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

Indirect defence via tritrophic interactions

Many plants interact with carnivores as an indirect defence against herbivores. The release of volatile organic compounds (VOCs) and the secretion of extrafloral nectar (EFN) are induced by insect feeding, a response that is mediated by the plant hormone, jasmonic acid. Although VOCs mainly attract predatory mites and parasitic wasps, while EFN mainly attracts ants, many more animal-plant interactions are influenced by these two traits. Other traits involved in defensive tritrophic interactions are cellular food bodies and domatia, which serve the nutrition and housing of predators. They are not known to respond to herbivory, while food body production can be induced by the presence of the mutualists. Interactions among the different defensive traits, and between them and other biotic and abiotic factors exist on the genetic, physiological, and ecological levels, but so far remain understudied. Indirect defences are increasingly being discussed as an environmentally-friendly crop protection strategy, but much more knowledge on their fitness effects under certain environmental conditions is required before we can understand their ecological and evolutionary relevance, and before tritrophic interactions can serve as a reliable tool in agronomy.

Nitric oxide-responsive genes and promoters in Arabidopsis thaliana: a bioinformatics approach

Due to its high reactivity and its ability to diffuse and permeate the cell membrane, nitric oxide (NO) and its exchangeable redox-activated species are unique biological messengers in animals and in plants. Although an increasing number of reports indicate that NO is an essential molecule in several physiological processes, there is not a clear picture of its method of action. Studies on the transcriptional changes induced by NO permitted identification of genes involved in different functional processes such as signal transduction, defence and cell death, transport, basic metabolism, and reactive oxygen species (ROS) production and degradation. The co-expression of these genes can be explained by the co-operation of a set of transcription factors that bind a common region in the promoter of the regulated genes. The present report describes the search for a common transcription factor-binding site (TFBS) in promoter regions of NO-regulated genes, based on microarray analyses. Using Genomatix Gene2Promotor and MatInspector, eight families of TFBSs were found to occur at least 15% more often in the promoter regions of the responsive genes in comparison with the promoter regions of 28,447 Arabidopsis control genes. Most of these TFBSs, such as ocs element-like sequences and WRKY, have already been reported to be involved in particular stress responses. Furthermore, the promoter regions of genes involved in jasmonic acid (JA) biosynthesis were analysed for a common TFBS module, since some genes responsible for JA biosynthesis are induced by NO, and an interaction between NO and JA signalling has already been described.

Priming defense genes and metabolites in hybrid poplar by the green leaf volatile cis-3-hexenyl acetate

* Herbivore-induced plant volatiles (HIPVs), in addition to attracting natural enemies of herbivores, can serve a signaling function within plants to induce or prime defenses. However, it is largely unknown, particularly in woody plants, which volatile compounds within HIPV blends can act as signaling molecules. * Leaves of hybrid poplar saplings were exposed in vivo to naturally wound-emitted concentrations of the green leaf volatile (GLV) cis-3-hexenyl acetate (z3HAC) and then subsequently fed upon by gypsy moth larvae. Volatiles were collected throughout the experiments, and leaf tissue was collected to measure phytohormone concentrations and expression of defense-related genes. * Relative to controls, z3HAC-exposed leaves had higher concentrations of jasmonic acid and linolenic acid following gypsy moth feeding. Furthermore, z3HAC primed transcripts of genes that mediate oxylipin signaling and direct defenses, as determined by both qRT-PCR and microarray analysis using the AspenDB 7 K expressed sequence tags (EST) microarray containing c. 5400 unique gene models. Moreover, z3HAC primed the release of terpene volatiles. * The widespread priming response suggests an adaptive benefit to detecting z3HAC as a wound signal. Thus, woody plants can detect and use z3HAC as a signal to prime defenses before actually experiencing damage. GLVs may therefore have important ecological functions in arboreal ecosystems.

Jasmonate signalling network in Arabidopsis thaliana: crucial regulatory nodes and new physiological scenarios

Plant development and stress responses are regulated by complex signalling networks that mediate specific and dynamic plant responses upon activation by various types of exogenous and endogenous signal. In this review, we focus on the latest published work on jasmonate (JA) signalling components and new regulatory nodes in the transcriptional network that regulates a number of diverse plant responses to developmental and environmental cues. Not surprisingly, the majority of the key revelations in the field have been made in Arabidopsis thaliana. However, for comparative reasons, we integrate information on Arabidopsis with recent reports for other plant species (when available). Recent findings on the regulation of plant responses to pathogens by JAs, as well as new evidence implicating JAs in the regulation of senescence, suggest a common mechanism of JA action in these responses via distinct groups of transcription factors. Moreover, a significant increase in the amount of evidence has allowed placing of specific mitogen-activated protein kinases (MAPKs) as crucial regulatory nodes in the defence signalling network. In addition, we report on new physiological scenarios for JA signalling, such as organogenesis of nitrogen-fixing nodules and anticancer therapy.

Caterpillar saliva interferes with induced Arabidopsis thaliana defence responses via the systemic acquired resistance pathway

Arabidopsis thaliana (L.) Heynh. genotypes limited in their ability to mount either octadecanoid-dependent induced resistance (IR(-)) or systemic acquired resistance (SAR(-)) were used to characterize the roles of these pathways in plant-herbivore interactions. Molecular and biochemical markers of IR were analysed in plants subject to herbivory by caterpillars of the beet armyworm, Spodoptera exigua Hübner, which had either intact or impaired salivary secretions since salivary enzymes, such as glucose oxidase, have been implicated in the ability of caterpillars to circumvent induced plant defences. Transcript expression of genes encoding laccase-like multicopper oxidase [AtLMCO4 (polyphenol oxidase)] and defensin (AtPDF1.2) showed salivary-specific patterns which were disrupted in the SAR(-) mutant plants. The activity of octadecanoid-associated anti-nutritive proteins, such as LMCO and trypsin inhibitor, showed similar patterns. Gene and protein changes parallel plant hormone levels where elevated jasmonic acid was observed in wild-type plants fed upon by caterpillars with impaired salivary secretions compared with plants subject to herbivory by normal caterpillars. This salivary-specific difference in jasmonic acid levels was alleviated in SAR(-) mutants. These results support the model that caterpillar saliva interferes with jasmonate-dependent plant defences by activating the SAR pathway.

