Phenylalanine treatment induces tomato resistance to Tuta absoluta via increased accumulation of benzenoid/phenylpropanoid volatiles serving as defense signals.

Tuta absoluta ("leafminer"), is a major pest of tomato crops worldwide. Controlling this insect is difficult due to its efficient infestation, rapid proliferation, and resilience to changing weather conditions. Furthermore, chemical pesticides have only a short-term effect due to rapid development of T. absoluta strains. Here, we show that a variety of tomato cultivars, treated with external phenylalanine solutions exhibit high resistance to T. absoluta, under both greenhouse and open field conditions, at different locations. A large-scale metabolomic study revealed that tomato leaves absorb and metabolize externally given Phe efficiently, resulting in a change in their volatile profile, and repellence of T. absoluta moths. The change in the volatile profile is due to an increase in three phenylalanine-derived benzenoid phenylpropanoid volatiles (BPVs), benzaldehyde, phenylacetaldehyde, and 2-phenylethanol. This treatment had no effect on terpenes and green leaf volatiles, known to contribute to the fight against insects. Phe-treated plants also increased the resistance of neighboring non-treated plants. RNAseq analysis of the neighboring non-treated plants revealed an exclusive upregulation of genes, with enrichment of genes related to the plant immune response system. Exposure of tomato plants to either benzaldehyde, phenylacetaldehyde, or 2-phenylethanol, resulted in induction of genes related to the plant immune system that were also induced due to neighboring Phe-treated plants. We suggest a novel role of phenylalanine-derived BPVs as mediators of plant-insect interactions, acting as inducers of the plant defense mechanisms.

Ketocarotenoid production in tomato triggers metabolic reprogramming and cellular adaptation: The quest for homeostasis.

Plants are sessile and therefore have developed an extraordinary capacity to adapt to external signals. Here, the focus is on the plasticity of the plant cell to respond to new intracellular cues. Ketocarotenoids are high-value natural red pigments with potent antioxidant activity. In the present study, system-level analyses have revealed that the heterologous biosynthesis of ketocarotenoids in tomato initiated a series of cellular and metabolic mechanisms to cope with the formation of metabolites that are non-endogenous to the plant. The broad multilevel changes were linked to, among others, (i) the remodelling of the plastidial membrane, where the synthesis and storage of ketocarotenoids occurs; (ii) the recruiting of core metabolic pathways for the generation of metabolite precursors and energy; and (iii) redox control. The involvement of the metabolites as regulators of cellular processes shown here reinforces their pivotal role suggested in the remodelled 'central dogma' concept. Furthermore, the role of metabolic reprogramming to ensure cellular homeostasis is proposed.

Non-Aqueous Isolation and Enrichment of Glandular Capitate Stalked and Sessile Trichomes from Cannabis sativa.

This paper presents a protocol for the convenient and high-throughput isolation and enrichment of glandular capitate stalked and sessile trichomes from Cannabis sativa. The biosynthetic pathways for cannabinoid and volatile terpene metabolism are localized primarily in the Cannabis trichomes, and isolated trichomes are beneficial for transcriptome analysis. The existing protocols for isolating glandular trichomes for transcriptomic characterization are inconvenient and deliver compromised trichome heads and a relatively low amount of isolated trichomes. Furthermore, they rely on expensive apparatus and isolation media containing protein inhibitors to avoid RNA degradation. The present protocol suggests combining three individual modifications to obtain a large amount of isolated glandular capitate stalked and sessile trichomes from C. sativa mature female inflorescences and fan leaves, respectively. The first modification involves substituting liquid nitrogen for the conventional isolation medium to facilitate the passage of trichomes through the micro-sieves. The second modification involves using dry ice to detach the trichomes from the plant source. The third modification involves passing the plant material consecutively through five micro-sieves of diminishing pore sizes. Microscopic imaging demonstrated the effectiveness of the isolation technique for both trichome types. In addition, the quality of RNA extracted from the isolated trichomes was appropriate for downstream transcriptomic analysis.

