Ergosterol triggers characteristic elicitation steps in Beta vulgaris leaf tissues

Sterols, pleiotropic players in plant-microbe interactions

Plant-microbe interactions (PMIs) are regulated through a wide range of mechanisms in which sterols from plants and microbes are involved in numerous ways, including recognition, transduction, communication, and/or exchanges between partners. Phytosterol equilibrium is regulated by PMIs through expression of genes involved in phytosterol biosynthesis, together with their accumulation. As such, PMI outcomes also include plasma membrane (PM) functionalization events, in which phytosterols have a central role, and activation of sterol-interacting proteins involved in cell signaling. In spite (or perhaps because) of such multifaceted abilities, an overall mechanism of sterol contribution is difficult to determine. However, promising approaches exploring sterol diversity, their contribution to PMI outcomes, and their localization would help us to decipher their crucial role in PMIs.

CuZn-superoxide dismutase is differentially modified in localization and expression by three abiotic stresses in miniature rose bushes

Three abiotic stresses, copper application (CS), mechanical rubbing (MS) and water deprivation (WS) applied on miniature rose bushes specifically activate the expression of the CuZn-Superoxide dismutase (SOD). The Cu/Zn-SOD protein immunodetected in the 4th internode was shown engaged in lignification in phloem, cambium and xylem cells. The SOD occurrence was detailed in the vessel associated cells (VACs), using immunogold labeling observed in transmission electron microscopy. The enzyme was detected in mitochondria, plastids, Golgi vesicles, endoplasmic reticulum and plasma membrane. In addition, in pit-fields without plasmodesmata linking vessel associated cells to vessels, the abiotic stresses increased the transfer apparatus volume. The content in unmethylatedpectins increased in wall ingrowths after CS and MS, but not in WS. In addition to the different localization, the SOD was differentially overexpressed according to the applied stress: an isoform detected at 17 kDa under CuSO4 application, two isoforms respectively detected at 20 and 17 kDa under MS and detected at 17 and 15 kDa under WS. Notably, the only 17 kDa isoform was detected in plasma membrane vesicles from plants submitted to the three stresses. Thus, by increasing the transfer apparatus development, the key role of VACs was emphasized in establishing an adaptative response to abiotic stresses, in miniature rose bushes. Additionally, it has been observed that the differential SOD localization under such stresses sustained the regulatory function of VACs in the transitory sink function of xylem.

The function of plant PR1 and other members of the CAP protein superfamily in plant-pathogen interactions

The pathogenesis-related (PR) proteins of plants have originally been identified as proteins that are strongly induced upon biotic and abiotic stress. These proteins fall into 17 distinct classes (PR1-PR17). The mode of action of most of these PR proteins has been well characterized, except for PR1, which belongs to a widespread superfamily of proteins that share a common CAP domain. Proteins of this family are not only expressed in plants but also in humans and in many different pathogens, including phytopathogenic nematodes and fungi. These proteins are associated with a diverse range of physiological functions. However, their precise mode of action has remained elusive. The importance of these proteins in immune defence is illustrated by the fact that PR1 overexpression in plants results in increased resistance against pathogens. However, PR1-like CAP proteins are also produced by pathogens and deletion of these genes results in reduced virulence, suggesting that CAP proteins can exert both defensive and offensive functions. Recent progress has revealed that plant PR1 is proteolytically cleaved to release a C-terminal CAPE1 peptide, which is sufficient to activate an immune response. The release of this signalling peptide is blocked by pathogenic effectors to evade immune defence. Moreover, plant PR1 forms complexes with other PR family members, including PR5, also known as thaumatin, and PR14, a lipid transfer protein, to enhance the host's immune response. Here, we discuss possible functions of PR1 proteins and their interactors, particularly in light of the fact that these proteins can bind lipids, which have important immune signalling functions.

Strain improvement of Trichoderma harzianum for enhanced biocontrol capacity: Strategies and prospects

In the control of plant diseases, biocontrol has the advantages of being efficient and safe for human health and the environment. The filamentous fungus Trichoderma harzianum and its closely related species can inhibit the growth of many phytopathogenic fungi, and have been developed as commercial biocontrol agents for decades. In this review, we summarize studies on T. harzianum species complex from the perspective of strain improvement. To elevate the biocontrol ability, the production of extracellular proteins and compounds with antimicrobial or plant immunity-eliciting activities need to be enhanced. In addition, resistance to various environmental stressors should be strengthened. Engineering the gene regulatory system has the potential to modulate a variety of biological processes related to biocontrol. With the rapidly developing technologies for fungal genetic engineering, T. harzianum strains with increased biocontrol activities are expected to be constructed to promote the sustainable development of agriculture.

