Green leaf volatiles: a plant's multifunctional weapon against herbivores and pathogens

Plants cannot avoid being attacked by an almost infinite number of microorganisms and insects. Consequently, they arm themselves with molecular weapons against their attackers. Plant defense responses are the result of a complex signaling network, in which the hormones jasmonic acid (JA), salicylic acid (SA) and ethylene (ET) are the usual suspects under the magnifying glass when researchers investigate host-pest interactions. However, Green Leaf Volatiles (GLVs), C₆ molecules, which are very quickly produced and/or emitted upon herbivory or pathogen infection by almost every green plant, also play an important role in plant defenses. GLVs are semiochemicals used by insects to find their food or their conspecifics. They have also been reported to be fundamental in indirect defenses and to have a direct effect on pests, but these are not the only roles of GLVs. These volatiles, being probably one of the fastest weapons exploited, are also able to directly elicit or prime plant defense responses. Moreover, GLVs, via crosstalk with phytohormones, mostly JA, can influence the outcome of the plant’s defense response against pathogens. For all these reasons GLVs should be considered as co-protagonists in the play between plants and their attackers.

E-2-hexenal promotes susceptibility to Pseudomonas syringae by activating jasmonic acid pathways in Arabidopsis

Green leaf volatiles (GLVs) are C6-molecules - alcohols, aldehydes, and esters - produced by plants upon herbivory or during pathogen infection. Exposure to this blend of volatiles induces defense-related responses in neighboring undamaged plants, thus assigning a role to GLVs in regulating plant defenses. Here we compared Arabidopsis thaliana ecotype Landsberg erecta (Ler) with a hydroperoxide lyase line, hpl1, unable to synthesize GLVs, for susceptibility to Pseudomonas syringae pv. tomato (DC3000). We found that the growth of DC3000 was significantly reduced in the hpl1 mutant. This phenomenon correlated with lower jasmonic acid (JA) levels and higher salicylic acid levels in the hpl1 mutant. Furthermore, upon infection, the JA-responsive genes VSP2 and LEC were only slightly or not induced, respectively, in hpl1. This suggests that the reduced growth of DC3000 in hpl1 plants is due to the constraint of JA-dependent responses. Treatment of hpl1 plants with E-2-hexenal, one of the more reactive GLVs, prior to infection with DC3000, resulted in increased growth of DC3000 in hpl1, thus complementing this mutant. Interestingly, the growth of DC3000 also increased in Ler plants treated with E-2-hexenal. This stronger growth was not dependent on the JA-signaling component MYC2, but on ORA59, an integrator of JA and ethylene signaling pathways, and on the production of coronatine by DC3000. GLVs may have multiple effects on plant-pathogen interactions, in this case reducing resistance to Pseudomonas syringae via JA and ORA59.

Rapid phosphatidic acid accumulation in response to low temperature stress in Arabidopsis is generated through diacylglycerol kinase

Phosphatidic acid (PtdOH) is emerging as an important signaling lipid in abiotic stress responses in plants. The effect of cold stress was monitored using (32)P-labeled seedlings and leaf discs of Arabidopsis thaliana. Low, non-freezing temperatures were found to trigger a very rapid (32)P-PtdOH increase, peaking within 2 and 5 min, respectively. In principle, PtdOH can be generated through three different pathways, i.e., (1) via de novo phospholipid biosynthesis (through acylation of lyso-PtdOH), (2) via phospholipase D hydrolysis of structural phospholipids, or (3) via phosphorylation of diacylglycerol (DAG) by DAG kinase (DGK). Using a differential (32)P-labeling protocol and a PLD-transphosphatidylation assay, evidence is provided that the rapid (32)P-PtdOH response was primarily generated through DGK. A simultaneous decrease in the levels of (32)P-PtdInsP, correlating in time, temperature dependency, and magnitude with the increase in (32)P-PtdOH, suggested that a PtdInsP-hydrolyzing PLC generated the DAG in this reaction. Testing T-DNA insertion lines available for the seven DGK genes, revealed no clear changes in (32)P-PtdOH responses, suggesting functional redundancy. Similarly, known cold-stress mutants were analyzed to investigate whether the PtdOH response acted downstream of the respective gene products. The hos1, los1, and fry1 mutants were found to exhibit normal PtdOH responses. Slight changes were found for ice1, snow1, and the overexpression line Super-ICE1, however, this was not cold-specific and likely due to pleiotropic effects. A tentative model illustrating direct cold effects on phospholipid metabolism is presented.