Distinctive foliar features and volatile profiles in three Ambrosia species (Asteraceae).

MAIN CONCLUSION: Ambrosia species differ both in their trichome types and in metabolic profiles of leaf volatiles. The current study provides tools for easier taxonomic identification of ragweed species. The genus Ambrosia (Asteraceae) includes some of the most noxious allergenic invasive weeds in the world. Due to high polymorphism in this genus, identification of species is often difficult. This study focuses on microscopic investigation of foliar features and GC-MS identification of the main leaf volatile components of three Ambrosia species currently found in Israel-invasive species Ambrosia confertiflora and A. tenuifolia, and transient A. grayi. A. confertiflora and A. tenuifolia have three trichome types: non-glandular trichomes, capitate glandular trichomes and linear glandular trichomes. Their non-glandular trichomes and capitate trichomes have distinct structures and can serve as taxonomic characters. A. grayi (the least successful invader) has only very dense covering trichomes. All three Ambrosia species have secretory structures in their leaf midrib. A. confertiflora, the most problematic invasive plant in Israel, had a ten times higher volatiles content than the other two species. In A. confertiflora, the most abundant volatiles were chrysanthenone (25.5%), borneol (18%), germacrene D and (E)-caryophyllene (both around 12%). In A. tenuifolia, the most abundant volatiles were ß-myrcene (32.9%), (2E)-hexenal (13%) and 1,8-cineole (11.7%). In A. grayi, the most abundant volatiles were ß-myrcene (17.9%), germacrene D (17.8%) and limonene (14%). The three examined species have distinct trichome types and metabolic profiles. Non-glandular trichomes show structural diversification between species and are a good descriptive character. Considering the anthropocentric significance of this highly problematic genus, the current study provides tools for easier identification of ragweed species.

Transcriptional up-regulation of host-specific terpene metabolism in aphid-induced galls of Pistacia palaestina.

Galling insects gain food and shelter by inducing specialized anatomical structures in their plant hosts. Such galls often accumulate plant defensive metabolites protecting the inhabiting insects from predation. We previously found that, despite a marked natural chemopolymorphism in natural populations of Pistacia palaestina, the monoterpene content in Baizongia pistaciae-induced galls is substantially higher than in leaves of their hosts. Here we show a general up-regulation of key structural genes in both the plastidial and cytosolic terpene biosynthetic pathways in galls as compared with non-colonized leaves. Novel prenyltransferases and terpene synthases were functionally expressed in Escherichia coli to reveal their biochemical function. Individual Pistacia trees exhibiting chemopolymorphism in terpene compositions displayed differential up-regulation of selected terpene synthase genes, and the metabolites generated by their gene products in vitro corresponded to the monoterpenes accumulated by each tree. Our results delineate molecular mechanisms responsible for the formation of enhanced monoterpene in galls and the observed intraspecific monoterpene chemodiversity displayed in P. palaestina. We demonstrate that gall-inhabiting aphids transcriptionally reprogram their host terpene pathways by up-regulating tree-specific genes, boosting the accumulation of plant defensive compounds for the protection of colonizing insects.

Genetic dissection of aroma biosynthesis in melon and its relationship with climacteric ripening.

Aroma is an essential trait in melon fruit quality, but its complexity and genetic basis are still poorly understood. The aim of this study was the identification of quantitative trait loci (QTLs) underlying volatile organic compounds (VOCs) biosynthesis in melon rind and flesh, using a Recombinant Inbred Line (RIL) population from the cross 'Piel de Sapo' (PS) × 'Védrantais' (VED), two commercial varieties segregating for ripening behavior. A total of 82 VOCs were detected by gas chromatography-mass spectrometry (GC-MS), and 166 QTLs were identified. The main QTL cluster was on chromosome 8, collocating with the previously described ripening-related QTL ETHQV8.1, with an important role in VOCs biosynthesis. QTL clusters involved in esters, lipid-derived volatiles and apocarotenoids were also identified, and candidate genes have been proposed for ethyl 3-(methylthio)propanoate and benzaldehyde biosynthesis. Our results provide genetic insights for deciphering fruit aroma in melon and offer new tools for flavor breeding.

