The Induction and Modulation of Plant Defense Responses by Bacterial Lipopolysaccharides

Roles of different forms of lipopolysaccharides in Ralstonia solanacearum pathogenesis

Lipopolysaccharides (LPS) are critical components for the fitness of most gram-negative bacteria. Ralstonia solanacearum causes a deadly wilting disease in many crops; however, the pathogenic roles of different forms of LPS and their pathways of biogenesis remain unknown. By screening for phage-resistant mutants of R. solanacearum Pss4, whose genome sequence is unavailable, mutants with various types of structural defects in LPS were isolated. Pathogenesis assays of the mutants revealed that production of rough LPS (R-LPS), which does not contain O-polysaccharides, was sufficient to cause necrosis on Nicotiana benthamiana and induce the hypersensitive response on N. tabacum. However, biosynthesis of smooth LPS (S-LPS), which contains O-polysaccharides, was required for bacterial proliferation at infection sites on N. benthamiana leaves and for proliferation and causing wilt on tomato. Complementation tests confirmed the involvement of the previously unidentified cluster RSc2201 to RSc2204 in the formation of R. solanacearum S-LPS. With these data and the availability of the annotated genomic sequence of strain GMI1000, certain loci involved in key steps of R. solanacearum LPS biosynthesis were identified. The strategy of this work could be useful for similar studies in other bacteria without available genome sequences.

Genome Sequencing of Xanthomonas vasicola Pathovar vasculorum Reveals Variation in Plasmids and Genes Encoding Lipopolysaccharide Synthesis, Type-IV Pilus and Type-III Secretion Effectors

Xanthomonas vasicola pathovar vasculorum (Xvv) is the bacterial agent causing gumming disease in sugarcane. Here, we compare complete genome sequences for five isolates of Xvv originating from sugarcane and one from maize. This identified two distinct types of lipopolysaccharide synthesis gene clusters among Xvv isolates: one is similar to that of Xanthomonas axonopodis pathovar citri (Xac) and is probably the ancestral type, while the other is similar to those of the sugarcane-inhabiting species, Xanthomonas sacchari. Four of six Xvv isolates harboured sequences similar to the Xac plasmid, pXAC47, and showed a distinct Type-IV pilus (T4P) sequence type, whereas the T4P locus of the other two isolates resembled that of the closely related banana pathogen, Xanthomonas campestris pathovar musacearum (Xcm). The Xvv isolate from maize has lost a gene encoding a homologue of the virulence effector, xopAF, which was present in all five of the sugarcane isolates, while xopL contained a premature stop codon in four out of six isolates. These findings shed new light on evolutionary events since the divergence of Xvv and Xcm, as well as further elucidating the relationships between the two closely related pathogens.

Plant immunity in plant-aphid interactions

Aphids are economically important pests that cause extensive feeding damage and transmit viruses. While some species have a broad host range and cause damage to a variety of crops, others are restricted to only closely related plant species. While probing and feeding aphids secrete saliva, containing effectors, into their hosts to manipulate host cell processes and promote infestation. Aphid effector discovery studies pointed out parallels between infection and infestation strategies of plant pathogens and aphids. Interestingly, resistance to some aphid species is known to involve plant resistance proteins with a typical NB-LRR domain structure. Whether these resistance proteins indeed recognize aphid effectors to trigger ETI remains to be elucidated. In addition, it was recently shown that unknown aphid derived elicitors can initiate reactive oxygen species (ROS) production and callose deposition and that these responses were dependent on BAK1 (BRASSINOSTERIOD INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1) which is a key component of the plant immune system. In addition, BAK-1 contributes to non-host resistance to aphids pointing to another parallel between plant-pathogen and - aphid interactions. Understanding the role of plant immunity and non-host resistance to aphids is essential to generate durable and sustainable aphid control strategies. Although insect behavior plays a role in host selection and non-host resistance, an important observation is that aphids interact with non-host plants by probing the leaf surface, but are unable to feed or establish colonization. Therefore, we hypothesize that aphids interact with non-host plants at the molecular level, but are potentially not successful in suppressing plant defenses and/or releasing nutrients.

