Role of beta-oxidation in jasmonate biosynthesis and systemic wound signaling in tomato

Perception, signaling and cross-talk of jasmonates and the seminal contributions of the Daoxin Xie's lab and the Chuanyou Li's lab

Jasmonates (JAs) are lipid-derived signals in plant responses to biotic and abiotic stresses and in development. The most active JA compound is (+)-7-iso-JA-Ile, a JA conjugate with isoleucine. Biosynthesis, metabolism and key components of perception and signal transduction have been identified and numerous JA-induced gene expression data collected. For JA-Ile perception, the SCF(COI1)-JAZ co-receptor complex has been identified and crystalized. Activators such as MYC2 and repressors such as JAZs including their targets were found. Involvement of JA-Ile in response to herbivores and pathogens and in root growth inhibition is among the most studied aspects of JA-Ile signaling. There are an increasing number of examples, where JA-Ile shows cross-talk with other plant hormones. Seminal contributions in JA/JA-Ile research were given by Daoxin Xie's lab and Chuanyou Li's lab, both in Beijing. Here, characterization was done regarding components of the JA-Ile receptor, such as COI1 (JAI1) and SCF, regarding activators (MYCs, MYBs) and repressors (JAV1, bHLH IIId's) of JA-regulated gene expression, as well as regarding components of auxin biosynthesis and action, such as the transcription factor PLETHORA active in the root stem cell niche. This overview reflects the work of both labs in the light of our present knowledge on biosynthesis, perception and signal transduction of JA/JA-Ile and its cross-talk to other hormones.

Terpene down-regulation triggers defense responses in transgenic orange leading to resistance against fungal pathogens

Terpenoid volatiles are isoprene compounds that are emitted by plants to communicate with the environment. In addition to their function in repelling herbivores and attracting carnivorous predators in green tissues, the presumed primary function of terpenoid volatiles released from mature fruits is the attraction of seed-dispersing animals. Mature oranges (Citrus sinensis) primarily accumulate terpenes in peel oil glands, with d-limonene accounting for approximately 97% of the total volatile terpenes. In a previous report, we showed that down-regulation of a d-limonene synthase gene alters monoterpene levels in orange antisense (AS) fruits, leading to resistance against Penicillium digitatum infection. A global gene expression analysis of AS versus empty vector (EV) transgenic fruits revealed that the down-regulation of d-limonene up-regulated genes involved in the innate immune response. Basal levels of jasmonic acid were substantially higher in the EV compared with AS oranges. Upon fungal challenge, salicylic acid levels were triggered in EV samples, while jasmonic acid metabolism and signaling were drastically increased in AS orange peels. In nature, d-limonene levels increase in orange fruit once the seeds are fully viable. The inverse correlation between the increase in d-limonene content and the decrease in the defense response suggests that d-limonene promotes infection by microorganisms that are likely involved in facilitating access to the pulp for seed-dispersing frugivores.

The essential role of jasmonic acid in plant-herbivore interactions--using the wild tobacco Nicotiana attenuata as a model

The plant hormone jasmonic acid (JA) plays a central role in plant defense against herbivores. Herbivore damage elicits a rapid and transient JA burst in the wounded leaves and JA functions as a signal to mediate the accumulation of various secondary metabolites that confer resistance to herbivores. Nicotiana attenuata is a wild tobacco species that inhabits western North America. More than fifteen years of study and its unique interaction with the specialist herbivore insect Manduca sexta have made this plant one of the best models for studying plant-herbivore interactions. Here we review the recent progress in understanding the elicitation of JA accumulation by herbivore-specific elicitors, the regulation of JA biosynthesis, JA signaling, and the herbivore-defense traits in N. attenuata.

Role of tomato lipoxygenase D in wound-induced jasmonate biosynthesis and plant immunity to insect herbivores

In response to insect attack and mechanical wounding, plants activate the expression of genes involved in various defense-related processes. A fascinating feature of these inducible defenses is their occurrence both locally at the wounding site and systemically in undamaged leaves throughout the plant. Wound-inducible proteinase inhibitors (PIs) in tomato (Solanum lycopersicum) provide an attractive model to understand the signal transduction events leading from localized injury to the systemic expression of defense-related genes. Among the identified intercellular molecules in regulating systemic wound response of tomato are the peptide signal systemin and the oxylipin signal jasmonic acid (JA). The systemin/JA signaling pathway provides a unique opportunity to investigate, in a single experimental system, the mechanism by which peptide and oxylipin signals interact to coordinate plant systemic immunity. Here we describe the characterization of the tomato suppressor of prosystemin-mediated responses8 (spr8) mutant, which was isolated as a suppressor of (pro)systemin-mediated signaling. spr8 plants exhibit a series of JA-dependent immune deficiencies, including the inability to express wound-responsive genes, abnormal development of glandular trichomes, and severely compromised resistance to cotton bollworm (Helicoverpa armigera) and Botrytis cinerea. Map-based cloning studies demonstrate that the spr8 mutant phenotype results from a point mutation in the catalytic domain of TomLoxD, a chloroplast-localized lipoxygenase involved in JA biosynthesis. We present evidence that overexpression of TomLoxD leads to elevated wound-induced JA biosynthesis, increased expression of wound-responsive genes and, therefore, enhanced resistance to insect herbivory attack and necrotrophic pathogen infection. These results indicate that TomLoxD is involved in wound-induced JA biosynthesis and highlight the application potential of this gene for crop protection against insects and pathogens.

Expression of α-DIOXYGENASE 1 in tomato and Arabidopsis contributes to plant defenses against aphids

Plant α-dioxygenases (α-DOX) are fatty acid-hydroperoxidases that contribute to the synthesis of oxylipins, a diverse group of compounds primarily generated through oxidation of linoleic (LA) and linolenic acid (LNA). Oxylipins are implicated in plant signaling against biotic and abiotic stresses. We report here that the potato aphid (Macrosiphum euphorbiae) induces Slα-DOX1 but not Slα-DOX2 expression in tomato (Solanum lycopersicum). Slα-DOX1 upregulation by aphids does not require either jasmonic acid (JA) or salicylic acid (SA) accumulation, since tomato mutants deficient in JA (spr2, acx1) or SA accumulation (NahG) still show Slα-DOX1 induction. Virus-induced gene silencing of Slα-DOX1 enhanced aphid population growth in wild-type (WT) plants, revealing that Slα-DOX1 contributes to basal resistance to aphids. Moreover, an even higher percent increase in aphid numbers occurred when Slα-DOX1 was silenced in spr2, a mutant line characterized by elevated LA levels, decreased LNA, and enhanced aphid resistance as compared with WT. These results suggest that aphid reproduction is influenced by oxylipins synthesized from LA by Slα-DOX1. In agreement with our experiments in tomato, two independent α-dox1 T-DNA insertion mutant lines in Arabidopsis thaliana also showed increased susceptibility to the green peach aphid (Myzus persicae), indicating that the role α-DOX is conserved in other plant-aphid interactions.