Deciphering the role of plant plasma membrane lipids in response to invasion patterns: how could biology and biophysics help?

Plants have to constantly face pathogen attacks. To cope with diseases, they have to detect the invading pathogen as early as possible via the sensing of conserved motifs called invasion patterns. The first step of perception occurs at the plasma membrane. While many invasion patterns are perceived by specific proteinaceous immune receptors, several studies have highlighted the influence of the lipid composition and dynamics of the plasma membrane in the sensing of invasion patterns. In this review, we summarize current knowledge on how some microbial invasion patterns could interact with the lipids of the plasma membrane, leading to a plant immune response. Depending on the invasion pattern, different mechanisms are involved. This review outlines the potential of combining biological with biophysical approaches to decipher how plasma membrane lipids are involved in the perception of microbial invasion patterns.

Sterol Extraction from Isolated Plant Plasma Membrane Vesicles Affects H+-ATPase Activity and H+-Transport

Plasma membrane H+-ATPase is known to be detected in detergent-resistant sterol-enriched fractions, also called "raft" domains. Studies on H+-ATPase reconstituted in artificial or native membrane vesicles have shown both sterol-mediated stimulations and inhibitions of its activity. Here, using sealed isolated plasma membrane vesicles, we investigated the effects of sterol depletion in the presence of methyl-β-cyclodextrin (MβCD) on H+-ATPase activity. The rate of ATP-dependent ∆µH+ generation and the kinetic parameters of ATP hydrolysis were evaluated. We show that the relative sterols content in membrane vesicles decreased gradually after treatment with MβCD and reached approximately 40% of their initial level in 30 mM probe solution. However, changes in the hydrolytic and H+-transport activities of the enzyme were nonlinear. The extraction of up to 20% of the initial sterols was accompanied by strong stimulation of ATP-dependent H+-transport in comparison with the hydrolytic activity of enzymes. Further sterol depletion led to a significant inhibition of active proton transport with an increase in passive H+-leakage. The solubilization of control and sterol-depleted vesicles in the presence of dodecyl maltoside negated the differences in the kinetics parameters of ATP hydrolysis, and all samples demonstrated maximal hydrolytic activities. The mechanisms behind the sensitivity of ATP-dependent H+-transport to sterols in the lipid environment of plasma membrane H+-ATPase are discussed.

Role of ethylene and light in chitosan-induced local and systemic defence responses of tomato plants

Plant defence responses can be triggered by the application of elicitors for example chitosan (β-1,4-linked glucosamine; CHT). It is well-known that CHT induces rapid, local production of reactive oxygen species (ROS) and nitric oxide (NO) resulting in fast stomatal closure. Systemic defence responses are based primarily on phytohormones such as ethylene (ET) and salicylic acid (SA), moreover on the expression of hormone-mediated defence genes and proteins. At the same time, these responses can be dependent also on external factors, such as light but its role was less-investigated. Based on our result in intact tomato plants (Solanum lycopersicum L.), CHT treatment not only induced significant ET emission and stomatal closure locally but also promoted significant production of superoxide which was also detectable in the distal, systemic leaves. However, these changes in ET and superoxide accumulation were detected only in wild type (WT) plants kept in light and were inhibited under darkness as well as in ET receptor Never ripe (Nr) mutants suggesting pivotal importance of ET and light in inducing resistance both locally and systemically upon CHT. Interestingly, CHT-induced NO production was mostly independent of ET or light. At the same time, expression of Pathogenesis-related 3 (PR3) was increased locally in both genotypes in the light and in WT leaves under darkness. This was also observed in distal leaves of WT plants. The CHT-induced endoplasmic reticulum (ER) stress, as well as unfolded protein response (UPR) were examined for the first time, via analysis of the lumenal binding protein (BiP). Whereas local expression of BiP was not dependent on the availability of light or ET, systemically it was mediated by ET.