AGROBEST: an efficient Agrobacterium-mediated transient expression method for versatile gene function analyses in Arabidopsis seedlings

BACKGROUND

Transient gene expression via Agrobacterium-mediated DNA transfer offers a simple and fast method to analyze transgene functions. Although Arabidopsis is the most-studied model plant with powerful genetic and genomic resources, achieving highly efficient and consistent transient expression for gene function analysis in Arabidopsis remains challenging.

RESULTS

We developed a highly efficient and robust Agrobacterium-mediated transient expression system, named AGROBEST (Agrobacterium-mediated enhanced seedling transformation), which achieves versatile analysis of diverse gene functions in intact Arabidopsis seedlings. Using β-glucuronidase (GUS) as a reporter for Agrobacterium-mediated transformation assay, we show that the use of a specific disarmed Agrobacterium strain with vir gene pre-induction resulted in homogenous GUS staining in cotyledons of young Arabidopsis seedlings. Optimization with AB salts in plant culture medium buffered with acidic pH 5.5 during Agrobacterium infection greatly enhanced the transient expression levels, which were significantly higher than with two existing methods. Importantly, the optimized method conferred 100% infected seedlings with highly increased transient expression in shoots and also transformation events in roots of ~70% infected seedlings in both the immune receptor mutant efr-1 and wild-type Col-0 seedlings. Finally, we demonstrated the versatile applicability of the method for examining transcription factor action and circadian reporter-gene regulation as well as protein subcellular localization and protein-protein interactions in physiological contexts.

CONCLUSIONS

AGROBEST is a simple, fast, reliable, and robust transient expression system enabling high transient expression and transformation efficiency in Arabidopsis seedlings. Demonstration of the proof-of-concept experiments elevates the transient expression technology to the level of functional studies in Arabidopsis seedlings in addition to previous applications in fluorescent protein localization and protein-protein interaction studies. In addition, AGROBEST offers a new way to dissect the molecular mechanisms involved in Agrobacterium-mediated DNA transfer.

Plant innate immunity: an updated insight into defense mechanism

Plants are invaded by an array of pathogens of which only a few succeed in causing disease. The attack by others is countered by a sophisticated immune system possessed by the plants. The plant immune system is broadly divided into two, viz. microbial-associated molecular-patterns-triggered immunity (MTI) and effector-triggered immunity (ETI). MTI confers basal resistance, while ETI confers durable resistance, often resulting in hypersensitive response. Plants also possess systemic acquired resistance (SAR), which provides long-term defense against a broad-spectrum of pathogens. Salicylic-acid-mediated systemic acquired immunity provokes the defense response throughout the plant system during pathogen infection at a particular site. Trans-generational immune priming allows the plant to heritably shield their progeny towards pathogens previously encountered. Plants circumvent the viral infection through RNA interference phenomena by utilizing small RNAs. This review summarizes the molecular mechanisms of plant immune system, and the latest breakthroughs reported in plant defense. We discuss the plant–pathogen interactions and integrated defense responses in the context of presenting an integral understanding in plant molecular immunity.

Metabolic survey of defense responses to a compatible hemibiotroph, Phytophthora parasitica var. nicotianae, in ethylene signaling-impaired tobacco

Reactive oxygen species (ROS) and ethylene play an important role in determining the resistance or susceptibility of plants to pathogen attack. A previous study of the response of tobacco cultivar ( Nicotiana tabacum L. cv. Wisconsin 38) to a compatible hemibiotroph, Phytophthora parasitica var. nicotianae (Ppn) showed that biphasic bursts of ROS and ethylene are positively associated with disease severity. The levels of ethylene and ROS might influence the susceptibility of plants to pathogens, with changing levels of metabolite related to disease resistance or susceptibility. In this study, to obtain more detailed information on the interaction of ROS and ethylene signaling related to resistance and/or susceptibility of plants to pathogen, Ppn-induced metabolic profiles from wild type (WT) and ethylene signaling-impaired transgenic plants that expressed Ein3 antisense (Ein3-AS) were compared using ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS). Nonredundant mass ions (576 in ESI+ mode and 336 in ESI- mode) were selected, and 56 mass ions were identified on the basis of their accurate mass ions and MS/MS spectra. Two-way hierarchical clustering analysis of the selected mass ions revealed that nicotine and phenylpropanoid-polyamine conjugates, such as caffeoyl-dihydrocaffeoyl-spermidine, dicaffeoyl-spermidine, caffeoyl-feruloyl-spermidine, and two bis(dihydrocaffeoyl)-spermine isomers, and their intermediates, such as arginine and putrecine, were present at lower levels in Ein3-AS transgenic plants during Ppn interaction than in WT, whereas galactolipid and oxidized free fatty acid levels were higher in Ein3-AS transgenic plants. Taken together, these results reveal a function for ethylene signaling in tobacco defense responses during Ppn interaction.

