Coi1-dependent expression of an Arabidopsis vegetative storage protein in flowers and siliques and in response to coronatine or methyl jasmonate

Jasmonate perception: Ligand-receptor interaction, regulation, and evolution

Phytohormones integrate external environmental and developmental signals with internal cellular responses for plant survival and multiplication in changing surroundings. Jasmonate (JA), which might originate from prokaryotes and benefit plant terrestrial adaptation, is a vital phytohormone that regulates diverse developmental processes and defense responses against various environmental stresses. In this review, we first provide an overview of ligand-receptor binding techniques used for the characterization of phytohormone-receptor interactions, then introduce the identification of the receptor COI1 and active JA molecules, and finally summarize recent advances on the regulation of JA perception and its evolution.

Micronutrients Affect Expression of Induced Resistance Genes in Hydroponically Grown Watermelon against Fusarium oxysporum f. sp. niveum and Meloidogyne incognita

The soil-borne pathogens, particularly Fusarium oxysporum f. sp. niveum (FON) and southern root-knot nematode (RKN, Meloidogyne incognita) are the major threats to watermelon production in the southeastern United States. The role of soil micronutrients on induced resistance (IR) to plant diseases is well-documented in soil-based media. However, soil-based media do not allow us to determine the contribution of individual micronutrients in the induction of IR. In this manuscript, we utilized hydroponics-medium to assess the effect of controlled application of micronutrients, including iron (Fe), manganese (Mn), and zinc (Zn) on the expression of important IR genes (PR1, PR5, and NPR1 from salicylic acid (SA) pathway, and VSP, PDF, and LOX genes from jasmonic acid (JA) pathway) in watermelon seedlings upon inoculation with either FON or RKN or both. A subset of micronutrient-treated plants was inoculated (on the eighth day of micronutrient application) with FON and RKN (single or mixed inoculation). The expression of the IR genes in treated and control samples was evaluated using qRT-PCR. Although, significant phenotypic differences were not observed with respect to the severity of wilt symptoms or RKN galling with any of the micronutrient treatments within the 30-day experimental period, differences in the induction of IR genes were considerably noticeable. However, the level of gene expression varied with sampling period, type and concentration of micronutrients applied, and pathogen inoculation. In the absence of pathogens, micronutrient applications on the seventh day, in general, downregulated the expression of the majority of the IR genes. However, pathogen inoculation preferentially either up- or down-regulated the expression levels of the IR genes at three days post-inoculation depending on the type and concentration of micronutrients. The results demonstrated here indicate that micronutrients in watermelon may potentially make watermelon plants susceptible to infection by FON and RKN. However, upon infection the IR genes are significantly up-regulated that they may potentially aid the prevention of further infection via SA- and JA-pathways. This is the first demonstration of the impact of micronutrients affecting IR in watermelon against FON and RKN infection.

Transcriptome Analysis Identified Gene Regulation Networks in Soybean Leaves Perturbed by the Coronatine Toxin

The non-host specific Pseudomonas syringae phytotoxin Coronatine (COR) causes chlorosis and promotes toxicity by inducing physiological changes in plants. We performed transcriptome analysis to better understand plants' transcriptional and metabolic response to COR. Toward this end, mock-treated and COR-treated soybean plants were analyzed by RNA-Seq. A total of 4,545 genes were differentially expressed between the two treatments, of which 2,170 were up-regulated whereas 2,375 were down-regulated in COR treated samples. Gene annotation and pathway analysis conducted using the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) databases revealed that the differential genes were involved in photosynthesis, jasmonic acid (JA) synthesis, signal transduction, and phenylpropane metabolism. This study will provide new insights into COR mediated responses and extend our understanding of COR function in plants.

Jasmonic acid-responsive RRTF1 transcription factor controls DTX18 gene expression in hydroxycinnamic acid amide secretion

Jasmonates (JAs) are plant hormones that regulate the biosynthesis of many secondary metabolites, such as hydroxycinnamic acid amides (HCAAs), through jasmonic acid (JA)-responsive transcription factors (TFs). HCAAs are renowned for their role in plant defense against pathogens. The multidrug and toxic compound extrusion transporter DETOXIFICATION18 (DTX18) has been shown to mediate the extracellular accumulation of HCAAs p-coumaroylagmatine (CouAgm) at the plant surface for defense response. However, little is known about the regulatory mechanism of DTX18 gene expression by TFs. Yeast one-hybrid screening using the DTX18 promoter as bait isolated the key positive regulator redox-responsive TF 1 (RRTF1), which is a member of the AP2/ethylene-response factor family of proteins. RRTF1 is a JA-responsive factor that is required for the transcription of the DTX18 gene, and it thus promotes CouAgm secretion at the plant surface. As a result, overexpression of RRTF1 caused increased resistance against the fungus Botrytis cinerea, whereas rrtf1 mutant plants were more susceptible. Using yeast two-hybrid screening, we identified the BTB/POZ-MATH (BPM) protein BPM1 as an interacting partner of RRTF1. The BPM family of proteins acts as substrate adaptors of CUL3-based E3 ubiquitin ligases, and we found that only BPM1 and BPM3 were able to interact with RRTF1. In addition, we demonstrated that RRTF1 was subjected to degradation through the 26S proteasome pathway and that JA stabilized RRTF1. Knockout of BPM1 and BPM3 in bpm1/3 double mutants enhanced RRTF1 accumulation and DTX18 gene expression, thus increasing resistance to the fungus B. cinerea. Our results provide a better understanding of the fine-tuned regulation of JA-induced TFs in HCAA accumulation.

Cell death resulted from loss of fumarylacetoacetate hydrolase in Arabidopsis is related to phytohormone jasmonate but not salicylic acid

Fumarylacetoacetate hydrolase (FAH) catalyzes the final step in Tyr degradation pathway essential to animals but not well understood in plants. Previously, we found that mutation of SSCD1 encoding Arabidopsis FAH causes cell death under short day, which uncovered an important role of Tyr degradation pathway in plants. Since phytohormones salicylic acid (SA) and jasmonate (JA) are involved in programmed cell death, in this study, we investigated whether sscd1 cell death is related to SA and JA, and found that (1) it is accompanied by up-regulation of JA- and SA-inducible genes as well as accumulation of JA but not SA; (2) it is repressed by breakdown of JA signaling but not SA signaling; (3) the up-regulation of reactive oxygen species marker genes in sscd1 is repressed by breakdown of JA signaling; (4) treatment of wild-type Arabidopsis with succinylacetone, an abnormal metabolite caused by loss of FAH, induces expression of JA-inducible genes whereas treatment with JA induces expression of some Tyr degradation genes with dependence of JA signaling. These results demonstrated that cell death resulted from loss of FAH in Arabidopsis is related to JA but not SA, and suggested that JA signaling positively regulates sscd1 cell death by up-regulating Tyr degradation.