Increased substrate availability reveals the potential of scentless lisianthus flowers in producing fragrant benzenoid-phenylpropanoids.

Lisianthus (Eustoma grandiflorum), a leading plant in the cut flower industry, is scentless. Here we show that lisianthus flowers have potential to produce several fragrant benzenoid-phenylpropanoids when substrate availability is not limited. To enable hyperaccumulation of substrates for the production of volatile benzenoid-phenylpropanoids, lisianthus commercial hybrid "Excalibur Pink" was transformed via floral dipping with a feedback-insensitive Escherichia coli DAHP synthase (AroG*) and Clarkia breweri benzyl alcohol acetyltransferase (BEAT), under constitutive promoters. The T1 progeny of "Excalibur Pink" plants segregated into four visual phenotypes, with pink or white colored petals and multiple or single petal layers. Interestingly, transformation with AroG* and BEAT caused no significant effect in the pigment composition among phenotypes, but did increase the levels of down-stream fragrant volatile benzenoids. All the transgenic lines exclusively accumulated methyl benzoate, a fragrant benzenoid, either in their petals or leaves. Furthermore, feeding with benzyl alcohol resulted in the accumulation of two novel benzenoids, benzyl acetate (the product of BEAT) and benzoate, as well as a dramatic increase in the concentrations of additional benzenoid-phenylpropanoid volatiles. Presumably, the degree of benzaldehyde overproduction after benzyl alcohol feeding in both leaves and flowers revealed their reverse conversion in lisianthus plants. These findings demonstrate the concealed capability of lisianthus plants to produce a wide array of fragrant benzenoid-phenylpropanoids, given high substrate concentrations, which could in turn open opportunities for future scent engineering.

A gain of function mutation in SlNRC4a enhances basal immunity resulting in broad-spectrum disease resistance.

Plants rely on innate immunity to perceive and ward off microbes and pests, and are able to overcome the majority of invading microorganisms. Even so, specialized pathogens overcome plant defenses, posing a persistent threat to crop and food security worldwide, raising the need for agricultural products with broad, efficient resistance. Here we report a specific mutation in a tomato (S. lycopersicum) helper nucleotide-binding domain leucine-rich repeat H-NLR, SlNRC4a, which results in gain of function constitutive basal defense activation, in absence of PRR activation. Knockout of the entire NRC4 clade in tomato was reported to compromise Rpi-blb2 mediated immunity. The SlNRC4a mutant reported here possesses enhanced immunity and disease resistance to a broad-spectrum of pathogenic fungi, bacteria and pests, while lacking auto-activated HR or negative effects on plant growth and crop yield, providing promising prospects for agricultural adaptation in the war against plant pathogens that decrease productivity.

Phenylalanine increases chrysanthemum flower immunity against Botrytis cinerea attack.