Stress responses: JAZ players deliver fusion and rhythm

How do stress-induced jasmonates cause genetic re-programming of plant cells? New research shows that the JAZ proteins provide the missing link.

Plant interactions with microbes and insects: from molecular mechanisms to ecology

Plants are members of complex communities and interact both with antagonists and beneficial organisms. An important question in plant defense-signaling research is how plants integrate signals induced by pathogens, beneficial microbes and insects into the most appropriate adaptive response. Molecular and genomic tools are now being used to uncover the complexity of the induced defense signaling networks that have evolved during the arms races between plants and their attackers. Molecular biologists and ecologists are joining forces to place molecular mechanisms of plant defense into an ecological perspective. Here, we review our current understanding of the molecular mechanisms of induced plant defense and their potential ecological relevance in nature.

The developmental selector AS1 is an evolutionarily conserved regulator of the plant immune response

The MYB-related gene ASYMMETRIC LEAVES 1 (AS1) and its orthologs have an evolutionarily conserved role in specification of leaf cell identity. AS1 is expressed in leaf founder cells, where it functions as a heterodimer with the structurally unrelated AS2 proteins to repress activity of KNOTTED 1-like homeobox (KNOX) genes. AS1 therefore confines KNOX activity to the shoot apical meristem, where it promotes stem cell function through the regulation of phytohormone activities. Here, we show that loss-of-function mutations in AS1 unexpectedly convey heightened protection against necrotrophic fungi. AS1 operates as a negative regulator of inducible resistance against these pathogens by selectively binding to the promoters of genes controlled by the immune activator, jasmonic acid (JA), damping the defense response. In contrast, AS1 is a positive regulator of salicylic acid (SA)-independent extracellular defenses against bacterial pathogens. Neither the absence of AS2 nor ERECTA function, which enhances the morphological phenotype of as1, nor the conditional or constitutive expression of KNOX genes impacted disease resistance. Thus, the function of AS1 in responses to phytopathogens is independent of its AS2-associated role in development. Loss of function in the AS1 orthologs PHAN in Antirrhinum majus and NSPHAN in Nicotiana sylvestris produced pathogen-response phenotypes similar to as1 plants, and therefore the defense function of AS1 is evolutionarily conserved in plant species with a divergence time of approximately 125 million years.

A nonclassical arabinogalactan protein gene highly expressed in vascular tissues, AGP31, is transcriptionally repressed by methyl jasmonic acid in Arabidopsis

In response to wounding and pathogens, jasmonate (JA) serves as a signal molecule for both induction and repression of gene expression. To examine defense-regulated gene repression in Arabidopsis (Arabidopsis thaliana), we have identified a nonclassical arabinogalactan protein (AGP) gene, AGP31, and show that its mRNA decreased to about 30% of its original level within 8 h in response to methyl JA (MeJA) treatment of whole 7-d-old seedlings. Wounding and abscisic acid treatment had similar effects. MeJA suppression primarily depends on the action of the JA-signaling protein, COI1, as shown by much lower MeJA suppression in coi1-1 mutant plants. The main mechanism of mRNA suppression by MeJA is repression of transcription, as shown by nuclear run-on experiments. The AGP31 protein shares features with several known and putative nonclassical AGPs from other species: a putative signal peptide, a histidine-rich region near the N terminus followed by a repetitive proline-rich domain, and a cysteine-rich C-terminal PAC (for proline-rich protein and AGP, containing cysteine) domain. Positive Yariv reagent interaction demonstrated that the protein is an AGP. Monosaccharide analysis of purified AGP31 indicated it is a galactose-rich AGP. Expression of an AGP31-enhanced green fluorescent protein fusion protein in transgenic cells revealed that the AGP31 protein was localized to the cell wall. AGP31 promoter-beta-glucuronidase reporter gene analysis showed expression in the vascular bundle throughout the plant, except in the flower. In the flower, beta-glucuronidase staining occurred throughout the pistil, except in the stigma. The strong preferential expression in vascular tissues suggests that AGP31 may be involved in vascular tissue function during both the defense response and development.

Signaling pathways controlling induced resistance to insect herbivores in Arabidopsis

Insect attack triggers changes in transcript level in plants that are mediated predominantly by jasmonic acid (JA). The implication of ethylene (ET), salicylic acid (SA), and other signals in this response is less understood and was monitored with a microarray containing insect- and defense-regulated genes. Arabidopsis thaliana mutants coi1-1, ein2-1, and sid2-1 impaired in JA, ET, and SA signaling pathways were challenged with the specialist small cabbage white (Pieris rapae) and the generalist Egyptian cotton worm (Spodoptera littoralis). JA was shown to be a major signal controlling the upregulation of defense genes in response to either insect but was found to suppress changes in transcript level only in response to P. rapae. Larval growth was affected by the JA-dependent defenses, but S. littoralis gained much more weight on coi1-1 than P. rapae. ET and SA mutants had an altered transcript profile after S. littoralis herbivory but not after P. rapae herbivory. In contrast, both insects yielded similar transcript signatures in the abscisic acid (ABA)-biosynthetic mutants aba2-1 and aba3-1, and ABA controlled transcript levels both negatively and positively in insect-attacked plants. In accordance with the transcript signature, S. littoralis larvae performed better on aba2-1 mutants. This study reveals a new role for ABA in defense against insects in Arabidopsis and identifies some components important for plant resistance to herbivory.

Defense against Sclerotinia sclerotiorum in Arabidopsis is dependent on jasmonic acid, salicylic acid, and ethylene signaling

Genotypic differences in susceptibility of Arabidopsis thaliana to Sclerotinia sclerotiorum have not been reported due to the extreme susceptibility of this cruciferous plant. To overcome this limitation, we have established inoculation conditions that enable evaluation of differences in susceptibility to S. sclerotiorum among Arabidopsis mutants and ecotypes. Two coil mutant alleles conferred hypersusceptibility to S. sclerotiorum. The plant defensin gene PDF1.2 was no longer induced after challenging the coi1-2 mutant with S. sclerotiorum. Hypersusceptibility of the coi1-2 mutant to S. sclerotiorum was not correlated with oxalate sensitivity. The mutants npr1 and ein2 were also hypersusceptible to S. sclerotiorum. Induction of PDF1.2 and the pathogenesis-related gene PR1 was reduced in ein2 and npr1 mutants, respectively. Actigard, a commercial formulation of the systemic acquired resistance inducer benzothiadiazole, reduced susceptibility to S. sclerotiorum. Based on histochemical analysis of oxalate-deficient and wild-type strains of S. sclerotiorum, oxalate caused a decrease in hydrogen peroxide production but no detectable changes in plant superoxide production or gene expression.