Identification of the targets of HbEIN3/EILs in genomic wide in Hevea brasiliensis

EIN3/EILs are key regulators in ET signaling pathway. In this work, 4 members of EIN3/EILs of Hevea brasiliensis (HbEIN3/EILs) showed interaction with two F box proteins, HbEBF1 and HbEBF2. HbEIN3 located in nucleus and exhibited strong transcriptional activity. HbEIN3 was induced by ET treatment in C-serum, but not in B-serum of latex. HbEIN3/EILs bound to G-box cis-element. To globally search the potential targets of HbEIN3/EILs, genomic sequences of H. brasiliensis was re-annotated and an HCES (Hevea Cis-Elements Scanning) program was developed ( www.h-brasiliensis.com ). HCES scanning results showed that ET- and JA- responsive cis-elements distribute overlapping in gene promoters. 3146 genes containing G-box in promoters are potential targets of HbEIN3, including 41 genes involved in biosynthesis and drainage of latex, of which 7 rate-limiting genes of latex production were regulated by both ET and JA, suggesting that ET and JA signaling pathways coordinated the latex biosynthesis and drainage in H. brasiliensis. Abbreviations: ABRE: ABA responsive elements; bHLH: basic helix-loop-helix; COG: Orthologous Groups; DRE: dehydration response element; ERE: ethylene responsive element; ET: Ethylene; GO: Gene Ontology; HCES: Hevea Cis-Elements Scanning; JA: jasmonates; JRE: Jasmonate-responsive element; KEGG: Kyoto Encyclopedia of Genes and Genomes; NR: non-redundant database; PLACE: Plant Cis-acting Regulatory DNA Elements; qRT-PCR: quantitative real-time RT-PCR.

The Arabidopsis thaliana lectin receptor kinase LecRK-I.9 is required for full resistance to Pseudomonas syringae and affects jasmonate signalling

On microbial attack, plants can detect invaders and activate plant innate immunity. For the detection of pathogen molecules or cell wall damage, plants employ receptors that trigger the activation of defence responses. Cell surface proteins that belong to large families of lectin receptor kinases are candidates to function as immune receptors. Here, the function of LecRK-I.9 (At5g60300), a legume-type lectin receptor kinase involved in cell wall-plasma membrane contacts and in extracellular ATP (eATP) perception, was studied through biochemical, gene expression and reverse genetics approaches. In Arabidopsis thaliana, LecRK-I.9 expression is rapidly, highly and locally induced on inoculation with avirulent strains of Pseudomonas syringae pv. tomato (Pst). Two allelic lecrk-I.9 knock-out mutants showed decreased resistance to Pst. Conversely, over-expression of LecRK-I.9 led to increased resistance to Pst. The analysis of defence gene expression suggests an alteration of both the salicylic acid (SA) and jasmonic acid (JA) signalling pathways. In particular, LecRK-I.9 expression during plant-pathogen interaction was dependent on COI1 (CORONATINE INSENSITIVE 1) and JAR1 (JASMONATE RESISTANT 1) components, and JA-responsive transcription factors (TFs) showed altered levels of expression in plants over-expressing LecRK-I.9. A similar misregulation of these TFs was obtained by JA treatment. This study identified LecRK-I.9 as necessary for full resistance to Pst and demonstrated its involvement in the control of defence against pathogens through a regulation of JA signalling components. The role of LecRK-I.9 is discussed with regard to the potential molecular mechanisms linking JA signalling to cell wall damage and/or eATP perception.

Quantitative Resistance: More Than Just Perception of a Pathogen

Molecular plant pathology has focused on studying large-effect qualitative resistance loci that predominantly function in detecting pathogens and/or transmitting signals resulting from pathogen detection. By contrast, less is known about quantitative resistance loci, particularly the molecular mechanisms controlling variation in quantitative resistance. Recent studies have provided insight into these mechanisms, showing that genetic variation at hundreds of causal genes may underpin quantitative resistance. Loci controlling quantitative resistance contain some of the same causal genes that mediate qualitative resistance, but the predominant mechanisms of quantitative resistance extend beyond pathogen recognition. Indeed, most causal genes for quantitative resistance encode specific defense-related outputs such as strengthening of the cell wall or defense compound biosynthesis. Extending previous work on qualitative resistance to focus on the mechanisms of quantitative resistance, such as the link between perception of microbe-associated molecular patterns and growth, has shown that the mechanisms underlying these defense outputs are also highly polygenic. Studies that include genetic variation in the pathogen have begun to highlight a potential need to rethink how the field considers broad-spectrum resistance and how it is affected by genetic variation within pathogen species and between pathogen species. These studies are broadening our understanding of quantitative resistance and highlighting the potentially vast scale of the genetic basis of quantitative resistance.

Key Components of Different Plant Defense Pathways Are Dispensable for Powdery Mildew Resistance of the Arabidopsis mlo2 mlo6 mlo12 Triple Mutant

Loss of function mutations of particular plant MILDEW RESISTANCE LOCUS O (MLO) genes confer durable and broad-spectrum penetration resistance against powdery mildew fungi. Here, we combined genetic, transcriptomic and metabolomic analyses to explore the defense mechanisms in the fully resistant Arabidopsis thaliana mlo2 mlo6 mlo12 triple mutant. We found that this genotype unexpectedly overcomes the requirement for indolic antimicrobials and defense-related secretion, which are critical for incomplete resistance of mlo2 single mutants. Comparative microarray-based transcriptome analysis of mlo2 mlo6 mlo12 mutants and wild type plants upon Golovinomyces orontii inoculation revealed an increased and accelerated accumulation of many defense-related transcripts. Despite the biotrophic nature of the interaction, this included the non-canonical activation of a jasmonic acid/ethylene-dependent transcriptional program. In contrast to a non-adapted powdery mildew pathogen, the adapted powdery mildew fungus is able to defeat the accumulation of defense-relevant indolic metabolites in a MLO protein-dependent manner. We suggest that a broad and fast activation of immune responses in mlo2 mlo6 mlo12 plants can compensate for the lack of single or few defense pathways. In addition, our results point to a role of Arabidopsis MLO2, MLO6, and MLO12 in enabling defense suppression during invasion by adapted powdery mildew fungi.

Transcriptome Analysis of the Signalling Networks in Coronatine-Induced Secondary Laticifer Differentiation from Vascular Cambia in Rubber Trees

The secondary laticifer in rubber tree (Hevea brasiliensis Muell. Arg.) is a specific tissue within the secondary phloem. This tissue differentiates from the vascular cambia, and its function is natural rubber biosynthesis and storage. Given that jasmonates play a pivotal role in secondary laticifer differentiation, we established an experimental system with jasmonate (JA) mimic coronatine (COR) for studying the secondary laticifer differentiation: in this system, differentiation occurs within five days of the treatment of epicormic shoots with COR. In the present study, the experimental system was used to perform transcriptome sequencing and gene expression analysis. A total of 67,873 unigenes were assembled, and 50,548 unigenes were mapped at least in one public database. Of these being annotated unigenes, 15,780 unigenes were differentially expressed early after COR treatment, and 19,824 unigenes were differentially expressed late after COR treatment. At the early stage, 8,646 unigenes were up-regulated, while 7,134 unigenes were down-regulated. At the late stage, the numbers of up- and down-regulated unigenes were 7,711 and 12,113, respectively. The annotation data and gene expression analysis of the differentially expressed unigenes suggest that JA-mediated signalling, Ca²⁺ signal transduction and the CLAVATA-MAPK-WOX signalling pathway may be involved in regulating secondary laticifer differentiation in rubber trees.