Flowers are the most vulnerable plant organ to infection by the necrotrophic fungus Botrytis cinerea. Here we show that pre-treatment of chrysanthemum (Chrysanthemum morifolium) flowers with phenylalanine (Phe) significantly reduces their susceptibility to B. cinerea. To comprehend how Phe treatment induces resistance, we monitored the dynamics of metabolites (by GC/LC-MS) and transcriptomes (by RNAseq) in flowers after Phe treatment and B. cinerea infection. Phe treatment resulted in accumulation of 3-phenyllactate and benzaldehyde, and in particular induced the expression of genes related to Ca2+ signaling and receptor kinases, implicating an induction of the defense response. Interestingly, the main effects of Phe treatment were observed in flowers exposed to B. cinerea infection, stabilizing the global fluctuations in the levels of metabolites and transcripts while reducing susceptibility to the fungus. We suggest that Phe-induced resistance is associated to cell priming, enabling rapid and targeted reprogramming of cellular defense responses to resist disease development. After Phe pre-treatment, the levels of the anti-fungal volatiles phenylacetaldehyde and eugenol were maintained and the level of coniferin, a plausible monolignol precursor in cell wall lignification, was strongly increased. In addition, Phe pre-treatment reduced ROS generation, prevented ethylene emission, and caused changes in the expression of a minor number of genes related to cell wall biogenesis, encoding the RLK THESEUS1, or involved in Ca2+ and hormonal signaling processes. Our findings point to Phe pre-treatment as a potential orchestrator of a broad-spectrum defense response which may not only provide an ecologically friendly pest control strategy but also offers a promising way of priming plants to induce defense responses against B. cinerea.

Prenyltransferases catalyzing geranyldiphosphate formation in tomato fruit.

Monoterpenes contribute either favorably or adversely to the flavor of tomato, yet modern tomato varieties generally lack monoterpenes in their fruit. The main immediate biosynthetic precursor of monoterpenes is geranyldiphosphate (GPP), produced by the action of GPP synthases (GPPSs). Plant GPPSs are often heteromeric enzymes consisting of a non-catalytic small subunit (GPPS.SSU) and a large subunit (GPPS.LSU), the latter similar to geranylgeranyldiphosphate synthases (GGPPSs) which generate longer prenylphosphate chains. We show here that LeGGPPS2, an enzyme previously reported to support carotenoid biosynthesis, can synthesize farnesyldiphosphate (FPP) and GPP in vitro, in addition to geranylgeranyldiphosphate, depending on the assay conditions. Moreover, GPP formation is favored in vitro by the interaction of LeGGPPS2 with GPPS.SSU from either Anthirrhinum majus (AmGPPS.SSU) or from a newly discovered GPPS.SSU ortholog present in the genome of M82 tomato. SlGPPS.SSU is not expressed in M82 tomato fruit but its orthologs are expressed in fruit of wild tomato relatives, such as Solanum pimpinelifollium and S. cheesmaniae that accumulate monoterpenes.

Concealed ester formation and amino acid metabolism to volatile compounds in table grape (Vitis vinifera L.) berries.

Volatile esters contribute to the aroma and flavor of many fruits but are normally absent in grape berries (Vitis vinifera L.). To examine the biosynthetic potential of grape berries to form volatile esters, berry sections were incubated with exogenous L-Phe, L-Leu or L-Met. In general, amino-acid incubation caused the accumulation of the respective aldehydes and alcohols. Moreover, L-Leu incubation resulted in the accumulation of 3-methylbutyl acetate and L-Phe incubation resulted in the accumulation 2-phenylethyl acetate in 'Muscat Hamburg' but not in the other grape accessions. Exogenous L-Met administration did not result in volatile esters accumulation but the accumulation of sulfur volatile compounds such as methional and dimethyl disulfide was prominent. Berry-derived cell-free extracts displayed differential alcohol acetyltransferase activities and supported the formation of 3-methylbutyl acetate and benzyl acetate. 2-Phenylethyl acetate was produced only in 'Muscat Hamburg' cell-free extracts. VvAAT2, a newly characterized gene, was preferentially expressed in 'Muscat Hamburg' berries and functionally expressed in E. coli. VvAAT2 possesses alcohol acetyltransferase activity utilizing benzyl alcohol, 2-phenylethanol, hexanol or 3-methylbutanol as substrates. Our study demonstrates that grape berries have a concealed potential to accumulate volatile esters and this process is limited by substrate availability.

Differential metabolism of L-phenylalanine in the formation of aromatic volatiles in melon (Cucumis melo L.) fruit.

