Herbivore-induced allene oxide synthase transcripts and jasmonic acid in Nicotiana attenuata

The tomato homolog of CORONATINE-INSENSITIVE1 is required for the maternal control of seed maturation, jasmonate-signaled defense responses, and glandular trichome development

Jasmonic acid (JA) is a fatty acid-derived signaling molecule that regulates a broad range of plant defense responses against herbivores and some microbial pathogens. Molecular genetic studies in Arabidopsis have established that JA also performs a critical role in anther and pollen development but is not essential for other developmental aspects of the plant's life cycle. Here, we describe the phenotypic and molecular characterization of a sterile mutant of tomato (jasmonic acid-insensitive1 [jai1]) that is defective in JA signaling. Although the mutant exhibited reduced pollen viability, sterility was caused by a defect in the maternal control of seed maturation, which was associated with the loss of accumulation of JA-regulated proteinase inhibitor proteins in reproductive tissues. jai1 plants exhibited several defense-related phenotypes, including the inability to express JA-responsive genes, severely compromised resistance to two-spotted spider mites, and abnormal development of glandular trichomes. We demonstrate that these defects are caused by the loss of function of the tomato homolog of CORONATINE-INSENSITIVE1 (COI1), an F-box protein that is required for JA-signaled processes in Arabidopsis. These findings indicate that the JA/COI1 signaling pathway regulates distinct developmental processes in different plants and suggest a role for JA in the promotion of glandular trichome-based defenses.

Antisense LOX expression increases herbivore performance by decreasing defense responses and inhibiting growth-related transcriptional reorganization in Nicotiana attenuata

Inhibition of jasmonic acid (JA) signaling has been shown to decrease herbivore resistance, but the responsible mechanisms are largely unknown because insect resistance is poorly understood in most model plant systems. We characterize three members of the lipoxygenase (LOX) gene family in the native tobacco plant Nicotiana attenuata and manipulate, by antisense expression, a specific, wound- and herbivory-induced isoform (LOX3) involved in JA biosynthesis. In three independent lines, antisense expression reduced wound-induced JA accumulation but not the release of green leaf volatiles (GLVs). The impaired JA signaling reduced two herbivore-induced direct defenses, nicotine and trypsin protease inhibitors (TPI), as well as the potent indirect defense, the release of volatile terpenes that attract generalist predators to feeding herbivores. All these defenses could be fully restored by methyl-JA (MeJA) treatment, with the exception of the increase in TPI activity, which was partially restored, suggesting the involvement of additional signals. The impaired ability to produce chemical defenses resulted in lower resistance to Manduca sexta attack, which could also be restored by MeJA treatment. Expression analysis using a cDNA microarray, specifically designed to analyze M. sexta-induced gene expression in N. attenuata, revealed a pivotal role for LOX3-produced oxylipins in upregulating defense genes (protease inhibitor, PI; xyloglucan endotransglucosylase/hydrolase, XTH; threonine deaminase, TD; hydroperoxide lyase, HPL), suppressing both downregulated growth genes (RUBISCO and photosystem II, PSII) and upregulated oxylipin genes (alpha-dioxygenase, alpha-DOX). By genetically manipulating signaling in a plant with a well-characterized ecology, we demonstrate that the complex phenotypic changes that mediate herbivore resistance are controlled by a specific part of the oxylipin cascade.