Jasmonates: Multifunctional Roles in Stress Tolerance

Jasmonates (JAs) [Jasmonic acid (JA) and methyl jasmonates (MeJAs)] are known to take part in various physiological processes. Exogenous application of JAs so far tested on different plants under abiotic stresses particularly salinity, drought, and temperature (low/high) conditions have proved effective in improving plant stress tolerance. However, its extent of effectiveness entirely depends on the type of plant species tested or its concentration. The effects of introgression or silencing of different JA- and Me-JA-related genes have been summarized in this review, which have shown a substantial role in improving crop yield and quality in different plants under stress or non-stress conditions. Regulation of JAs synthesis is impaired in stressed as well as unstressed plant cells/tissues, which is believed to be associated with a variety of metabolic events including signal transduction. Although, mitogen activated protein kinases (MAPKs) are important components of JA signaling and biosynthesis pathways, nitric oxide, ROS, calcium, ABA, ethylene, and salicylic acid are also important mediators of plant growth and development during JA signal transduction and synthesis. The exploration of other signaling molecules can be beneficial to examine the details of underlying molecular mechanisms of JA signal transduction. Much work is to be done in near future to find the proper answers of the questions like action of JA related metabolites, and identification of universal JA receptors etc. Complete signaling pathways involving MAPKs, CDPK, TGA, SIPK, WIPK, and WRKY transcription factors are yet to be investigated to understand the complete mechanism of action of JAs.

The ERF transcription factor EPI1 is a negative regulator of dark-induced and jasmonate-stimulated senescence in Arabidopsis

Identification of the factors involved in the regulation of senescence and the analysis of their function are important for both a biological understanding of the senescence mechanism and the improvement of agricultural productivity. In this study, we identified an ERF gene termed "ERF gene conferring Postharvest longevity Improvement 1" (EPI1) as a possible regulator of senescence in Arabidopsis. We found that EPI1 possesses transcriptional repression activity and that the transgenic plants overexpressing EPI1 and expressing its chimeric repressor, EPI1-SRDX, commonly suppressed the darkness-induced senescence in their excised aerial parts. These transgenic plants additionally maintained a high level of chlorophyll, even after the methyl jasmonate (MeJA) treatment, which stimulated senescence in the dark. In addition, we found that senescence-induced and -reduced genes are down- and upregulated, respectively, in the MeJA-treated transgenic plants under darkness. Our results suggest that EPI1 functions as a negative regulator of the dark-induced and JA-stimulated senescence.

The Mechanistic Underpinnings of an ago1-Mediated, Environmentally Dependent, and Stochastic Phenotype

The crucial role of microRNAs in plant development is exceedingly well supported; their importance in environmental robustness is studied in less detail. Here, we describe a novel, environmentally dependent phenotype in hypomorphic argonaute1 (ago1) mutants and uncover its mechanistic underpinnings in Arabidopsis (Arabidopsis thaliana). AGO1 is a key player in microRNA-mediated gene regulation. We observed transparent lesions on embryonic leaves of ago1 mutant seedlings. These lesions increased in frequency in full-spectrum light. Notably, the lesion phenotype was most environmentally responsive in ago1-27 mutants. This allele is thought to primarily affect translational repression, which has been linked with the response to environmental perturbation. Using several lines of evidence, we found that these lesions represent dead and dying tissues due to an aberrant hypersensitive response. Although all three canonical defense hormone pathways (salicylic acid, jasmonate, and jasmonate/ethylene pathways) were up-regulated in ago1 mutants, we demonstrate that jasmonate perception drives the lesion phenotype. Double mutants of ago1 and coronatine insensitive1, the jasmonate receptor, showed greatly decreased frequency of affected seedlings. The chaperone HEAT SHOCK PROTEIN 90 (HSP90), which maintains phenotypic robustness in the face of environmental perturbations, is known to facilitate AGO1 function. HSP90 perturbation has been shown previously to up-regulate jasmonate signaling and to increase plant resistance to herbivory. Although single HSP90 mutants showed subtly elevated levels of lesions, double mutant analysis disagreed with a simple epistatic model for HSP90 and AGO1 interaction; rather, both appeared to act nonadditively in producing lesions. In summary, our study identifies AGO1 as a major, largely HSP90-independent, factor in providing environmental robustness to plants.

Reduced Biosynthesis of Digalactosyldiacylglycerol, a Major Chloroplast Membrane Lipid, Leads to Oxylipin Overproduction and Phloem Cap Lignification in Arabidopsis

DIGALACTOSYLDIACYLGLYCEROL SYNTHASE1 (DGD1) is a chloroplast outer membrane protein responsible for the biosynthesis of the lipid digalactosyldiacylglycerol (DGDG) from monogalactosyldiacylglycerol (MGDG). The Arabidopsis thaliana dgd1 mutants have a greater than 90% reduction in DGDG content, reduced photosynthesis, and altered chloroplast morphology. However, the most pronounced visible phenotype is the extremely short inflorescence stem, but how deficient DGDG biosynthesis causes this phenotype is unclear. We found that, in dgd1 mutants, phloem cap cells were lignified and jasmonic acid (JA)-responsive genes were highly upregulated under normal growth conditions. The coronative insensitive1 dgd1 and allene oxide synthase dgd1 double mutants no longer exhibited the short inflorescence stem and lignification phenotypes but still had the same lipid profile and reduced photosynthesis as dgd1 single mutants. Hormone and lipidomics analyses showed higher levels of JA, JA-isoleucine, 12-oxo-phytodienoic acid, and arabidopsides in dgd1 mutants. Transcript and protein level analyses further suggest that JA biosynthesis in dgd1 is initially activated through the increased expression of genes encoding 13-lipoxygenases (LOXs) and phospholipase A-Iγ3 (At1g51440), a plastid lipase with a high substrate preference for MGDG, and is sustained by further increases in LOX and allene oxide cyclase mRNA and protein levels. Our results demonstrate a link between the biosynthesis of DGDG and JA.

Analysis of Differentially Expressed Genes Associated with Coronatine-Induced Laticifer Differentiation in the Rubber Tree by Subtractive Hybridization Suppression

The secondary laticifer in the secondary phloem is differentiated from the vascular cambia of the rubber tree (Hevea brasiliensis Muell. Arg.). The number of secondary laticifers is closely related to the rubber yield potential of Hevea. Pharmacological data show that jasmonic acid and its precursor linolenic acid are effective in inducing secondary laticifer differentiation in epicormic shoots of the rubber tree. In the present study, an experimental system of coronatine-induced laticifer differentiation was developed to perform SSH identification of genes with differential expression. A total of 528 positive clones were obtained by blue-white screening, of which 248 clones came from the forward SSH library while 280 clones came from the reverse SSH library. Approximately 215 of the 248 clones and 171 of the 280 clones contained cDNA inserts by colony PCR screening. A total of 286 of the 386 ESTs were detected to be differentially expressed by reverse northern blot and sequenced. Approximately 147 unigenes with an average length of 497 bp from the forward and 109 unigenes with an average length of 514 bp from the reverse SSH libraries were assembled and annotated. The unigenes were associated with the stress/defense response, plant hormone signal transduction and structure development. It is suggested that Ca²⁺ signal transduction and redox seem to be involved in differentiation, while PGA and EIF are associated with the division of cambium initials for COR-induced secondary laticifer differentiation in the rubber tree.

Characterization of Brachypodium distachyon as a nonhost model against switchgrass rust pathogen Puccinia emaculata

BACKGROUND

Switchgrass rust, caused by Puccinia emaculata, is an important disease of switchgrass, a potential biofuel crop in the United States. In severe cases, switchgrass rust has the potential to significantly affect biomass yield. In an effort to identify novel sources of resistance against switchgrass rust, we explored nonhost resistance against P. emaculata by characterizing its interactions with six monocot nonhost plant species. We also studied the genetic variations for resistance among Brachypodium inbred accessions and the involvement of various defense pathways in nonhost resistance of Brachypodium.