Studies on the active pathways and the genes involved in the biosynthesis of L-phenylalanine-derived volatiles in fleshy fruits are sparse. Melon fruit rinds converted stable-isotope labeled L-phe into more than 20 volatiles. Phenylpropanes, phenylpropenes and benzenoids are apparently produced via the well-known phenylpropanoid pathway involving phenylalanine ammonia lyase (PAL) and being (E)-cinnamic acid a key intermediate. Phenethyl derivatives seemed to be derived from L-phe via a separate biosynthetic route not involving (E)-cinnamic acid and PAL. To explore for a biosynthetic route to (E)-cinnamaldehyde in melon rinds, soluble protein cell-free extracts were assayed with (E)-cinnamic acid, CoA, ATP, NADPH and MgSO4, producing (E)-cinnamaldehyde in vitro. In this context, we characterized CmCNL, a gene encoding for (E)-cinnamic acid:coenzyme A ligase, inferred to be involved in the biosynthesis of (E)-cinnamaldehyde. Additionally we describe CmBAMT, a SABATH gene family member encoding a benzoic acid:S-adenosyl-L-methionine carboxyl methyltransferase having a role in the accumulation of methyl benzoate. Our approach leads to a more comprehensive understanding of L-phe metabolism into aromatic volatiles in melon fruit.

Deciphering genetic factors that determine melon fruit-quality traits using RNA-Seq-based high-resolution QTL and eQTL mapping.

Combined quantitative trait loci (QTL) and expression-QTL (eQTL) mapping analysis was performed to identify genetic factors affecting melon (Cucumis melo) fruit quality, by linking genotypic, metabolic and transcriptomic data from a melon recombinant inbred line (RIL) population. RNA sequencing (RNA-Seq) of fruit from 96 RILs yielded a highly saturated collection of > 58 000 single-nucleotide polymorphisms, identifying 6636 recombination events that separated the genome into 3663 genomic bins. Bin-based QTL analysis of 79 RILs and 129 fruit-quality traits affecting taste, aroma and color resulted in the mapping of 241 QTL. Thiol acyltransferase (CmThAT1) gene was identified within the QTL interval of its product, S-methyl-thioacetate, a key component of melon fruit aroma. Metabolic activity of CmThAT1-encoded protein was validated in bacteria and in vitro. QTL analysis of flesh color intensity identified a candidate white-flesh gene (CmPPR1), one of two major loci determining fruit flesh color in melon. CmPPR1 encodes a member of the pentatricopeptide protein family, involved in processing of RNA in plastids, where carotenoid and chlorophyll pigments accumulate. Network analysis of > 12 000 eQTL mapped for > 8000 differentially expressed fruit genes supported the role of CmPPR1 in determining the expression level of plastid targeted genes. We highlight the potential of RNA-Seq-based QTL analysis of small to moderate size, advanced RIL populations for precise marker-assisted breeding and gene discovery. We provide the following resources: a RIL population genotyped with a unique set of SNP markers, confined genomic segments that harbor QTL governing 129 traits and a saturated set of melon eQTLs.

Corrected and Republished from: Activation Status-Coupled Transient S-Acylation Determines Membrane Partitioning of a Plant Rho-Related GTPase.

ROPs or RACs are plant Rho-related GTPases implicated in the regulation of a multitude of signaling pathways that function at the plasma membrane via posttranslational lipid modifications. The relationships between ROP activation status and membrane localization has not been established. Here, we show that endogenous ROPs, as well as a transgenic His6-green fluorescent protein (GFP)-Arabidopsis thaliana ROP6 (AtROP6) fusion protein, were partitioned between Triton X-100-soluble and -insoluble membranes. In contrast, the His6-GFP-Atrop6CA activated mutant accumulated exclusively in detergent-resistant membranes. GDP induced accumulation of ROPs in Triton-soluble membranes, whereas GTPγS induced accumulation of ROPs in detergent-resistant membranes. Recombinant wild-type and constitutively active AtROP6 proteins were purified from Arabidopsis plants, and in turn, their lipids were cleaved and analyzed by gas chromatography-coupled mass spectrometry. In Triton-soluble membranes, the wild-type AtROP6 was only prenylated, primarily by geranylgeranyl. The activated AtROP6 that accumulated in detergent-resistant membranes was modified by prenyl and acyl lipids, identified as palmitic and stearic acids. Consistently, activated His6-GFP-Atrop6CAmS156, in which C156 was mutated into serine, accumulated in Triton-soluble membranes. These findings show that upon GTP binding and activation, AtROP6, and possibly other ROPs, are transiently S-acylated, inducing their partitioning into detergent-resistant membranes.

