The role of octadecanoids and functional mimics in soybean defense responses

Oxylipins and Reactive Carbonyls as Regulators of the Plant Redox and Reactive Oxygen Species Network under Stress

Reactive oxygen species (ROS), and in particular H2O2, serve as essential second messengers at low concentrations. However, excessive ROS accumulation leads to severe and irreversible cell damage. Hence, control of ROS levels is needed, especially under non-optimal growth conditions caused by abiotic or biotic stresses, which at least initially stimulate ROS synthesis. A complex network of thiol-sensitive proteins is instrumental in realizing tight ROS control; this is called the redox regulatory network. It consists of sensors, input elements, transmitters, and targets. Recent evidence revealed that the interplay of the redox network and oxylipins–molecules derived from oxygenation of polyunsaturated fatty acids, especially under high ROS levels–plays a decisive role in coupling ROS generation and subsequent stress defense signaling pathways in plants. This review aims to provide a broad overview of the current knowledge on the interaction of distinct oxylipins generated enzymatically (12-OPDA, 4-HNE, phytoprostanes) or non-enzymatically (MDA, acrolein) and components of the redox network. Further, recent findings on the contribution of oxylipins to environmental acclimatization will be discussed using flooding, herbivory, and establishment of thermotolerance as prime examples of relevant biotic and abiotic stresses.

Modulation of NaCl-induced osmotic, cytogenetic, oxidative and anatomic damages by coronatine treatment in onion (Allium cepa L.)

Coronatine (COR), a bacterial phytotoxin produced by Pseudomonas syringae, plays important roles in many plant growth processes. Onion bulbs were divided four groups to investigate the effects of COR against sodium chloride (NaCl) stress exposure in Allium cepa L. root tips. While control group bulbs were soaked in tap water medium, treatment group bulbs were grown in 0.15 M NaCl, 0.01 µM COR and 0.01 µM COR + 0.15 M NaCl medium, respectively. NaCl stress seriously inhibited the germination, root lenght, root number and fresh weight of the bulbs. It significantly decreased the mitotic index (MI), whereas dramatically increased the micronucleus (MN) frequency and chromosomal aberrations (CAs). Moreover, in order to determine the level of lipid peroxidation occurring in the cell membrane, malondialdehyde (MDA) content was measured and it was determined that it was at the highest level in the group germinated in NaCl medium alone. Similarly, it was revealed that the superoxide dismutase (SOD), catalase (CAT) and free proline contents in the group germinated in NaCl medium alone were higher than the other groups. On the other hand, NaCl stress caused significant injuries such as epidermis/cortex cell damage, MN formation in epidermis/cortex cells, flattened cells nuclei, unclear vascular tissue, cortex cell wall thickening, accumulation of certain chemical compounds in cortex cells and necrotic areas in the anatomical structure of bulb roots. However, exogenous COR application significantly alleviated the negative effects of NaCl stress on bulb germination and growth, antioxidant defense system, cytogenetic and anatomical structure. Thus, it has been proven that COR can be used as a protective agent against the harmful effects of NaCl on onion.

Screening of Indanoyl-Type Compounds as Elicitors of Isoflavonoid Phytoalexins in Colombian Common Bean Cultivars

Eleven indanoyl derivatives were synthesized and, along with methyl jasmonate, evaluated as isoflavonoid-phytoalexin elicitors in two cultivars of common bean (Phaseolus vulgaris L. cvs. ICA-Cerinza and Uribe Rosado, tolerant and susceptible to anthracnose, respectively). Indanoyl derivatives (an ester, two amides, and eight indanoyl-amino acid conjugates) were obtained from 1-oxo-indane-4-carboxylic acid. In general, the accumulation of isoflavonoid-type phytoalexins, such as isoflavones (genistein, daidzein, and 2′-hydroxygenistein), isoflavanones (dalbergioidin and kievitone), isoflavan (phaseollinisoflavan), coumestrol, and pterocarpans (phaseollidin and phaseollin), was dependent on the common bean cultivar, the post-induction time, and the elicitor structure. Isoflavones, dalbergioidin, and coumestrol reached their highest amounts during the first 48 to 72 h, whereas kievitone, phaseollinisoflavano, and the pterocarpans reached maximum levels between 72 and 96 h. The 1-oxo-indanoyl-L-isoleucine methyl ester elicited the highest levels of phytoalexins (similar to those elicited by the methyl jasmonate) and showed no significant phytotoxic effects on common bean seedlings. The indanoyl-type synthetic elicitor, 1-oxo-indanoyl-L-isoleucine methyl ester, may represent a promising agronomic alternative for disease control in common bean by enhancing the accumulation of antimicrobial isoflavonoid phytoalexins.

Detection and Imaging of the Plant Pathogen Response by Near-Infrared Fluorescent Polyphenol Sensors

Plants use secondary metabolites such as polyphenols for chemical defense against pathogens and herbivores. Despite their importance in plant pathogen interactions and tolerance to diseases, it remains challenging to detect polyphenols in complex plant tissues. Here, we create molecular sensors for plant polyphenol imaging that are based on near-infrared (NIR) fluorescent single-wall carbon nanotubes (SWCNTs). We identified polyethylene glycol-phospholipids that render (6,5)-SWCNTs sensitive (K_d =90 nM) to plant polyphenols (tannins, flavonoids, …), which red-shift (up to 20 nm) and quench their emission (ca. 1000 nm). These sensors report changes in total polyphenol level after herbivore or pathogen challenge in crop plant systems (Soybean Glycine max) and leaf tissue extracts (Tococa spp.). We furthermore demonstrate remote chemical imaging of pathogen-induced polyphenol release from roots of soybean seedlings over the time course of 24 h. This approach allows in situ visualization and understanding of the chemical plant defense in real time and paves the way for plant phenotyping for optimized polyphenol secretion.