RESULTS

We characterized P. emaculata interactions with six monocot nonhost species and identified Brachypodium distachyon (Bd21) as a suitable nonhost model to study switchgrass rust. Interestingly, screening of Brachypodium accessions identified natural variations in resistance to switchgrass rust. Brachypodium inbred accessions Bd3-1 and Bd30-1 were identified as most and least resistant to switchgrass rust, respectively, when compared to tested accessions. Transcript profiling of defense-related genes indicated that the genes which were induced in Bd21after P. emaculata inoculation also had higher basal transcript abundance in Bd3-1 when compared to Bd30-1 and Bd21 indicating their potential involvement in nonhost resistance against switchgrass rust.

CONCLUSION

In the present study, we identified Brachypodium as a suitable nonhost model to study switchgrass rust which exhibit type I nonhost resistance. Variations in resistance response were also observed among tested Brachypodium accessions. Brachypodium nonhost resistance against P. emaculata may involve various defense pathways as indicated by transcript profiling of defense related genes. Overall, this study provides a new avenue to utilize novel sources of nonhost resistance in Brachypodium against switchgrass rust.

Friends or foes: new insights in jasmonate and ethylene co-actions

One strategy for sessile plants to adapt to their surrounding environment involves the modulation of their various internal phytohormone signaling and distributions when the plants sense environmental change. There are currently dozens of identified phytohormones in plant cells and they act in concert to regulate plant growth, development, metabolism and defense. It has been determined that phytohormones often act together to achieve certain physiological functions. Thus, the study of hormone-hormone interactions is becoming a competitive research field for deciphering the underlying regulatory mechanisms. Among phytohormones, jasmonate and ethylene present a fascinating case of synergism and antagonism. They are commonly recognized as defense hormones that act synergistically. Plants impaired in jasmonate and/or ethylene signaling are susceptible to infections by necrotrophic fungi, suggesting that these two hormones are both required for defense. Moreover, jasmonate and ethylene also act antagonistically, such as in the regulation of apical hook development and wounding responses. Here, we highlight the recent breakthroughs in the understanding of jasmonate-ethylene co-actions and point out the potential power of studying protein-protein interactions for systematically exploring signal cross-talk.

Jasmonate-activated MYC2 represses ETHYLENE INSENSITIVE3 activity to antagonize ethylene-promoted apical hook formation in Arabidopsis

The apical hook is an essential structure that enables epigeal plants to protrude through the soil. Arabidopsis thaliana HOOKLESS1 (HLS1) is reported to be a key regulator of hook development and a direct target gene of the ethylene (ET)-activated transcription factors ETHYLENE INSENSITIVE3 (EIN3) and its close homolog EIN3-Like1. Previous research has shown that the phytohormones jasmonate (JA) and ET antagonistically regulate apical hook development, although the underlying molecular mechanism is largely unknown. Here, we report that JA represses hook formation by reducing HLS1 expression. Our results further reveal that the JA-activated transcription factor MYC2 represses EIN3 function to reduce HLS1 expression through at least the following two layers of regulation: (1) MYC2 binds to the promoter of an F-box gene, EIN3 BINDING F-BOX PROTEIN1, to induce its expression and thus promote EIN3 degradation; and (2) MYC2 physically interacts with EIN3 and inhibits its DNA binding activity. Collectively, our findings shed light on the molecular mechanism underlying the antagonism between JA and ET during apical hook development and provide insight into the coaction of multiple phytohormones in the regulation of plant growth and development.

Nicotianamine synthase gene family as central components in heavy metal and phytohormone response in maize

Nicotianamine (NA) is an important divalent metal chelator and the main precursor of phytosiderophores. NA is synthesized from S-adenosylmethionine in a process catalyzed by nicotianamine synthase (NAS). In this study, a set of structural and phylogenetic analyses have been applied to identify the maize NAS genes based on the maize genome sequence release. Ten maize NAS genes have been mapped; seven of them have not been reported to date. Phylogenetic analysis and expression pattern from microarray data led to their classification into two different orthologous groups. C-terminal fusion of ZmNAS3 with GFP was found in the cytoplasm of Arabidopsis leaf protoplast. Expression analysis by reverse transcription polymerase chain reaction revealed ZmNAS genes are responsive to heavy metal ions (Ni, Fe, Cu, Mn, Zn, and Cd), and all 10 ZmNAS genes were only observed in the root tissue except of ZmNAS6. The promoter of ZmNAS genes was analyzed for the presence of different cis-element response to all kinds of phytohormones and environment stresses. We found that the ZmNAS gene expression of maize seedlings was regulated by jasmonic acid, abscisic acid, and salicylic acid. Microarray data demonstrated that the ZmNAS genes show differential, organ-specific expression patterns in the maize developmental steps. The integrated comparative analysis can improve our current view of ZmNAS genes and facilitate the functional characterization of individual members.

Cesium Inhibits Plant Growth through Jasmonate Signaling in Arabidopsis thaliana

It has been suggested that cesium is absorbed from the soil through potassium uptake machineries in plants; however, not much is known about perception mechanism and downstream response. Here, we report that the jasmonate pathway is required in plant response to cesium. Jasmonate biosynthesis mutant aos and jasmonate-insensitive mutant coi1-16 show clear resistance to root growth inhibition caused by cesium. However, the potassium and cesium contents in these mutants are comparable to wild-type plants, indicating that jasmonate biosynthesis and signaling are not involved in cesium uptake, but involved in cesium perception. Cesium induces expression of a high-affinity potassium transporter gene HAK5 and reduces potassium content in the plant body, suggesting a competitive nature of potassium and cesium uptake in plants. It has also been found that cesium-induced HAK5 expression is antagonized by exogenous application of methyl-jasmonate. Taken together, it has been indicated that cesium inhibits plant growth via induction of the jasmonate pathway and likely modifies potassium uptake machineries.

TGA transcription factors and jasmonate-independent COI1 signalling regulate specific plant responses to reactive oxylipins

Jasmonates and phytoprostanes are oxylipins that regulate stress responses and diverse physiological and developmental processes. 12-Oxo-phytodienoic acid (OPDA) and phytoprostanes are structurally related electrophilic cyclopentenones, which activate similar gene expression profiles that are for the most part different from the action of the cyclopentanone jasmonic acid (JA) and its biologically active amino acid conjugates. Whereas JA-isoleucine signals through binding to COI1, the bZIP transcription factors TGA2, TGA5, and TGA6 are involved in regulation of gene expression in response to phytoprostanes. Here root growth inhibition and target gene expression were compared after treatment with JA, OPDA, or phytoprostanes in mutants of the COI1/MYC2 pathway and in different TGA factor mutants. Inhibition of root growth by phytoprostanes was dependent on COI1 but independent of jasmonate biosynthesis. In contrast, phytoprostane-responsive gene expression was strongly dependent on TGA2, TGA5, and TGA6, but not dependent on COI1, MYC2, TGA1, and TGA4. Different mutant and overexpressing lines were used to determine individual contributions of TGA factors to cyclopentenone-responsive gene expression. Whereas OPDA-induced expression of the cytochrome P450 gene CYP81D11 was primarily regulated by TGA2 and TGA5, the glutathione S-transferase gene GST25 and the OPDA reductase gene OPR1 were regulated by TGA5 and TGA6, but less so by TGA2. These results support the model that phytoprostanes and OPDA regulate differently (i) growth responses, which are COI1 dependent but jasmonate independent; and (ii) lipid stress responses, which are strongly dependent on TGA2, TGA5, and TGA6. Identification of molecular components in cyclopentenone signalling provides an insight into novel oxylipin signal transduction pathways.