Identification of two compounds able to improve flax resistance towards Fusarium oxysporum infection

Plants are surrounded by a diverse range of microorganisms that causes serious crop losses and requires the use of pesticides. Flax is a major crop in Normandy used for its fibres and is regularly challenged by the pathogenic fungus Fusarium oxysporum (Fo) f. sp. lini. To protect themselves, plants use "innate immunity" as a first line of defense level against pathogens. Activation of plant defense with elicitors could be an alternative for crop plant protection. A previous work was conducted by screening a chemical library and led to the identification of compounds able to activate defense responses in Arabidopsis thaliana. Four compounds were tested for their abilities to improve resistance of two flax varieties against Fo. Two of them, one natural (holaphyllamine or HPA) and one synthetic (M4), neither affected flax nor Fo growth. HPA and M4 induced oxidative burst and callose deposition. Furthermore, HPA and M4 caused changes in the expression patterns of defense-related genes coding a glucanase and a chitinase-like. Finally, plants pre-treated with HPA or M4 exhibited a significant decrease in the disease symptoms. Together, these findings demonstrate that HPA and M4 are able to activate defense responses in flax and improve its resistance against Fo infection.

Identification of plant genes putatively involved in the perception of fungal ergosterol-squalene

Trichoderma biocontrol strains establish a complex network of interactions with plants, in which diverse fungal molecules are involved in the recognition of these fungi as nonpathogenic organisms. These molecules act as microbial-associated molecular patterns that trigger plant responses. Previous studies have reported the importance of ergosterol produced by Trichoderma spp. for the ability of these fungi to induce plant growth and defenses. In addition, squalene, a sterol biosynthetic intermediate, seems to play an important role in these interactions. Here, we analyzed the effect of different concentrations of ergosterol and squalene on tomato (Solanum lycopersicum) growth and on the transcription level of defense- and growth-related genes. We used an RNA-seq strategy to identify several tomato genes encoding predicted pattern recognition receptor proteins or WRKY transcription factors, both of which are putatively involved in the perception and response to ergosterol and squalene. Finally, an analysis of Arabidopsis thaliana mutants lacking the genes homologous to these tomato candidates led to the identification of a WRKY40 transcription factor that negatively regulates salicylic acid-related genes and positively regulates ethylene- and jasmonate-related genes in the presence of ergosterol and squalene.

Palmitvaccenic Acid (Δ11-cis-hexadecenoic acid) Is Synthesized by an OLE1-like Desaturase in the Arbuscular Mycorrhiza Fungus Rhizophagus irregularis

Arbuscular mycorrhiza (AM) fungi deliver mineral nutrients to the plant host in exchange for reduced carbon in the form of sugars and lipids. Colonization with AM fungi upregulates a specific host lipid synthesis pathway resulting in the production of fatty acids. Predominantly palmitic acid (16:0) and the unusual palmitvaccenic acid (16:1^Δ11cis) accumulate in the fungus Rhizophagus irregularis. Here, we present the isolation and characterization of RiOLE1-LIKE, the desaturase involved in palmitvaccenic acid synthesis, by heterologous expression in yeast and plants. Results are in line with the scenario in which RiOLE1-LIKE encodes an acyl-CoA desaturase with substrate specificity for C15-C18 acyl groups, in particular C16. Phylogenetic analysis of RiOLE1-LIKE-related sequences revealed that this gene is conserved in AM fungi from the Glomales and Diversisporales but is absent from nonsymbiotic Mortierellaceae and Mucoromycotina fungi, suggesting that 16:1^Δ11cis provides a specific function during AM colonization.

Apoplastic invasion patterns triggering plant immunity: plasma membrane sensing at the frontline

Plants are able to effectively cope with invading pathogens by activating an immune response based on the detection of invasion patterns (IPs) originating from the pathogen or released by the plant after infection. At a first level, this perception takes place at the plasma membrane through cell surface immune receptors and although the involvement of proteinaceous pattern recognition receptors (PRRs) is well established, increasing data are also pointing out the role of membrane lipids in the sensing of IPs. In this review, we discuss the evolution of various conceptual models describing plant immunity and present an overview of well-characterized IPs from different natures and origins. We summarize the current knowledge on how they are perceived by plants at the plasma membrane, highlighting the increasingly apparent diversity of sentinel-related systems in plants.

Analysis of Barley Leaf Epidermis and Extrahaustorial Proteomes During Powdery Mildew Infection Reveals That the PR5 Thaumatin-Like Protein TLP5 Is Required for Susceptibility Towards Blumeria graminis f. sp. hordei