Plant-microbial interaction: The mechanism and the application of microbial elicitor induced secondary metabolites biosynthesis in medicinal plants

Plants and microbes interact with each other via different chemical signaling pathways. At the risophere level, the microbes can secrete molecules, called elicitors, which act on their receptors located in plant cells. The so-called elicitor molecules as well as their actions differ according to the mcirobes and induce different bilogical responses in plants such as the synthesis of secondary metabolites. Microbial compounds induced phenotype changes in plants are known as elicitors and signaling pathways which integrate elicitor's signals in plants are called elicitation. In this review, the impact of microbial elicitors on the synthesis and the secretion of secondary metabolites in plants was highlighted. Moreover, biological properties of these bioactive compounds were also highlighted and discussed. Indeed, several bacteria, fungi, and viruses release elicitors which bind to plant cell receptors and mediate signaling pathways involved in secondary metabolites synthesis. Different phytochemical classes such as terpenoids, phenolic acids and flavonoids were synthesized and/or increased in medicinal plants via the action of microbial elicitors. Moreover, these compounds compounds exhibit numerous biological activities and can therefore be explored in drugs discovery.

The phytotoxin COR induces transcriptional reprogramming of photosynthetic, hormonal and defence networks in tomato

Coronatine (COR) is a non-host specific phytotoxin secreted by Pseudomonas syringae pv. tomato that can induce leaf chlorosis and increase the virulence of pathogens during plant-pathogen interactions. Studies have shown that COR can regulate multiple physiological processes in plants, but its involvement in bacterial pathogenesis and plant growth regulation is not well understood. In this study, transcriptome sequencing was carried out on 4-week-old tomato leaves that were either mock-treated or treated with COR. Transcriptome sequencing led to the identification of 6144 differentially expressed genes (DEGs), of which 4361 genes were downregulated and 1783 genes were upregulated upon COR treatment. To obtain functional information on the DEGs, we annotated these genes using GO and KEGG databases. Functional classification analysis showed that the DEGs were primarily involved in photosynthesis, chlorophyll and carotenoid biosynthesis, jasmonic acid (JA) synthesis and phenylpropane metabolism. A total of 23 genes related to chlorophyll biosynthesis had significant changes, of which 22 genes were downregulated and one gene was upregulated, indicating that chlorophyll biosynthesis was inhibited upon COR treatment. A total of 17 photosystem I related genes and 22 photosystem II related genes involving 20 protein subunits were also downregulated. In the JA synthesis pathway, 25 genes were up regulated, and six genes were downregulated in COR treated samples. COR was also involved in the regulation of multiple secondary metabolites. The identified DEGs will help us better understand the virulence effects and physiological functions of COR and provide a theoretical basis for breeding resistance into economically important crops.

Increased accumulation of isoflavonoids in common bean (Phaseolus vulgaris L.) tissues treated with 1-oxo-indane-4-carboxylic acid derivatives

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Eleven indanoyl derivatives as potential elicitors were synthesized and characterized.

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Accumulation of nine isoflavonoid phytoalexins in two common bean (P. vulgaris L.) cultivars grown in Colombia was analyzed.

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Dose-response and time-course experiments were performed on cotyledons and hypocotyls-roots of common bean treated with the potential elicitors.

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Application of indanoyl-type elicitors increased the concentration of isoflavonoids in tissues of common bean.

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Accumulation of isoflavonoid phytoalexins was dependent on the cultivar, the tissue type, the elicitor structure and the post-induction time.

Isoflavonoid phytoalexins (isoflavones: genistein, 2′-hydroxygenistein, and daidzein; isoflavanones: dalbergioidin and kievitone; coumestrol; pterocarpans: phaseollidin and phaseollin; and the isoflavan: phaseollinisoflavan) production in response to the application of eleven 1-oxo-indane-4-carboxylic acid derivatives (indanoyl esters and indanoyl amino acids conjugates), in cotyledons and hypocotyl/root of two common bean (Phaseolus vulgaris L.) cultivars was evaluated. The content of isoflavonoids depended on the cultivar, the treated tissue, the time after induction, the structure and concentration of the elicitor. The highest isoflavonoid contents were found when 1-oxo-indanoyl-amino acids conjugates were used as elicitors. Cotyledons and hypocotyl/root of the anthracnose-resistant cultivar produced significantly higher isoflavonoid contents as compared to the susceptible one. Maximum levels of phaseollin were obtained using 0.66 mM 1-oxo-indanoyl-l-isoleucyl methyl ester and between 72 and 96 h post-induction. So, 1-oxo-indane-4-carboxylic acid derivatives, may be used to enhance the amount of isoflavonoid phytoalexins in common bean and protect crops from phytopathogenic microorganisms.

Recent Advances in Synthetic Chemical Inducers of Plant Immunity

Different from the conventional biocidal agrochemicals, synthetic chemical inducers of plant immunity activate, bolster, or prime plant defense machineries rather than directly acting on the pathogens. Advances in combinatorial synthesis and high-throughput screening methods have led to the discovery of various synthetic plant immune activators as well as priming agents. The availability of their structures and recent progress in the mechanistic understanding of plant immune responses have opened up the possibility of identifying new or more potent chemical inducers through rational design. In this review, we first summarize the chemical inducers identified through large-scale screening and then discuss the emerging trends in the identification and development of novel plant immune inducers including natural elicitor based chemical derivation, bifunctional combination, and computer-aided design.

Identification of rate-limiting enzymes involved in paclitaxel biosynthesis pathway affected by coronatine and methyl-β-cyclodextrin in Taxus baccata L. cell suspension cultures

Background

Paclitaxel is a potent antitumor alkaloid widely used for the treatment of several cancer types. This valuable secondary metabolite naturally exists in the inner bark of Taxus species in very low amounts. The small-scale production of paclitaxel in Taxus cell cultures requires utilization of several elicitors.

Objective

The main objective of this work was to identify key genes that encode rate-limiting enzymes in paclitaxel biosynthesis pathway by investigating the possible relationship between paclitaxel production and a set of 13 involved genes’ relative expression in Taxus baccata L. cell suspension cultures affected by coronatine and methyl-β-cyclodextrin.

Methods

In the present research, the most important key genes were identified using gene expression profiling evaluation and paclitaxel production assessment in Taxus baccata L. cell cultures affected by mentioned elicitors.