Lipopolysaccharides elicit an oxidative burst as a component of the innate immune system in the seagrass Thalassia testudinum

This study represents the first report characterizing the biological effects of a lipopolysaccharide (LPS) immune modulator on a marine vascular plant. LPS was shown to serve as a strong elicitor of the early defense response in the subtropical seagrass Thalassia testudinum Banks ex König and was capable of inducing an oxidative burst identified at the single cell level. The formation of reactive oxygen species (ROS), detected by a redox-sensitive fluorescent probe and luminol-based chemiluminescence, included a diphenyleneiodonium sensitive response, suggesting the involvement of an NADPH oxidase. A 900 bp cDNA fragment coding for this enzyme was sequenced and found to encode a NAD binding pocket domain with extensive homology to the Arabidopsis thaliana rbohF (respiratory burst oxidase homolog) gene. The triggered release of ROS occurred at 20 min post-elicitation and was dose-dependent, requiring a minimal threshold of 50 μg/mL LPS. Pharmacological dissection of the early events preceding ROS emission indicated that the signal transduction chain of events involved extracellular alkalinization, G-proteins, phospholipase A2, as well as K(+), Ca(2+), and anion channels. Despite exclusively thriving in a marine environment, seagrasses contain ROS-generating machinery and signal transduction components that appear to be evolutionarily conserved with the well-characterized defense response systems found in terrestrial plants.

Salmonella colonization activates the plant immune system and benefits from association with plant pathogenic bacteria

Despite increasing incidences of human salmonellosis caused by consumption of contaminated vegetables, relatively little is known about how the plant immune system responds to and may inhibit Salmonella colonization. Here we show that Salmonella Typhimurium activates the plant immune system primarily due to its recognition of the flg22 region in Salmonella flagellin. Several previously identified plant genes that play a role in immunity were found to affect the host response to Salmonella. The Salmonella flg22 (Seflg22) peptide induced the immune response in leaves which effectively restricted the growth of Salmonella as well as the plant pathogenic bacterium, Pseudomonas syringae pv. tomato. Induction of immune responses by Seflg22 was dependent on the plant FLS2 receptor. Salmonella multiplied poorly on plant tissues similar to other bacteria which are non-pathogenic to plants. However, Salmonella populations increased significantly when co-inoculated with P. syringae pv. tomato but not when co-inoculated with a type III secretion system mutant of this pathogen. Our results suggest that Salmonella benefits from the immune-suppressing effects of plant pathogenic bacteria, and this growth enhancement may increase the risk of salmonellosis.

The receptor-like protein ReMAX of Arabidopsis detects the microbe-associated molecular pattern eMax from Xanthomonas

As part of their immune system, plants have pattern recognition receptors (PRRs) that can detect a broad range of microbe-associated molecular patterns (MAMPs). Here, we identified a PRR of Arabidopsis thaliana with specificity for the bacterial MAMP eMax from xanthomonads. Response to eMax seems to be restricted to the Brassicaceae family and also varied among different accessions of Arabidopsis. In crosses between sensitive accessions and the insensitive accession Shakhdara, eMax perception mapped to receptor-like protein1 (RLP1). Functional complementation of rlp1 mutants required gene constructs that code for a longer version of RLP1 that we termed ReMAX (for receptor of eMax). ReMAX/RLP1 is a typical RLP with structural similarity to the tomato (Solanum lycopersicum) RLP Eix2, which detects fungal xylanase as a MAMP. Attempts to demonstrate receptor function by interfamily transfer of ReMAX to Nicotiana benthamiana were successful after using hybrid receptors with the C-terminal part of ReMAX replaced by that of Eix2. These results show that ReMAX determines specificity for eMax. They also demonstrate hybrid receptor technology as a promising tool to overcome problems that impede interfamily transfer of PRRs to enhance pathogen detection in crop plants.