The bHLH-type transcription factor AtAIB positively regulates ABA response in Arabidopsis

The phytohormone ABA was known to play a vital role in modulating plant responses to drought stress. Here, we report that a nuclear-localized basic helix-loop-helix (bHLH)-type protein, AtAIB, positively regulates ABA response in Arabidopsis. The expression of AtAIB was transitorily induced by ABA and PEG, although its transcripts were accumulated in various organs. We provided evidence showing that AtAIB has transcriptional activation activity in yeast. Knockdown of AtAIB expression caused reduced sensitivity to ABA, whereas overexpression of this gene led to elevated sensitivity to ABA in cotyledon greening and seedling root growth. Furthermore, soil-grown plants overexpressing AtAIB showed increased drought tolerance. Taken together, these results suggested that AtAIB functions as a transcription activator involved in the regulation of ABA signaling in Arabidopsis.

Jasmonates: an update on biosynthesis, signal transduction and action in plant stress response, growth and development

BACKGROUND

Jasmonates are ubiquitously occurring lipid-derived compounds with signal functions in plant responses to abiotic and biotic stresses, as well as in plant growth and development. Jasmonic acid and its various metabolites are members of the oxylipin family. Many of them alter gene expression positively or negatively in a regulatory network with synergistic and antagonistic effects in relation to other plant hormones such as salicylate, auxin, ethylene and abscisic acid.

SCOPE

This review summarizes biosynthesis and signal transduction of jasmonates with emphasis on new findings in relation to enzymes, their crystal structure, new compounds detected in the oxylipin and jasmonate families, and newly found functions.

CONCLUSIONS

Crystal structure of enzymes in jasmonate biosynthesis, increasing number of jasmonate metabolites and newly identified components of the jasmonate signal-transduction pathway, including specifically acting transcription factors, have led to new insights into jasmonate action, but its receptor(s) is/are still missing, in contrast to all other plant hormones.

The promoter of the TLC1.1 retrotransposon from Solanum chilense is activated by multiple stress-related signaling molecules

The LTR retrotransposons are the most abundant mobile elements in the plant genome and seem to play an important role in genome reorganization induced by environmental challenges. Their success in this function depends on the ability of their promoters to respond to different signaling pathways that regulate plant adaptation to biotic and abiotic stresses. The promoter of the TLC1.1 retrotransposon from Solanum chilense contains two primary ethylene-responsive elements (PERE boxes) that are essential for its response to ethylene and for the stress-induced expression. Here, we describe that a 270 bp fragment (P270), derivative of this retroelement promoter, is also able to activate the transcription of the GUS reporter gene in transgenic plants in response to salicylic acid (SA), abscisic acid (ABA), methyl jasmonate (MeJA), hydrogen peroxide (H2O2) and the synthetic auxin 2,4-D. PERE box-dependent and independent routes are involved in the response of P270 to these signal molecules. MeJA, H2O2 and 2,4-D activate this promoter through cis-acting elements other than PERE boxes, whereas ABA and SA act via a PERE box-independent pathway but require this element for maximal activation. Three putative cis-acting elements MRE, GCN4 and GT1/TCA identified in the P270 promoter may be involved in the PERE box-independent activation pathway. These results suggest that the promoter of TLC1.1 may act as an integrator of different signal transduction pathways, allowing this member of the TLC1 retrotransposon family to be activated in response to multiples challenges.

The HvNAC6 transcription factor: a positive regulator of penetration resistance in barley and Arabidopsis

Pathogens induce the expression of many genes encoding plant transcription factors, though specific knowledge of the biological function of individual transcription factors remains scarce. NAC transcription factors are encoded in plants by a gene family with proposed functions in both abiotic and biotic stress adaptation, as well as in developmental processes. In this paper, we provide convincing evidence that a barley NAC transcription factor has a direct role in regulating basal defence. The gene transcript was isolated by differential display from barley leaves infected with the biotrophic powdery mildew fungus, Blumeria graminis f.sp. hordei (Bgh). The full-length cDNA clone was obtained using 5'-RACE and termed HvNAC6, due to its high similarity to the rice homologue, OsNAC6. Gene silencing of HvNAC6 during Bgh inoculation compromises penetration resistance in barley epidermal cells towards virulent Bgh. Complementing the effect of HvNAC6 gene silencing, transient overexpression of HvNAC6 increases the occurrence of penetration resistant cells towards Bgh attack. Quantitative RT-PCR shows the early and transient induction of HvNAC6 in barley epidermis upon Bgh infection. Additionally, our results show that the Arabidopsis HvNAC6 homologue ATAF1 is also induced by Bgh and the ataf1-1 mutant line shows decreased penetration resistance to this non-host pathogen. Collectively, these data suggest a conserved role of HvNAC6 and ATAF1 in the regulation of penetration resistance in monocots and dicots, respectively.

Identification of 118 Arabidopsis transcription factor and 30 ubiquitin-ligase genes responding to chitin, a plant-defense elicitor

Chitin, found in the cell walls of true fungi and the exoskeleton of insects and nematodes, is a well-established elicitor of plant defense responses. In this study, we analyzed the expression patterns of Arabidopsis thaliana transcription factor (TF) and ubiquitin-ligase genes in response to purified chitooctaose at different treatment times (15, 30, 60, 90, and 120 min after treatment), using both quantitative polymerase chain reaction and the Affymetrix Arabidopsis whole-genome array. A total of 118 TF genes and 30 ubiquitin-ligase genes were responsive to the chitin treatment. Among these genes, members from the following four TF families were overrepresented: APETALA2/ethylene-reponsive element binding proteins (27), C2H2 zinc finger proteins (14), MYB domain-containing proteins (11), and WRKY domain transcription factors (14). Transcript variants from a few of these genes were found to respond differentially to chitin, suggesting transcript-specific regulation of these TF genes.