Powdery mildews are biotrophic pathogens causing fungal diseases in many economically important crops, including cereals, which are affected by Blumeria graminis. Powdery mildews only invade the epidermal cell layer of leaf tissues, in which they form haustorial structures. Haustoria are at the center of the biotrophic interaction by taking up nutrients from the host and by delivering effectors in the invaded cells to jeopardize plant immunity. Haustoria are composed of a fungal core delimited by a haustorial plasma membrane and cell wall. Surrounding these is the extrahaustorial complex, of which the extrahaustorial membrane is of plant origin. Although haustoria transcriptomes and proteomes have been investigated for Blumeria, the proteomes of barley epidermis upon infection and the barley components of the extrahaustorial complex remains unexplored. When comparing proteomes of infected and non-infected epidermis, several classical pathogenesis-related (PR) proteins were more abundant in infected epidermis. These included peroxidases, chitinases, cysteine-rich venom secreted proteins/PR1 and two thaumatin-like PR5 protein isoforms, of which TLP5 was previously shown to interact with the Blumeria effector BEC1054 (CSEP0064). Against expectations, transient TLP5 gene silencing suggested that TLP5 does not contribute to resistance but modulates susceptibility towards B. graminis. In a second proteomics comparison, haustorial structures were enriched from infected epidermal strips to identify plant proteins closely associated with the extrahaustorial complex. In these haustoria-enriched samples, relative abundances were higher for several V-type ATP synthase/ATPase subunits, suggesting the generation of proton gradients in the extrahaustorial space. Other haustoria-associated proteins included secreted or membrane proteins such as a PIP2 aquaporin, an early nodulin-like protein 9, an aspartate protease and other proteases, a lipase, and a lipid transfer protein, all of which are potential modulators of immunity, or the targets of pathogen effectors. Moreover, the ER BIP-like HSP70, may link ER stress responses and the idea of ER-like properties previously attributed to the extrahaustorial membrane. This initial investigation exploring the barley proteomes of Blumeria-infected tissues and haustoria, associated with a transient gene silencing approach, is invaluable to gain first insight of key players of resistance and susceptibility.

Involvement of Phosphatidylserine and Triacylglycerol in the Response of Sweet Potato Leaves to Salt Stress

Lipid remodeling plays an important role in the adaptation of plants to environmental factors, but the mechanism by which lipid remodeling mediates salt stress response remains unclear. In this study, we compared the root and leaf lipidome profiles of salt-tolerant and salt-sensitive sweet potato cultivars (Xu 22 and Xu 32, respectively) under salinity stress. After salt treatment, the leaf lipidome showed more significant remodeling than the root lipidome in both cultivars. Compared with Xu 32 leaves, Xu 22 leaves generally maintained higher abundance of phospholipids, glycolipids, sphingolipids, sterol derivatives, and diacylglycerol under salinity conditions. Interestingly, salinity stress significantly increased phosphatidylserine (PS) abundance in Xu 22 leaves by predominantly triggering the increase of PS (20:5/22:6). Furthermore, Xu 32 leaves accumulated higher triacylglycerol (TG) level than Xu 22 leaves under salinity conditions. The exogenous application of PS delayed salt-induced leaf senescence in Xu 32 by reducing salt-induced K+ efflux and upregulating plasma membrane H+-ATPase activity. However, the inhibition of TG mobilization in salinized-Xu 22 leaves disturbed energy and K+/Na+ homeostasis, as well as plasma membrane H+-ATPase activity. These results demonstrate alterations in the leaf lipidome of sweet potato under salinity condition, underscoring the importance of PS and TG in mediating salt-defensive responses in sweet potato leaves.

Identification of Candidate Ergosterol-Responsive Proteins Associated with the Plasma Membrane of Arabidopsis thaliana

The impact of fungal diseases on crop production negatively reflects on sustainable food production and overall economic health. Ergosterol is the major sterol component in fungal membranes and regarded as a general elicitor or microbe-associated molecular pattern (MAMP) molecule. Although plant responses to ergosterol have been reported, the perception mechanism is still unknown. Here, Arabidopsis thaliana protein fractions were used to identify those differentially regulated following ergosterol treatment; additionally, they were subjected to affinity-based chromatography enrichment strategies to capture and categorize ergosterol-interacting candidate proteins using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Mature plants were treated with 250 nM ergosterol over a 24 h period, and plasma membrane-associated fractions were isolated. In addition, ergosterol was immobilized on two different affinity-based systems to capture interacting proteins/complexes. This resulted in the identification of defense-related proteins such as chitin elicitor receptor kinase (CERK), non-race specific disease resistance/harpin-induced (NDR1/HIN1)-like protein, Ras-related proteins, aquaporins, remorin protein, leucine-rich repeat (LRR)- receptor like kinases (RLKs), G-type lectin S-receptor-like serine/threonine-protein kinase (GsSRK), and glycosylphosphatidylinositol (GPI)-anchored protein. Furthermore, the results elucidated unknown signaling responses to this MAMP, including endocytosis, and other similarities to those previously reported for bacterial flagellin, lipopolysaccharides, and fungal chitin.