Results and Conclusion

Gene expression levels were variably increased using methyl-β-cyclodextrin, and in some cases, a synergistic effect on transcript accumulation was observed when culture medium was supplemented with both elicitors. It was revealed that DBAT, BAPT, and DBTNBT are the most important rate-limiting enzymes in paclitaxel biosynthesis pathway in Taxus baccata L. cell suspension cultures under coronatine and methyl-β-cyclodextrin elicitation condition. Moreover, PAM was identified as one of the important key genes especially in the absence of β-phenylalanine. In cell cultures affected by these elicitors, paclitaxel was found largely in the culture media (more than 90%). The secretion of this secondary metabolite suggests a limited feedback inhibition and reduced paclitaxel toxicity for producer cells. It is the result of the ABC gene relative expression level increment under methyl-β-cyclodextrin elicitation and highly depends on methyl-β-cyclodextrin’s special property (complex formation with hydrophobic compounds). Paclitaxel biosynthesis was obviously increased due to the effect of coronatine and methyl-β-cyclodextrin elicitation, leading to the production level of 5.62 times higher than that of the untreated cultures.

Graphical abstract

Increasing the synthesis of bioactive abietane diterpenes in Salvia sclarea hairy roots by elicited transcriptional reprogramming

Transcriptional activation of genes belonging to the plastidial MEP-derived isoprenoid pathway by elicitation with methyl jasmonate and coronatine enhanced the content of bioactive abietane diterpenes in Salvia sclarea hairy roots. We have shown that aethiopinone, an abietane diterpene synthesized in Salvia sclarea roots is cytotoxic and induces apoptosis in human melanoma cells. To develop a production platform for this compound and other abietane diterpenes, hairy root technology was combined with the elicitation of methyl jasmonate (MeJA) or the phytotoxin coronatine (Cor). Both MeJA and Cor induced a significant accumulation of aethiopinone, but prolonged exposure to MeJA irremediably caused inhibition of hairy root growth, which was unaffected by Cor treatment. Considering together the fold increase in aethiopinone content and the final hairy root biomass, the best combination was a Cor treatment for 28 days, which allowed to obtain up to 105.34 ± 2.30 mg L^-1 of this compound to be obtained, corresponding to a 24-fold increase above the basal content in untreated hairy roots. MeJA or Cor elicitation also enhanced the synthesis of other bioactive abietane-quinone diterpenes. The elicitor-dependent steering effect was due to a coordinated transcriptional activation of several biosynthetic genes belonging to the plastidial MEP-derived isoprenoid pathway. High correlations between aethiopinone content and MeJA or Cor-elicited level of gene transcripts were found for DXS2 (r ² = 0.99), DXR (r ² = 0.99), and GGPPS (r ² = 0.98), encoding enzymes acting upstream of GGPP, the common precursor of diterpenes and other plastidial-derived terpenes, as well as CPPS (r ² = 0.99), encoding the enzyme involved in the first cyclization steps leading to copalyl-diphosphate, the precursor of abietane-like diterpenes. These results point to these genes as possible targets of metabolic engineering approaches to establish a more efficient production platform for such promising anti-proliferative plant-derived compounds.

Elicitation, an Effective Strategy for the Biotechnological Production of Bioactive High-Added Value Compounds in Plant Cell Factories

Plant in vitro cultures represent an attractive and cost-effective alternative to classical approaches to plant secondary metabolite (PSM) production (the “Plant Cell Factory” concept). Among other advantages, they constitute the only sustainable and eco-friendly system to obtain complex chemical structures biosynthesized by rare or endangered plant species that resist domestication. For successful results, the biotechnological production of PSM requires an optimized system, for which elicitation has proved one of the most effective strategies. In plant cell cultures, an elicitor can be defined as a compound introduced in small concentrations to a living system to promote the biosynthesis of the target metabolite. Traditionally, elicitors have been classified in two types, abiotic or biotic, according to their chemical nature and exogenous or endogenous origin, and notably include yeast extract, methyl jasmonate, salicylic acid, vanadyl sulphate and chitosan. In this review, we summarize the enhancing effects of elicitors on the production of high-added value plant compounds such as taxanes, ginsenosides, aryltetralin lignans and other types of polyphenols, focusing particularly on the use of a new generation of elicitors such as coronatine and cyclodextrins.

Enhanced extracellular production of trans-resveratrol in Vitis vinifera suspension cultured cells by using cyclodextrins and coronatine

In the present work the effect of cyclodextrin and coronatine on both trans-resveratrol production and the expression of stilbene biosynthetic genes in Vitis vinifera L. cv Monastrell suspension cultured cells were evaluated. The results showed the maximum level of trans-resveratrol produced by cells and secreted to the culture medium with 50 mM cyclodextrins and 1 μM coronatine. Since the levels of trans-resveratrol produced in the combined treatment were higher than the sum of the individual treatments, a synergistic effect between both elicitors was assumed. In addition, all the analysed genes were induced by cyclodextrins and/or coronatine. The expression of the phenylalanine ammonia lyase and stilbene synthase genes was greatly enhanced by coronatine although an increase in the amount of trans-resveratrol in the spent medium was not detected. Therefore, despite the fact that trans-resveratrol production is related with the expression of genes involved in the biosynthetic process, other factors may be involved, such as post-transcriptional and post-traductional regulation. The expression maximal levels of cinnamate 4-hydroxylase and 4-coumarate-CoA ligase genes were found with cyclodextrins alone or in combination with coronatine suggesting that the activity of these enzymes could be not only important for the formation of intermediates of trans-R biosynthesis but also for those intermediates involved in the biosynthesis of lignins and/or flavonoids.

Additional evidence against jasmonate-induced jasmonate induction hypothesis

Jasmonates are phytohormones involved in development and stress reactions. The most prominent jasmonate is jasmonic acid, however, the bioactive jasmonate is (+)-7-iso-jasmonoyl-L-isoleucine (JA-Ile). Biosynthesis of jasmonates is long time known; compartmentalization, enzymes and corresponding genes are well studied. Because all genes encoding these biosynthetic enzymes are jasmonate inducible, a hypothesis of jasmonate-induced-jasmonate-biosynthesis is widely accepted. Here, this hypothesis was revisited by employing the synthetic JA-Ile mimic coronalon to intact and wounded leaves, which excludes structural cross-contamination with endogenous jasmonates. At an effective concentration that induced various jasmonate-responsive genes in Arabidopsis, neither accumulation of endogenous jasmonic acid, JA-Ile, nor of their hydroxylated metabolites was detected. Results indicate that in spite of jasmonate-induced biosynthetic gene expression, no jasmonate biosynthesis/accumulation takes place supporting a post-translational regulation.