Pathological hormone imbalances

Plant hormones play important roles in regulating developmental processes and signalling networks involved in plant responses to a wide range of biotic and abiotic stresses. Salicylic acid (SA), jasmonates (JA) and ethylene (ET) are well known to play crucial roles in plant disease and pest resistance. However, the roles of other hormones such as abscisic acid (ABA), auxin, gibberellin (GA), cytokinin (CK) and brassinosteroid (BL) in plant defence are less well known. Much progress has been made in understanding plant hormone signalling and plant disease resistance. However, these studies have mostly proceeded independently of each other, and there is limited knowledge regarding interactions between plant hormone-mediated signalling and responses to various pathogens. Here, we review the roles of hormones other than SA, JA and ET in plant defence and the interactions between hormone-mediated signalling, plant defence and pathogen virulence. We propose that these hormones may influence disease outcomes through their effect on SA or JA signalling.

Defense suppression by virulence effectors of bacterial phytopathogens

Phytopathogenic bacteria and plants are locked in molecular struggles that determine the outcome of an infection. Bacteria make effector molecules that can induce defenses if recognized by specific host resistance (R) proteins. In susceptible hosts, however, effectors frequently promote virulence by suppressing host defenses. Defense-inducing and defense-suppressing activities are often related, as virulence-associated host modifications can elicit R protein activation. Thus, understanding of how an effector elicits defenses can translate into understanding of how it promotes virulence and vice versa. To control host cell functions, such as defense gene expression and vesicle trafficking, effectors use various biochemical activities, including protein modification, transcriptional regulation, and hormone mimicry. Progress with individual effectors will lead to an integrated view of how the activities of a collection of effectors intersect with genetically variable host plants to regulate susceptibility and resistance.

The JAZ family of repressors is the missing link in jasmonate signalling

Jasmonates are essential phytohormones for plant development and survival. However, the molecular details of their signalling pathway remain largely unknown. The identification more than a decade ago of COI1 as an F-box protein suggested the existence of a repressor of jasmonate responses that is targeted by the SCF(COI1) complex for proteasome degradation in response to jasmonate. Here we report the identification of JASMONATE-INSENSITIVE 3 (JAI3) and a family of related proteins named JAZ (jasmonate ZIM-domain), in Arabidopsis thaliana. Our results demonstrate that JAI3 and other JAZs are direct targets of the SCF(COI1) E3 ubiquitin ligase and jasmonate treatment induces their proteasome degradation. Moreover, JAI3 negatively regulates the key transcriptional activator of jasmonate responses, MYC2. The JAZ family therefore represents the molecular link between the two previously known steps in the jasmonate pathway. Furthermore, we demonstrate the existence of a regulatory feed-back loop involving MYC2 and JAZ proteins, which provides a mechanistic explanation for the pulsed response to jasmonate and the subsequent desensitization of the cell.

JAZ repressor proteins are targets of the SCF(COI1) complex during jasmonate signalling

Jasmonate and related signalling compounds have a crucial role in both host immunity and development in plants, but the molecular details of the signalling mechanism are poorly understood. Here we identify members of the jasmonate ZIM-domain (JAZ) protein family as key regulators of jasmonate signalling. JAZ1 protein acts to repress transcription of jasmonate-responsive genes. Jasmonate treatment causes JAZ1 degradation and this degradation is dependent on activities of the SCF(COI1) ubiquitin ligase and the 26S proteasome. Furthermore, the jasmonoyl-isoleucine (JA-Ile) conjugate, but not other jasmonate-derivatives such as jasmonate, 12-oxo-phytodienoic acid, or methyl-jasmonate, promotes physical interaction between COI1 and JAZ1 proteins in the absence of other plant proteins. Our results suggest a model in which jasmonate ligands promote the binding of the SCF(COI1) ubiquitin ligase to and subsequent degradation of the JAZ1 repressor protein, and implicate the SCF(COI1)-JAZ1 protein complex as a site of perception of the plant hormone JA-Ile.

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.

MYC2 differentially modulates diverse jasmonate-dependent functions in Arabidopsis

The Arabidopsis thaliana basic helix-loop-helix Leu zipper transcription factor (TF) MYC2/JIN1 differentially regulates jasmonate (JA)-responsive pathogen defense (e.g., PDF1.2) and wound response (e.g., VSP) genes. In this study, genome-wide transcriptional profiling of wild type and mutant myc2/jin1 plants followed by functional analyses has revealed new roles for MYC2 in the modulation of diverse JA functions. We found that MYC2 negatively regulates Trp and Trp-derived secondary metabolism such as indole glucosinolate biosynthesis during JA signaling. Furthermore, MYC2 positively regulates JA-mediated resistance to insect pests, such as Helicoverpa armigera, and tolerance to oxidative stress, possibly via enhanced ascorbate redox cycling and flavonoid biosynthesis. Analyses of MYC2 cis binding elements and expression of MYC2-regulated genes in T-DNA insertion lines of a subset of MYC2-regulated TFs suggested that MYC2 might modulate JA responses via differential regulation of an intermediate spectrum of TFs with activating or repressing roles in JA signaling. MYC2 also negatively regulates its own expression, and this may be one of the mechanisms used in fine-tuning JA signaling. Overall, these results provide new insights into the function of MYC2 and the transcriptional coordination of the JA signaling pathway.

Before gene expression: early events in plant-insect interaction

Successful defense depends on the ability of the plant to recognize an attacking 'enemy' as early as possible. Early defense responses require enemy-initiated signaling cascades. Their activation ensures an induced response that is quantitative, timely and coordinated with other activities of the host cells. Damage-induced ion imbalances and modulations of channel activities are the first events occurring in the plasma membrane and result in rapid perturbations of the plasma membrane potential (V(m)) involving variations of cytosolic Ca(2+) concentrations. Interacting downstream networks of kinases and phytohormones mediate the signal and result in concerted gene activation. Here we review and discuss early events occurring before herbivore attack-related gene expression that are responsible for cascades of events and signal transductions, eventually leading to indirect and direct plant responses.