Recent progress in understanding PAMP- and effector-triggered immunity against the rice blast fungus Magnaporthe oryzae

Rice blast, caused by the fungal pathogen Magnaporthe oryzae, is one of the most destructive diseases of rice worldwide. The rice-M. oryzae pathosystem has become a model in the study of plant-fungal interactions because of its scientific advancement and economic importance. Recent studies have identified a number of new pathogen-associated molecular patterns (PAMPs) and effectors from the blast fungus that trigger rice immune responses upon perception. Interaction analyses between avirulence effectors and their cognate resistance proteins have provided new insights into the molecular basis of plant-fungal interactions. In this review, we summarize the recent research on the characterization of those genes in both M. oryzae and rice that are important for the PAMP- and effector-triggered immunity recognition and signaling processes. We also discuss future directions for research that will further our understanding of this pathosystem.

Ralstonia solanacearum RSc0411 (lptC) is a determinant for full virulence and has a strain-specific novel function in the T3SS activity

Previously, we have identified an avirulent Ralstonia solanacearum mutant carrying a transposon insertion in RSc0411, a gene homologous to the Escherichia coli LPS-transporting protein LptC. However, how the disruption of RSc0411 affects the bacterium-plant interactions and leads to decreased pathogenicity was not known. Here we show that the disruption of RSc0411 leads to pleiotropic defects, including reducing bacterial motility, biofilm formation, root attachment, rough-form LPS production and virulence in tomato and increasing membrane permeability. Disruption of the orthologous RSc0411 present in other R. solanacearum strains proves that most of these functions are conserved in the species. In contrast, trans-complementation analyses show that only RSc0411 orthologues from closely related bacteria can rescue the defects of the disruption mutant. These results enable us to propose a function for RSc0411, and for the clustered genes, in LPS biogenesis, and for the first time, to our knowledge, also a role of a gene from the DUF1239 gene family in bacterial pathogenicity. In addition and notably, the RSc0411 mutant displays a strain-specific phenotype for hypersensitive response (HR), in which the RSc0411 disruption impairs the HR caused by strain Pss190 but not that by strain Pss1308. Consistent with this strain-specific defect, the mutation clearly affects expression of the type III secretion system (T3SS) in Pss190 but not in other strains, suggesting that the HR-deficient phenotype of the RSc0411 mutant in Pss190 is due to impairment of the T3SS and thus RSc0411 has a strain-specific role in the T3SS activity of R. solanacearum.

Microbial recognition and evasion of host immunity

Plants are able to detect microbes by pattern recognition receptors in the host cells that, upon recognition of the enemy, activate effective immune responses in the invaded tissue. Recognition of microbes occurs by common conserved structures called microbe-associated molecular patterns (MAMPs). Plant pathogens and beneficial soil-borne microbes live in close contact with their host. Hence, prevention of the host's defence programme is essential for their survival. Active suppression of host defences by microbial effector proteins is a well-known strategy employed by many successful plant-associated microbes. Evasion of host immune recognition is less well studied but is emerging as another important strategy. Escape from recognition by the host's immune system can be caused by alterations in the structure of the recognized MAMPs, or by active intervention of ligand-receptor recognition. This paper reviews the structure and recognition of common MAMPs and the ways that plant-associated microbes have evolved to prevent detection by their host.

Requirement of the lipopolysaccharide O-chain biosynthesis gene wxocB for type III secretion and virulence of Xanthomonas oryzae pv. Oryzicola