Synthetic plant defense elicitors

To defend themselves against invading pathogens plants utilize a complex regulatory network that coordinates extensive transcriptional and metabolic reprogramming. Although many of the key players of this immunity-associated network are known, the details of its topology and dynamics are still poorly understood. As an alternative to forward and reverse genetic studies, chemical genetics-related approaches based on bioactive small molecules have gained substantial popularity in the analysis of biological pathways and networks. Use of such molecular probes can allow researchers to access biological space that was previously inaccessible to genetic analyses due to gene redundancy or lethality of mutations. Synthetic elicitors are small drug-like molecules that induce plant defense responses, but are distinct from known natural elicitors of plant immunity. While the discovery of some synthetic elicitors had already been reported in the 1970s, recent breakthroughs in combinatorial chemical synthesis now allow for inexpensive high-throughput screens for bioactive plant defense-inducing compounds. Along with powerful reverse genetics tools and resources available for model plants and crop systems, comprehensive collections of new synthetic elicitors will likely allow plant scientists to study the intricacies of plant defense signaling pathways and networks in an unparalleled fashion. As synthetic elicitors can protect crops from diseases, without the need to be directly toxic for pathogenic organisms, they may also serve as promising alternatives to conventional biocidal pesticides, which often are harmful for the environment, farmers and consumers. Here we are discussing various types of synthetic elicitors that have been used for studies on the plant immune system, their modes-of-action as well as their application in crop protection.

A metabolic profiling strategy for the dissection of plant defense against fungal pathogens

Here we present a metabolic profiling strategy employing direct infusion Orbitrap mass spectrometry (MS) and gas chromatography-mass spectrometry (GC/MS) for the monitoring of soybean's (Glycine max L.) global metabolism regulation in response to Rhizoctonia solani infection in a time-course. Key elements in the approach are the construction of a comprehensive metabolite library for soybean, which accelerates the steps of metabolite identification and biological interpretation of results, and bioinformatics tools for the visualization and analysis of its metabolome. The study of metabolic networks revealed that infection results in the mobilization of carbohydrates, disturbance of the amino acid pool, and activation of isoflavonoid, α-linolenate, and phenylpropanoid biosynthetic pathways of the plant. Components of these pathways include phytoalexins, coumarins, flavonoids, signaling molecules, and hormones, many of which exhibit antioxidant properties and bioactivity helping the plant to counterattack the pathogen's invasion. Unraveling the biochemical mechanism operating during soybean-Rhizoctonia interaction, in addition to its significance towards the understanding of the plant's metabolism regulation under biotic stress, provides valuable insights with potential for applications in biotechnology, crop breeding, and agrochemical and food industries.

Jasmonate-dependent induction of polyphenol oxidase activity in tomato foliage is important for defense against Spodoptera exigua but not against Manduca sexta

BACKGROUND

Jasmonates are involved in plant defense, participating in the timely induction of defense responses against insect herbivores from different feeding guilds and with different degrees of host specialization. It is less clear to what extent the induction of plant defense is controlled by different members of the jasmonate family and how specificity of the response is achieved. Using transgenic plants blocked in jasmonic acid (JA) biosynthesis, we previously showed that JA is required for the formation of glandular trichomes and trichome-borne metabolites as constitutive defense traits in tomato, affecting oviposition and feeding behavior of the specialist Manduca sexta. In contrast, JA was not required for the local induction of defense gene expression after wounding. In JA-deficient plants, the JA precursor oxophytodienoic acid (OPDA) substituted as a regulator of defense gene expression maintaining considerable resistance against M. sexta larvae. In this study, we investigate the contribution of JA and OPDA to defense against the generalist herbivore Spodoptera exigua.

RESULTS

S. exigua preferred JA-deficient over wild-type tomato plants as a host for both oviposition and feeding. Feeding preference for JA-deficient plants was caused by constitutively reduced levels of repellent terpenes. Growth and development of the larvae, on the other hand, were controlled by additional JA-dependent defense traits, including the JA-mediated induction of foliar polyphenol oxidase (PPO) activity. PPO induction was more pronounced after S. exigua herbivory as compared to mechanical wounding or M. sexta feeding. The difference was attributed to an elicitor exclusively present in S. exigua oral secretions.

CONCLUSIONS

The behavior of M. sexta and S. exigua during oviposition and feeding is controlled by constitutive JA/JA-Ile-dependent defense traits involving mono- and sesquiterpenes in both species, and cis-3-hexenal as an additional chemical cue for M. sexta. The requirement of jasmonates for resistance of tomato plants against caterpillar feeding differs for the two species. While the OPDA-mediated induction of local defense is sufficient to restrict growth and development of M. sexta larvae in absence of JA/JA-Ile, defense against S. exigua relied on additional JA/JA-Ile dependent factors, including the induction of foliar polyphenol oxidase activity in response to S. exigua oral secretions.

Identification of Alternaria alternata mycotoxins by LC-SPE-NMR and their cytotoxic effects to soybean (Glycine max) cell suspension culture

This present work describes the application of liquid chromatography-solid phase extraction-nuclear magnetic resonance spectroscopy to analyse Alternaria alternata crude extracts. Altenusin (1), alternariol (2), 3'-hydroxyalternariol monomethyl ether (3), and alternariol monomethyl ether (4), were separated and identified. High-resolution mass spectrometry confirmed the proposed structures. The cytotoxic effects of these compounds towards plants were determined using soybean (Glycine max) cell cultures as a model. EC(50) values which range from 0.11 (± 0.02) to 4.69 (± 0.47) μM showed the high cytotoxicity of these compounds.

Coronatine, a more powerful elicitor for inducing taxane biosynthesis in Taxus media cell cultures than methyl jasmonate

Coronatine is a toxin produced by the pathogen Pseudomonas syringae. This compound has received much attention recently for its potential to act as a plant growth regulator and elicitor of plant secondary metabolism. To gain more insight into the mechanism by which elicitors can affect the biosynthesis of paclitaxel (Px) and related taxanes, the effect of coronatine (Cor) and methyl jasmonate (MeJA) on Taxus media cell cultures has been studied. For this study, a two-stage cell culture was established, in which cells were first cultured for 14 days in a medium optimised for growth, after which the cells were transferred to medium optimised for secondary metabolite production. The two elicitors were added to the medium at the beginning of the second stage. Total taxane production in the cell suspension was significantly enhanced by both elicitors, increasing from a maximum level of 8.14mg/L in control conditions to 21.48mg/L (day 12) with MeJA and 77.46mg/L (day 16) with Cor. Expression analysis indicated that the txs, t13oh, t2oh, t7oh, dbat, pam, bata and dbtnbt genes were variably induced by the presence of the elicitors. Genes encoding enzymes involved in the formation of the polihydroxylated hypothetical intermediate (TXS, T13OH, T2OH, T7OH) and the phenylalanoil CoA chain (PAM) were stronger induced than those encoding enzymes catalysing the last steps of the Px biosynthetic pathway (DBAT, BAPT and DBTNBT). Notably, although taxane accumulation differed qualitatively and quantitatively following MeJA- or Cor-elicitation, gene expression induction patterns were similar, inferring that both elicitors may involve distinct but yet uncharacterised regulatory mechanisms.

