrogen. However, many of the environmental factors may reach levels which represent stress for the plant. The fluctuations can range between moderate and unfavorable, and the factors can be of biotic or abiotic origin. Among the biotic factors influencing plant life are pathogens and herbivores. In case of bacteria and fungi, symbiotic interactions such as nitrogen-fixating nodules and mycorrhiza, respectively, may be established. In case of insects, a tritrophic interaction of herbivores, carnivores, and plants may occur mutualistically or parasitically. Among the numerous abiotic factors are low temperature, frost, heat, high light conditions, ultraviolet light, darkness, oxidation stress, hypoxia, wind, touch, nutrient imbalance, salt stress, osmotic adjustment, water deficit, and desiccation. In the last decade jasmonates were recognized as being signals in plant responses to most of these biotic and abiotic factors. Signaling via jasmonates was found to occur intracellularly, intercellularly, and systemically as well as interorganismically. Jasmonates are a group of ubiquitously occurring plant growth regulators originally found as the major constituents in the etheric oil of jasmine, and were first suggested to play a role in senescence due to a strong senescence-promoting effect. Subsequently, numerous developmental processes were described in which jasmonates exhibited hormone-like properties. Recent knowledge is reviewed here on jasmonates and their precursors, the octadecanoids. After discussing occurrence and biosynthesis, emphasis is placed upon the signal transduction pathways in plant stress responses in which jasmonates act as a signal. Finally, examples are described on the role of jasmonates in developmental processes.

Cross talk between signaling pathways in pathogen defense

Plant defense in response to microbial attack is regulated through a complex network of signaling pathways that involve three signaling molecules: salicylic acid (SA), jasmonic acid (JA) and ethylene. The SA and JA signaling pathways are mutually antagonistic. This regulatory cross talk may have evolved to allow plants to fine-tune the induction of their defenses in response to different plant pathogens.

Fatty acid-derived signals in plants

Plants synthesize many fatty acid derivatives, several of which play important regulatory roles. Jasmonates are the best characterized examples. Jasmonate-insensitive mutants and mutants with a constitutive jasmonate response have given us new insights into jasmonate signalling. The jasmonate biosynthesis mutant opr3 allowed the dissection of cyclopentanone and cyclopentenone signalling, thus defining specific roles for these molecules. Jasmonate signalling is a complex network of individual signals and recent findings on specific activities of methyl jasmonate and (Z)-jasmone add to this picture. In addition, there are keto, hydroxy and hydroperoxy fatty acids that might be involved in cell death and the expression of stress-related genes. Finally, there are bruchins and volicitin, signal molecules from insects that are perceived by plants in the picomole to femtomole range. They highlight the importance of fatty acid-derived molecules in interspecies communication and in plant defence.

Fusion genetic analysis of jasmonate-signalling mutants in Arabidopsis

Jasmonates induce plant-defence responses and act to regulate defence-related genes including positive feedback of the lipoxygenase 2 (LOX2) gene involved in jasmonate synthesis. To identify jasmonate-signalling mutants, we used a fusion genetic strategy in which the firefly luciferase (FLUC) and Escherichia coli beta-glucuronidase (GUS) reporters were expressed under control of the jasmonate-responsive LOX2 promoter. Spatial and temporal patterns of reporter expression were determined initially, and revealed that JA-responsive expression from the LOX2 promoter required de novo protein synthesis. Reporter activity was also induced by the protein kinase inhibitor staurosporine and antagonized by the protein phosphatase inhibitor okadaic acid. FLUC bio-imaging, RNA gel-blot analysis and progeny analyses identified three recessive mutants that underexpress the FLUC reporter, designated jue1, 2 and 3, as well as two recessive mutants, designated joe1 and 2, that overexpress the reporter. Genetic analysis indicated that reporter overexpression in the joe mutants requires COI. joe1 responded to MeJA with increased anthocyanin accumulation, while joe2 responded with decreased root growth inhibition. In addition, reporter induction and endogenous LOX2 expression by staurosporine was absent in joe2.

The oxylipin pathway in Arabidopsis

Oxylipins are acyclic or cyclic oxidation products derived from the catabolism of fatty acids which regulate many defense and developmental pathways in plants. The dramatic increase in the volume of publications and reviews on these compounds since 1997 documents the increasing interest in this compound and its role in plants. Research on this topic has solidified our understanding of the chemistry and biosynthetic pathways for oxylipin production. However, more information is still needed on how free fatty acids are produced and the role of beta-oxidation in the biosynthetic pathway for oxylipins. It is also becoming apparent that oxylipin content and composition changes during growth and development and during pathogen or insect attack. Oxylipins such as jasmonic acid (JA) or 12-oxo-phytodienoic acid modulate the expression of numerous genes and influence specific aspects of plant growth, development and responses to abiotic and biotic stresses. Although oxylipins are believed to act alone, several examples were presented to illustrate that JA-induced responses are modulated by the type and the nature of crosstalk with other signaling molecules such as ethylene and salicylic acid. How oxylipins cause changes in gene expression and instigate a physiological response is becoming understood with the isolation of mutations in both positive and negative regulators in the jasmonate signaling pathway and the use of cDNA microarrays.