ABA is an essential signal for plant resistance to pathogens affecting JA biosynthesis and the activation of defenses in Arabidopsis

Analyses of Arabidopsis thaliana defense response to the damping-off oomycete pathogen Pythium irregulare show that resistance to P. irregulare requires a multicomponent defense strategy. Penetration represents a first layer, as indicated by the susceptibility of pen2 mutants, followed by recognition, likely mediated by ERECTA receptor-like kinases. Subsequent signaling of inducible defenses is predominantly mediated by jasmonic acid (JA), with insensitive coi1 mutants showing extreme susceptibility. In contrast with the generally accepted roles of ethylene and salicylic acid cooperating with or antagonizing, respectively, JA in the activation of defenses against necrotrophs, both are required to prevent disease progression, although much less so than JA. Meta-analysis of transcriptome profiles confirmed the predominant role of JA in activation of P. irregulare-induced defenses and uncovered abscisic acid (ABA) as an important regulator of defense gene expression. Analysis of cis-regulatory sequences also revealed an unexpected overrepresentation of ABA response elements in promoters of P. irregulare-responsive genes. Subsequent infections of ABA-related and callose-deficient mutants confirmed the importance of ABA in defense, acting partly through an undescribed mechanism. The results support a model for ABA affecting JA biosynthesis in the activation of defenses against this oomycete.

Ubiquitin, hormones and biotic stress in plants

BACKGROUND

The covalent attachment of ubiquitin to a substrate protein changes its fate. Notably, proteins typically tagged with a lysine48-linked polyubiquitin chain become substrates for degradation by the 26S proteasome. In recent years many experiments have been performed to characterize the proteins involved in the ubiquitylation process and to identify their substrates, in order to understand better the mechanisms that link specific protein degradation events to regulation of plant growth and development.

SCOPE

This review focuses on the role that ubiquitin plays in hormone synthesis, hormonal signalling cascades and plant defence mechanisms. Several examples are given of how targeted degradation of proteins affects downstream transcriptional regulation of hormone-responsive genes in the auxin, gibberellin, abscisic acid, ethylene and jasmonate signalling pathways. Additional experiments suggest that ubiquitin-mediated proteolysis may also act upstream of the hormonal signalling cascades by regulating hormone biosynthesis, transport and perception. Moreover, several experiments demonstrate that hormonal cross-talk can occur at the level of proteolysis. The more recently established role of the ubiquitin/proteasome system (UPS) in defence against biotic threats is also reviewed.

CONCLUSIONS

The UPS has been implicated in the regulation of almost every developmental process in plants, from embryogenesis to floral organ production probably through its central role in many hormone pathways. More recent evidence provides molecular mechanisms for hormonal cross-talk and links the UPS system to biotic defence responses.

ABA is an essential signal for plant resistance to pathogens affecting JA biosynthesis and the activation of defenses in Arabidopsis

Analyses of Arabidopsis thaliana defense response to the damping-off oomycete pathogen Pythium irregulare show that resistance to P. irregulare requires a multicomponent defense strategy. Penetration represents a first layer, as indicated by the susceptibility of pen2 mutants, followed by recognition, likely mediated by ERECTA receptor-like kinases. Subsequent signaling of inducible defenses is predominantly mediated by jasmonic acid (JA), with insensitive coi1 mutants showing extreme susceptibility. In contrast with the generally accepted roles of ethylene and salicylic acid cooperating with or antagonizing, respectively, JA in the activation of defenses against necrotrophs, both are required to prevent disease progression, although much less so than JA. Meta-analysis of transcriptome profiles confirmed the predominant role of JA in activation of P. irregulare-induced defenses and uncovered abscisic acid (ABA) as an important regulator of defense gene expression. Analysis of cis-regulatory sequences also revealed an unexpected overrepresentation of ABA response elements in promoters of P. irregulare-responsive genes. Subsequent infections of ABA-related and callose-deficient mutants confirmed the importance of ABA in defense, acting partly through an undescribed mechanism. The results support a model for ABA affecting JA biosynthesis in the activation of defenses against this oomycete.

Impairment of cellulose synthases required for Arabidopsis secondary cell wall formation enhances disease resistance

Cellulose is synthesized by cellulose synthases (CESAs) contained in plasma membrane-localized complexes. In Arabidopsis thaliana, three types of CESA subunits (CESA4/IRREGULAR XYLEM5 [IRX5], CESA7/IRX3, and CESA8/IRX1) are required for secondary cell wall formation. We report that mutations in these proteins conferred enhanced resistance to the soil-borne bacterium Ralstonia solanacearum and the necrotrophic fungus Plectosphaerella cucumerina. By contrast, susceptibility to these pathogens was not altered in cell wall mutants of primary wall CESA subunits (CESA1, CESA3/ISOXABEN RESISTANT1 [IXR1], and CESA6/IXR2) or POWDERY MILDEW-RESISTANT5 (PMR5) and PMR6 genes. Double mutants indicated that irx-mediated resistance was independent of salicylic acid, ethylene, and jasmonate signaling. Comparative transcriptomic analyses identified a set of common irx upregulated genes, including a number of abscisic acid (ABA)-responsive, defense-related genes encoding antibiotic peptides and enzymes involved in the synthesis and activation of antimicrobial secondary metabolites. These data as well as the increased susceptibility of ABA mutants (abi1-1, abi2-1, and aba1-6) to R. solanacearum support a direct role of ABA in resistance to this pathogen. Our results also indicate that alteration of secondary cell wall integrity by inhibiting cellulose synthesis leads to specific activation of novel defense pathways that contribute to the generation of an antimicrobial-enriched environment hostile to pathogens.

A gain-of-function allele of TPC1 activates oxylipin biogenesis after leaf wounding in Arabidopsis

Jasmonates, potent lipid mediators of defense gene expression in plants, are rapidly synthesized in response to wounding. These lipid mediators also stimulate their own production via a positive feedback circuit, which depends on both JA synthesis and JA signaling. To date, molecular components regulating the activation of jasmonate biogenesis and its feedback loop have been poorly characterized. We employed a genetic screen capable of detecting the misregulated activity of 13-lipoxygenase, which operates at the entry point of the jasmonate biosynthesis pathway. Leaf extracts from the Arabidopsis fou2 (fatty acid oxygenation upregulated 2) mutant displayed an increased capacity to catalyze the synthesis of lipoxygenase (LOX) metabolites. Quantitative oxylipin analysis identified less than twofold increased jasmonate levels in healthy fou2 leaves compared to wild-type; however, wounded fou2 leaves strongly increased jasmonate biogenesis compared to wounded wild-type. Furthermore, the plants displayed enhanced resistance to the fungus Botrytis cinerea. Higher than wild-type LOX activity and enhanced resistance in the fou2 mutant depend fully on a functional jasmonate response pathway. The fou2 mutant carries a missense mutation in the putative voltage sensor of the Two Pore Channel 1 gene (TPC1), which encodes a Ca(2+)-permeant non-selective cation channel. Patch-clamp analysis of fou2 vacuolar membranes showed faster time-dependent conductivity and activation of the mutated channel at lower membrane potentials than wild-type. The results indicate that cation fluxes exert strong control over the positive feedback loop whereby JA stimulates its own synthesis.