Hexanoic acid is a resistance inducer that protects tomato plants against Pseudomonas syringae by priming the jasmonic acid and salicylic acid pathways

Hexanoic acid-induced resistance (Hx-IR) is effective against several pathogens in tomato plants. Our study of the mechanisms implicated in Hx-IR against Pseudomonas syringae pv. tomato DC3000 suggests that hexanoic acid (Hx) treatment counteracts the negative effect of coronatine (COR) and jasmonyl-isoleucine (JA-Ile) on the salicylic acid (SA) pathway. In Hx-treated plants, an increase in the expression of jasmonic acid carboxyl methyltransferase (JMT) and the SA marker genes PR1 and PR5 indicates a boost in this signalling pathway at the expense of a decrease in JA-Ile. Moreover, Hx treatment potentiates 12-oxo-phytodienoic acid accumulation, which suggests that this molecule might play a role per se in Hx-IR. These results support a positive relationship between the SA and JA pathways in Hx-primed plants. Furthermore, one of the mechanisms of virulence mediated by COR is stomatal re-opening on infection with P. syringae. In this work, we observed that Hx seems to inhibit stomatal opening in planta in the presence of COR, which suggests that, on infection in tomato, this treatment suppresses effector action to prevent bacterial entry into the mesophyll.

Stress-induced expression of the transcription factor RERJ1 is tightly regulated in response to jasmonic acid accumulation in rice

The plant hormone jasmonic acid (JA) regulates various developmental processes and plant defence responses to environmental stresses. We previously reported that RERJ1, a JA-inducible transcription factor in rice, is up-regulated by exposure to wounding and drought stress. Here, we demonstrated that the expression of RERJ1 after wounding is regulated in a JA-dependent manner in rice, based on histochemical analysis of RERJ1 promoter-GUS transgenic plants. RERJ1 expression was induced only at the region of injury after wounding, whereas expression was induced in the entire leaf after drought. According to JA measurements of stressed leaves, high accumulation of endogenous JA was only detected around the wound site in a rice leaves, whereas the drought treatment led to uniform accumulation of JA in the entire leaf, suggesting that RERJ1 will be a useful marker gene for studies on localization of JA in rice. Nuclear localization and transactivation ability of RERJ1 were also demonstrated. These results suggest that RERJ1 plays a role as a transcriptional activator for regulating stress-inducible gene expression, with a strong correlation to JA accumulation in the stressed region.

Role of plant peroxisomes in the production of jasmonic acid-based signals

Jasmonates are a family of oxylipins derived from linolenic acid that control plant responses to biotic and abiotic stress factors and also regulate plant growth and development. Jasmonic acid (JA) is synthesized through the octadecanoid pathway that involves the translocation of lipid intermediates from the chloroplast membranes to the cytoplasm and later on into peroxisomes. The peroxisomal steps of the pathway involve the reduction of cis-(+)-12-oxophytodienoic acid (12-OPDA) and dinor-OPDA, which are the final products of the choroplastic phase of the biosynthetic pathway acting on 18:3 and 16:3 fatty acids, respectively. Further shortening of the carbon side-chain by successive rounds of β-oxidation reactions are required to complete JA biosynthesis. After peroxisomal reactions are completed, (+)-7-iso-JA is synthesized and then transported to the cytoplasm where is conjugated to the amino acid isoleucine to form the bioactive form of the hormone (+)-7-iso-JA-Ile (JA-Ile). Further regulatory activity of JA-Ile triggering gene activation in the jasmonate-dependent signaling cascades is exerted through a process mediated by the perception via the E3 ubiquitin ligase COI1 and further ligand-activated interaction with the family of JAZ repressor proteins. Upon interaction, JAZ are ubiquitinated and degraded by the proteasome, thus releasing transcription factors such as MYC2 from repression and allowing the activation of JA-responsive genes.

Analysis of ripening-related gene expression in papaya using an Arabidopsis-based microarray

BACKGROUND

Papaya (Carica papaya L.) is a commercially important crop that produces climacteric fruits with a soft and sweet pulp that contain a wide range of health promoting phytochemicals. Despite its importance, little is known about transcriptional modifications during papaya fruit ripening and their control. In this study we report the analysis of ripe papaya transcriptome by using a cross-species (XSpecies) microarray technique based on the phylogenetic proximity between papaya and Arabidopsis thaliana.

RESULTS

Papaya transcriptome analyses resulted in the identification of 414 ripening-related genes with some having their expression validated by qPCR. The transcription profile was compared with that from ripening tomato and grape. There were many similarities between papaya and tomato especially with respect to the expression of genes encoding proteins involved in primary metabolism, regulation of transcription, biotic and abiotic stress and cell wall metabolism. XSpecies microarray data indicated that transcription factors (TFs) of the MADS-box, NAC and AP2/ERF gene families were involved in the control of papaya ripening and revealed that cell wall-related gene expression in papaya had similarities to the expression profiles seen in Arabidopsis during hypocotyl development.

CONCLUSION

The cross-species array experiment identified a ripening-related set of genes in papaya allowing the comparison of transcription control between papaya and other fruit bearing taxa during the ripening process.

Involvement of OsJAZ8 in jasmonate-induced resistance to bacterial blight in rice

The plant hormone jasmonic acid (JA) has a crucial role in both host immunity and development in plants. Here, we report the importance of JA signaling in the defense system of rice. Exogenous application of JA conferred resistance to bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) in rice. Expression of OsJAZ8, a rice jasmonate ZIM-domain protein, was highly up-regulated by JA. OsJAZ8 interacted with a putative OsCOI1, which is a component of the SCF(COI1) E3 ubiquitin ligase complex, in a coronatine-dependent manner. OsJAZ8 also formed heterodimers with other OsJAZ proteins but did not form homodimer. JA treatment caused OsJAZ8 degradation and this degradation was dependent on the 26S proteasome pathway. Furthermore, the JA-dependent OsJAZ8 degradation was mediated by the Jas domain. Transgenic rice plants overexpressing OsJAZ8ΔC, which lacks the Jas domain, exhibited a JA-insensitive phenotype. A large-scale analysis using a rice DNA microarray revealed that overexpression of OsJAZ8ΔC altered the expression of JA-responsive genes, including defense-related genes, in rice. Furthermore, OsJAZ8ΔC negatively regulated the JA-induced resistance to Xoo in rice. On the basis of these data, we conclude that JA plays an important role in resistance to Xoo, and OsJAZ8 acts as a repressor of JA signaling in rice.

Chemical and genetic exploration of jasmonate biosynthesis and signaling paths

Jasmonates are lipid-derived compounds that act as signals in plant stress responses and developmental processes. Enzymes participating in biosynthesis of jasmonic acid (JA) and components of JA signaling have been extensively characterized by biochemical and molecular-genetic tools. Mutants have helped to define the pathway for synthesis of jasmonoyl-L-isoleucine (JA-Ile), the bioactive form of JA, and to identify the F-box protein COI1 as central regulatory unit. Details on the molecular mechanism of JA signaling were recently unraveled by the discovery of JAZ proteins that together with the adaptor protein NINJA and the general co-repressor TOPLESS form a transcriptional repressor complex. The current model of JA perception and signaling implies the SCF(COI1) complex operating as E3 ubiquitin ligase that upon binding of JA-Ile targets JAZ proteins for degradation by the 26S proteasome pathway, thereby allowing MYC2 and other transcription factors to activate gene expression. Chemical strategies, as integral part of jasmonate research, have helped the establishment of structure-activity relationships and the discovery of (+)-7-iso-JA-L-Ile as the major bioactive form of the hormone. The transient nature of its accumulation highlights the need to understand catabolism and inactivation of JA-Ile and recent studies indicate that oxidation of JA-Ile by cytochrome P450 monooxygenase is the major mechanism for turning JA signaling off. Plants contain numerous JA metabolites, which may have pronounced and differential bioactivity. A major challenge in the field of plant lipid signaling is to identify the cognate receptors and modes of action of these bioactive jasmonates/oxylipins.