Evaluation of λ-Carrageenan, CpG-ODN, Glycine Betaine, Spirulina platensis, and Ergosterol as Elicitors for Control of Zymoseptoria tritici in Wheat

Wheat crops are constantly challenged by the pathogen Zymoseptoria tritici, responsible for Septoria tritici Blotch (STB) disease. The present study reports the evaluation of five elicitor compounds (λ-carrageenan, cytosine-phosphate-guanine oligodesoxynucleotide motifs [CpG ODN], glycine betaine, Spirulina platensis, and ergosterol) for the protection of wheat against STB in order to offer new alternative tools to farmers for sustainable crop protection. Screening of elicitors of wheat defenses was carried out through a succession of experiments: biocidal in vitro tests enabled checking for any fungicidal activities, glasshouse experiments allowed determination of the efficacy of a given compound in protecting wheat against STB, and quantitative reverse-transcription polymerase chain reaction biomolecular tests investigated the relative expression of 23 defense genes in treated versus untreated plants. Therefore, we demonstrated that λ-carrageenan, CpG-ODN, glycine betaine, S. platensis, and ergosterol are potential elicitors of wheat defenses. Foliar treatment with these compounds conferred protection of wheat by up to approximately 70% against Z. tritici under semicontrolled conditions and induced both salicylic acid- and jasmonic acid-dependent signaling pathways in the plant. These findings contribute to extending the narrow list of potential elicitors of wheat defenses against Z. tritici.

Holaphyllamine, a steroid, is able to induce defense responses in Arabidopsis thaliana and increases resistance against bacterial infection

MAIN CONCLUSION

A chemical screen of plant-derived compounds identified holaphyllamine, a steroid, able to trigger defense responses in Arabidopsis thaliana and improve resistance against the pathogenic bacterium Pseudomonas syringae pv tomato DC3000. A chemical screen of 1600 plant-derived compounds was conducted and allowed the identification of a steroid able to activate defense responses in A. thaliana at a concentration of 1 µM without altering growth. The identified compound is holaphyllamine (HPA) whose chemical structure is similar to steroid pregnanes of mammals. Our data show that HPA, which is not constitutively present in A. thaliana, is able to trigger the formation of reactive oxygen species, deposition of callose and expression of several pathogenesis-related genes of the salicylic and jasmonic acid pathways. In addition, the results show that pre-treatment of A. thaliana seedlings with HPA before infection with the pathogenic bacterium Pseudomonas syringae pv tomato DC3000 results in a significant reduction of symptoms (i.e., reduction of bacterial colonies). Using A. thaliana mutants, we have found that the activation of defense responses by HPA does not depend on BRI1/BAK1 receptor kinases. Finally, a structure/function study reveals that the minimal structure required for activity is a 5-pregnen-20-one steroid with an equatorial nucleophilic group in C-3. Together, these findings demonstrate that HPA can activate defense responses that lead to improved resistance against bacterial infection in A. thaliana.

Rapid turnover of effectors in grass powdery mildew (Blumeria graminis)

Background

Grass powdery mildew (Blumeria graminis, Ascomycota) is a major pathogen of cereal crops and has become a model organism for obligate biotrophic fungal pathogens of plants. The sequenced genomes of two formae speciales (ff.spp.), B.g. hordei and B.g. tritici (pathogens of barley and wheat), were found to be enriched in candidate effector genes (CEGs). Similar to other filamentous pathogens, CEGs in B. graminis are under positive selection. Additionally, effectors are more likely to have presence-absence polymorphisms than other genes among different strains.

Results

Here we identified effectors in the genomes of three additional host-specific lineages of B. graminis (B.g. poae, B.g. avenae and B.g. infecting Lolium) which diverged between 24 and 5 million years ago (Mya). We found that most CEGs in B. graminis are clustered in families and that most families are present in both reference genomes (B.g. hordei and B.g. tritici) and in the genomes of all three newly annotated lineages. We identified conserved protein domains including a novel lipid binding domain. The phylogenetic analysis showed that frequent gene duplications and losses shaped the diversity of the effector repertoires of the different lineages through their evolutionary history. We observed several lineage-specific expansions where large clades of CEGs originated in only one lineage from a single gene through repeated gene duplications. When we applied a birth-death model we found that the turnover rate (the rate at which genes are deleted and duplicated) of CEG families is much higher than for non-CEG families. The analysis of genomic context revealed that the immediate surroundings of CEGs are enriched in transposable elements (TE) which could play a role in the duplication and deletion of CEGs.