Xanthomonas oryzae pv. oryzicola causes bacterial leaf streak of rice. A mutant disrupted in wxocB, predicted to encode an enzyme for lipopolysaccharide (LPS) synthesis, was previously shown to suffer reduced virulence. Here, we confirm a role for wxocB in virulence and demonstrate its requirement for LPS O-chain assembly. Structure analysis indicated that wild-type LPS contains a polyrhamnose O chain with irregular, variant residues and a core oligosaccharide identical to that of other Xanthomonas spp. and that the wxocB mutant lacks the O chain. The mutant also showed moderate impairment in exopolysaccharide (EPS) production, but comparison with an EPS-deficient mutant demonstrated that this impairment could not account entirely for the reduced virulence. The wxocB mutant was not detectably different from the wild type in its induction of pathogenesis-related rice genes, type II secretion competence, flagellar motility, or resistance to two phytoalexins or resveratrol, and it was more, not less, resistant to oxidative stress and a third phytoalexin, indicating that none of these properties is involved. The mutant was more sensitive to SDS and to novobiocin, so increased sensitivity to some host-derived antimicrobials cannot be ruled out. However, the mutant showed a marked decrease in type III secretion into plant cells. This was not associated with any change in expression of genes for type III secretion or the ability to attach to plant cells in suspension. Thus, virulence of the wxocB mutant is likely reduced due primarily to a direct, possibly structural, effect of the loss of the O chain on type III delivery of effector proteins.

MAMP (microbe-associated molecular pattern) triggered immunity in plants

Plants are sessile organisms that are under constant attack from microbes. They rely on both preformed defenses, and their innate immune system to ward of the microbial pathogens. Preformed defences include for example the cell wall and cuticle, which act as physical barriers to microbial colonization. The plant immune system is composed of surveillance systems that perceive several general microbe elicitors, which allow plants to switch from growth and development into a defense mode, rejecting most potentially harmful microbes. The elicitors are essential structures for pathogen survival and are conserved among pathogens. The conserved microbe-specific molecules, referred to as microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs), are recognized by the plant innate immune systems pattern recognition receptors (PRRs). General elicitors like flagellin (Flg), elongation factor Tu (EF-Tu), peptidoglycan (PGN), lipopolysaccharides (LPS), Ax21 (Activator of XA21-mediated immunity in rice), fungal chitin, and β-glucans from oomycetes are recognized by plant surface localized PRRs. Several of the MAMPs and their corresponding PRRs have, in recent years, been identified. This review focuses on the current knowledge regarding important MAMPs from bacteria, fungi, and oomycetes, their structure, the plant PRRs that recognizes them, and how they induce MAMP-triggered immunity (MTI) in plants.

Positive crosstalk of MAMP signaling pathways in rice cells

Plants have evolved efficient defense mechanisms known as priming and synergy, both of which can mobilize defense responses more extensively against successive pathogen invasion or simultaneous stimulation by different signal molecules. However, the mechanisms underlying these phenomena were largely unknown. In the present study, we used cultured rice cells and combination of purified MAMP molecules as a model system to study the mechanisms of these phenomena. We found that the pretreatment of rice cells with a low concentration of bacterial lipopolysaccharide (LPS) apparently primed the defense responses induced by successive N-acetylchitooctaose (GN8) treatment. On the other hand, simultaneous treatment with GN8 and LPS also resulted in the similar enhancement of defense responses observed for the LPS-induced priming, indicating that the synergistic effects of these MAMPs are basically responsible for such enhancement of defense responses, though the effect could be interpreted as "priming" under some experimental conditions. These results also indicate that such a positive crosstalk of signaling cascade downstream of MAMP receptors seems to occur very rapidly, probably at early step(s) of signaling pathway. Comprehensive analysis of phytohormones revealed a specific enhancement of the synthesis of jasmonic acid (JA), both in the LPS pretreatment and also simultaneous treatment, indicating a role of JA in the enhancement of downstream responses.

Nitric oxide-mediated maintenance of redox homeostasis contributes to NPR1-dependent plant innate immunity triggered by lipopolysaccharides

The perception of lipopolysaccharides (LPS) by plant cells can lead to nitric oxide (NO) production and defense gene induction. However, the signaling cascades underlying these cellular responses have not yet been resolved. This work investigated the biosynthetic origin of NO and the role of NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) to gain insight into the mechanism involved in LPS-induced resistance of Arabidopsis (Arabidopsis thaliana). Analysis of inhibitors and mutants showed that LPS-induced NO synthesis was mainly mediated by an arginine-utilizing source of NO generation. Furthermore, LPS-induced NO caused transcript accumulation of alternative oxidase genes and increased antioxidant enzyme activity, which enhanced antioxidant capacity and modulated redox state. We also analyzed the subcellular localization of NPR1 to identify the mechanism for protein-modulated plant innate immunity triggered by LPS. LPS-activated defense responses, including callose deposition and defense-related gene expression, were found to be regulated through an NPR1-dependent pathway. In summary, a significant NO synthesis induced by LPS contributes to the LPS-induced defense responses by up-regulation of defense genes and modulation of cellular redox state. Moreover, NPR1 plays an important role in LPS-triggered plant innate immunity.