Transportation of de novo synthesized jasmonoyl isoleucine in tomato

In plants, jasmonic acid (JA) and its derivatives are thought to be involved in mobile forms of defense against biotic and abiotic stresses. In this study, the distal transport of JA-isoleucine (JA-Ile) that is synthesized de novo in response to leaf wounding in tomato (Solanum lycopersicum) plants was investigated. JA-[¹³C₆]Ile was recovered in distal untreated leaves after wounded leaves were treated with [¹³C₆]Ile. However, as [¹³C₆]Ile was also recovered in the distal untreated leaves, whether JA-Ile was synthesized in the wounded or in the untreated leaves was unclear. Hence, stem exudates were analyzed to obtain more detailed information. When [¹³C₆]Ile and [²H₆]JA were applied separately into the wounds on two different leaves, JA-[¹³C₆]Ile and [²H₆]JA-Ile were detected in the stem exudates but [²H₆]JA-[¹³C₆]Ile was not, indicating that JA was conjugated with Ile in the wounded leaf and that the resulting JA-Ile was then transported into systemic tissues. The [²H₃]JA-Ile that was applied exogenously to the wounded tissues reached distal untreated leaves within 10 min. Additionally, applying [²H₃]JA-Ile to the wounded leaves at concentrations of 10 and 60 nmol/two leaves induced the accumulation of PIN II, LAP A, and JAZ3 mRNA in the distal untreated leaves of the spr2 mutant S. lycopersicum plants. These results demonstrate the transportation of de novo synthesized JA-Ile and suggest that JA-Ile may be a mobile signal.

ALLENE OXIDE CYCLASE (AOC) gene family members of Arabidopsis thaliana: tissue- and organ-specific promoter activities and in vivo heteromerization

Jasmonates are important signals in plant stress responses and plant development. An essential step in the biosynthesis of jasmonic acid (JA) is catalysed by ALLENE OXIDE CYCLASE (AOC) which establishes the naturally occurring enantiomeric structure of jasmonates. In Arabidopsis thaliana, four genes encode four functional AOC polypeptides (AOC1, AOC2, AOC3, and AOC4) raising the question of functional redundancy or diversification. Analysis of transcript accumulation revealed an organ-specific expression pattern, whereas detailed inspection of transgenic lines expressing the GUS reporter gene under the control of individual AOC promoters showed partially redundant promoter activities during development: (i) In fully developed leaves, promoter activities of AOC1, AOC2, and AOC3 appeared throughout all leaf tissue, but AOC4 promoter activity was vascular bundle-specific; (ii) only AOC3 and AOC4 showed promoter activities in roots; and (iii) partially specific promoter activities were found for AOC1 and AOC4 in flower development. In situ hybridization of flower stalks confirmed the GUS activity data. Characterization of single and double AOC loss-of-function mutants further corroborates the hypothesis of functional redundancies among individual AOCs due to a lack of phenotypes indicative of JA deficiency (e.g. male sterility). To elucidate whether redundant AOC expression might contribute to regulation on AOC activity level, protein interaction studies using bimolecular fluorescence complementation (BiFC) were performed and showed that all AOCs can interact among each other. The data suggest a putative regulatory mechanism of temporal and spatial fine-tuning in JA formation by differential expression and via possible heteromerization of the four AOCs.

The jasmonate receptor COI1 plays a role in jasmonate-induced lateral root formation and lateral root positioning in Arabidopsis thaliana

Jasmonic acid (JA) regulates a broad range of plant defense and developmental responses. COI1 has been recently found to act as JA receptor. In this report, we show that low micromolar concentrations of JA inhibited primary root (PR) growth and promoted lateral root (LR) formation in Arabidopsis wild-type (WT) seedlings. It was observed that the coi1-1 mutant was less sensitive to JA on pericycle cell activation to induce lateral root primordia (LRP) formation and presented alterations in lateral root positioning and lateral root emergence on bends. To investigate JA-auxin interactions important for remodeling of root system (RS) architecture, we tested the expression of auxin-inducible markers DR5:uidA and BA3:uidA in WT and coi1-1 seedlings in response to indole-3-acetic acid (IAA) and JA and analyzed the RS architecture of a suite of auxin-related mutants under JA treatments. We found that JA did not affect DR5:uidA and BA3:uidA expression in WT and coi1-1 seedlings. Our data also showed that PR growth inhibition in response to JA was likely independent of auxin signaling and that the induction of LRP required ARF7, ARF19, SLR, TIR1, AFB2, AFB3 and AXR1 loci. We conclude that JA regulation of postembryonic root development involves both auxin-dependent and independent mechanisms.

Altered chloroplast development and delayed fruit ripening caused by mutations in a zinc metalloprotease at the lutescent2 locus of tomato

The chloroplast is the site of photosynthesis in higher plants but also functions as the center of synthesis for primary and specialized metabolites including amino acids, fatty acids, starch, and diverse isoprenoids. Mutants that disrupt aspects of chloroplast function represent valuable tools for defining structural and biochemical regulation of the chloroplast and its interplay with whole-plant structure and function. The lutescent1 (l1) and l2 mutants of tomato (Solanum lycopersicum) possess a range of chlorophyll-deficient phenotypes including reduced rates of chlorophyll synthesis during deetiolation and enhanced rates of chlorophyll loss in leaves and fruits as they age, particularly in response to high-light stress and darkness. In addition, the onset of fruit ripening is delayed in lutescent mutants by approximately 1 week although once ripening is initiated they ripen at a normal rate and accumulation of carotenoids is not impaired. The l2 locus was mapped to the long arm of chromosome 10 and positional cloning revealed the existence of a premature stop codon in a chloroplast-targeted zinc metalloprotease of the M50 family that is homologous to the Arabidopsis (Arabidopsis thaliana) gene ETHYLENE-DEPENDENT GRAVITROPISM DEFICIENT AND YELLOW-GREEN1. Screening of tomato germplasm identified two additional l2 mutant alleles. This study suggests a role for the chloroplast in mediating the onset of fruit ripening in tomato and indicates that chromoplast development in fruit does not depend on functional chloroplasts.