The mitogen-activated protein kinase cascade MKK3-MPK6 is an important part of the jasmonate signal transduction pathway in Arabidopsis

The plant hormone jasmonic acid (JA) plays a key role in the environmental stress responses and developmental processes of plants. Although ATMYC2/JASMONATE-INSENSITIVE1 (JIN1) is a major positive regulator of JA-inducible gene expression and essential for JA-dependent developmental processes in Arabidopsis thaliana, molecular mechanisms underlying the control of ATMYC2/JIN1 expression remain largely unknown. Here, we identify a mitogen-activated protein kinase (MAPK) cascade, MAPK KINASE 3 (MKK3)-MAPK 6 (MPK6), which is activated by JA in Arabidopsis. We also show that JA negatively controls ATMYC2/JIN1 expression, based on quantitative RT-PCR and genetic analyses using gain-of-function and loss-of-function mutants of the MKK3-MPK6 cascade. These results indicate that this kinase unit plays a key role in JA-dependent negative regulation of ATMYC2/JIN1 expression. Both positive and negative regulation by JA may be used to fine-tune ATMYC2/JIN1 expression to control JA signaling. Moreover, JA-regulated root growth inhibition is affected by mutations in the MKK3-MPK6 cascade, which indicates important roles in JA signaling. We provide a model explaining how MPK6 can convert three distinct signals - JA, pathogen, and cold/salt stress - into three different sets of responses in Arabidopsis.

Arabidopsis systemic immunity uses conserved defense signaling pathways and is mediated by jasmonates

In the absence of adaptive immunity displayed by animals, plants respond locally to biotic challenge via inducible basal defense networks activated through recognition and response to conserved pathogen-associated molecular patterns. In addition, immunity can be induced in tissues remote from infection sites by systemic acquired resistance (SAR), initiated after gene-for-gene recognition between plant resistance proteins and microbial effectors. The nature of the mobile signal and remotely activated networks responsible for establishing SAR remain unclear. Salicylic acid (SA) participates in the local and systemic response, but SAR does not require long-distance translocation of SA. Here, we show that, despite the absence of pathogen-associated molecular pattern contact, systemically responding leaves rapidly activate a SAR transcriptional signature with strong similarity to local basal defense. We present several lines of evidence that suggest jasmonates are central to systemic defense, possibly acting as the initiating signal for classic SAR. Jasmonic acid (JA), but not SA, rapidly accumulates in phloem exudates of leaves challenged with an avirulent strain of Pseudomonas syringae. In systemically responding leaves, transcripts associated with jasmonate biosynthesis are up-regulated within 4 h, and JA increases transiently. SAR can be mimicked by foliar JA application and is abrogated in mutants impaired in jasmonate synthesis or response. We conclude that jasmonate signaling appears to mediate long-distance information transmission. Moreover, the systemic transcriptional response shares extraordinary overlap with local herbivory and wounding responses, indicating that jasmonates may be pivotal to an evolutionarily conserved signaling network that decodes multiple abiotic and biotic stress signals.

ATTED-II: a database of co-expressed genes and cis elements for identifying co-regulated gene groups in Arabidopsis

Publicly available database of co-expressed gene sets would be a valuable tool for a wide variety of experimental designs, including targeting of genes for functional identification or for regulatory investigation. Here, we report the construction of an Arabidopsis thaliana trans-factor and cis-element prediction database (ATTED-II) that provides co-regulated gene relationships based on co-expressed genes deduced from microarray data and the predicted cis elements. ATTED-II () includes the following features: (i) lists and networks of co-expressed genes calculated from 58 publicly available experimental series, which are composed of 1388 GeneChip data in A.thaliana; (ii) prediction of cis-regulatory elements in the 200 bp region upstream of the transcription start site to predict co-regulated genes amongst the co-expressed genes; and (iii) visual representation of expression patterns for individual genes. ATTED-II can thus help researchers to clarify the function and regulation of particular genes and gene networks.

Yeast increases resistance in Arabidopsis against Pseudomonas syringae and Botrytis cinerea by salicylic acid-dependent as well as -independent mechanisms

Cell-wall and glucopeptide components of yeast have been reported to exhibit elicitor activity. The mode of action of defense activation by yeast is not known so far. In this study, we used the model plant Arabidopsis to investigate the activation of defense responses by yeast, the effect on resistance against different pathogens, and the mode of action. Treatment of Arabidopsis plants with an autoclaved yeast suspension induced the expression of systemic acquired resistance-related genes and accumulation of the phytoalexin camalexin. Symptom development and bacterial growth after infection with a virulent strain of the pathogen Pseudomonas syringae was reduced in yeast-pretreated plants. No protection was detectable in mutants affected in the salicylate pathway, while mutants in the jasmonate or camalexin pathway were protected by yeast, indicating that the salicylate pathway is necessary for the yeast-induced resistance against P. syringae. Yeast also reduced symptom development after challenge with Botrytis cinerea. This protection was detectable in all mutants tested, indicating that it is independent of the salicylate, jasmonate, and camalexin pathway.