Phenylpyruvate Contributes to the Synthesis of Fragrant Benzenoid-Phenylpropanoids in Petunia × hybrida Flowers.

Phenylalanine (Phe) is a precursor for a large group of plant specialized metabolites, including the fragrant volatile benzenoid-phenylpropanoids (BPs). In plants, the main pathway leading to production of Phe is via arogenate, while the pathway via phenylpyruvate (PPY) is considered merely an alternative route. Unlike plants, in most microorganisms the only pathway leading to the synthesis of Phe is via PPY. Here we studied the effect of increased PPY production in petunia on the formation of BPs volatiles and other specialized metabolites originating from Phe both in flowers and leaves. Stimulation of the pathway via PPY was achieved by transforming petunia with PheA∗ , a gene encoding a bacterial feedback insensitive bi-functional chorismate mutase/prephenate dehydratase enzyme. PheA∗ overexpression caused dramatic increase in the levels of flower BP volatiles such as phenylacetaldehyde, benzaldehyde, benzyl acetate, vanillin, and eugenol. All three BP pathways characterized in petunia flowers were stimulated in PheA∗ flowers. In contrast, PheA∗ overexpression had only a minor effect on the levels of amino acids and non-volatile metabolites both in the leaves and flowers. The one exception is a dramatic increase in the level of rosmarinate, a conjugate between Phe-derived caffeate and Tyr-derived 3,4-dihydroxyphenylacetate, in PheA∗ leaves. PheA∗ petunia flowers may serve as an excellent system for revealing the role of PPY in the production of BPs, including possible routes directly converting PPY to the fragrant volatiles. This study emphasizes the potential of the PPY route in achieving fragrance enhancement in flowering plants.

Differences in Monoterpene Biosynthesis and Accumulation in Pistacia palaestina Leaves and Aphid-Induced Galls.

Certain insect species can induce gall formation on numerous plants species. Although the mechanism of gall development is largely unknown, it is clear that insects manipulate their hosts' anatomy, physiology, and chemistry for their own benefit. It is well known that insect-induced galls often contain vast amounts of plant defensive compounds as compared to non-colonized tissues, but it is not clear if defensive compounds can be produced in situ in the galled tissues. To answer this question, we analyzed terpene accumulation patterns and possible independent biosynthetic potential of galls induced by the aphid Baizongia pistaciae L. on the terminal buds of Pistacia palaestina Boiss. We compared monoterpene levels and monoterpene synthase enzyme activity in galls and healthy leaves from individual trees growing in a natural setting. At all developmental stages, monoterpene content and monoterpene synthase activity were consistently (up to 10 fold on a fresh weight basis) higher in galls than in intact non-colonized leaves. A remarkable tree to tree variation in the products produced in vitro from the substrate geranyl diphosphate by soluble protein extracts derived from individual trees was observed. Furthermore, galls and leaves from the same trees displayed enhanced and often distinct biosynthetic capabilities. Our results clearly indicate that galls possess independent metabolic capacities to produce and accumulate monoterpenes as compared to leaves. Our study indicates that galling aphids manipulate the enzymatic machinery of their host plant, intensifying their own defenses against natural enemies.

Characterization of Morphology, Volatile Profiles, and Molecular Markers in Edible Desert Truffles from the Negev Desert.