Convergent responses to stress. Solar ultraviolet-B radiation and Manduca sexta herbivory elicit overlapping transcriptional responses in field-grown plants of Nicotiana longiflora

The effects of solar ultraviolet (UV)-B (280-315 nm) on plants have been studied intensively over the last 2 decades in connection with research on the biological impacts of stratospheric ozone depletion. However, the molecular mechanisms that mediate plant responses to solar (ambient) UV-B and their interactions with response mechanisms activated by other stressors remain for the most part unclear. Using a microarray enriched in wound- and insect-responsive sequences, we examined expression responses of 241 genes to ambient UV-B in field-grown plants of Nicotiana longiflora Cav. Approximately 20% of the sequences represented on the array showed differential expression in response to solar UV-B. The expression responses to UV-B had parallels with those elicited by simulated Manduca sexta herbivory. The most obvious similarities were: (a) down-regulation of several photosynthesis-related genes, and (b) up-regulation of genes involved in fatty acid metabolism and oxylipin biosynthesis such as HPL (hydroperoxide lyase), alpha-DIOX (alpha-dioxygenase), LOX (13-lipoxygenase), and AOS (allene oxide synthase). Genes encoding a WRKY transcription factor, a ferredoxin-dependent glutamate-synthase, and several other insect-responsive genes of unknown function were also similarly regulated by UV-B and insect herbivory treatments. Our results suggest that UV-B and caterpillar herbivory activate common regulatory elements and provide a platform for understanding the mechanisms of UV-B impacts on insect herbivory that have been documented in recent field studies.

Ethylene insensitivity modulates ozone-induced cell death in birch

We have used genotypic variation in birch (Betula pendula Roth) to investigate the roles of ozone (O(3))-induced ethylene (ET), jasmonic acid, and salicylic acid in the regulation of tissue tolerance to O(3). Of these hormones, ET evolution correlated best with O(3)-induced cell death. Disruption of ET perception by transformation of birch with the dominant negative mutant allele etr1-1 of the Arabidopsis ET receptor gene ETR1 or blocking of ET perception with 1-methylcyclopropene reduced but did not completely prevent the O(3)-induced cell death, when inhibition of ET biosynthesis with aminooxyacetic acid completely abolished O(3) lesion formation. This suggests the presence of an ET-signaling-independent but ET biosynthesis-dependent component in the ET-mediated stimulation of cell death in O(3)-exposed birch. Functional ET signaling was required for the O(3) induction of the gene encoding beta-cyanoalanine synthase, which catalyzes detoxification of the cyanide formed during ET biosynthesis. The results suggest that functional ET signaling is required to protect birch from the O(3)-induced cell death and that a decrease in ET sensitivity together with a simultaneous, high ET biosynthesis can potentially cause cell death through a deficient detoxification of cyanide.

Molecular interactions between the specialist herbivore Manduca sexta (lepidoptera, sphingidae) and its natural host Nicotiana attenuata: V. microarray analysis and further characterization of large-scale changes in herbivore-induced mRNAs

We extend our analysis of the transcriptional reorganization that occurs when the native tobacco, Nicotiana attenuata, is attacked by Manduca sexta larvae by cloning 115 transcripts by mRNA differential display reverse transcription-polymerase chain reaction and subtractive hybridization using magnetic beads (SHMB) from the M. sexta-responsive transcriptome. These transcripts were spotted as cDNA with eight others, previously confirmed to be differentially regulated by northern analysis on glass slide microarrays, and hybridized with Cy3- and Cy5-labeled probes derived from plants after 2, 6, 12, and 24 h of continuous attack. Microarray analysis proved to be a powerful means of verifying differential expression; 73 of the cloned genes (63%) were differentially regulated (in equal proportions from differential display reverse transcription-polymerase chain reaction and SHMB procedures), and of these, 24 (32%) had similarity to known genes or putative proteins (more from SHMB). The analysis provided insights into the signaling and transcriptional basis of direct and indirect defenses used against herbivores, suggesting simultaneous activation of salicylic acid-, ethylene-, cytokinin-, WRKY-, MYB-, and oxylipin-signaling pathways and implicating terpenoid-, pathogen-, and cell wall-related transcripts in defense responses. These defense responses require resources that could be made available by decreases in four photosynthetic-related transcripts, increases in transcripts associated with protein and nucleotide turnover, and increases in transcripts associated with carbohydrate metabolism. This putative up-regulation of defense-associated and down-regulation of growth-associated transcripts occur against a backdrop of altered transcripts for RNA-binding proteins, putative ATP/ADP translocators, chaperonins, histones, and water channel proteins, responses consistent with a major metabolic reconfiguration that underscores the complexity of response to herbivore attack.