The Snf1-related protein kinases SnRK2.4 and SnRK2.10 are involved in maintenance of root system architecture during salt stress

The sucrose non-fermenting-1-related protein kinase 2 (SnRK2) family represents a unique family of plant-specific protein kinases implicated in cellular signalling in response to osmotic stress. In our studies, we observed that two class 1 SnRK2 kinases, SnRK2.4 and SnRK2.10, are rapidly and transiently activated in Arabidopsis roots after exposure to salt. Under saline conditions, snrk2.4 knockout mutants had a reduced primary root length, while snrk2.10 mutants exhibited a reduction in the number of lateral roots. The reduced lateral root density was found to be a combinatory effect of a decrease in the number of lateral root primordia and an increase in the number of arrested lateral root primordia. The phenotypes were in agreement with the observed expression patterns of genomic yellow fluorescent protein (YFP) fusions of SnRK2.10 and -2.4, under control of their native promoter sequences. SnRK2.10 was found to be expressed in the vascular tissue at the base of a developing lateral root, whereas SnRK2.4 was expressed throughout the root, with higher expression in the vascular system. Salt stress triggered a rapid re-localization of SnRK2.4-YFP from the cytosol to punctate structures in root epidermal cells. Differential centrifugation experiments of isolated Arabidopsis root proteins confirmed recruitment of endogenous SnRK2.4/2.10 to membranes upon exposure to salt, supporting their observed binding affinity for the phospholipid phosphatidic acid. Together, our results reveal a role for SnRK2.4 and -2.10 in root growth and architecture in saline conditions.

Regulators of floral fragrance production and their target genes in petunia are not exclusively active in the epidermal cells of petals

In which cells of the flower volatile biosynthesis takes place is unclear. In rose and snapdragon, some enzymes of the volatile phenylpropanoid/benzenoid pathway have been shown to be present in the epidermal cells of petals. It is therefore generally believed that the production of these compounds occurs in these cells. However, whether the entire pathway is active in these cells and whether it is exclusively active in these cells remains to be proven. Cell-specific transcription factors activating these genes will determine in which cells they are expressed. In petunia, the transcription factor EMISSION OF BENZENOIDS II (EOBII) activates the ODORANT1 (ODO1) promoter and the promoter of the biosynthetic gene isoeugenol synthase (IGS). The regulator ODO1 in turn activates the promoter of the shikimate gene 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). Here the identification of a new target gene of ODO1, encoding an ABC transporter localized on the plasma membrane, PhABCG1, which is co-expressed with ODO1, is described. PhABCG1 expression is up-regulated in petals overexpressing ODO1 through activation of the PhABCG1 promoter. Interestingly, the ODO1, PhABCG1, and IGS promoters were active in petunia protoplasts originating from both epidermal and mesophyll cell layers of the petal, suggesting that the volatile phenylpropanoid/benzenoid pathway in petunia is active in these different cell types. Since volatile release occurs from epidermal cells, trafficking of (volatile) compounds between cell layers must be involved, but the exact function of PhABCG1 remains to be resolved.

A model for combinatorial regulation of the petunia R2R3-MYB transcription factor ODORANT1

The emission of floral volatiles requires coordinated expression of biosynthetic genes. In the regulatory network of the volatile benzenoid/phenylpropanoid pathway in Petunia hybrida two master regulators of the pathway have been identified. The R2R3-MYB transcription factor EMISSION OF BENZENOIDS II (EOBII) utilizes a specific MYB binding site to activate the expression of the R2R3-MYB ODORANT1 (ODO1). However, because EOBII is expressed early in flower development, when ODO1 is not, there must be other factors that play a role in regulating expression of ODO1. Through functional analyses of ODO1 promoter fragments from fragrant and non-fragrant flowers, we provide evidence for additional players and present a model for combinatorial regulation of ODO1 expression in Petunia.