Conclusions

The CEG repertoires of related pathogens diverged dramatically in short evolutionary times because of rapid turnover and of positive selection fixing non-synonymous mutations. While signatures of positive selection on effector sequences are the expected outcome of the evolutionary “arms race” between pathogen and plant immune system, it is more difficult to infer the mechanisms and evolutionary forces that maintained an extreme turnover rate in CEG families of B. graminis for several millions of years.

Electronic supplementary material

The online version of this article (10.1186/s12862-017-1064-2) contains supplementary material, which is available to authorized users.

Triterpene derivatives from Euphorbia enhance resistance against Verticillium wilt of tomato

Oxidation of α-euphorbol and 31-norlanostenol, two triterpenic compounds isolated from the latex of Euphorbia resinifera and Euphorbia officinarum respectively, yielded four products named 3β-tosyloxy-4α,14α-dimethyl-5α-cholesta-7,9-diene; 4α,14α-dimethyl-5α-cholesta-7,9-dien-3β-ol; 24-methylen-elemo-lanosta-8,24-dien-3-one and elemo-lanost-8-en-3,11,24-trione. They were evaluated for protection of tomato plants against Verticillium dahliae in a greenhouse. The four semisynthesized products were phytotoxic at higher concentrations as they completely inhibited tomato germination at 100 and 500 μg/ml. However at lower concentrations (10 and 50 μg/ml) germination and root length were not affected. Disease resistance against Verticillium wilt was assessed in tomato plants derived from seeds that germinated in the presence of 10 and 50 μg/ml of the four products. All of them were able to reduce significantly disease severity, with 10 μg/ml being more effective than 50 μg/ml. Reduction of leaf alteration index and of stunting index ranged from 52 to 68% and from 43 to 67%, respectively, while vessel discoloration was reduced by at least 95%. The compounds were also able to elicit H₂O₂ accumulation before and after fungal inoculation and to significantly enhance peroxidase and polyphenol oxidase activities. These results suggest that the hemisynthetized triterpenes can be used as elicitors of disease resistance.

Lipids in plant-microbe interactions

Bacteria and fungi can undergo symbiotic or pathogenic interactions with plants. Membrane lipids and lipid-derived molecules from the plant or the microbial organism play important roles during the infection process. For example, lipids (phospholipids, glycolipids, sphingolipids, sterol lipids) are involved in establishing the membrane interface between the two organisms. Furthermore, lipid-derived molecules are crucial for intracellular signaling in the plant cell, and lipids serve as signals during plant-microbial communication. These signal lipids include phosphatidic acid, diacylglycerol, lysophospholipids, and free fatty acids derived from phospholipase activity, apocarotenoids, and sphingolipid breakdown products such as ceramide, ceramide-phosphate, long chain base, and long chain base-phosphate. Fatty acids are the precursors for oxylipins, including jasmonic acid, and for azelaic acid, which together with glycerol-3-phosphate are crucial for the regulation of systemic acquired resistance. This article is part of a Special Issue titled "Plant Lipid Biology," guest editors Kent Chapman and Ivo Feussner.

Trichodiene Production in a Trichoderma harzianum erg1-Silenced Strain Provides Evidence of the Importance of the Sterol Biosynthetic Pathway in Inducing Plant Defense-Related Gene Expression

Trichoderma species are often used as biocontrol agents against plant-pathogenic fungi. A complex molecular interaction occurs among the biocontrol agent, the antagonistic fungus, and the plant. Terpenes and sterols produced by the biocontrol fungus have been found to affect gene expression in both the antagonistic fungus and the plant. The terpene trichodiene (TD) elicits the expression of genes related to tomato defense and to Botrytis virulence. We show here that TD itself is able to induce the expression of Botrytis genes involved in the synthesis of botrydial (BOT) and also induces terpene gene expression in Trichoderma spp. The terpene ergosterol, in addition to its role as a structural component of the fungal cell membranes, acts as an elicitor of defense response in plants. In the present work, using a transformant of T. harzianum, which is silenced in the erg1 gene and accumulates high levels of squalene, we show that this ergosterol precursor also acts as an important elicitor molecule of tomato defense-related genes and induces Botrytis genes involved in BOT biosynthesis, in both cases, in a concentration-dependent manner. Our data emphasize the importance of a balance of squalene and ergosterol in fungal interactions as well as in the biocontrol activity of Trichoderma spp.