ML3: a novel regulator of herbivory-induced responses in Arabidopsis thaliana

ML (MD2-related lipid recognition) proteins are known to enhance innate immune responses in mammals. This study reports the analysis of the putative ML gene family in Arabidopsis thaliana and suggests a role for the ML3 gene in herbivory-associated responses in plants. Feeding by larvae of the Lepidopteran generalist herbivore Spodoptera littoralis and larvae of the specialist herbivore Plutella xylostella activated ML3 transcription in leaf tissues. ML3 loss-of-function Arabidopsis plants were compromised in the upregulation of herbivory-induced genes and displayed a semi-dwarf phenotype. Herbivory bioassays showed that larvae of S. littoralis fed on ml3 mutant plants gained more weight compared to larvae fed on wild-type plants while larvae of P. xylostella did not show any significant difference. Virus-induced gene silencing of ML3 expression in plants compromised in jasmonic acid (JA) and salicylic acid (SA) signalling revealed a complex role of ML3 in JA/defence signalling affecting both JA- and SA-dependent responses. The data suggest that ML3 is involved in herbivory-mediated responses in Arabidopsis and that it has a potential role in herbivory-associated molecular pattern recognition.

Arabidopsis seedling flood-inoculation technique: a rapid and reliable assay for studying plant-bacterial interactions

BACKGROUND

The Arabidopsis thaliana-Pseudomonas syringae model pathosystem is one of the most widely used systems to understand the mechanisms of microbial pathogenesis and plant innate immunity. Several inoculation methods have been used to study plant-pathogen interactions in this model system. However, none of the methods reported to date are similar to those occurring in nature and amicable to large-scale mutant screens.

RESULTS

In this study, we developed a rapid and reliable seedling flood-inoculation method based on young Arabidopsis seedlings grown on MS medium. This method has several advantages over conventional soil-grown plant inoculation assays, including a shorter growth and incubation period, ease of inoculation and handling, uniform infection and disease development, requires less growth chamber space and is suitable for high-throughput screens. In this study we demonstrated the efficacy of the Arabidopsis seedling assay to study 1) the virulence factors of P. syringae pv. tomato DC3000, including type III protein secretion system (TTSS) and phytotoxin coronatine (COR); 2) the effector-triggered immunity; and 3) Arabidopsis mutants affected in salicylic acid (SA)- and pathogen-associated molecular pattern (PAMPs)-mediated pathways. Furthermore, we applied this technique to study nonhost resistance (NHR) responses in Arabidopsis using nonhost pathogens, such as P. syringae pv. tabaci, pv. glycinea and pv. tomato T1, and confirmed the functional role of FLAGELLIN-SENSING 2 (FLS2) in NHR.

CONCLUSIONS

The Arabidopsis seedling flood-inoculation assay provides a rapid, efficient and economical method for studying Arabidopsis-Pseudomonas interactions with minimal growth chamber space and time. This assay could also provide an excellent system for investigating the virulence mechanisms of P. syringae. Using this method, we demonstrated that FLS2 plays a critical role in conferring NHR against nonhost pathovars of P. syringae, but not to Xanthomonas campestris pv. vesicatoria. This method is potentially ideal for high-throughput screening of both Arabidopsis and pathogen mutants.

GsTIFY10, a novel positive regulator of plant tolerance to bicarbonate stress and a repressor of jasmonate signaling

Recent discoveries show that TIFY family genes are plant-specific genes involved in the response to several abiotic stresses, also acting as key regulators of jasmonate signaling in Arabidopsis thaliana. However, there is limited information about this gene family in wild soybean, nor is its role in plant bicarbonate stress adaptation completely understood. Here, we isolated and characterized a novel TIFY family gene, GsTIFY10, from Glycine soja. GsTIFY10 could be induced by bicarbonate, salinity stress and the phytohormone JA, both in the leaves and roots of wild soybean. Over-expression of GsTIFY10 in Arabidopsis resulted in enhanced plant tolerance to bicarbonate stress during seed germination, early seedling and adult seedling developmental stages, and the expression levels of some bicarbonate stress response and stress-inducible marker genes were significantly higher in the GsTIFY10 overexpression lines than in wild-type plants. It was also found that GsTIFY10 could repress JA signal transduction. The roots of plants overexpressing GsTIFY10 grew longer than wild-type in the presence of MeJA, and some JA response and JA biosynthesis marker genes were suppressed in the GsTIFY10 overexpression lines. Subcellular localization studies using a GFP fusion protein showed that GsTIFY10 is localized to the nucleus. These results suggest that the newly isolated wild soybean GsTIFY10 is a positive regulator of plant bicarbonate stress tolerance and is also a repressor of jasmonate signaling, from hormone perception to transcriptional activity.

Stability of AtVSP in the insect digestive canal determines its defensive capability

We have previously demonstrated that Arabidopsis vegetative storage protein (AtVSP) is an acid phosphatase that has anti-insect activity in in vitro feeding assays [Liu et al., 2005. Plant Physiology 139, 1545-1556]. To investigate the functionality of AtVSP in planta as an anti-insect defense protein, we produced AtVSP-overexpressing as well as AtVSP-silenced transgenic Arabidopsis lines, and evaluated impact on the polyphagous American grasshopper Schistocerca americana. Grasshoppers showed no significant difference in weight gain and growth rate when feeding on wild type, overexpressing, or silenced lines, respectively. In addition, AtVSP protein was undetectable in either the midgut or frass of grasshoppers reared on transgenic plants suggesting that AtVSP was unable to withstand proteolytic degradation. To determine the stability of the AtVSP protein in grasshopper digestive canal, midgut extracts from various nymphal stages were incubated with bacterially expressed AtVSP for different periods of time. AtVSP was hydrolyzed rapidly by grasshopper midgut extract, in stark contrast with its fate when incubated with cowpea bruchid midgut extract. Multiple proteases have been detected in the midgut of grasshoppers, which may play important roles in determining the insect response to AtVSP. Results indicate that stability of an anti-insect protein in insect guts is a crucial property integral to the defense protein.

The role of Arabidopsis Rubisco activase in jasmonate-induced leaf senescence

Leaf senescence, as the last stage of leaf development, is regulated by diverse developmental and environmental factors. Jasmonates (JAs) have been shown to induce leaf senescence in several plant species; however, the molecular mechanism for JA-induced leaf senescence remains unknown. In this study, proteomic, genetic, and physiological approaches were used to reveal the molecular basis of JA-induced leaf senescence in Arabidopsis (Arabidopsis thaliana). We identified 35 coronatine-insensitive 1 (COI1)-dependent JA-regulated proteins using two-dimensional difference gel electrophoresis in Arabidopsis. Among these 35 proteins, Rubisco activase (RCA) was a COI1-dependent JA-repressed protein. We found that RCA was down-regulated at the levels of transcript and protein abundance by JA in a COI1-dependent manner. We further found that loss of RCA led to typical senescence-associated features and that the COI1-dependent JA repression of RCA played an important role in JA-induced leaf senescence.