Glyceollin, a soybean phytoalexin with medicinal properties

This review covers the biosynthesis of glyceollin and its biological activities including antiproliferative/antitumor action (toward B16 melanoma cells, LNCaP prostate cancer cells, and BG-1 ovarian cancer cells), anti-estrogenic action (through estrogen receptors α- and β-), antibacterial action (toward Erwinia carotovora, Escherichia coli, Bradyrhizobium japonicum, Sinorhizobium fredii ), antinematode activity, and antifungal activity (toward Fusarium solani, Phakospora pachyrhizi, Diaporthe phaseolorum, Macrophomina phaseolina, Sclerotina sclerotiorum, Phytophthora sojae, Cercospora sojina, Phialophora gregata, and Rhizoctonia solani). Other activities include insulinotropic action and attenuation of vascular contractions in rat aorta.

Identification of prenylated pterocarpans and other isoflavonoids in Rhizopus spp. elicited soya bean seedlings by electrospray ionisation mass spectrometry

Phytoalexins from soya are mainly characterised as prenylated pterocarpans, the glyceollins. Extracts of non-soaked and soaked soya beans, as well as that of soya seedlings, grown in the presence of Rhizopus microsporus var. oryzae, were screened for the presence of prenylated flavonoids with a liquid chromatography/mass spectrometry (LC/MS)-based screening method. The glyceollins I-III and glyceollidins I-II, belonging to the isoflavonoid subclass of the pterocarpans, were tentatively assigned. The formation of these prenylated pterocarpans was accompanied by that of other prenylated isoflavonoids of the subclasses of the isoflavones and the coumestans. It was estimated that approx. 40% of the total isoflavonoid content in Rhizopus-challenged soya bean seedlings were prenylated pterocarpans, whereas 7% comprised prenylated isoflavones and prenylated coumestans. The site of prenylation (A-ring or B-ring) of the prenylated isoflavones was tentatively annotated using positive-ion mode MS by comparing the (1,3) A(+) retro-Diels-Alder (RDA) fragments of prenylated and non-prenylated isoflavones. Furthermore, the fragmentation pathways of the five pterocarpans in negative-ion (NI) mode were proposed, which involved the cleavage of the C-ring and/or D-ring. The absence of the ring-closed prenyl (pyran or furan) gave exclusively -H(2) O(x,y) RDA fragments, whereas its presence gave predominantly the common RDA fragments.

Flavone synthase II (CYP93B16) from soybean (Glycine max L.)

Flavonoids are a very diverse group of plant secondary metabolites with a wide array of activities in plants, as well as in nutrition and health. All flavonoids are derived from a limited number of flavanone intermediates, which serve as substrates for a variety of enzyme activities, enabling the generation of diversity in flavonoid structures. Flavonoids can be characteristic metabolites, like isoflavonoids for legumes. Others, like flavones, occur in nearly all plants. Interestingly, there exist two fundamentally different enzymatic systems able to directly generate flavones from flavanones, flavone synthase (FNS) I and II. We describe an inducible flavone synthase activity from soybean (Glycine max) cell cultures, generating 7,4'-dihydroxyflavone (DHF), which we classified as FNS II. The corresponding full-length cDNA (CYP93B16) was isolated using known FNS II sequences from other plants. Functional expression in yeast allowed the detailed biochemical characterization of the catalytic activity of FNS II. A direct conversion of flavanones such as liquiritigenin, naringenin, and eriodictyol into the corresponding flavones DHF, apigenin and luteolin, respectively, was demonstrated. The enzymatic reaction of FNSII was stereoselective, favouring the (S)- over the (R)-enantiomer. Phylogenetic analyses of the subfamily of plant CYP93B enzymes indicate the evolution of a gene encoding a flavone synthase which originally catalyzed the direct conversion of flavanones into flavones, via early gene duplication into a less efficient enzyme with an altered catalytic mechanism. Ultimately, this allowed the evolution of the legume-specific isoflavonoid synthase activity.

Spodoptera littoralis-induced lectin expression in tobacco

The induced defense response in plants towards herbivores is mainly regulated by jasmonates and leads to the accumulation of so-called jasmonate-induced proteins. Recently, a jasmonate (JA) inducible lectin called Nicotiana tabacum agglutinin or NICTABA was discovered in tobacco (N. tabacum cv Samsun) leaves. Tobacco plants also accumulate the lectin after insect attack by caterpillars. To study the functional role of NICTABA, the accumulation of the JA precursor 12-oxophytodienoic acid (OPDA), JA as well as different JA metabolites were analyzed in tobacco leaves after herbivory by larvae of the cotton leafworm (Spodoptera littoralis) and correlated with NICTABA accumulation. It was shown that OPDA, JA as well as its methyl ester can trigger NICTABA accumulation. However, hydroxylation of JA and its subsequent sulfation and glucosylation results in inactive compounds that have lost the capacity to induce NICTABA gene expression. The expression profile of NICTABA after caterpillar feeding was recorded in local as well as in systemic leaves, and compared to the expression of several genes encoding defense proteins, and genes encoding a tobacco systemin and the allene oxide cyclase, an enzyme in JA biosynthesis. Furthermore, the accumulation of NICTABA was quantified after S. littoralis herbivory and immunofluorescence microscopy was used to study the localization of NICTABA in the tobacco leaf.