Plant lipid environment and membrane enzymes: the case of the plasma membrane H+-ATPase

Several lipid classes constitute the universal matrix of the biological membranes. With their amphipathic nature, lipids not only build the continuous barrier that confers identity to every cell and organelle, but they are also active actors that modulate the activity of the proteins immersed in the lipid bilayer. The plasma membrane H(+)-ATPase, an enzyme from plant cells, is an excellent example of a transmembrane protein whose activity is influenced by the hydrophilic compartments at both sides of the membrane and by the hydrophobic domains of the lipid bilayer. As a result, an extensive documentation of the effect of numerous amphiphiles in the enzyme activity can be found. Detergents, membrane glycerolipids, and sterols can produce activation or inhibition of the enzyme activity. In some cases, these effects are associated with the lipids of the membrane bulk, but in others, a direct interaction of the lipid with the protein is involved. This review gives an account of reports related to the action of the membrane lipids on the H(+)-ATPase activity.

Influence of Rhizoctonia solani and Trichoderma spp. in growth of bean (Phaseolus vulgaris L.) and in the induction of plant defense-related genes

Many Trichoderma species are well-known for their ability to promote plant growth and defense. We study how the interaction of bean plants with R. solani and/or Trichoderma affect the plants growth and the level of expression of defense-related genes. Trichoderma isolates were evaluated in vitro for their potential to antagonize R. solani. Bioassays were performed in climatic chambers and development of the plants was evaluated. The effect of Trichoderma treatment and/or R. solani infection on the expression of bean defense-related genes was analyzed by real-time PCR and the production of ergosterol and squalene was quantified. In vitro growth inhibition of R. solani was between 86 and 58%. In in vivo assays, the bean plants treated with Trichoderma harzianum T019 always had an increased size respect to control and the plants treated with this isolate did not decrease their size in presence of R. solani. The interaction of plants with R. solani and/or Trichoderma affects the level of expression of seven defense-related genes. Squalene and ergosterol production differences were found among the Trichoderma isolates, T019 showing the highest values for both compounds. T. harzianum T019 shows a positive effect on the level of resistance of bean plants to R. solani. This strain induces the expression of plant defense-related genes and produces a higher level of ergosterol, indicating its ability to grow at a higher rate in the soil, which would explain its positive effects on plant growth and defense in the presence of the pathogen.

Ergosterol, an orphan fungal microbe-associated molecular pattern (MAMP)

Fungal pathogens continue to pose a significant threat to crop production and food supply. The early stages of plant-fungus interactions are mostly mediated by microbe-associated molecular pattern (MAMP) molecules, perceived by plant pattern recognition receptors (PRRs). Currently, the identified fungal MAMP molecules include chitin, chitosan, β-glucans, elicitins and ergosterol. Although the molecular battles between host plants and infecting fungal phytopathogens have been studied extensively, many aspects still need to be investigated to obtain a holistic understanding of the intrinsic mechanisms, which is paramount in combating fungal plant diseases. Here, an overview is given of the most recent findings concerning an 'orphan' fungal MAMP molecule, ergosterol, and we present what is currently known from a synopsis of different genes, proteins and metabolites found to play key roles in induced immune responses in plant-fungus interactions. Clearly, integrative investigations are still needed to provide a comprehensive systems-based understanding of the dynamics associated with molecular mechanisms in plant-ergosterol interactions and associated host responses.

Novel aspinolide production by Trichoderma arundinaceum with a potential role in Botrytis cinerea antagonistic activity and plant defence priming

Harzianum A (HA), a trichothecene produced by Trichoderma arundinaceum, has recently been described to have antagonistic activity against fungal plant pathogens and to induce plant defence genes. In the present work, we have shown that a tri5 gene-disrupted mutant that lacks HA production overproduces two polyketides, aspinolides B and C, which were not detected in the wild-type strain. Furthermore, four new aspinolides (D-G) were characterized. These compounds confirm that a terpene-polyketide cross-pathway exists in T. arundinaceum, and they may be responsible for the antifungal activity and the plant sensitization effect observed with the tri5-disrupted mutant. In addition, the molecular changes involving virulence factors in the phytopathogenic fungus Botrytis cinerea 98 (Bc98) during interaction with T. arundinaceum were investigated. The expression of genes involved in the production of botrydial by Bc98 was relatively repressed by HA, whereas other virulence genes of this pathogen were induced by the presence of T. arundinaceum, for example atrB and pg1 which encode for an ABC transporter and endopolygalacturonase 1 respectively. In addition, the interaction with Bc98 significantly repressed the production of HA by T. arundinaceum, indicating that a bidirectional transcriptional regulation is established between these two antagonistic fungi.