Plant peroxisomes: biogenesis and function

Peroxisomes are eukaryotic organelles that are highly dynamic both in morphology and metabolism. Plant peroxisomes are involved in numerous processes, including primary and secondary metabolism, development, and responses to abiotic and biotic stresses. Considerable progress has been made in the identification of factors involved in peroxisomal biogenesis, revealing mechanisms that are both shared with and diverged from non-plant systems. Furthermore, recent advances have begun to reveal an unexpectedly large plant peroxisomal proteome and have increased our understanding of metabolic pathways in peroxisomes. Coordination of the biosynthesis, import, biochemical activity, and degradation of peroxisomal proteins allows for highly dynamic responses of peroxisomal metabolism to meet the needs of a plant. Knowledge gained from plant peroxisomal research will be instrumental to fully understanding the organelle's dynamic behavior and defining peroxisomal metabolic networks, thus allowing the development of molecular strategies for rational engineering of plant metabolism, biomass production, stress tolerance, and pathogen defense.

Loss of function of FATTY ACID DESATURASE7 in tomato enhances basal aphid resistance in a salicylate-dependent manner

We report here that disruption of function of the ω-3 FATTY ACID DESATURASE7 (FAD7) enhances plant defenses against aphids. The suppressor of prosystemin-mediated responses2 (spr2) mutation in tomato (Solanum lycopersicum), which eliminates the function of FAD7, reduces the settling behavior, survival, and fecundity of the potato aphid (Macrosiphum euphorbiae). Likewise, the antisense suppression of LeFAD7 expression in wild-type tomato plants reduces aphid infestations. Aphid resistance in the spr2 mutant is associated with enhanced levels of salicylic acid (SA) and mRNA encoding the pathogenesis-related protein P4. Introduction of the Naphthalene/salicylate hydroxylase transgene, which suppresses SA accumulation, restores wild-type levels of aphid susceptibility to spr2. Resistance in spr2 is also lost when we utilize virus-induced gene silencing to suppress the expression of NONEXPRESSOR OF PATHOGENESIS-RELATED PROTEINS1 (NPR1), a positive regulator of many SA-dependent defenses. These results indicate that FAD7 suppresses defenses against aphids that are mediated through SA and NPR1. Although loss of function of FAD7 also inhibits the synthesis of jasmonate (JA), the effects of this desaturase on aphid resistance are not dependent on JA; other mutants impaired in JA synthesis (acx1) or perception (jai1-1) show wild-type levels of aphid susceptibility, and spr2 retains aphid resistance when treated with methyl jasmonate. Thus, FAD7 may influence JA-dependent defenses against chewing insects and SA-dependent defenses against aphids through independent effects on JA synthesis and SA signaling. The Arabidopsis (Arabidopsis thaliana) mutants Atfad7-2 and Atfad7-1fad8 also show enhanced resistance to the green peach aphid (Myzus persicae) compared with wild-type controls, indicating that FAD7 influences plant-aphid interactions in at least two plant families.

Role of cis-12-oxo-phytodienoic acid in tomato embryo development

Oxylipins including jasmonates are signaling compounds in plant growth, development, and responses to biotic and abiotic stresses. In Arabidopsis (Arabidopsis thaliana) most mutants affected in jasmonic acid (JA) biosynthesis and signaling are male sterile, whereas the JA-insensitive tomato (Solanum lycopersicum) mutant jai1 is female sterile. The diminished seed formation in jai1 together with the ovule-specific accumulation of the JA biosynthesis enzyme allene oxide cyclase (AOC), which correlates with elevated levels of JAs, suggest a role of oxylipins in tomato flower/seed development. Here, we show that 35S::SlAOC-RNAi lines with strongly reduced AOC in ovules exhibited reduced seed set similarly to the jai1 plants. Investigation of embryo development of wild-type tomato plants showed preferential occurrence of AOC promoter activity and AOC protein accumulation in the developing seed coat and the embryo, whereas 12-oxo-phytodienoic acid (OPDA) was the dominant oxylipin occurring nearly exclusively in the seed coat tissues. The OPDA- and JA-deficient mutant spr2 was delayed in embryo development and showed an increased programmed cell death in the developing seed coat and endosperm. In contrast, the mutant acx1a, which accumulates preferentially OPDA and residual amount of JA, developed embryos similar to the wild type, suggesting a role of OPDA in embryo development. Activity of the residual amount of JA in the acx1a mutant is highly improbable since the known reproductive phenotype of the JA-insensitive mutant jai1 could be rescued by wound-induced formation of OPDA. These data suggest a role of OPDA or an OPDA-related compound for proper embryo development possibly by regulating carbohydrate supply and detoxification.

Postharvest temperature influences volatile lactone production via regulation of acyl-CoA oxidases in peach fruit

The biosynthesis of volatile compounds in plants is affected by environmental conditions. Lactones are considered to be peach-like aroma volatiles; however, no enzymes or genes associated with their biosynthesis have been characterized. White-fleshed (cv. Hujingmilu) and yellow-fleshed (cv. Jinxiu) melting peach (Prunus persica L. Batsch) fruit were used as materials in two successive seasons and responses measured to four different temperature treatments. Five major lactones accumulated during postharvest peach fruit ripening at 20 °C. Peach fruit at 5 °C, which induces chilling injury (CI), had the lowest lactone content during subsequent shelf life after removal, while 0 °C and a low-temperature conditioning (LTC) treatment alleviated development of CI and maintained significantly higher lactone contents. Expression of PpACX1 and activity of acyl-CoA oxidase (ACX) with C16-CoA tended to increase during postharvest ripening both at 20 °C and during shelf life after removal from cold storage when no CI was developed. There was a positive correlation between ACX and lactones in peach fruit postharvest. Changes in lactone production in response to temperatures are suggested to be a consequence of altered expression of PpACX1 and long-chain ACX activity.

Plant immunity to necrotrophs

Plants inhabit environments crowded with infectious microbes that pose constant threats to their survival. Necrotrophic pathogens are notorious for their aggressive and wide-ranging virulence strategies that promote host cell death and acquire nutrients for growth and reproduction from dead cells. This lifestyle constitutes the axis of their pathogenesis and virulence strategies and marks contrasting immune responses to biotrophic pathogens. The diversity of virulence strategies in necrotrophic species corresponds to multifaceted host immune response mechanisms. When effective, the plant immune system disarms the infectious necrotroph of its pathogenic arsenal or attenuates its effect, restricting further ingress and disease symptom development. Simply inherited resistance traits confer protection against host-specific necrotrophs (HSNs), whereas resistance to broad host-range necrotrophs (BHNs) is complex. Components of host genetic networks, as well as the molecular and cellular processes that mediate host immune responses to necrotrophs, are being identified. In this review, recent advances in our understanding of plant immune responses to necrotrophs and comparison with responses to biotrophic pathogens are summarized, highlighting common and contrasting mechanisms.

Synthetic molecular mimics of naturally occurring cyclopentenones exhibit antifungal activity towards pathogenic fungi

The naturally occurring reactive electrophilic species 12-oxo-phytodienoic acid (12-oxo-PDA) is a potent antifungal agent, whereas the plant growth regulator jasmonic acid, which is synthesized from 12-oxo-PDA, is ineffective. To address what structural features of the molecule endow it with antifungal activity, we synthesized a series of molecular mimics of 12-oxo-PDA varying in the length of the alkyl chain at its C-4 ring position. The octyl analogue (4-octyl cyclopentenone) was the most effective at suppressing spore germination and subsequent mycelial growth of a range of fungal pathogens and was particularly effective against Cladosporium herbarum and Botrytis cinerea, with minimum fungicidal concentrations in the range 100–200 µM. Introduction of a carboxyl group to the end of the chain, mimicking natural fatty acids, markedly reduced antifungal efficacy. Electrolyte leakage, indicative of membrane perturbation, was evident in both C. herbarum and B. cinerea exposed to 4-octyl cyclopentenone. Lipid composition analysis of the fungal spores revealed that those species with a high oil content, namely Fusarium oxysporum and Alternaria brassicicola, were less sensitive to 4-octyl cyclopentenone. The comparable hydrophobicity of 4-octyl cyclopentenone and 12-oxo-PDA accounts for the similar spore suppression activity of these two compounds. The relative ease of synthesis of 4-octyl cyclopentenone makes it an attractive compound for potential use as an antifungal agent.