COI1, a jasmonate receptor, is involved in ethylene-induced inhibition of Arabidopsis root growth in the light

Plant response to stress is orchestrated by hormone signalling pathways including those activated by jasmonates (JAs) and by ethylene, both of which stunt root growth. COI1 is a JA receptor and is required for the known responses to this hormone. It was observed that the coi1 mutant, which is largely unresponsive to growth inhibition by JAs, was also partially unresponsive to growth inhibition by ethylene and by its immediate precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), in the light but not in the dark. Although COI1 was required for this response to ACC, other components of the JA signal perception pathway were not. Mutants selected for insensitivity to ethylene, including etr1, ein2, and ein3, showed greater ACC-induced root growth inhibition in the light than in the dark. However, the double mutants etr1;coi1, ein2;coi1, and ein3;coi1, and coi1 seedlings treated with silver ions to block the ethylene receptors showed almost complete unresponsiveness to ACC-induced root growth inhibition in the light. The light requirement for the COI1-mediated growth inhibition by ACC was for long photoperiods, and the ACC response was not abolished by mutations in the known photoreceptors. The complementation assay indicated that SCF complex assembly was not required for COI1 function in the ACC response, in contrast to the JA response. It is concluded that COI1 is required for the light-dependent, JA-independent, root growth inhibition by ethylene.

Jasmonate and Phytochrome A Signaling inArabidopsisWound 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-β-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.

Chloroplastic phosphoadenosine phosphosulfate metabolism regulates basal levels of the prohormone jasmonic acid in Arabidopsis leaves

Levels of the enzymes that produce wound response mediators have to be controlled tightly in unwounded tissues. The Arabidopsis (Arabidopsis thaliana) fatty acid oxygenation up-regulated8 (fou8) mutant catalyzes high rates of alpha -linolenic acid oxygenation and has higher than wild-type levels of the alpha -linolenic acid-derived wound response mediator jasmonic acid (JA) in undamaged leaves. fou8 produces a null allele in the gene SAL1 (also known as FIERY1 or FRY1). Overexpression of the wild-type gene product had the opposite effect of the null allele, suggesting a regulatory role of SAL1 acting in JA synthesis. The biochemical phenotypes in fou8 were complemented when the yeast (Saccharomyces cerevisiae) sulfur metabolism 3'(2'), 5'-bisphosphate nucleotidase MET22 was targeted to chloroplasts in fou8. The data are consistent with a role of SAL1 in the chloroplast-localized dephosphorylation of 3'-phospho-5'-adenosine phosphosulfate to 5'-adenosine phosphosulfate or in a closely related reaction (e.g. 3',5'-bisphosphate dephosphorylation). Furthermore, the fou8 phenotype was genetically suppressed in a triple mutant (fou8 apk1 apk2) affecting chloroplastic 3'-phospho-5'-adenosine phosphosulfate synthesis. These results show that a nucleotide component of the sulfur futile cycle regulates early steps of JA production and basal JA levels.

Regulation of gene expression by jasmonate hormones

Plants possess inducible defense systems to oppose attack by pathogens and herbivores. Jasmonates are important signaling molecules produced by plants which regulate in positive or negative crosstalk with ethylene subsets of genes involved in defense against necrotrophic microorganisms or herbivorous insects, respectively. This review presents an overview of promoter sequences and transcription factors involved in jasmonate-responsive gene expression with the most important components summarized here. Frequently occurring jasmonate-responsive promoter sequences are the GCC motif, which is commonly found in promoters activated synergistically by jasmonate and ethylene, and the G-box, which is commonly found in promoters activated by jasmonates and repressed by ethylene. Important transcription factors conferring jasmonate-responsive gene expression in Arabidopsis are ORA59 and AtMYC2. ORA59 interacts with the GCC motif and controls the expression of genes that are synergistically induced by jasmonates and ethylene, whereas AtMYC2 interacts with the G-box and related sequences, and controls genes activated by jasmonate alone. AtMYC2 can interact with JAZ proteins, which are hypothesized to act as repressors. The bioactive jasmonate (+)-7-iso-JA-l-Ile promotes the interaction between the ubiquitin ligase complex SCF(COI1) and JAZ proteins, resulting in their degradation by the 26S proteasome, thereby liberating AtMYC2 from repression according to the prevailing model. Literature up to 1 June 2009 was used for this review.

Hormone interactions in stomatal function

Research in recent years on the biology of guard cells has shown that these specialized cells integrate both extra- and intra-cellular signals in the control of stomatal apertures. Among the phytohormones, abscisic acid (ABA) is one of the key players regulating stomatal function. In addition, auxin, cytokinin, ethylene, brassinosteroids, jasmonates, and salicylic acid also contribute to stomatal aperture regulation. The interaction of multiple hormones can serve to determine the size of stomatal apertures in a condition-specific manner. Here, we discuss the roles of different phytohormones and the effects of their interactions on guard cell physiology and function.

Wound-induced endogenous jasmonates stunt plant growth by inhibiting mitosis

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

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.

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.

A role for gene duplication and natural variation of gene expression in the evolution of metabolism

Background

Most eukaryotic genomes have undergone whole genome duplications during their evolutionary history. Recent studies have shown that the function of these duplicated genes can diverge from the ancestral gene via neo- or sub-functionalization within single genotypes. An additional possibility is that gene duplicates may also undergo partitioning of function among different genotypes of a species leading to genetic differentiation. Finally, the ability of gene duplicates to diverge may be limited by their biological function.

Methodology/Principal Findings

To test these hypotheses, I estimated the impact of gene duplication and metabolic function upon intraspecific gene expression variation of segmental and tandem duplicated genes within Arabidopsis thaliana. In all instances, the younger tandem duplicated genes showed higher intraspecific gene expression variation than the average Arabidopsis gene. Surprisingly, the older segmental duplicates also showed evidence of elevated intraspecific gene expression variation albeit typically lower than for the tandem duplicates. The specific biological function of the gene as defined by metabolic pathway also modulated the level of intraspecific gene expression variation. The major energy metabolism and biosynthetic pathways showed decreased variation, suggesting that they are constrained in their ability to accumulate gene expression variation. In contrast, a major herbivory defense pathway showed significantly elevated intraspecific variation suggesting that it may be under pressure to maintain and/or generate diversity in response to fluctuating insect herbivory pressures.

Conclusion

These data show that intraspecific variation in gene expression is facilitated by an interaction of gene duplication and biological activity. Further, this plays a role in controlling diversity of plant metabolism.

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.