Elucidation of signaling molecules involved in ergosterol perception in tobacco

Ergosterol, a principal compound of the fungal plasma membrane, is regarded as a pathogen-associated molecular pattern. In the present study, the role of salicylic acid (SA), jasmonic acid (JA) and spermine signaling pathways after ergosterol elicitation were evaluated. SA, JA and spermine production, as well as accumulation of transcripts for a lipoxygenase (NaLOX3) gene, the phenylalanine-ammonia lyase gene, selected pathogenesis-related genes (PR1, PR5), and peroxidase tPOXC1 were determined in tobacco (Nicotiana tabacum L. cv. Xanthi) in response to ergosterol elicitation. To understand the sequence of the signaling cascade, several representative steps involved in the synthesis of crucial signaling molecules were targeted using specific inhibitors. SA signaling pathway, together with calmodulin-dependent protein kinases and nitric oxide, was demonstrated to play an important role in the induction of defense-related genes following ergosterol treatment. The results suggested that nitric oxide participates in defense-related gene activation following ergosterol treatment but does not directly participate in activation of reactive oxygen species production. The induction of PR5 and tPOXC1 transcripts was found to be not fully dependent on calmodulin/Ca2+ and SA signaling, contrary to the PR1a transcript. A possible candidate for this SA-independent pathway is the spermine pathway, as elevated spermine levels were detected following ergosterol treatment.

Biotechnological approaches for field applications of chitooligosaccharides (COS) to induce innate immunity in plants

Plants have evolved mechanisms to recognize a wide range of pathogen-derived molecules and to express induced resistance against pathogen attack. Exploitation of induced resistance, by application of novel bioactive elicitors, is an attractive alternative for crop protection. Chitooligosaccharide (COS) elicitors, released during plant fungal interactions, induce plant defenses upon recognition. Detailed analyses of structure/function relationships of bioactive chitosans as well as recent progress towards understanding the mechanism of COS sensing in plants through the identification and characterization of their cognate receptors have generated fresh impetus for approaches that would induce innate immunity in plants. These progresses combined with the application of chitin/chitosan/COS in disease management are reviewed here. In considering the field application of COS, however, efficient and large-scale production of desired COS is a challenging task. The available methods, including chemical or enzymatic hydrolysis and chemical or biotechnological synthesis to produce COS, are also reviewed.

Fungal hemolysins

Hemolysins are a class of proteins defined by their ability to lyse red cells but have been described to exhibit pleiotropic functions. These proteins have been extensively studied in bacteria and more recently in fungi. Within the last decade, a number of studies have characterized fungal hemolysins and revealed a fascinating yet diverse group of proteins. The purpose of this review is to provide a synopsis of the known fungal hemolysins with an emphasis on those belonging to the aegerolysin protein family. New insight and perspective into fungal hemolysins in biotechnology and health are additionally presented.

Cytosolic calcium rises and related events in ergosterol-treated Nicotiana cells

The typical fungal membrane component ergosterol was previously shown to trigger defence responses and protect plants against pathogens. Most of the elicitors mobilize the second messenger calcium, to trigger plant defences. We checked the involvement of calcium in response to ergosterol using Nicotiana plumbaginifolia and Nicotiana tabacum cv Xanthi cells expressing apoaequorin in the cytosol. First, it was verified if ergosterol was efficient in these cells inducing modifications of proton fluxes and increased expression of defence-related genes. Then, it was shown that ergosterol induced a rapid and transient biphasic increase of free [Ca²⁺](cyt) which intensity depends on ergosterol concentration in the range 0.002-10 μM. Among sterols, this calcium mobilization was specific for ergosterol and, ergosterol-induced pH and [Ca²⁺](cyt) changes were specifically desensitized after two subsequent applications of ergosterol. Specific modulators allowed elucidating some events in the signalling pathway triggered by ergosterol. The action of BAPTA, LaCl₃, nifedipine, verapamil, neomycin, U73122 and ruthenium red suggested that the first phase was linked to calcium influx from external medium which subsequently triggered the second phase linked to calcium release from internal stores. The calcium influx and the [Ca²⁺](cyt) increase depended on upstream protein phosphorylation. The extracellular alkalinization and ROS production depended on calcium influx but, the ergosterol-induced MAPK activation was calcium-independent. ROS were not involved in cytosolic calcium rise as described in other models, indicating that ROS do not systematically participate in the amplification of calcium signalling. Interestingly, ergosterol-induced ROS production is not linked to cell death and ergosterol does not induce any calcium elevation in the nucleus.