Fatty Acid-derived signals in plant defense

Fatty acids (FAs) consist of long hydrophobic, often unbranched chains of hydrocarbons, with hydrophilic carboxylic acid groups at one end. They are an important source of reserve energy and essential components of membrane lipids in all living organisms. In plants, FA metabolic pathways play significant roles in pathogen defense. Historically, FAs were only assigned passive roles in plant defense such as biosynthetic precursors for cuticular components or the phytohormone jasmonic acid. However, recent discoveries demonstrate more direct roles for FAs and their breakdown products in inducing various modes of plant defenses. Both 16- and 18-carbon FAs participate in defense to modulate basal, effector-triggered, and systemic immunity in plants. Studies of FA metabolic mutants also reveal an active signaling role for the cuticle in plant defense. This review summarizes the current knowledge of the involvement of FAs, FA-derived oxylipins, and enzymes catalyzing FA metabolism in plant defense.

Investigation of downstream signals of the soybean autoregulation of nodulation receptor kinase GmNARK

The Glycine max nodule autoregulation receptor kinase (GmNARK) plays a central role in the systemic signal transduction pathway controlling nodulation in soybean. We used transcriptional profiling to identify potential downstream signals of this receptor kinase. These studies revealed that GmNARK-mediated signaling controls the expression of genes involved in the jasmonic acid (JA) pathway. Genes encoding the key enzymes controlling JA biosynthesis as well as JA-response genes were regulated systemically but not locally by root inoculation with Bradyrhizobium japonicum. This systemic regulation was abolished in Gmnark mutant plants, indicating that their expression was specifically controlled by signaling events associated with this receptor kinase. Foliar application of a JA biosynthesis inhibitor significantly reduced nodulation specifically in supernodulating mutant plants. These results indicate that the receptor-mediated regulation of JA signaling plays an important role in the AON signal transduction pathway. A second class of genes was identified that were controlled by GmNARK in a rhizobia-independent manner. These candidates provide insight on additional, nonsymbiotic signaling pathways that are likely regulated by GmNARK, such as those involved in root growth and defense. The discovery of downstream components of the GmNARK receptor kinase advances our understanding of the systemic control of nodule development and its association with other signaling networks.

Microbial oligosaccharides differentially induce volatiles and signalling components in Medicago truncatula

Plants perceive biotic stimuli by recognising a multitude of different signalling compounds originating from the interacting organisms. Some of these substances represent pathogen-associated molecular patterns, which act as general elicitors of defence reactions. But also beneficial microorganisms like rhizobia take advantage of compounds structurally related to certain elicitors, i.e. Nod-factors, to communicate their presence to the host plant. In a bioassay-based study we aimed to determine to what extent distinct oligosaccharidic signals are able to elicit overlapping responses, including the emission of volatile organic compounds which is mainly considered a typical mode of inducible indirect defence against herbivores. The model legume Medicago truncatula Gaertn. was challenged with pathogen elicitors (beta-(1,3)-beta-(1,6)-glucans and N,N',N'',N'''-tetraacetylchitotetraose) and two Nod-factors, with one of them being able to induce a nodulation response in M. truncatula. Single oligosaccharidic elicitors caused the emission of volatile organic compounds, mainly sesquiterpenoids. The volatile blends detected were quite characteristic for the applied compounds, which could be pinpointed by multivariate statistical methods. As potential mediators of this response, the levels of jasmonic acid and salicylic acid were determined. Strikingly, neither of these phytohormones exhibited changing levels correlating with enhanced volatile emission. All stimuli tested caused an overproduction of reactive oxygen species, whereas nitric oxide accumulation was only effected by elicitors that were equally able to induce volatile emission. Thus, all signalling compounds tested elicited distinct reaction patterns. However, similarities between defence reactions induced by herbivory and pathogen-derived elicitors could be ascertained; but also Nod-factors were able to trigger defence-related reactions.

Identification of a multigene family encoding putative beta-glucan-binding proteins in Medicago truncatula

Branched 1,6-1,3-beta-glucans from Phytophthora sojae cell walls represent pathogen-associated molecular patterns (PAMPs) that have been shown to mediate the activation of plant defence reactions in many legumes. In soybean, a receptor protein complex containing a high affinity beta-glucan-binding protein (GBP) was identified and investigated in detail. In the model legume Medicago truncatula, used for functional genomic studies of various plant-microbe interactions, a high-affinity beta-glucan-binding site was characterized biochemically. However, to date, none of the genes encoding GBPs from M. truncatula have been described. Here, we report the identification of four full-length clones encoding putative beta-glucan-binding proteins from M. truncatula, MtGBP1, 2, 3, and 4, composing a multigene family encoding GBP-related proteins in this plant. Differences in expression patterns as well as in regulation on treatment with two different biotic elicitors are demonstrated for the members of the GBP family and for a selection of defence-related genes.

Activation of members of a MAPK module in beta-glucan elicitor-mediated non-host resistance of soybean

Plants recognize microbial pathogens by discriminating pathogen-associated molecular patterns from self-structures. We study the non-host disease resistance of soybean (Glycine max L.) to the oomycete, Phytophthora sojae. Soybean senses a specific molecular pattern consisting of a branched heptaglucoside that is present in the oomycetal cell walls. Recognition of this elicitor may be achieved through a beta-glucan-binding protein, which forms part of a proposed receptor complex. Subsequently, soybean mounts a complex defense response, which includes the increase of the cytosolic calcium concentration, the production of reactive oxygen species, and the activation of genes responsible for the synthesis of phytoalexins. We now report the identification of two mitogen-activated protein kinases (MAPKs) and one MAPK kinase (MAPKK) that may function as signaling elements in triggering the resistance response. The use of specific antisera enabled the identification of GmMPKs 3 and 6 whose activity is enhanced within the signaling pathway leading to defense reactions. Elicitor specificity of MAPK activation as well as the sensitivity against inhibitors suggested these kinases as part of the beta-glucan signal transduction pathway. An upstream GmMKK1 was identified based on sequence similarity to other plant MAPKKs and its interaction with the MAPKs was analyzed. Recombinant GmMKK1 interacted predominantly with GmMPK6, with concomitant phosphorylation of the MAPK protein. Moreover, a preferential physical interaction between GmMKK1 and GmMPK6 was demonstrated in yeast. These results suggest a role of a MAPK cascade in mediating beta-glucan signal transduction in soybean, similar to other triggers that activate MAPKs during innate immune responses in plants.