Desert truffles are mycorrhizal, hypogeous fungi considered a delicacy. On the basis of morphological characters, we identified three desert truffle species that grow in the same habitat in the Negev desert. These include Picoa lefebvrei (Pat.), Tirmania nivea (Desf.) Trappe, and Terfezia boudieri (Chatain), all associated with Helianthemum sessiliflorum. Their taxonomy was confirmed by PCR-RFLP. The main volatiles of fruit bodies of T. boudieri and T. nivea were 1-octen-3-ol and hexanal; however, volatiles of the latter species further included branched-chain amino acid derivatives such as 2-methylbutanal and 3-methylbutanal, phenylalanine derivatives such as benzaldehyde and benzenacetaldehyde, and methionine derivatives such as methional and dimethyl disulfide. The least aromatic truffle, P. lefebvrei, contained low levels of 1-octen-3-ol as the main volatile. Axenic mycelia cultures of T. boudieri displayed a simpler volatile profile compared to its fruit bodies. This work highlights differences in the volatile profiles of desert truffles and could hence be of interest for selecting and cultivating genotypes with the most likable aroma.

Use of the Endophytic Fungus Daldinia cf. concentrica and Its Volatiles as Bio-Control Agents.

Endophytic fungi are organisms that spend most of their life cycle within plant tissues without causing any visible damage to the host plant. Many endophytes were found to secrete specialized metabolites and/or emit volatile organic compounds (VOCs), which may be biologically active and assist fungal survival inside the plant as well as benefit their hosts. We report on the isolation and characterization of a VOCs-emitting endophytic fungus, isolated from an olive tree (Olea europaea L.) growing in Israel; the isolate was identified as Daldinia cf. concentrica. We found that the emitted VOCs were active against various fungi from diverse phyla. Results from postharvest experiments demonstrated that D. cf. concentrica prevented development of molds on organic dried fruits, and eliminated Aspergillus niger infection in peanuts. Gas chromatography-mass spectrometry analysis of the volatiles led to identification of 27 VOCs. On the basis of these VOCs we prepared two mixtures that displayed a broad spectrum of antifungal activity. In postharvest experiments these mixtures prevented development of molds on wheat grains, and fully eliminated A. niger infection in peanuts. In light of these findings, we suggest use of D. cf. concentrica and/or its volatiles as an alternative approach to controlling phytopathogenic fungi in the food industry and in agriculture.

Fruit carotenoid-deficient mutants in tomato reveal a function of the plastidial isopentenyl diphosphate isomerase (IDI1) in carotenoid biosynthesis.

Isoprenoids consist of a large class of compounds that are present in all living organisms. They are derived from the 5C building blocks isopentenyl diphosphate (IDP) and its isomer dimethylallyl diphosphate (DMADP). In plants, IDP is synthesized in the cytoplasm from mevalonic acid via the MVA pathway, and in plastids from 2-C-methyl-d-erythritol-4-phosphate through the MEP pathway. The enzyme IDP isomerase (IDI) catalyzes the interconversion between IDP and DMADP. Most plants contain two IDI enzymes, the functions of which are characteristically compartmentalized in the cells. Carotenoids are isoprenoids that play essential roles in photosynthesis and provide colors to flowers and fruits. They are synthesized in the plastids via the MEP pathway. Fruits of Solanum lycopersicum (tomato) accumulate high levels of the red carotene lycopene. We have identified mutations in tomato that reduce overall carotenoid accumulation in fruits. Four alleles of a locus named FRUIT CAROTENOID DEFICIENT 1 (fcd1) were characterized. Map-based cloning of fcd1 indicated that this gene encodes the plastidial enzyme IDI1. Lack of IDI1 reduced the concentration of carotenoids in fruits, flowers and cotyledons, but not in mature leaves. These results indicate that the plastidial IDI plays an important function in carotenoid biosynthesis, thus highlighting its role in optimizing the ratio between IDP and DMADP as precursors for different downstream isoprenoid pathways.