Herbivore-induced resistance against microbial pathogens in Arabidopsis

Caterpillars of the herbivore Pieris rapae stimulate the production of jasmonic acid (JA) and ethylene (ET) in Arabidopsis (Arabidopsis thaliana) and trigger a defense response that affects insect performance on systemic tissues. To investigate the spectrum of effectiveness of P. rapae-induced resistance, we examined the level of resistance against different pathogens. Although the necrotrophic fungus Alternaria brassicicola is sensitive to JA-dependent defenses, herbivore-induced resistance was not effective against this pathogen. By contrast, caterpillar feeding significantly reduced disease caused by the bacterial pathogens Pseudomonas syringae pv tomato and Xanthomonas campestris pv armoraciae. However, this effect was apparent only locally in caterpillar-damaged tissue. Arabidopsis mutants jar1, coi1, ein2, sid2, eds5, and npr1 showed wild-type levels of P. rapae-induced protection against P. syringae pv tomato, suggesting that this local, herbivore-induced defense response does not depend exclusively on either JA, ET, or salicylic acid (SA). Resistance against the biotroph Turnip crinkle virus (TCV) requires SA, but not JA and ET. Nevertheless, herbivore feeding strongly affected TCV multiplication and TCV lesion formation, also in systemic tissues. Wounding alone was not effective, but application of P. rapae regurgitate onto the wounds induced a similar level of protection. Analysis of SA-induced PATHOGENESIS RELATED-1 (PR-1) expression revealed that P. rapae grazing primed Arabidopsis leaves for augmented expression of SA-dependent defenses. Pharmacological experiments showed that ET acts synergistically on SA-induced PR-1, suggesting that the increased production of ET upon herbivore feeding sensitizes the tissue to respond faster to SA, thereby contributing to an enhanced defensive capacity toward pathogens, such as TCV, that trigger SA-dependent defenses upon infection.

A Basic Leucine Zipper Transcription Factor, G-box-binding Factor 1, Regulates Blue Light-mediated Photomorphogenic Growth in Arabidopsis

Several transcriptional regulators have been identified and demonstrated to play either positive or negative regulatory roles in seedling development. However, the regulatory coordination between hypocotyl elongation and cotyledon expansion during early seedling development in plants remains unknown. We report the identification of a Z-box binding factor (ZBF2) and its functional characterization in cryptochrome-mediated blue light signaling. ZBF2 encodes a G-box binding factor (GBF1), which is a basic leucine zipper transcription factor. Our DNA-protein interaction studies reveal that ZBF2/GBF1 also interacts with the Z-box light-responsive element of light-regulated promoters. Genetic analyses of gbf1 mutants and overexpression studies suggest that GBF1 acts as a repressor of blue light-mediated inhibition in hypocotyl elongation, however, it acts as a positive regulator of cotyledon expansion during photomorphogenic growth. Furthermore, whereas GBF1 acts as a positive regulator of lateral root formation, it differentially regulates the expression of light-inducible genes. Taken together, these results demonstrate that GBF1 is a unique transcriptional regulator of photomorphogenesis in blue light.

Crosstalk between abiotic and biotic stress responses: a current view from the points of convergence in the stress signaling networks

Plants have evolved a wide range of mechanisms to cope with biotic and abiotic stresses. To date, the molecular mechanisms that are involved in each stress has been revealed comparatively independently, and so our understanding of convergence points between biotic and abiotic stress signaling pathways remain rudimentary. However, recent studies have revealed several molecules, including transcription factors and kinases, as promising candidates for common players that are involved in crosstalk between stress signaling pathways. Emerging evidence suggests that hormone signaling pathways regulated by abscisic acid, salicylic acid, jasmonic acid and ethylene, as well as ROS signaling pathways, play key roles in the crosstalk between biotic and abiotic stress signaling.

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.

Arabidopsis MAP kinase 4 regulates salicylic acid- and jasmonic acid/ethylene-dependent responses via EDS1 and PAD4

Arabidopsis MPK4 has been implicated in plant defense regulation because mpk4 knockout plants exhibit constitutive activation of salicylic acid (SA)-dependent defenses, but fail to induce jasmonic acid (JA) defense marker genes in response to JA. We show here that mpk4 mutants are also defective in defense gene induction in response to ethylene (ET), and that they are more susceptible than wild-type (WT) to Alternaria brassicicola that induces the ET/JA defense pathway(s). Both SA-repressing and ET/JA-(co)activating functions depend on MPK4 kinase activity and involve the defense regulators EDS1 and PAD4, as mutations in these genes suppress de-repression of the SA pathway and suppress the block of the ET/JA pathway in mpk4. EDS1/PAD4 thus affect SA-ET/JA signal antagonism as activators of SA but as repressors of ET/JA defenses, and MPK4 negatively regulates both of these functions. We also show that the MPK4-EDS1/PAD4 branch of ET defense signaling is independent of the ERF1 transcription factor, and use comparative microarray analysis of ctr1, ctr1/mpk4, mpk4 and WT to show that MPK4 is required for induction of a small subset of ET-regulated genes. The regulation of some, but not all, of these genes involves EDS1 and PAD4.

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.

Influx of extracellular Ca2+ involved in jasmonic-acid-induced elevation of [Ca2+]cyt and JR1 expression in Arabidopsis thaliana

The changes in cytosolic Ca2+ levels play important roles in the signal transduction pathways of many environmental and developmental stimuli in plants and animals. We demonstrated that the increase in cytosolic free Ca2+ concentration ([Ca2+]cyt) of Arabidopsis thaliana leaf cells was induced by exogenous application of jasmonic acid (JA). The elevation of [Ca2+]cyt was detected within 1 min after JA treatment by the fluorescence intensity using laser scanning confocal microscopy, and the elevated level of fluorescence was maintained during measuring time. With pretreatment of nifedipine (Nif), a nonpermeable L-type channel blocker, the fluorescence of [Ca2+]cyt induced by JA was inhibited in a dose-dependent manner. In contrast, verapamil, another L-type channel blocker, had no significant effect. Furthermore, Nif repressed JA-induced gene expression of JR1 but verapamil did not. JA-induced gene expression could be mimicked by higher concentration of extracellular Ca2+. W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide], an antagonist of calmodulin (CaM), blocked the JA induction of JR1 expression while W-5 [N-(6-aminohexyl)-1-naphthalenesulfonamide], its inactive antagonist, had no apparent effect. These data provide the evidence that the influx of extracellular Ca2+ through Nif sensitive plasma membrane Ca2+ channel may be responsible for JA-induced elevation of [Ca2+]cyt and downstream gene expression, CaM may be also involved in JA signaling pathway.