A simple and efficient micrografting method for stably transformed Nicotiana attenuata plants to examine shoot-root signaling

To adjust their development to the environment, plants rely on specific signals that travel from shoot to root and vice versa. Here we describe an efficient micrografting protocol for Nicotiana attenuata, a useful tool for identifying these signals and understanding their functions. Additionally we analyzed transcript accumulation profiles of scions and rootstocks of grafts performed with wild-type and stably transformed N. attenuata. Our results are consistent with the source-to-sink movement of an sRNA silencing signal.

Profiling hydroxycinnamoyl-coenzyme A thioesters: unlocking the back door of phenylpropanoid metabolism

In plants, 20 to 30% of photosynthetically fixed carbon is directed toward lignin and other phenylpropanoid compounds for which hydroxycinnamoyl-coenzyme A (CoA) esters are key intermediates. CoA thioesters, ubiquitous metabolites found in all living cells (often at trace levels), have traditionally been challenging to measure. Here we report a hydrophilic interaction liquid chromatography (HILIC) method, coupled with tandem mass spectrometry (MS/MS), that allows simultaneous sensitive quantification of previously undetectable hydroxycinnamoyl-CoA esters and an extended range of acyl-CoAs from plant tissues. This method provides rapid liquid chromatography (LC) analysis (10 min/sample) and the ability for qualitative assessment of acyl-CoAs by MS/MS precursor ion scanning.

Silencing NOA1 elevates herbivory-induced jasmonic acid accumulation and compromises most of the carbon-based defense metabolites in Nicotiana attenuata(F)

Nitric oxide-associated protein 1 (NOA1) is involved in various abiotic stress responses and is required for plant resistance to pathogen infections. However, the role of NOA1 in plant-herbivore interactions has not been explored. We created NOA1-silenced Nicotiana attenuata plants (irNaNOA1). Compared with wild-type (WT) plants, irNaNOA1 plants had highly decreased photosynthesis rates. We further examined various traits important for plant defense against its specialist herbivore Manduca sexta by treating WT and irNaNOA1 plants with mechanical wounding and M. sexta oral secretions (OS). NOA1-silenced plants showed elevated levels of herbivory-induced jasmonic acid (JA), but decreased JA-isoleucine conjugate (JA-Ile) levels. The decreased JA-Ile levels did not result from compromised JAR (jasmonic acid resistant) activity in irNOA1 plants. Moreover, nitrogen-rich defensive compounds, nicotine and trypsin proteinase inhibitors, did not differ between WT and irNaNOA1 plants. In contrast, concentrations of most carbon-based defensive compounds were lower in these plants than in WT plants, although the levels of chlorogenic acid were not changed. Therefore, silencing NOA1 alters the allocation of carbon resources within the phenylpropanoid pathway. These data suggest the involvement of NOA1 in N. attenuata's defense against M. sexta attack, and highlight its role in photosynthesis, and biosynthesis of jasmonates and secondary metabolites.

S-Nitrosoglutathione reductase (GSNOR) mediates the biosynthesis of jasmonic acid and ethylene induced by feeding of the insect herbivore Manduca sexta and is important for jasmonate-elicited responses in Nicotiana attenuata

S-nitrosoglutathione reductase (GSNOR) reduces the nitric oxide (NO) adduct S-nitrosoglutathione (GSNO), an essential reservoir for NO bioactivity. In plants, GSNOR has been found to be important in resistance to bacterial and fungal pathogens, but whether it is also involved in plant-herbivore interactions was not known. Using a virus-induced gene silencing (VIGS) system, the activity of GSNOR in a wild tobacco species, Nicotiana attenuata, was knocked down and the function of GSNOR in defence against the insect herbivore Manduca sexta was examined. Silencing GSNOR decreased the herbivory-induced accumulation of jasmonic acid (JA) and ethylene, two important phytohormones regulating plant defence levels, without compromising the activity of two mitogen-activated protein kinases (MAPKs), salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK). Decreased activity of trypsin proteinase inhibitors (TPIs) were detected in GSNOR-silenced plants after simulated M. sexta feeding and bioassays indicated that GSNOR-silenced plants have elevated susceptibility to M. sexta attack. Furthermore, GSNOR is required for methyl jasmonate (MeJA)-induced accumulation of defence-related secondary metabolites (TPI, caffeoylputrescine, and diterpene glycosides) but is not needed for the transcriptional regulation of JAZ3 (jasmonate ZIM-domain 3) and TD (threonine deaminase), indicating that GSNOR mediates certain but not all jasmonate-inducible responses. This work highlights the important role of GSNOR in plant resistance to herbivory and jasmonate signalling and suggests the potential involvement of NO in plant-herbivore interactions. Our data also suggest that GSNOR could be a target of genetic modification for improving crop resistance to herbivores.

Systemin/Jasmonate-mediated systemic defense signaling in tomato

Wound-inducible proteinase inhibitors (PIs) in tomato plants provide a useful model system to elucidate the signal transduction pathways that regulate systemic defense response. Among the proposed intercellular signals for wound-induced PIs expression are the peptide systemin and the oxylipin-derived phytohormone jasmonic acid (JA). An increasing body of evidence indicates that systemin and JA work in the same signaling pathway to activate the expression of PIs and other defense-related genes. However, relatively less is known about how these signals interact to promote cell-to-cell communication over long distances. Genetic analysis of the systemin/JA signaling pathway in tomato plants provides a unique opportunity to study, in a single experimental system, the mechanism by which peptide and oxylipin signals interact to coordinate systemic expression of defense-related genes. Previously, it has been proposed that systemin is the long-distance mobile signal for defense gene expression. Recently, grafting experiments with tomato mutants defective in JA biosynthesis and signaling provide new evidence that JA, rather than systemin, functions as the systemic wound signal, and that the biosynthesis of JA is regulated by the peptide systemin. Further understanding of the systemin/JA signaling pathway promises to provide new insights into the basic mechanisms governing plant defense to biotic stress.