RNA-seq discovery, functional characterization, and comparison of sesquiterpene synthases from Solanum lycopersicum and Solanum habrochaites trichomes

Solanum lycopersicum and Solanum habrochaites (f. typicum) accession PI127826 emit a variety of sesquiterpenes. To identify terpene synthases involved in the production of these volatile sesquiterpenes, we used massive parallel pyrosequencing (RNA-seq) to obtain the transcriptome of the stem trichomes from these plants. This approach resulted initially in the discovery of six sesquiterpene synthase cDNAs from S. lycopersicum and five from S. habrochaites. Searches of other databases and the S. lycopersicum genome resulted in the discovery of two additional sesquiterpene synthases expressed in trichomes. The sesquiterpene synthases from S. lycopersicum and S. habrochaites have high levels of protein identity. Several of them appeared to encode for non-functional proteins. Functional recombinant proteins produced germacrenes, β-caryophyllene/α-humulene, viridiflorene and valencene from (E,E)-farnesyl diphosphate. However, the activities of these enzymes do not completely explain the differences in sesquiterpene production between the two tomato plants. RT-qPCR confirmed high levels of expression of most of the S. lycopersicum sesquiterpene synthases in stem trichomes. In addition, one sesquiterpene synthase was induced by jasmonic acid, while another appeared to be slightly repressed by the treatment. Our data provide a foundation to study the evolution of terpene synthases in cultivated and wild tomato.

The transcription factor EMISSION OF BENZENOIDS II activates the MYB ODORANT1 promoter at a MYB binding site specific for fragrant petunias

Fragrance production in petunia flowers is highly regulated. Two transcription factors, ODORANT1 (ODO1) and EMISSION OF BENZENOIDS II (EOBII) have recently been identified as regulators of the volatile benzenoid/phenylpropanoid pathway in petals. Unlike the non-fragrant Petunia hybrida cultivar R27, the fragrant cultivar Mitchell highly expresses ODO1. Using stable reporter lines, we identified the 1.2-kbp ODO1 promoter from Mitchell that is sufficient for tissue-specific, developmental and rhythmic expression. This promoter fragment can be activated in non-fragrant R27 petals, indicating that the set of trans-acting factors driving ODO1 expression is conserved in these two petunias. Conversely, the 1.2-kbp ODO1 promoter of R27 is much less active in Mitchell petals. Transient transformation of 5' deletion and chimeric Mitchell and R27 ODO1 promoter reporter constructs in petunia petals identified an enhancer region, which is specific for the fragrant Mitchell cultivar and contains a putative MYB binding site (MBS). Mutations in the MBS of the Mitchell promoter decreased overall promoter activity by 50%, highlighting the importance of the enhancer region. We show that EOBII binds and activates the ODO1 promoter via this MBS, establishing a molecular link between these two regulators of floral fragrance biosynthesis in petunia.

Identification of genes in the phenylalanine metabolic pathway by ectopic expression of a MYB transcription factor in tomato fruit

Altering expression of transcription factors can be an effective means to coordinately modulate entire metabolic pathways in plants. It can also provide useful information concerning the identities of genes that constitute metabolic networks. Here, we used ectopic expression of a MYB transcription factor, Petunia hybrida ODORANT1, to alter Phe and phenylpropanoid metabolism in tomato (Solanum lycopersicum) fruits. Despite the importance of Phe and phenylpropanoids to plant and human health, the pathway for Phe synthesis has not been unambiguously determined. Microarray analysis of ripening fruits from transgenic and control plants permitted identification of a suite of coregulated genes involved in synthesis and further metabolism of Phe. The pattern of coregulated gene expression facilitated discovery of the tomato gene encoding prephenate aminotransferase, which converts prephenate to arogenate. The expression and biochemical data establish an arogenate pathway for Phe synthesis in tomato fruits. Metabolic profiling and ¹³C flux analysis of ripe fruits further revealed large increases in the levels of a specific subset of phenylpropanoid compounds. However, while increased levels of these human nutrition-related phenylpropanoids may be desirable, there were no increases in levels of Phe-derived flavor volatiles.