Phosphoinositide and inositolpolyphosphate signalling in defense responses of Arabidopsis thaliana challenged by mechanical wounding

Various biochemical signals are implicated in Arabidopsis wound signalling, including jasmonic acid (JA), salicylic acid, auxin, and Ca2+. Here, we report on cross-talk of phytohormones with phosphoinositide signals not previously implicated in plant wound responses. Within 30 min of mechanical wounding of Arabidopsis rosette-leaves, the levels of the lipid-derived soluble inositolpolyphosphate, inositol 1,4,5-trisphosphate (InsP(3)), increased four to five-fold. Concomitantly, the precursor lipids, phosphatidylinositol 4,5-bisphosphate, phosphatidylinositol 4-phosphate and phosphatidylinositol transiently depleted, followed by re-synthesis after 30-60 min of stimulation. Increased InsP(3) levels with wounding coincided with JA increases over the first hours of stimulation. In dde2-2-mutant plants deficient in JA biosynthesis, no InsP(3) increase was observed upon wounding, indicating that JA was required for InsP(3) formation, and InsP(3) levels increased in wild-type plants challenged with sorbitol, increasing endogenous JA levels. In InsP 5-ptase plants with attenuated phosphoinositide signalling, the induction of wounding-inducible genes was diminished compared with wild-type plants, suggesting a role for phosphoinositide signalling in mediating plant wound responses. The gene-expression patterns suggest that phosphoinositides contribute to both JA-dependent and JA-independent aspects of wound signalling. Weight gain of Plutella xylostella caterpillars feeding on InsP 5-ptase plants was increased compared with that of caterpillars feeding on wild-type plants. The ecophysiological relevance of phosphoinositide signals in plant defense responses to herbivory is discussed in light of recent findings of inositolpolyphosphate involvement in phytohormone-receptor function.

Gibberellins, jasmonate and abscisic acid modulate the sucrose-induced expression of anthocyanin biosynthetic genes in Arabidopsis

Anthocyanins are secondary metabolites, which play an important role in the physiology of plants. Both sucrose and hormones regulate anthocyanin synthesis. Here, the interplay between sucrose and plant hormones was investigated in the expression of sucrose-regulated genes coding for anthocyanin biosynthetic enzymes in Arabidopsis seedlings. The expression pattern of 14 genes involved in the anthocyanin biosynthetic pathway, including two transcription factors (PAP1, PAP2), was analysed by real-time reverse transcriptase polymerase chain reaction (RT-PCR) in Arabidopsis seedlings treated with sucrose and plant hormones. Sucrose-induction of the anthocyanin synthesis pathway was repressed by the addition of gibberellic acid (GA) whereas jasmonate (JA) and abscisic acid (ABA) had a synergic effect with sucrose. The gai mutant was less sensitive to GA-dependent repression of dihydroflavonol reductase. This would seem to prove that GAI signalling is involved in the crosstalk between sucrose and GA in wild-type Arabidopsis seedlings. Conversely, the inductive effect of sucrose was not strictly ABA mediated. Sucrose induction of anthocyanin genes required the COI1 gene, but not JAR1, which suggests a possible convergence of the jasmonate- and sucrose-signalling pathways. The results suggest the existence of a crosstalk between the sucrose and hormone signalling pathways in the regulation of the anthocyanin biosynthetic pathway.

Diverse stress signals activate the C1 subgroup MAP kinases of Arabidopsis

Mitogen-activated protein kinase (MAPK) cascades play an important role in mediating stress responses in plants. In Arabidopsis, 20 MAPKs have been identified and classified into four major groups (A-D). Little is known about the role of group C MAPKs. We have studied the activation of Arabidopsis subgroup C1 MAPKs (AtMPK1/AtMPK2) in response to mechanical injury. An increase in their kinase activity was detected in response to wounding that was blocked by cycloheximide. Jasmonic acid (JA) activated AtMPK1/AtMPK2 in the absence of wounding. Wound and JA-induction of AtMPK1/2 kinase activity was not prevented in the JA-insensitive coi1 mutant. Other stress signals, such as abscisic acid (ABA) and hydrogen peroxide, activated AtMPK1/2. This report shows for the first time that regulation of AtMPK1/2 kinase activity in Arabidopsis might be under the control of signals involved in different kinds of stress.

The coronatine-insensitive 1 mutation reveals the hormonal signaling interaction between abscisic acid and methyl jasmonate in Arabidopsis guard cells. Specific impairment of ion channel activation and second messenger production

Methyl jasmonate (MeJA) elicits stomatal closing similar to abscisic acid (ABA), but whether the two compounds use similar or different signaling mechanisms in guard cells remains to be clarified. We investigated the effects of MeJA and ABA on second messenger production and ion channel activation in guard cells of wild-type Arabidopsis (Arabidopsis thaliana) and MeJA-insensitive coronatine-insensitive 1 (coi1) mutants. The coi1 mutation impaired MeJA-induced stomatal closing but not ABA-induced stomatal closing. MeJA as well as ABA induced production of reactive oxygen species (ROS) and nitric oxide (NO) in wild-type guard cells, whereas MeJA did not induce production of ROS and NO in coi1 guard cells. The experiments using an inhibitor and scavengers demonstrated that both ROS and NO are involved in MeJA-induced stomatal closing as well as ABA-induced stomatal closing. Not only ABA but also MeJA activated slow anion channels and Ca(2+) permeable cation channels in the plasma membrane of wild-type guard cell protoplasts. However, in coi1 guard cell protoplasts, MeJA did not elicit either slow anion currents or Ca(2+) permeable cation currents, but ABA activated both types of ion channels. Furthermore, to elucidate signaling interaction between ABA and MeJA in guard cells, we examined MeJA signaling in ABA-insensitive mutant ABA-insensitive 2 (abi2-1), whose ABA signal transduction cascade has some disruption downstream of ROS production and NO production. MeJA also did not induce stomatal closing but stimulated production of ROS and NO in abi2-1. These results suggest that MeJA triggers stomatal closing via a receptor distinct from the ABA receptor and that the coi1 mutation disrupts MeJA signaling upstream of the blanch point of ABA signaling and MeJA signaling in Arabidopsis guard cells.

The transcription factors WRKY11 and WRKY17 act as negative regulators of basal resistance in Arabidopsis thaliana

Transcription factors are believed to play a pivotal role in the activation and fine-tuning of plant defense responses, but little is known about the exact function of individual transcription factors in this process. We analyzed the role of the IId subfamily of WRKY transcription factors in the regulation of basal resistance to Pseudomonas syringae pv tomato (Pst). The expression of four members of the subfamily was induced upon challenge with virulent and avirulent strains of Pst. Mutant analyses revealed that loss of WRKY11 function increased resistance toward avirulent and virulent Pst strains and that resistance was further enhanced in wrky11 wrky17 double mutant plants. Thus, WRKY11 and WRKY17 act as negative regulators of basal resistance to Pst. Genome-wide expression analysis and expression studies of selected genes in single and double mutants demonstrated that both transcription factors modulate transcriptional changes in response to pathogen challenge. Depending on the target gene, WRKY11 and WRKY17 act either specifically or in a partially redundant manner. We demonstrate complex cross-regulation within the IId WRKY subfamily and provide evidence that both WRKY transcription factors are involved in the regulation of Pst-induced jasmonic acid-dependent responses. These results provide genetic evidence for the importance of WRKY11 and WRKY17 in plant defense.

The Arabidopsis thaliana JASMONATE INSENSITIVE 1 gene is required for suppression of salicylic acid-dependent defenses during infection by Pseudomonas syringae

Many plant pathogens suppress antimicrobial defenses using virulence factors that modulate endogenous host defenses. The Pseudomonas syringae phytotoxin coronatine (COR) is believed to promote virulence by acting as a jasmonate analog, because COR-insensitive 1 (coil) Arabidopsis thaliana and tomato mutants are impaired in jasmonate signaling and exhibit reduced susceptibility to P. syringae. To further investigate the role of jasmonate signaling in disease development, we analyzed several jasmonate-insensitive A. thaliana mutants for susceptibility to P. syringae pv. tomato strain DC3000 and sensitivity to COR. Jasmonate-insensitive 1 (jin1) mutants exhibit both reduced susceptibility to P. syringae pv. tomato DC3000 and reduced sensitivity to COR, whereas jasmonate-resistant 1 (jar1) plants exhibit wild-type responses to both COR and P. syringae pv. tomato DC3000. A jin1 jar1 double mutant does not exhibit enhanced jasmonate insensitivity, suggesting that JIN1 functions downstream of jasmonic acid-amino acid conjugates synthesized by JAR1. Reduced disease susceptibility in jin1 mutants is correlated with elevated expression of pathogenesis-related 1 (PR-1) and is dependent on accumulation of salicylic acid (SA). We also show that JIN1 is required for normal P. syringae pv. tomato DC3000 symptom development through an SA-independent mechanism. Thus, P. syringae pv. tomato DC3000 appears to utilize COR to manipulate JIN1-dependent jasmonate signaling both to suppress SA-mediated defenses and to promote symptom development.