Intronic T-DNA insertion renders Arabidopsis opr3 a conditional jasmonic acid-producing mutant

Jasmonic acid and its derived metabolites (JAs) orchestrate plant defense against insects and fungi. 12-Oxo-phytodienoic acid (OPDA), a JA precursor, has also been implicated in plant defense. We sought to define JAs and OPDA functions through comparative defense susceptibility characteristics of three Arabidopsis (Arabidopsis thaliana) genotypes: aos, lacking JAs and OPDA; opda reductase3 (opr3), deficient in JA production but can accumulate OPDA; and transgenics that overexpress OPR3. opr3, like aos, is susceptible to cabbage loopers (Trichoplusia ni) but, relative to aos, opr3 has enhanced resistance to a necrotrophic fungus. Gas chromatography-mass spectrometry reveals that opr3 produces OPDA but no detectable JAs following wounding and looper infestation; unexpectedly, substantial levels of JAs accumulate in opr3 upon fungal infection. Full-length OPR3 transcripts accumulate in fungal-infected opr3, potentially through splicing of the T-DNA containing intron. Fungal resistance correlates with levels of JAs not OPDA; therefore, opr3 resistance to some pests is likely due to JA accumulation, and signaling activities ascribed to OPDA should be reassessed because opr3 can produce JAs. Together these data (1) reinforce the primary role JAs play in plant defense against insects and necrotrophic fungi, (2) argue for a reassessment of signaling activities ascribed to OPDA, and (3) provide evidence that mutants with intron insertions can retain gene function.

Cell-specific visualization of jasmonates in wounded tomato and Arabidopsis leaves using jasmonate-specific antibodies

Jasmonates are well-characterized signals in the development of plants and their response to abiotic and biotic stresses, such as touch and wounding by herbivores. A gap in our knowledge on jasmonate-induced processes, however, is the cellular localization of jasmonates. Here, a novel antibody-based approach was developed to visualize jasmonates in cross-sections of plant material. Antibodies raised in rabbits against BSA-coupled jasmonic acid (JA) are specific for JA, its methyl ester and isoleucine conjugate. They do not bind to 12-oxophytodienoic acid, 12-hydoxy-JA or coronatine. These antibodies were used in combination with newly established fixation and embedding methods. Jasmonates were rapidly and uniformly distributed within all cells near the site of damage of a mechanically wounded tomato (Solanum lycopersicum) leaf. Leaf tissue distally located to the wound site exhibited identical distribution, but had a lower signal intensity. The occurrence of jasmonates in all cell types of a wounded leaf was accompanied by transcript accumulation of early JA-induced genes visualized by in situ hybridization. With these new antibodies, a powerful tool is available to detect cell-specifically the occurrence of jasmonates in any jasmonate-dependent stress response or developmental process of plants.

Regulation of jasmonate metabolism and activation of systemic signaling in Solanum nigrum: COI1 and JAR4 play overlapping yet distinct roles

• Jasmonates are ubiquitous messengers in land plants essential for the activation of defense responses. However, their signaling properties, accumulation and metabolism vary substantially among species. Solanum nigrum is a wild Solanaceous species developed as a model to study defense responses. • Solanum nigrum plants transformed to silence the expression of key genes in jasmonate production (SnLOX3), conjugation (SnJAR4) and perception (SnCOI1) were generated to analyze the function of these genes in jasmonate accumulation and metabolism (studied by a combination of LC-MS/MS and (13) C-isotope labeling methods) and in signaling [studied by the systemic elicitation of leucine aminopeptidase (LAP) activity]. • In contrast with the early single jasmonic acid (JA) burst induced by wounding in wild-type (WT) plants, elicitation with insect oral secretions induced a later, second burst that was essential for the induction of systemic LAP activity, as demonstrated by ablation experiments. This induction was dependent on SnLOX3 and SnCOI1, but not on SnJAR4. In addition, the local accumulation of JA-glucose and JA-isoleucine was dependent on SnCOI1, whereas the accumulation of hydroxylated jasmonates was dependent on both SnCOI1 and SnJAR4. • The results demonstrate that SnLOX3, SnCOI1 and SnJAR4 have overlapping yet distinct roles in jasmonate signaling, differentially controlling jasmonate metabolism and the production of a systemic signal.

Distal transport of exogenously applied jasmonoyl-isoleucine with wounding stress

Determining the mobile signal used by plants to defend against biotic and abiotic stresses has proved elusive, but jasmonic acid (JA) and its derivatives appear to be involved. Using deuterium-labeled analogs, we investigated the distal transport of JA and jasmonoyl-isoleucine (JA-Ile) in response to leaf wounding in tobacco (Nicotiana tabacum) and tomato (Solanum lycopersicum) plants. We recovered [(2)H(2)-2]JA ([(2)H(2)]JA) and [(2)H(3)-12]JA-Ile ([(2)H(3)]JA-Ile) in distal leaves of N. tabacum and S. lycopersicum after treating wounded leaves with [(2)H(2)]JA or [(2)H(3)]JA-Ile. We found that JA-Ile had a greater mobility than JA, despite its lower polarity, and that application of exogenous JA-Ile to wounded leaves of N. tabacum led to a higher accumulation of JA and JA-Ile in distal leaves compared with wounded control plants. We also found that exudates from the stem of S. lycopersicum plants with damaged leaflets contained JA and JA-Ile at higher levels than in an undamaged plant, and a significant difference in the levels of JA-Ile was observed 30 min after wounding. Based on these results, it was found that JA-Ile is a transportable compound, which suggests that JA-Ile is a signaling cue involved in the resistance to biotic and abiotic stresses in plants.

The COP9 signalosome controls jasmonic acid synthesis and plant responses to herbivory and pathogens

The COP9 signalosome (CSN) is a multi-protein complex that regulates the activities of cullin-RING E3 ubiquitin ligases (CRLs). CRLs ubiquitinate proteins in order to target them for proteasomal degradation. The CSN is required for proper plant development. Here we show that the CSN also has a profound effect on plant defense responses. Silencing of genes for CSN subunits in tomato plants resulted in a mild morphological phenotype and reduced expression of wound-responsive genes in response to mechanical wounding, attack by Manduca sexta larvae, and Prosystemin over-expression. In contrast, expression of pathogenesis-related genes was increased in a stimulus-independent manner in these plants. The reduced wound response in CSN-silenced plants corresponded with reduced synthesis of jasmonic acid (JA), but levels of salicylic acid (SA) were unaltered. As a consequence, these plants exhibited reduced resistance against herbivorous M. sexta larvae and the necrotrophic fungal pathogen Botrytis cinerea. In contrast, susceptibility to tobacco mosaic virus (TMV) was not altered in CSN-silenced plants. These data demonstrate that the CSN orchestrates not only plant development but also JA-dependent plant defense responses.

New insights into plant responses to the attack from insect herbivores

Plants have evolved sophisticated systems to cope with herbivore challenges. When plants perceive herbivore-derived physical and chemical cues, such as elicitors in insects' oral secretions and compounds in oviposition fluids, plants dramatically reshape their transcriptomes, proteomes, and metabolomes. All these herbivory-induced changes are mediated by elaborate signaling networks, which include receptors/sensors, Ca(2+) influxes, kinase cascades, reactive oxygen species, and phytohormone signaling pathways. Furthermore, herbivory induces defense responses not only in the wounded regions but also in undamaged regions in the attacked leaves and in distal intact (systemic) leaves. Here, we review recent progress in understanding plant perception of herbivory and oviposition, and the herbivory-induced early signaling events and their biological functions. We consider the intraspecific phenotypic diversity of plant responses to herbivory and discuss the underlying genetic variation. We also discuss new tools and technical challenges in studying plant-herbivore interactions.