The role of lipopolysaccharide and peptidoglycan, two glycosylated bacterial microbe-associated molecular patterns (MAMPs), in plant innate immunity

In an environment that is rich in potentially pathogenic microorganisms, the survival of higher eukaryotic organisms depends on efficient pathogen sensing and rapidly mounted defence responses. Such protective mechanisms are found in all multicellular organisms, and are collectively referred to as 'innate immunity'. Innate immunity is the first line of defence against invading microorganisms in vertebrates and the only line of defence in invertebrates and plants. Bacterial glycoconjugates, such as lipopolysaccharides (LPSs) from the outer membrane of Gram-negative bacteria and peptidoglycan (PGN) from the cell walls of both Gram-positive and Gram-negative bacteria, have been found to act as elicitors of plant innate immunity. These conserved, indispensable, microbe-specific molecules are also referred to as 'microbe-associated molecular patterns' (MAMPs). MAMPs are recognized by the plant innate immune system through the action of pattern recognition receptors (PRRs). A greater insight into the mechanisms of MAMP recognition and the description of PRRs for different microbial glycoconjugates will have considerable impact on the improvement of plant health and disease resistance. Here, the current knowledge about LPS and PGN as MAMPs is reviewed.

Exopolysaccharide production is required for biofilm formation and plant colonization by the nitrogen-fixing endophyte Gluconacetobacter diazotrophicus

The genome of the endophytic diazotrophic bacterial species Gluconacetobacter diazotrophicus PAL5 (PAL5) revealed the presence of a gum gene cluster. In this study, the gumD gene homologue, which is predicted to be responsible for the first step in exopolysaccharide (EPS) production, was insertionally inactivated and the resultant mutant (MGD) was functionally studied. The mutant MGD presented normal growth and nitrogen (N(2)) fixation levels but did not produce EPS when grown on different carbon sources. MGD presented altered colony morphology on soft agar plates (0.3% agar) and was defective in biofilm formation on glass wool. Most interestingly, MGD was defective in rice root surface attachment and in root surface and endophytic colonization. Genetic complementation reverted all mutant phenotypes. Also, the addition of EPS purified from culture supernatants of the wild-type strain PAL5 to the mutant MGD was effective in partially restoring wild-type biofilm formation and plant colonization. These data provide strong evidence that the PAL5 gumD gene is involved in EPS biosynthesis and that EPS biosynthesis is required for biofilm formation and plant colonization. To our knowledge, this is the first report of a role of EPS in the endophytic colonization of graminaceous plants by a nitrogen-fixing bacterium.

Unraveling plant responses to bacterial pathogens through proteomics

Plant pathogenic bacteria cause diseases in important crops and seriously and negatively impact agricultural production. Therefore, an understanding of the mechanisms by which plants resist bacterial infection at the stage of the basal immune response or mount a successful specific R-dependent defense response is crucial since a better understanding of the biochemical and cellular mechanisms underlying these interactions will enable molecular and transgenic approaches to crops with increased biotic resistance. In recent years, proteomics has been used to gain in-depth understanding of many aspects of the host defense against pathogens and has allowed monitoring differences in abundance of proteins as well as posttranscriptional and posttranslational processes, protein activation/inactivation, and turnover. Proteomics also offers a window to study protein trafficking and routes of communication between organelles. Here, we summarize and discuss current progress in proteomics of the basal and specific host defense responses elicited by bacterial pathogens.