Tomato-produced 7-epizingiberene and R-curcumene act as repellents to whiteflies

How whiteflies (Bemisia tabaci) make the choice for a host plant prior to landing, is not precisely known. Here we investigated whether they respond to specific volatiles of tomato. Zingiberene and curcumene were purified from Solanum habrochaites (PI127826), characterised by NMR and X-ray analysis and identified as 7-epizingiberene and R-curcumene. In contrast, oil from Zingiber officinalis contained the stereoisomers zingiberene and S-curcumene, respectively. Using a combination of free-choice bio-assays and electroantennography, 7-epizingiberene and its dehydrogenated derivative R-curcumene were shown to be active as semiochemicals to B. tabaci adults, whereas the stereoisomers from ginger were not. In addition, R-curcumene elicited the strongest electroantennographic response. Bio-assays showed that a cultivated tomato could be made less attractive to B. tabaci than its neighbouring siblings by the addition of the tomato stereoisomer 7-epizingiberene or its derivative R-curcumene. These sesquiterpenes apparently repel adult whiteflies prior to landing, presumably because it informs them that after landing they, or their offspring, may be exposed to higher and lethal concentrations of the same compounds.

Mutations in γ-aminobutyric acid (GABA) transaminase genes in plants or Pseudomonas syringae reduce bacterial virulence

Pseudomonas syringae pv. tomato DC3000 is a bacterial pathogen of Arabidopsis and tomato that grows in the apoplast. The non-protein amino acid γ-amino butyric acid (GABA) is produced by Arabidopsis and tomato and is the most abundant amino acid in the apoplastic fluid of tomato. The DC3000 genome harbors three genes annotated as gabT GABA transaminases. A DC3000 mutant lacking all three gabT genes was constructed and found to be unable to utilize GABA as a sole carbon and nitrogen source. In complete minimal media supplemented with GABA, the mutant grew less well than wild-type DC3000 and showed strongly reduced expression of hrpL and avrPto, which encode an alternative sigma factor and effector, respectively, associated with the type III secretion system. The growth of the gabT triple mutant was weakly reduced in Arabidopsis ecotype Landberg erecta (Ler) and strongly reduced in the Ler pop2-1 GABA transaminase-deficient mutant that accumulates higher levels of GABA. Much of the ability to grow on GABA-amended minimal media or in Arabidopsis pop2-1 leaves could be restored to the gabT triple mutant by expression in trans of just gabT2. The ability of DC3000 to elicit the hypersensitive response (HR) in tobacco leaves is dependent upon deployment of the type III secretion system, and the gabT triple mutant was less able than wild-type DC3000 to elicit this HR when bacteria were infiltrated along with GABA at levels of 1 mm or more. GABA may have multiple effects on P. syringae-plant interactions, with elevated levels increasing disease resistance.

The role of specific tomato volatiles in tomato-whitefly interaction

Bemisia tabaci (whitefly) infestations and the subsequent transfer of viruses are the cause of severe losses in crop production and horticultural practice. To improve biological control of B. tabaci, we investigated repellent properties of plant-produced semiochemicals. The mix of headspace volatiles, collected from naturally repellent wild tomato accessions, influenced B. tabaci initial choice behavior, indicating a role for plant semiochemicals in locating host plants. A collection of wild tomato accessions and introgression lines (Solanum pennellii LA716 x Solanum lycopersicum 'Moneyberg') were extensively screened for attractiveness to B. tabaci, and their headspace profiles were determined by means of gas chromatography-mass spectrometry. Correlation analysis revealed that several terpenoids were putatively involved in tomato-whitefly interactions. Several of these candidate compounds conferred repellence to otherwise attractive tomato plants when applied to the plant's branches on paper cards. The sesquiterpenes zingiberene and curcumene and the monoterpenes p-cymene, alpha-terpinene, and alpha-phellandrene had the strongest effects in free-choice bioassays. These terpenes also elicited a response of receptors on the insect's antennae as determined by electroantennography. Conversely, the monoterpene beta-myrcene showed no activity in both assays. B. tabaci apparently uses, besides visual cues, specific plant volatile cues for the initial selection of a host. Altering whitefly choice behavior by manipulation of the terpenoid composition of the host headspace may therefore be feasible.

A plant thiolase involved in benzoic acid biosynthesis and volatile benzenoid production

The exact biosynthetic pathways leading to benzoic acid (BA) formation in plants are not known, but labeling experiments indicate the contribution of both beta-oxidative and non-beta-oxidative pathways. In Petunia hybrida BA is a key precursor for the production of volatile benzenoids by its flowers. Using functional genomics, we identified a 3-ketoacyl-CoA thiolase, PhKAT1, which is involved in the benzenoid biosynthetic pathway and the production of BA. PhKAT1 is localised in the peroxisomes, where it is important for the formation of benzoyl-CoA-related compounds. Silencing of PhKAT1 resulted in a major reduction in BA and benzenoid formation, leaving the production of other phenylpropanoid-related volatiles unaffected. During the night, when volatile benzenoid production is highest, it is largely the beta-oxidative pathway that contributes to the formation of BA and benzenoids. Our studies add the benzenoid biosynthetic pathway to the list of pathways in which 3-ketoacyl-CoA thiolases are involved in plants.