Ecological trade-offs between jasmonic acid-dependent direct and indirect plant defences in tritrophic interactions

Recent studies on plants genetically modified in jasmonic acid (JA) signalling support the hypothesis that the jasmonate family of oxylipins plays an important role in mediating direct and indirect plant defences. However, the interaction of two modes of defence in tritrophic systems is largely unknown. In this study, we examined the preference and performance of a herbivorous leafminer (Liriomyza huidobrensis) and its parasitic wasp (Opius dissitus) on three tomato genotypes: a wild-type (WT) plant, a JA biosynthesis (spr2) mutant, and a JA-overexpression 35S::prosys plant. Their proteinase inhibitor production and volatile emission were used as direct and indirect defence factors to evaluate the responses of leafminers and parasitoids. Here, we show that although spr2 mutant plants are compromised in direct defence against the larval leafminers and in attracting parasitoids, they are less attractive to adult flies compared with WT plants. Moreover, in comparison to other genotypes, the 35S::prosys plant displays greater direct and constitutive indirect defences, but reduced success of parasitism by parasitoids. Taken together, these results suggest that there are distinguished ecological trade-offs between JA-dependent direct and indirect defences in genetically modified plants whose fitness should be assessed in tritrophic systems and under natural conditions.

The tomato odorless-2 mutant is defective in trichome-based production of diverse specialized metabolites and broad-spectrum resistance to insect herbivores

Glandular secreting trichomes of cultivated tomato (Solanum lycopersicum) produce a wide array of volatile and nonvolatile specialized metabolites. Many of these compounds contribute to the characteristic aroma of tomato foliage and constitute a key part of the language by which plants communicate with other organisms in natural environments. Here, we describe a novel recessive mutation called odorless-2 (od-2) that was identified on the basis of an altered leaf-aroma phenotype. od-2 plants exhibit pleiotrophic phenotypes, including alterations in the morphology, density, and chemical composition of glandular trichomes. Type VI glandular trichomes isolated from od-2 leaves accumulate only trace levels of monoterpenes, sesquiterpenes, and flavonoids. Other foliar defensive compounds, including acyl sugars, glycoalkaloids, and jasmonate-regulated proteinase inhibitors, are produced in od-2 leaves. Growth of od-2 plants under natural field conditions showed that the mutant is highly susceptible to attack by an indigenous flea beetle, Epitrix cucumeris, and the Colorado potato beetle, Leptinotarsa decemlineata. The increased susceptibility of od-2 plants to Colorado potato beetle larvae and to the solanaceous specialist Manduca sexta was verified in no-choice bioassays. These findings indicate that Od-2 is essential for the synthesis of diverse trichome-borne compounds and further suggest that these compounds influence host plant selection and herbivore community composition under natural conditions.

The multifunctional protein in peroxisomal beta-oxidation: structure and substrate specificity of the Arabidopsis thaliana protein MFP2

Plant fatty acids can be completely degraded within the peroxisomes. Fatty acid degradation plays a role in several plant processes including plant hormone synthesis and seed germination. Two multifunctional peroxisomal isozymes, MFP2 and AIM1, both with 2-trans-enoyl-CoA hydratase and l-3-hydroxyacyl-CoA dehydrogenase activities, function in mouse ear cress (Arabidopsis thaliana) peroxisomal beta-oxidation, where fatty acids are degraded by the sequential removal of two carbon units. A deficiency in either of the two isozymes gives rise to a different phenotype; the biochemical and molecular background for these differences is not known. Structure determination of Arabidopsis MFP2 revealed that plant peroxisomal MFPs can be grouped into two families, as defined by a specific pattern of amino acid residues in the flexible loop of the acyl-binding pocket of the 2-trans-enoyl-CoA hydratase domain. This could explain the differences in substrate preferences and specific biological functions of the two isozymes. The in vitro substrate preference profiles illustrate that the Arabidopsis AIM1 hydratase has a preference for short chain acyl-CoAs compared with the Arabidopsis MFP2 hydratase. Remarkably, neither of the two was able to catabolize enoyl-CoA substrates longer than 14 carbon atoms efficiently, suggesting the existence of an uncharacterized long chain enoyl-CoA hydratase in Arabidopsis peroxisomes.

A genome-wide survey of maize lipid-related genes: candidate genes mining, digital gene expression profiling and co-location with QTL for maize kernel oil

Lipids play an important role in plants due to their abundance and their extensive participation in many metabolic processes. Genes involved in lipid metabolism have been extensively studied in Arabidopsis and other plant species. In this study, a total of 1003 maize lipid-related genes were cloned and annotated, including 42 genes with experimental validation, 732 genes with full-length cDNA and protein sequences in public databases and 229 newly cloned genes. Ninety-seven maize lipid-related genes with tissue-preferential expression were discovered by in silico gene expression profiling based on 1984483 maize Expressed Sequence Tags collected from 182 cDNA libraries. Meanwhile, 70 QTL clusters for maize kernel oil were identified, covering 34.5% of the maize genome. Fifty-nine (84%) QTL clusters co-located with at least one lipid-related gene, and the total number of these genes amounted to 147. Interestingly, thirteen genes with kernel-preferential expression profiles fell within QTL clusters for maize kernel oil content. All the maize lipid-related genes identified here may provide good targets for maize kernel oil QTL cloning and thus help us to better understand the molecular mechanism of maize kernel oil accumulation.

Exploring the impact of wounding and jasmonates on ascorbate metabolism

Vitamin C (ascorbate, AsA) is the most abundant water-soluble antioxidant in plants. Ascorbate provides the first line of defense against damaging reactive oxygen species (ROS), and helps protect plant cells from many factors that induce oxidative stress, including wounding, ozone, high salinity, and pathogen attack. Plant defenses against these stresses are also dependent upon jasmonates (JAs), a class of plant hormones that promote ROS accumulation. Here, we review evidence showing that wounding and JAs influence AsA accumulation in various plant species, and we report new data from Arabidopsis and tomato testing the influence of JAs on AsA levels in wounded and unwounded plants. In both species, certain mutations that impair JA metabolism and signaling influence foliar AsA levels, suggesting that endogenous JAs may regulate steady-state AsA. However, the impact of wounding on AsA accumulation was similar in JA mutants and wild type controls, indicating that this wound response does not require JAs. Our findings also indicate that the effects of wounding and JAs on AsA accumulation differ between species; these factors both enhanced AsA accumulation in Arabidopsis, but depressed AsA levels in tomato. These results underscore the importance of obtaining data from more than one model species, and demonstrate the complexity of AsA regulation.