Identification of a peroxisomal acyl-activating enzyme involved in the biosynthesis of jasmonic acid in Arabidopsis

Jasmonic acid (JA) is a lipid-derived signal that regulates a wide variety of developmental and defense-related processes in higher plants. JA is synthesized from linolenic acid via an enzymatic pathway that initiates in the plastid and terminates in peroxisomes. The C18 JA precursor 12-oxo-phytodienoic acid (OPDA) is converted in the peroxisome to 3-oxo-2-(2'-[Z]-pentenyl)cyclopentane-1-octanoic acid (OPC-8:0), which subsequently undergoes three rounds of beta-oxidation to yield JA. Although most JA biosynthetic enzymes have been identified, several key steps in the pathway remain to be elucidated. To address this knowledge gap, we employed co-expression analysis to identify genes that are coordinately regulated with known JA biosynthetic components in Arabidopsis. Among the candidate genes uncovered by this approach was a 4-coumarate-CoA ligase-like member of the acyl-activating enzyme (AAE) gene family, which we have named OPC-8:0 CoA Ligase1 (OPCL1). In response to wounding, opcl1 null mutants exhibited reduced levels of JA and hyperaccumulation of OPC-8:0. Recombinant OPCL1 was active against both OPDA and OPC-8:0, as well as medium-to-long straight-chain fatty acids. Subcellular localization studies with green fluorescent protein-tagged OPCL1 showed that the protein is targeted to peroxisomes. These findings establish a physiological role for OPCL1 in the activation of JA biosynthetic precursors in leaf peroxisomes, and further indicate that OPC-8:0 is a physiological substrate for the activation step. The results also demonstrate the utility of co-expression analysis for identification of factors that contribute to jasmonate homeostasis.

Enhancement of lignan biosynthesis in suspension cultures of Linum nodiflorum by coronalon, indanoyl-isoleucine and methyl jasmonate

The effect of the two synthetic elicitors coronalon and indanoyl-isoleucine and of methyl jasmonate (MeJA) on the accumulation and biosynthesis of lignans by cell suspension cultures of Linum nodiflorum (Linaceae) was investigated. The production of 6-methoxypodophyllotoxin (MPTOX) could be increased more than tenfold, the maximal content reaching up to over 2.5% of the cell dry weight. The highest yield was achieved by administering 50 microM of the synthetic elicitors on the fourth day and extracting the products on the tenth day of the culture period. An additional lignan accumulated in elicitor-treated cultures. Its structure was elucidated by extensive 1D and 2D NMR measurements, revealing its identity as 5'-demethoxy-MPTOX (5'-dMPTOX). Its average content amounted up to over 5% of the cell dry weight. Growth was only slightly affected by the addition of the elicitors. Methyl jasmonate exerted a moderate stimulating effect on the L. nodiflorum cells with MPTOX and 5'-dMPTOX contents going up to 1.4 and 2.1% of the cell dry weight, respectively. The activities of deoxypodophyllotoxin 6-hydroxylase and beta-peltatin 6-O-methyltransferase, two enzymes involved in MPTOX biosynthesis, were increased up to 21.9-fold and 14.6-fold, respectively, in the treated cultures.

Elicitor signal transduction leading to production of plant secondary metabolites

Plant secondary metabolites are unique sources for pharmaceuticals, food additives, flavors, and other industrial materials. Accumulation of such metabolites often occurs in plants subjected to stresses including various elicitors or signal molecules. Understanding signal transduction paths underlying elicitor-induced production of secondary metabolites is important for optimizing their commercial production. This paper summarizes progress made on several aspects of elicitor signal transduction leading to production of plant secondary metabolites, including: elicitor signal perception by various receptors of plants; avirulence determinants and corresponding plant R proteins; heterotrimeric and small GTP binding proteins; ion fluxes, especially Ca2+ influx, and Ca2+ signaling; medium alkalinization and cytoplasmic acidification; oxidative burst and reactive oxygen species; inositol trisphosphates and cyclic nucleotides (cAMP and cGMP); salicylic acid and nitric oxide; jasmonate, ethylene, and abscisic acid signaling; oxylipin signals such as allene oxide synthase-dependent jasmonate and hydroperoxide lyase-dependent C12 and C6 volatiles; as well as other lipid messengers such as lysophosphatidylcholine, phosphatidic acid, and diacylglycerol. All these signal components are employed directly or indirectly by elicitors for induction of plant secondary metabolite accumulation. Cross-talk between different signaling pathways is very common in plant defense response, thus the cross-talk amongst these signaling pathways, such as elicitor and jasmonate, jasmonate and ethylene, and each of these with reactive oxygen species, is discussed separately. This review also highlights the integration of multiple signaling pathways into or by transcription factors, as well as the linkage of the above signal components in elicitor signaling network through protein phosphorylation and dephosphorylation. Some perspectives on elicitor signal transduction and plant secondary metabolism at the transcriptome and metabolome levels are also presented.

Isolation of an N-alkylated benzylamine derivative from Pseudomonas putida BTP1 as elicitor of induced systemic resistance in bean

Root treatment of Phaseolus vulgaris with the nonpathogenic Pseudomonas putida BTP1 led to significant reduction of the disease caused by the pathogen Botrytis cinerea on leaves. The molecular determinant of P. putida BTP1 mainly responsible for the induced systemic resistance (ISR) was isolated from cell-free culture fluid after growth of the strain in the iron-poor casamino acid medium. Mass spectrometry analyses performed on both the bacterial product and synthetic analogues revealed a polyalkylated benzylamine structure, with the quaternary ammonium substituted by methyl, ethyl, and C13 aliphatic groups responsible for the relative hydrophobicity of the molecule. The specific involvement of the N-alkylated benzylamine derivative (NABD) in ISR elicitation was first evidenced by testing the purified compound that mimicked the protective effect afforded by crude supernatant samples. The evidence was supported by the loss of elicitor activity of mutants impaired in NABD biosynthesis. Our experiments also showed that other iron-regulated metabolites secreted by the strain are not involved in ISR stimulation. Thus, these results indicate a wider variety of Pseudomonas determinants for ISR than reported to date.