Reassessing the role of phospholipase D in the Arabidopsis wounding response

Plants respond to wounding by means of a multitude of reactions, with the purpose of stifling herbivore assault. Phospholipase D (PLD) has previously been implicated in the wounding response. Arabidopsis (Arabidopsis thaliana) AtPLDalpha1 has been proposed to be activated in intact cells, and the phosphatidic acid (PA) it produces to serve as a precursor for jasmonic acid (JA) synthesis and to be required for wounding-induced gene expression. Independently, PLD activity has been reported to have a bearing on wounding-induced MAPK activation. However, which PLD isoforms are activated, where this activity takes place (in the wounded or non-wounded cells) and what exactly the consequences are is a question that has not been comprehensively addressed. Here, we show that PLD activity during the wounding response is restricted to the ruptured cells using (32)P(i)-labelled phospholipid analyses of Arabidopsis pld knock-out mutants and PLD-silenced tomato cell-suspension cultures. pldalpha1 knock-out lines have reduced wounding-induced PA production, and the remainder is completely eliminated in a pldalpha1/delta double knock-out line. Surprisingly, wounding-induced protein kinase activation, AtLOX2 gene expression and JA biosynthesis were not affected in these knock-out lines. Moreover, larvae of the Cabbage White butterfly (Pieris rapae) grew equally well on wild-type and the pld knock-out mutants.

Multiple PLDs required for high salinity and water deficit tolerance in plants

High salinity and drought have received much attention because they severely affect crop production worldwide. Analysis and comprehension of the plant's response to excessive salt and dehydration will aid in the development of stress-tolerant crop varieties. Signal transduction lies at the basis of the response to these stresses, and numerous signaling pathways have been implicated. Here, we provide further evidence for the involvement of phospholipase D (PLD) in the plant's response to high salinity and dehydration. A tomato (Lycopersicon esculentum) alpha-class PLD, LePLDalpha1, is transcriptionally up-regulated and activated in cell suspension cultures treated with salt. Gene silencing revealed that this PLD is indeed involved in the salt-induced phosphatidic acid production, but not exclusively. Genetically modified tomato plants with reduced LePLDalpha1 protein levels did not reveal altered salt tolerance. In Arabidopsis (Arabidopsis thaliana), both AtPLDalpha1 and AtPLDdelta were found to be activated in response to salt stress. Moreover, pldalpha1 and plddelta single and double knock-out mutants exhibited enhanced sensitivity to high salinity stress in a plate assay. Furthermore, we show that both PLDs are activated upon dehydration and the knock-out mutants are hypersensitive to hyperosmotic stress, displaying strongly reduced growth.

Intraspecific variation in a generalist herbivore accounts for differential induction and impact of host plant defences

Plants and herbivores are thought to be engaged in a coevolutionary arms race: rising frequencies of plants with anti-herbivore defences exert pressure on herbivores to resist or circumvent these defences and vice versa. Owing to its frequency-dependent character, the arms race hypothesis predicts that herbivores exhibit genetic variation for traits that determine how they deal with the defences of a given host plant phenotype. Here, we show the existence of distinct variation within a single herbivore species, the spider mite Tetranychus urticae, in traits that lead to resistance or susceptibility to jasmonate (JA)-dependent defences of a host plant but also in traits responsible for induction or repression of JA defences. We characterized three distinct lines of T. urticae that differentially induced JA-related defence genes and metabolites while feeding on tomato plants (Solanum lycopersicum). These lines were also differently affected by induced JA defences. The first line, which induced JA-dependent tomato defences, was susceptible to those defences; the second line also induced JA defences but was resistant to them; and the third, although susceptible to JA defences, repressed induction. We hypothesize that such intraspecific variation is common among herbivores living in environments with a diversity of plants that impose diverse selection pressure.