Distinct carbohydrate and lipid-based molecular patterns within lipopolysaccharides from Burkholderia cepacia contribute to defense-associated differential gene expression in Arabidopsis thaliana

Lipopolysaccharides are structural components within the cell walls of Gram-negative bacteria. The LPSs as microbe-associated molecular pattern (MAMP) molecules can trigger defense-related responses involved in MAMP-triggered immunity and basal resistance in plants, presumably from an initial perception event. LPS from Burkholderia cepacia as well as two fragments, the glycolipid, lipid A and the polysaccharide (OPS-core) chain, were used to treat Arabidopsis thaliana seedlings to evaluate the eliciting activities of the individual LPS sub-domains by means of Annealing Control Primer-based Differential Display transcript profiling. Genes found to be up-regulated encode for proteins involved in signal perception and transduction, transcriptional regulation and defense - and stress responses. Furthermore, genes encoding proteins involved in chaperoning, secretion, protein-protein interactions and protein degradation were differentially expressed. It is concluded that intact LPS, as well as the two sub-components, induced the expression of a broad range of genes associated with perception and defense as well as metabolic reprogramming of cellular activities in support of immunity and basal resistance. Whilst the lipid A and OPS moieties were able to up-regulate sub-sets of defense-associated genes over the same spectrum of categories as intact LPS, the up-regulation observed with intact LPS was the more comprehensive, suggesting that the lipid A and glycan molecular patterns of the molecule act as partial agonists, but that the intact LPS structure is required for full agonist activity.

Pathway for lipid A biosynthesis in Arabidopsis thaliana resembling that of Escherichia coli

The lipid A moiety of Escherichia coli lipopolysaccharide is a hexa-acylated disaccharide of glucosamine that makes up the outer monolayer of the outer membrane. Arabidopsis thaliana contains nuclear genes encoding orthologs of key enzymes of bacterial lipid A biosynthesis, including LpxA, LpxC, LpxD, LpxB, LpxK and KdtA. Although structurally related lipid A molecules are found in most other gram-negative bacteria, lipid A and its precursors have not been directly detected in plants previously. However, homozygous insertional knockout mutations or RNAi knock-down constructs of Arabidopsis lpx and kdtA mutants revealed accumulation (or disappearance) of the expected monosaccharide or disaccharide lipid A precursors by mass spectrometry of total lipids extracted from 10-day old seedlings of these mutants. In addition, fluorescence microscopy of lpx-gfp fusions in transgenic Arabidopsis plants suggests that the Lpx and KdtA proteins are expressed and targeted to mitochondria. Although the structure of the lipid A end product generated by plants is still unknown, our work demonstrates that plants synthesize lipid A precursors using the same enzymatic pathway present in E. coli.

Structural analysis and involvement in plant innate immunity of Xanthomonas axonopodis pv. citri lipopolysaccharide

Xanthomonas axonopodis pv. citri (Xac) causes citrus canker, provoking defoliation and premature fruit drop with concomitant economical damage. In plant pathogenic bacteria, lipopolysaccharides are important virulence factors, and they are being increasingly recognized as major pathogen-associated molecular patterns for plants. In general, three domains are recognized in a lipopolysaccharide: the hydrophobic lipid A, the hydrophilic O-antigen polysaccharide, and the core oligosaccharide, connecting lipid A and O-antigen. In this work, we have determined the structure of purified lipopolysaccharides obtained from Xanthomonas axonopodis pv. citri wild type and a mutant of the O-antigen ABC transporter encoded by the wzt gene. High pH anion exchange chromatography and matrix-assisted laser desorption/ionization mass spectrum analysis were performed, enabling determination of the structure not only of the released oligosaccharides and lipid A moieties but also the intact lipopolysaccharides. The results demonstrate that Xac wild type and Xacwzt LPSs are composed mainly of a penta- or tetra-acylated diglucosamine backbone attached to either two pyrophosphorylethanolamine groups or to one pyrophosphorylethanolamine group and one phosphorylethanolamine group. The core region consists of a branched oligosaccharide formed by Kdo₂Hex₆GalA₃Fuc3NAcRha₄ and two phosphate groups. As expected, the presence of a rhamnose homo-oligosaccharide as O-antigen was determined only in the Xac wild type lipopolysaccharide. In addition, we have examined how lipopolysaccharides from Xac function in the pathogenesis process. We analyzed the response of the different lipopolysaccharides during the stomata aperture closure cycle, the callose deposition, the expression of defense-related genes, and reactive oxygen species production in citrus leaves, suggesting a functional role of the O-antigen from Xac lipopolysaccharides in the basal response.