Transcriptional regulators of stamen development in Arabidopsis identified by transcriptional profiling

In Arabidopsis, jasmonate is required for stamen and pollen maturation. Mutants deficient in jasmonate synthesis, such as opr3, are male-sterile but become fertile when jasmonate is applied to developing flower buds. We have used ATH1 oligonucleotide arrays to follow gene expression in opr3 stamens for 22 h following jasmonate treatment. In these experiments, a total of 821 genes were specifically induced by jasmonate and 480 genes were repressed. Comparisons with data from previous studies indicate that these genes constitute a stamen-specific jasmonate transcriptome, with a large proportion (70%) of the genes expressed in the sporophytic tissue but not in the pollen. Bioinformatics tools allowed us to associate many of the induced genes with metabolic pathways that are probably upregulated during jasmonate-induced maturation. Our pathway analysis led to the identification of specific genes within larger families of homologues that apparently encode stamen-specific isozymes. Extensive additional analysis of our dataset identified 13 transcription factors that may be key regulators of the stamen maturation processes triggered by jasmonate. Two of these transcription factors, MYB21 and MYB24, are the only members of subgroup 19 of the R2R3 family of MYB proteins. A myb21 mutant obtained by reverse genetics exhibited shorter anther filaments, delayed anther dehiscence and greatly reduced male fertility. A myb24 mutant was phenotypically wild-type, but production of a myb21myb24 double mutant indicated that introduction of the myb24 mutation exacerbated all three aspects of the myb21 phenotype. Exogenous jasmonate could not restore fertility to myb21 or myb21myb24 mutant plants. Together with the data from transcriptional profiling, these results indicate that MYB21 and MYB24 are induced by jasmonate and mediate important aspects of the jasmonate response during stamen development.

Crosstalk between jasmonic acid, ethylene and Nod factor signaling allows integration of diverse inputs for regulation of nodulation

Plant hormones interact at many different levels to form a network of signaling pathways connected by antagonistic and synergistic interactions. Ethylene and jasmonic acid both act to regulate the plant's responsiveness to a common set of biotic stimuli. In addition ethylene has been shown to negatively regulate the plant's response to the rhizobial bacterial signal, Nod factor. This regulation occurs at an early step in the Nod factor signal transduction pathway, at or above Nod factor-induced calcium spiking. Here we show that jasmonic acid also inhibits the plant's responses to rhizobial bacteria, with direct effects on Nod factor-induced calcium spiking. However, unlike ethylene, jasmonic acid not only inhibits spiking but also suppresses the frequency of calcium oscillations when applied at lower concentrations. This effect of jasmonic acid is amplified in the ethylene-insensitive mutant skl, indicating an antagonistic interaction between these two hormones for regulation of Nod factor signaling. The rapidity of the effects of ethylene and jasmonic acid on Nod factor signaling suggests direct crosstalk between these three signal transduction pathways. This work provides a model by which crosstalk between signaling pathways can rapidly integrate environmental, developmental and biotic stimuli to coordinate diverse plant responses.

The role of salicylic acid and jasmonic acid in pathogen defence

Phytohormones are not only instrumental in regulating developmental processes in plants but also play important roles for the plant's responses to biotic and abiotic stresses. In particular, abscisic acid, ethylene, jasmonic acid, and salicylic acid have been shown to possess crucial functions in mediating or orchestrating stress responses in plants. Here, we review the role of salicylic acid and jasmonic acid in pathogen defence responses with special emphasis on their function in the solanaceous plant potato.

Jasmonate biosynthesis in Arabidopsis thaliana--enzymes, products, regulation

Among the plant hormones jasmonic acid and related derivatives are known to mediate stress responses and several developmental processes. Biosynthesis, regulation, and metabolism of jasmonic acid in Arabidopsis thaliana are reviewed, including properties of mutants of jasmonate biosynthesis. The individual signalling properties of several jasmonates are described.

Ethylene as a modulator of disease resistance in plants

The role of ethylene in the hormonal regulation of plant development has been well established. In addition, it has been implicated in biotic stress, both as a virulence factor of fungal and bacterial pathogens and as a signaling compound in disease resistance. This apparent discrepancy has stimulated research on the effects of various types of pathogens on mutant and transgenic plants that are impaired in ethylene production or perception. It has become clear that ethylene differentially affects resistance against pathogens with different lifestyles and plays an important role in mediating different types of induced resistance.

Heterotrimeric G proteins facilitate Arabidopsis resistance to necrotrophic pathogens and are involved in jasmonate signaling

Heterotrimeric G proteins have been previously linked to plant defense; however a role for the Gbetagamma dimer in defense signaling has not been described to date. Using available Arabidopsis (Arabidopsis thaliana) mutants lacking functional Galpha or Gbeta subunits, we show that defense against the necrotrophic pathogens Alternaria brassicicola and Fusarium oxysporum is impaired in Gbeta-deficient mutants while Galpha-deficient mutants show slightly increased resistance compared to wild-type Columbia ecotype plants. In contrast, responses to virulent (DC3000) and avirulent (JL1065) strains of Pseudomonas syringae appear to be independent of heterotrimeric G proteins. The induction of a number of defense-related genes in Gbeta-deficient mutants were severely reduced in response to A. brassicicola infection. In addition, Gbeta-deficient mutants exhibit decreased sensitivity to a number of methyl jasmonate-induced responses such as induction of the plant defensin gene PDF1.2, inhibition of root elongation, seed germination, and growth of plants in sublethal concentrations of methyl jasmonate. In all cases, the behavior of the Galpha-deficient mutants is coherent with the classic heterotrimeric mechanism of action, indicating that jasmonic acid signaling is influenced by the Gbetagamma functional subunit but not by Galpha. We hypothesize that Gbetagamma acts as a direct or indirect enhancer of the jasmonate signaling pathway in plants.

Transcriptional regulatory networks in cellular responses and tolerance to dehydration and cold stresses

Plant growth and productivity are greatly affected by environmental stresses such as drought, high salinity, and low temperature. Expression of a variety of genes is induced by these stresses in various plants. The products of these genes function not only in stress tolerance but also in stress response. In the signal transduction network from perception of stress signals to stress-responsive gene expression, various transcription factors and cis-acting elements in the stress-responsive promoters function for plant adaptation to environmental stresses. Recent progress has been made in analyzing the complex cascades of gene expression in drought and cold stress responses, especially in identifying specificity and cross talk in stress signaling. In this review article, we highlight transcriptional regulation of gene expression in response to drought and cold stresses, with particular emphasis on the role of transcription factors and cis-acting elements in stress-inducible promoters.

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.