The Arabidopsis her1 mutant implicates GABA in E-2-hexenal responsiveness

When wounded or attacked by herbivores or pathogens, plants produce a blend of six-carbon alcohols, aldehydes and esters, known as C6-volatiles. Undamaged plants, when exposed to C6-volatiles, respond by inducing defense-related genes and secondary metabolites, suggesting that C6-volatiles can act as signaling molecules regulating plant defense responses. However, to date, the molecular mechanisms by which plants perceive and respond to these volatiles are unknown. To elucidate such mechanisms, we decided to isolate Arabidopsis thaliana mutants in which responses to C6-volatiles were altered. We observed that treatment of Arabidopsis seedlings with the C6-volatile E-2-hexenal inhibits root elongation. Among C6-volatiles this response is specific to E-2-hexenal, and is not dependent on ethylene, jasmonic and salicylic acid. Using this bioassay, we isolated 18 E-2-hexenal-response (her) mutants that showed sustained root growth after E-2-hexenal treatment. Here, we focused on the molecular characterization of one of these mutants, her1. Microarray and map-based cloning revealed that her1 encodes a gamma-amino butyric acid transaminase (GABA-TP), an enzyme that degrades GABA. As a consequence of the mutation, her1 plants accumulate high GABA levels in all their organs. Based on the observation that E-2-hexenal treatment induces GABA accumulation, and that high GABA levels confer resistance to E-2-hexenal, we propose a role for GABA in mediating E-2-hexenal responses.

Geranyl diphosphate synthase is required for biosynthesis of gibberellins

Geranyl diphosphate synthase (GPS) is generally considered to be responsible for the biosynthesis of monoterpene precursors only. However, reduction of LeGPS expression in tomato (Lycopersicon esculentum) by virus-induced gene silencing resulted in severely dwarfed plants. Further analysis of these dwarfed plants revealed a decreased gibberellin content, whereas carotenoid and chlorophyll levels were unaltered. Accordingly, the phenotype could be rescued by application of gibberellic acid. The dwarfed phenotype was also obtained in Arabidopsis thaliana plants transformed with RNAi constructs of AtGPS. These results link geranyl diphosphate (GPP) to the gibberellin biosynthesis pathway. They also demand a re-evaluation of the role of GPS in precursor synthesis for other di-, tri-, tetra- and/or polyterpenes and their derivatives.

The intragenic approach as a new extension to traditional plant breeding

The novel intragenic approach to genetic engineering improves existing varieties by eliminating undesirable features and activating dormant traits. It transforms plants with native expression cassettes to fine-tune the activity and/or tissue specificity of target genes. Any intragenic modification of traits could, at least in theory, also be accomplished by traditional breeding and transgenic modification. However, the new approach is unique in avoiding the transfer of unknown or foreign DNA. By consequently eliminating various potential risk factors, this method represents a relatively safe approach to crop improvement. Therefore, we argue that intragenic crops should be cleared through the regulatory process in a timely and cost-effective manner.

Tomato linalool synthase is induced in trichomes by jasmonic acid

Tomato (Lycopersicon esculentum) plants emit a blend of volatile organic compounds, which mainly consists of terpenes. Upon herbivory or wounding, the emission of several terpenes increases. We have identified and characterized the first two tomato monoterpene synthases, LeMTS1 and LeMTS2. Although these proteins were highly homologous, recombinant LeMTS1 protein produced (R)-linalool from geranyl diphosphate (GPP) and (E)-nerolidol from farnesyl diphosphate (FPP), while recombinant LeMTS2 produced beta-phellandrene, beta-myrcene, and sabinene from GPP. In addition, these genes were expressed in different tissues: LeMTS1 was expressed in flowers, young leaves, stems, and petioles, while LeMTS2 was strongest expressed in stems and roots. LeMTS1 expression in leaves was induced by spider mite-infestation, wounding and jasmonic acid (JA)-treatment, while LeMTS2 did not respond to these stimuli. The expression of LeMTS1 in stems and petioles was predominantly detected in trichomes and could be induced by JA. Because JA treatment strongly induced emission of linalool and overexpression of LeMTS1 in tomato resulted in increased production of linalool, we propose that LeMTS1 is a genuine linalool synthase. Our results underline the importance of trichomes in JA-induced terpene emission in tomato.