Allene oxide cyclase dependence of the wound response and vascular bundle-specific generation of jasmonates in tomato - amplification in wound signalling

The allene oxide cyclase (AOC)-catalyzed step in jasmonate (JA) biosynthesis is important in the wound response of tomato. As shown by treatments with systemin and its inactive analog, and by analysis of 35S::prosysteminsense and 35S::prosysteminantisense plants, the AOC seems to be activated by systemin (and JA) leading to elevated formation of JA. Data are presented on the local wound response following activation of AOC and generation of JA, both in vascular bundles. The tissue-specific occurrence of AOC protein and generation of JA is kept upon wounding or other stresses, but is compromised in 35S::AOCsense plants, whereas 35S::AOCantisense plants exhibited residual AOC expression, a less than 10% rise in JA, and no detectable expression of wound response genes. The (i). activation of systemin-dependent AOC and JA biosynthesis occurring only upon substrate generation, (ii). the tissue-specific occurrence of AOC in vascular bundles, where the prosystemin gene is expressed, and (iii). the tissue-specific generation of JA suggest an amplification in the wound response of tomato leaves allowing local and rapid defense responses.

Functional genomics of P450s

Plant systems utilize a diverse array of cytochrome P450 monooxygenases (P450s) in their biosynthetic and detoxicative pathways. Those P450s in biosynthetic pathways play critical roles in the synthesis of lignins, UV protectants, pigments, defense compounds, fatty acids, hormones, and signaling molecules. Those in catabolic pathways participate in the breakdown of endogenous compounds and toxic compounds encountered in the environment. Because of their roles in this wide diversity of metabolic processes, plant P450 proteins and transcripts can serve as downstream reporters for many different biochemical pathways responding to chemical, developmental, and environmental cues. This review focuses initially on defining P450 biochemistries, nomenclature systems, and the relationships between genes in the extended P450 superfamily that exists in all plant species. Subsequently, it focuses on outlining the many approaches being used to assign function to individual P450 proteins and gene loci. The examples of assigned P450 activities that are spread throughout this review highlight the importance of understanding and utilizing P450 sequences as markers for linking biochemical pathway responses to physiological processes.

Identification of a jasmonate-regulated allene oxide synthase that metabolizes 9-hydroperoxides of linoleic and linolenic acids

Allene oxide synthase (AOS) is a cytochrome P-450 (CYP74A) that catalyzes the first step in the conversion of 13-hydroperoxy linolenic acid to jasmonic acid and related signaling molecules in plants. Here, we report the molecular cloning and characterization of a novel AOS-encoding cDNA (LeAOS3) from Lycopersicon esculentum whose predicted amino acid sequence classifies it as a member of the CYP74C subfamily of enzymes that was hitherto not known to include AOSs. Recombinant LeAOS3 expressed in Escherichia coli showed spectral characteristics of a P-450. The enzyme transformed 9- and 13-hydroperoxides of linoleic and linolenic acid to alpha-ketol, gamma-ketol, and cyclopentenone compounds that arise from spontaneous hydrolysis of unstable allene oxides, indicating that the enzyme is an AOS. Kinetic assays demonstrated that LeAOS3 was approximately 10-fold more active against 9-hydroperoxides than the corresponding 13-isomers. LeAOS3 transcripts accumulated in roots, but were undetectable in aerial parts of mature plants. In contrast to wild-type plants, LeAOS3 expression was undetectable in roots of a tomato mutant that is defective in jasmonic acid signaling. These findings suggest that LeAOS3 plays a role in the metabolism of 9-lipoxygenase-derived hydroperoxides in roots, and that this branch of oxylipin biosynthesis is regulated by the jasmonate signaling cascade.