A tomato enzyme synthesizes (+)-7-iso-jasmonoyl-L-isoleucine in wounded leaves

Jasmonoyl-L-isoleucine (JA-Ile) is a key jasmonate signal that probably functions in all plant species. The JASMONATE RESISTANT 1 (JAR1) enzyme synthesizes JA-Ile in Arabidopsis [Arabidopsis thaliana (L.) Heynh.], but a similar enzyme from tomato [Solanum lycopersicum (L.)] was not previously described. Tomato SlJAR1 has 66% sequence identity with Arabidopsis JAR1 and the SlJAR1-GST fusion protein purified from Escherichia coli catalyzed the formation of JA-amino acid conjugates in vitro. Kinetic analysis showed the enzyme has a strong preference for Ile over Leu and Val and it was about 10-fold more active with (+)-7-iso-JA than with its epimer (-)-JA. Leaf wounding rapidly increased JA-Ile 50-fold to about 450 pmol g(-1) FW at 30 min after wounding, while conjugates with Leu, Phe, Val and Met were only marginally increased or not detected. Nearly all of the endogenous JA-Ile was the bioactive epimer (+)-7-iso-JA-Ile and there was no evidence for its conversion to (-)-JA-Ile up to 6 h after wounding. A transgenic RNAi approach was used to suppress SlJAR1 transcript that reduced JA-Ile accumulation by 50-75%, suggesting that other JA conjugating enzymes may be present. These results show that SlJAR1 synthesizes the bioactive conjugate (+)-7-iso-JA-Ile and this is the predominant isomer accumulated in wounded tomato leaves.

Novel bifunctional nucleases, OmBBD and AtBBD1, are involved in abscisic acid-mediated callose deposition in Arabidopsis

Screening of the expressed sequence tag library of the wild rice species Oryza minuta revealed an unknown gene that was rapidly and strongly induced in response to attack by a rice fungal pathogen (Magnaporthe oryzae) and an insect (Nilaparvata lugens) and by wounding, abscisic acid (ABA), and methyl jasmonate treatments. Its recombinant protein was identified as a bifunctional nuclease with both RNase and DNase activities in vitro. This gene was designated OmBBD (for O. minuta bifunctional nuclease in basal defense response). Overexpression of OmBBD in an Arabidopsis (Arabidopsis thaliana) model system caused the constitutive expression of the PDF1.2, ABA1, and AtSAC1 genes, which are involved in priming ABA-mediated callose deposition. This activation of defense responses led to an increased resistance against Botrytis cinerea. atbbd1, the knockout mutant of the Arabidopsis ortholog AtBBD1, was susceptible to attack by B. cinerea and had deficient callose deposition. Overexpression of either OmBBD or AtBBD1 in atbbd1 plants complemented the susceptible phenotype of atbbd1 against B. cinerea as well as the deficiency of callose deposition. We suggest that OmBBD and AtBBD1 have a novel regulatory role in ABA-mediated callose deposition.

Jasmonates: structural requirements for lipid-derived signals active in plant stress responses and development

Jasmonates are lipid-derived signals that mediate plant stress responses and development processes. Enzymes participating in biosynthesis of jasmonic acid (JA) (1, 2) and components of JA signaling have been extensively characterized by biochemical and molecular-genetic tools. Mutants of Arabidopsis and tomato have helped to define the pathway for synthesis of jasmonoyl-isoleucine (JA-Ile), the active form of JA, and to identify the F-box protein COI1 as central regulatory unit. However, details of the molecular mechanism of JA signaling have only recently been unraveled by the discovery of JAZ proteins that function in transcriptional repression. The emerging picture of JA perception and signaling cascade implies the SCF(COI1) complex operating as E3 ubiquitin ligase that upon binding of JA-Ile targets JAZ repressors for degradation by the 26S-proteasome pathway, thereby allowing the transcription factor MYC2 to activate gene expression. The fact that only one particular stereoisomer, (+)-7-iso-JA-l-Ile (4), shows high biological activity suggests that epimerization between active and inactive diastereomers could be a mechanism for turning JA signaling on or off. The recent demonstration that COI1 directly binds (+)-7-iso-JA-l-Ile (4) and thus functions as JA receptor revealed that formation of the ternary complex COI1-JA-Ile-JAZ is an ordered process. The pronounced differences in biological activity of JA stereoisomers also imply strict stereospecific control of product formation along the JA biosynthetic pathway. The pathway of JA biosynthesis has been unraveled, and most of the participating enzymes are well-characterized. For key enzymes of JA biosynthesis the crystal structures have been established, allowing insight into the mechanisms of catalysis and modes of substrate binding that lead to formation of stereospecific products.

Plants on early alert: glandular trichomes as sensors for insect herbivores

The ability of caterpillar or moth 'footsteps' to elicit defenses in the tomato (Solanum lycopersicum) plant was examined. Although touch responses frequently have been observed in plants, the role of herbivore 'touch' in eliciting antiherbivore defenses has not been adequately examined. A combination of methods, including in situ hybridization, reverse transcriptase-polymerase chain reaction, quantitative real-time polymerase chain reaction and gas chromatography-mass spectrometry, was used to determine the role of trichomes in mediating these touch responses. Mutants compromised in jasmonic acid and glandular trichomes were used to test whether both of these were required for these touch responses. We demonstrated that the rupture of foliar glandular trichomes by caterpillar or moth contact induced the expression of defense transcripts (e.g. proteinase inhibitor 2, or PIN2) regulated by jasmonic acid. Neither chewing nor the release of salivary components was required to initiate this induced response. Jasmonic acid and the genes encoding proteins involved in its biosynthesis were identified in the trichomes. Using mutants, we showed that both jasmonic acid and trichomes were required for the contact-induced expression of PIN2. In addition, hydrogen peroxide, formed on the leaf surface, was required for PIN2 expression. Because these defenses would be activated before egg hatch, this early detection system for herbivores may be of considerable ecological significance.

Genome analysis and genetic enhancement of tomato

The Solanaceae is an important family of vegetable crops, ornamentals and medicinal plants. Tomato has served as a model member of this family largely because of its enriched cytogenetic, genetic, as well as physical, maps. Mapping has helped in cloning several genes of importance such as Pto, responsible for resistance against bacterial speck disease, Mi-1.2 for resistance against nematodes, and fw2.2 QTL for fruit weight. A high-throughput genome-sequencing program has been initiated by an international consortium of 10 countries. Since heterochromatin has been found to be concentrated near centromeres, the consortium is focusing on sequencing only the gene-rich euchromatic region. Genomes of the members of Solanaceae show a significant degree of synteny, suggesting that the tomato genome sequence would help in the cloning of genes for important traits from other Solanaceae members as well. ESTs from a large number of cDNA libraries have been sequenced, and microarray chips, in conjunction with wide array of ripening mutants, have contributed immensely to the understanding of the fruit-ripening phenomenon. Work on the analysis of the tomato proteome has also been initiated. Transgenic tomato plants with improved abiotic stress tolerance, disease resistance and insect resistance, have been developed. Attempts have also been made to develop tomato as a bioreactor for various pharmaceutical proteins. However, control of fruit quality and ripening remains an active and challenging area of research. Such efforts should pave the way to improve not only tomato, but also other solanaceous crops.

Peroxisome biogenesis and function

Peroxisomes are small and single membrane-delimited organelles that execute numerous metabolic reactions and have pivotal roles in plant growth and development. In recent years, forward and reverse genetic studies along with biochemical and cell biological analyses in Arabidopsis have enabled researchers to identify many peroxisome proteins and elucidate their functions. This review focuses on the advances in our understanding of peroxisome biogenesis and metabolism, and further explores the contribution of large-scale analysis, such as in sillco predictions and proteomics, in augmenting our knowledge of peroxisome function In Arabidopsis.