Induction of a leaf specific geranylgeranyl pyrophosphate synthase and emission of (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene in tomato are dependent on both jasmonic acid and salicylic acid signaling pathways

Two cDNAs encoding geranylgeranyl pyrophosphate (GGPP) synthases from tomato (Lycopersicon esculentum) have been cloned and functionally expressed in Escherichia coli. LeGGPS1 was predominantly expressed in leaf tissue and LeGGPS2 in ripening fruit and flower tissue. LeGGPS1 expression was induced in leaves by spider mite (Tetranychus urticae)-feeding and mechanical wounding in wild type tomato but not in the jasmonic acid (JA)-response mutant def-1 and the salicylic acid (SA)-deficient transgenic NahG line. Furthermore, LeGGPS1 expression could be induced in leaves of wild type tomato plants by JA- or methyl salicylate (MeSA)-treatment. In contrast, expression of LeGGPS2 was not induced in leaves by spider mite-feeding, wounding, JA- or MeSA-treatment. We show that emission of the GGPP-derived volatile terpenoid (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT) correlates with expression of LeGGPS1. An exception was MeSA-treatment, which resulted in induction of LeGGPS1 but not in emission of TMTT. We show that there is an additional layer of regulation, because geranyllinalool synthase, catalyzing the first dedicated step in TMTT biosynthesis, was induced by JA but not by MeSA.

Regulation of terpenoid and benzenoid production in flowers

The production and emission of fragrant molecules by flowers are strictly regulated during the floral lifespan and often peak when pollinators are active. The best-studied classes of floral volatiles are benzenoids and terpenoids. The production of these molecules appears to be primarily regulated at the level of precursor biosynthesis. The genes from the petunia floral shikimate pathway, which provides the precursors for the formation of benzenoids, have recently been shown to be regulated by a MYB transcription factor. The floral terpenoids of snapdragon appear to be derived exclusively from the methyl-erythritol-phosphate pathway in plastids. This pathway controls precursor levels for geranyl diphosphate synthase, which in turn is transcriptionally regulated.

Mutations in the NB-ARC domain of I-2 that impair ATP hydrolysis cause autoactivation

Resistance (R) proteins in plants confer specificity to the innate immune system. Most R proteins have a centrally located NB-ARC (nucleotide-binding adaptor shared by APAF-1, R proteins, and CED-4) domain. For two tomato (Lycopersicon esculentum) R proteins, I-2 and Mi-1, we have previously shown that this domain acts as an ATPase module that can hydrolyze ATP in vitro. To investigate the role of nucleotide binding and hydrolysis for the function of I-2 in planta, specific mutations were introduced in conserved motifs of the NB-ARC domain. Two mutations resulted in autoactivating proteins that induce a pathogen-independent hypersensitive response upon expression in planta. These mutant forms of I-2 were found to be impaired in ATP hydrolysis, but not in ATP binding, suggesting that the ATP- rather than the ADP-bound state of I-2 is the active form that triggers defense signaling. In addition, upon ADP binding, the protein displayed an increased affinity for ADP suggestive of a change of conformation. Based on these data, we propose that the NB-ARC domain of I-2, and likely of related R proteins, functions as a molecular switch whose state (on/off) depends on the nucleotide bound (ATP/ADP).

LePLDbeta1 activation and relocalization in suspension-cultured tomato cells treated with xylanase

Phospholipase D (PLD) has been implicated in various cellular processes including membrane degradation, vesicular trafficking and signal transduction. Previously, we described a PLD gene family in tomato (Lycopersicon esculentum) and showed that expression of one of these genes, LePLDbeta1, was induced by treatment with the fungal elicitor xylanase. To further investigate the function of this PLD, a gene-specific RNAi construct was used to knock down levels of LePLDbeta1 transcript in suspension-cultured tomato cells. Silenced cells exhibited a strong decrease in xylanase-induced PLD activity and responded to xylanase treatment with a disproportionate oxidative burst. Furthermore, LePLDbeta1-silenced cell-suspension cultures were found to have increased polyphenol oxidase activity, to secrete less of the beta-d-xylosidase LeXYL2 and to secrete and express more of the xyloglucan-specific endoglucanase inhibitor protein XEGIP. Using an LePLDbeta1-green fluorescent protein (GFP) fusion protein for confocal laser scanning microscopy-mediated localization studies, untreated cells displayed a cytosolic localization, whereas treatment with xylanase induced relocalization to punctuate structures within the cytosol. Possible functions for PLDbeta in plant-pathogen interactions are discussed.