Genome-wide transcriptional analysis of the Arabidopsis thaliana interaction with the plant pathogen Pseudomonas syringae pv. tomato DC3000 and the human pathogen Escherichia coli O157:H7

Pseudomonas syringae pv. tomato DC3000 (Pst) is a virulent pathogen that causes disease on tomato and Arabidopsis. The type III secretion system (TTSS) plays a key role in pathogenesis by translocating virulence effectors from the bacteria into the plant host cell, while the phytotoxin coronatine (COR) contributes to virulence and disease symptom development. Recent studies suggest that both the TTSS and COR are involved in the suppression of host basal defenses. However, little is known about the interplay between the host gene expression changes associated with basal defenses and the virulence activities of the TTSS and COR during infection. In this study, we used the Affymetrix full genome chip to determine the Arabidopsis transcriptome associated with basal defense to Pst DC3000 hrp mutants and the human pathogenic bacterium Escherichia coli O157:H7. We then used Pst DC3000 virulence mutants to characterize Arabidopsis transcriptional responses to the action of hrp-regulated virulence factors (e.g. TTSS and COR) during bacterial infection. Additionally, we used bacterial fliC mutants to assess the role of the pathogen-associated molecular pattern flagellin in induction of basal defense-associated transcriptional responses. In total, our global gene expression analysis identified 2800 Arabidopsis genes that are reproducibly regulated in response to bacterial pathogen inoculation. Regulation of these genes provides a molecular signature for Arabidopsis basal defense to plant and human pathogenic bacteria, and illustrates both common and distinct global virulence effects of the TTSS, COR, and possibly other hrp-regulated virulence factors during Pst DC3000 infection.

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.

The Pseudomonas syringae phytotoxin coronatine promotes virulence by overcoming salicylic acid-dependent defences in Arabidopsis thaliana

SUMMARY Successful pathogen infection likely involves the suppression of general antimicrobial host defences. One Pseudomonas syringae virulence factor proposed to act in this manner is coronatine (COR), a phytotoxin believed to function as an analogue of one or more jasmonates, a family of plant growth regulators. COR biosynthetic (COR(-)) mutants of P. syringae pv. tomato strain DC3000 exhibit reduced virulence on Arabidopsis thaliana and tomato. In the present study, three genetically and biochemically defined COR(-) mutants of DC3000 were used to explore potential effects of COR and its precursors, coronafacic acid (CFA) and coronamic acid (CMA), on defence signalling pathways in A. thaliana. Inoculation with wild-type DC3000 resulted in the accumulation of several jasmonate-responsive transcripts, whereas infection with a mutant strain that accumulates CFA, which is structurally similar to methyl jasmonate (MeJA), did not. Thus, COR, but not CFA, stimulates jasmonate signalling during P. syringae infection of A. thaliana. The ability of the COR(-) mutants to grow to high levels in planta was fully restored in A. thaliana lines deficient for salicylic acid (SA) accumulation. Although the COR(-) mutants grew to high levels in SA-deficient plants, disease symptoms were reduced in these plants. Collectively, these results indicate that COR is required both for overcoming or suppressing SA-dependent defences during growth in plant tissue and for normal disease symptom development in A. thaliana.

12-oxo-phytodienoic acid triggers expression of a distinct set of genes and plays a role in wound-induced gene expression in Arabidopsis

Jasmonic acid (JA) and methyl jasmonate (MeJA), collectively known as JAs, regulate diverse physiological processes in plants, including the response to wounding. Recent reports suggest that a cyclopentenone precursor of JA, 12-oxo-phytodienoic acid (OPDA), can also induce gene expression. However, little is known about the physiological significance of OPDA-dependent gene expression. We used microarray analysis of approximately 21,500 Arabidopsis (Arabidopsis thaliana) genes to compare responses to JA, MeJA, and OPDA treatment. Although many genes responded identically to both OPDA and JAs, we identified a set of genes (OPDA-specific response genes [ORGs]) that specifically responded to OPDA but not to JAs. ORGs primarily encoded signaling components, transcription factors, and stress response-related genes. One-half of the ORGs were induced by wounding. Analysis using mutants deficient in the biosynthesis of JAs revealed that OPDA functions as a signaling molecule in the wounding response. Unlike signaling via JAs, OPDA signaling was CORONATINE INSENSITIVE 1 independent. These results indicate that an OPDA signaling pathway functions independently of JA/MeJA signaling and is required for the wounding response in Arabidopsis.

12-oxo-phytodienoic acid triggers expression of a distinct set of genes and plays a role in wound-induced gene expression in Arabidopsis

Jasmonic acid (JA) and methyl jasmonate (MeJA), collectively known as JAs, regulate diverse physiological processes in plants, including the response to wounding. Recent reports suggest that a cyclopentenone precursor of JA, 12-oxo-phytodienoic acid (OPDA), can also induce gene expression. However, little is known about the physiological significance of OPDA-dependent gene expression. We used microarray analysis of approximately 21,500 Arabidopsis (Arabidopsis thaliana) genes to compare responses to JA, MeJA, and OPDA treatment. Although many genes responded identically to both OPDA and JAs, we identified a set of genes (OPDA-specific response genes [ORGs]) that specifically responded to OPDA but not to JAs. ORGs primarily encoded signaling components, transcription factors, and stress response-related genes. One-half of the ORGs were induced by wounding. Analysis using mutants deficient in the biosynthesis of JAs revealed that OPDA functions as a signaling molecule in the wounding response. Unlike signaling via JAs, OPDA signaling was CORONATINE INSENSITIVE 1 independent. These results indicate that an OPDA signaling pathway functions independently of JA/MeJA signaling and is required for the wounding response in Arabidopsis.

Expression profiling reveals COI1 to be a key regulator of genes involved in wound- and methyl jasmonate-induced secondary metabolism, defence, and hormone interactions

The Arabidopsis gene COI1 is required for jasmonic acid (JA)-induced growth inhibition, resistance to insect herbivory, and resistance to pathogens. In addition, COI1 is also required for transcription of several genes induced by wounding or by JA. Here, we use microarray gene transcription profiling of wild type and coi1 mutant plants to examine the extent of the requirement of COI1 for JA-induced and wound-induced gene transcription. We show that COI1 is required for expression of approximately 84% of 212 genes induced by JA, and for expression of approximately 44% of 153 genes induced by wounding. Surprisingly, COI1 was also required for repression of 53% of 104 genes whose expression was suppressed by JA, and for repression of approximately 46% of 83 genes whose expression was suppressed by wounding. These results indicate that COI1 plays a pivotal role in wound- and JA signalling.