Characterization and cDNA-microarray expression analysis of 12-oxophytodienoate reductases reveals differential roles for octadecanoid biosynthesis in the local versus the systemic wound response

12-Oxophytodienoate reductases (OPRs) belong to a family of flavin-dependent oxidoreductases. With two new tomato isoforms reported here, three OPRs have now been characterized in both tomato and Arabidopsis. Only one of these isoforms (OPR3) participates directly in the octadecanoid pathway for jasmonic acid biosynthesis, as only OPR3 reduces the 9S,13S-stereoisomer of 12-oxophytodienoic acid, the biological precursor of jasmonic acid. The subcellular localization of OPRs was analyzed in tomato and Arabidopsis. The OPR3 protein and activity were consistently found in peroxisomes where they co-localize with the enzymes of beta-oxidation which catalyze the final steps in the formation of jasmonic acid. The octadecanoid pathway is thus confined to plastids and peroxisomes and, in contrast to previous assumptions, does not involve the cytosolic compartment. The expression of tomato (Lycopersicon esculentum,Le) OPR3 was analyzed in the context of defense-related genes using a microarray comprising 233 cDNA probes. LeOPR3 was found to be up-regulated after wounding with induction kinetics resembling those of other octadecanoid pathway enzymes. In contrast to the induction of genes for wound response proteins (e.g. proteinase inhibitors), the accumulation of octadecanoid pathway transcripts was found to be more rapid and transient in wounded leaves, but hardly detectable in unwounded, systemic leaves. Consistent with the expression data, OPDA and JA were found to accumulate locally but not systemically in the leaves of wounded tomato plants. The transcriptional activation of the octadecanoid pathway and the accumulation of JA to high levels are, thus not required for the activation of defense gene expression in systemic tissues.

A saponin-detoxifying enzyme mediates suppression of plant defences

Plant disease resistance can be conferred by constitutive features such as structural barriers or preformed antimicrobial secondary metabolites. Additional defence mechanisms are activated in response to pathogen attack and include localized cell death (the hypersensitive response). Pathogens use different strategies to counter constitutive and induced plant defences, including degradation of preformed antimicrobial compounds and the production of molecules that suppress induced plant defences. Here we present evidence for a two-component process in which a fungal pathogen subverts the preformed antimicrobial compounds of its host and uses them to interfere with induced defence responses. Antimicrobial saponins are first hydrolysed by a fungal saponin-detoxifying enzyme. The degradation product of this hydrolysis then suppresses induced defence responses by interfering with fundamental signal transduction processes leading to disease resistance.

Octadecanoid signaling component "burst" in rice (Oryza sativa L.) seedling leaves upon wounding by cut and treatment with fungal elicitor chitosan

Octadecanoid pathway components, 12-oxo-phytodieonic acid (OPDA) and jasmonic acid (JA), are key biologically active regulators of plant self-defense response(s). However, to date these compounds have been studied mostly in dicots, and used large (1-10 g fresh weight, FW) samples for quantification, even when examined in mature rice plants, which is a drawback considering their rapid responsiveness to stress. Focusing on rice--a monocot cereal crop research model--this work describes an efficient and simultaneous quantification of both OPDA and JA using a minimum amount of 200mg FW seedling leaf tissue upon wounding (by cut) and treatment with fungal elicitor, chitosan (CT) by high-pressure liquid chromatography-turboionspray tandem mass spectrometry. Transient OPDA/JA "burst" was consistently and reproducibly detected within 3 min in wounded and CT treated leaves. OPDA peaked dramatically around 5 min and returned to its basal level within 15 min, whereas JA induction upon wounding and CT treatment were in parallel to OPDA production, peaking at 30 and 60 min, respectively. Present results mark a major advance in our understanding of key inducible octadecanoid pathway components in rice, and strongly suggest a role for the octadecanoid pathway downstream of perception of at least these two fundamentally different extracellular stimuli.