Transcriptional profiling of sorghum induced by methyl jasmonate, salicylic acid, and aminocyclopropane carboxylic acid reveals cooperative regulation and novel gene responses

We have conducted a large-scale study of gene expression in the C4 monocot sorghum (Sorghum bicolor) L. Moench cv BTx623 in response to the signaling compounds salicylic acid (SA), methyl jasmonate (MeJA), and the ethylene precursor aminocyclopropane carboxylic acid. Expression profiles were generated from seedling root and shoot tissue at 3 and 27 h, using a microarray containing 12,982 nonredundant elements. Data from 102 slides and quantitative reverse transcription-PCR data on mRNA abundance from 171 genes were collected and analyzed and are here made publicly available. Numerous gene clusters were identified in which expression was correlated with particular signaling compound and tissue combinations. Many genes previously implicated in defense responded to the treatments, including numerous pathogenesis-related genes and most members of the phenylpropanoid pathway, and several other genes that may represent novel activities or pathways. Genes of the octadecanoic acid pathway of jasmonic acid (JA) synthesis were induced by SA as well as by MeJA. The resulting hypothesis that increased SA could lead to increased endogenous JA production was confirmed by measurement of JA content. Comparison of responses to SA, MeJA, and combined SA+MeJA revealed patterns of one-way and mutual antagonisms, as well as synergistic effects on regulation of some genes. These experiments thus help further define the transcriptional results of cross talk between the SA and JA pathways and suggest that a subset of genes coregulated by SA and JA may comprise a uniquely evolved sector of plant signaling responsive cascades.

The oxylipin signal jasmonic acid is activated by an enzyme that conjugates it to isoleucine in Arabidopsis

Despite its importance in a variety of plant defense responses, our understanding of how jasmonic acid (JA) functions at the biochemical level is limited. Several amino acid conjugates of JA were tested for their ability to complement the JA-insensitive Arabidopsis thaliana mutant jar1-1. Unlike free JA, JA-Ile inhibited root growth in jar1-1 to the same extent as in the wild type, whereas JA-Val, JA-Leu, and JA-Phe were ineffective inhibitors in both genotypes. Thin-layer chromatography and gas chromatography-mass spectrometry (GC-MS) analysis of products produced in vitro by recombinant JAR1 demonstrated that this enzyme forms JA-amido conjugates with several amino acids, including JA-Ile. JA-Val, -Leu, -Ile, and -Phe were each quantified in Arabidopsis seedlings by GC-MS. JA-Ile was found at 29.6 pmole g(-1) fresh weight (FW) in the wild type but was more than sevenfold lower in two jar1 alleles. JA-Leu, -Val, and -Phe were present at only low levels in both genotypes. Expression of wild-type JAR1 in transgenic jar1-1 plants restored sensitivity to JA and elevated JA-Ile to the same level as in the wild type. The ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) conjugated to JA was also found in plant tissue at 18.4 pmole g(-1) FW. JA-ACC was determined not be an effective jasmonate root inhibitor, and surprisingly, was twofold higher in the mutants than in the wild type. This suggests that another JA-conjugating enzyme(s) is present in Arabidopsis. Synthesis of JA-ACC might provide a mechanism to coregulate the availability of JA and ACC for conversion to the active hormones JA-Ile and ethylene, respectively. We conclude that JAR1 is a JA-amino synthetase that is required to activate JA for optimal signaling in Arabidopsis. Plant hormone activation by conjugation to amino acids and the enzymes involved in their formation were previously unknown.

Biotic and heavy metal stress response in plants: evidence for common signals

In higher plants, biotic stress (e.g., herbivore or pathogen attack) as well as abiotic stress (in particular heavy metals) often induce the synthesis and accumulation of the same defense-related secondary metabolites. This well-known finding still awaits an explanation regarding the common features of both stress types. In this study, a mechanism is proposed that links reactive oxygen species (ROS) generation with lipid oxidation processes, ultimately resulting in the formation of similar, highly active signalling compounds. The generation of ROS is a common event in both heavy metal treatment and biotic stress although it can depend on quite different, enzymatic and non-enzymatic reactions. Regardless, ROS are involved in the oxidation of unsaturated fatty acids which initiate the formation of oxylipins, a highly variable class of lipid-derived compounds in plants. Oxylipins represent new endogenous signals involved in biotic- and abiotic-induced stress responses.

Coronalon: a powerful tool in plant stress physiology

Coronalon, a synthetic 6-ethyl indanoyl isoleucine conjugate, has been designed as a highly active mimic of octadecanoid phytohormones that are involved in insect and disease resistance. The spectrum of biological activities that is affected by coronalon was investigated in nine different plant systems specifically responding to jasmonates and/or 12-oxo-phytodienoic acid. In all bioassays analyzed, coronalon demonstrated a general strong activity at low micromolar concentrations. The results obtained showed the induction of (i) defense-related secondary metabolite accumulation in both cell cultures and plant tissues, (ii) specific abiotic and biotic stress-related gene expression, and (iii) root growth retardation. The general activity of coronalon in the induction of plant stress responses together with its simple and efficient synthesis suggests that this compound might serve as a valuable tool in the examination of various aspects in plant stress physiology. Moreover, coronalon might become employed in agriculture to elicit plant resistance against various aggressors.

Diverse environmental cues transiently regulate OsOPR1 of the “octadecanoid pathway” revealing its importance in rice defense/stress and development

Previously, we reported rice (Oryza sativa L. japonica type cv. Nipponbare) allene oxide synthase (OsAOS) and cyclase (OsAOC) genes of the octadecanoid pathway. Here we have isolated a 12-oxo-phytodienoic acid reductase gene, called OsOPR1, encoding the last committed enzymatic step on the octadecanoid pathway leading to jasmonic acid (JA) biosynthesis. OsOPR1 encodes a 380 amino acid long polypeptide with a predicted molecular mass of 42465.02 and pI of 5.79, and belongs to a gene family in the rice genome. Transcriptional profiling using our established two-week-old rice seedling model system against a variety of environmental factors such as wounding, global signals (including JA), osmotic stresses, heavy metals, UV-C irradiation, fungal elicitor, protein phosphatase inhibitors, and gaseous pollutants indicated that OsOPR1 transcript was rapidly, transiently and differentially up-regulated within 30 min in leaves. Surprisingly, co-application of signaling molecules JA, salicylate and ethylene, resulted in a massive accumulation of the OsOPR1 transcript at 30 min and remained elevated with time, a new observation. Furthermore, transient expression of OsOPR1, most likely regulated by a de novo synthesized negative trans-acting factor(s), was evidenced by the use of cycloheximide. Finally, the endogenous OsOPR1 expression varied with the stage of plant development. These results strongly suggest a regulatory role for OsOPR1 in rice plant defense/stress response pathway(s) and reproduction.