A genome-wide survey for Arabidopsis leucine-rich repeat receptor kinases implicated in plant immunity

Receptor-like kinases (RLK) are among the largest gene families encoded by plant genomes. Common structural features of plant RLKs are an extracellular ligand binding domain, a membrane spanning domain, and an intracellular protein kinase domain. The largest subfamily of plant RLKs is characterized by extracellular leucine-rich repeat (LRR-RLK) structures that are known biochemical modules for mediating ligand binding and protein-protein interactions. In the frame of the Arabidopsis Functional Genomics Network initiative of the German Research Foundation (DFG) we have conducted a comprehensive survey for and functional characterization of LRR-RLKs potentially implicated in Arabidopsis thaliana immunity to microbial infection. Arabidopsis gene expression patterns suggested an important role of this class of proteins in biotic stress adaptation. Detailed biochemical and physiological characterization of the brassinosteroid insensitive 1-associated receptor kinase 1 (BAK1) revealed brassinolide-independent roles of this protein in plant immunity, in addition to its well-established function in plant development. The LRR-RLK BAK1 has further been shown to form heteromeric complexes with various other LRR-RLKs in a ligand-dependent manner, suggesting a role as adapter or co-receptor in plant receptor complexes. Here, we review the current status of BAK1 and BAK1-interacting LRR-RLKs in plant immunity.

Lipopolysaccharide mobility in leaf tissue of Arabidopsis thaliana

Bacterial lipopolysaccharides (LPS) are triggers of defence responses in plants, and induce local as well as systemic acquired resistance. Arabidopsis thaliana plants pretreated with LPS show an increased resistance to the virulent bacterial plant pathogen Pseudomonas syringae pv. tomato DC3000. To investigate the mobilization and transport of LPS in Arabidopsis leaves, fluorescently labelled LPS (Alexa Fluor® 488 conjugate) from Salmonella minnesota was used. Leaves were pressure infiltrated with fluorescein-labelled LPS and fluorescence microscopy was used to follow the movement and localization of LPS as a function of time. The observation of leaves 1 h after supplementation with fluorescein-labelled LPS revealed a fluorescent signal in the intercellular space. Capillary zone electrophoresis was used for the detection and analysis of the labelled LPS in directly treated leaves and systemic leaves. In addition, gel electrophoresis was used to confirm LPS mobilization. The results indicated that LPS mobilization/translocation occurs through the xylem from local, treated leaves to systemic, untreated leaves. Consequently, care should be taken when ascribing the observed biochemical responses and induced resistance from LPS perception as being uniquely local or systemic, as these responses might overlap because of the mobility of LPS in the plant vascular system.

Deciphering the structural and biological properties of the lipid A moiety of lipopolysaccharides from Burkholderia cepacia strain ASP B 2D, in Arabidopsis thaliana

Lipopolysaccharides (LPSs) are major, indispensable cell surface components of Gram-negative bacteria that have diverse roles in bacterial pathogenesis of plants. Environmental strains of Burkholderia cepacia have been described as phytopathogens, growth promotors, biocontrol agents and bioremediation agents. We have previously shown that LPSs from B. cepacia can be recognized as microbe-associated molecular pattern molecules, to elicit defense responses in plants. Recent findings suggest that the lipid A moiety might be partially responsible for LPSs perception. These studies were extended by analysis of the structure and biological activity of the lipid A moiety of LPSs of B. cepacia(.) The full structure was determined by a combination of negative/positive-ion matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) on intact and partially degraded substrates. B. cepacia lipid A was found to contain a tetra- or penta-acylated, 1,4'-diphosphorylated, β-(1-6)-linked D-GlcN disaccharide and further substituted by L-Ara4N in position 4'. As primary fatty acids, R-configurated 16:0(3-OH) (amide-linked in 2 and 2') and 14:0(3-OH) (ester-linked in 3 and 3', nonstoichiometric) were identified. A secondary 14:0 was located at position 2'. Its biological activity to elicit defense-related responses was subsequently investigated by monitoring the changes in the transcriptome of Arabidopsis thaliana seedlings. Genes found to be upregulated code for proteins involved in signal perception and transduction, transcriptional regulation, defense and stress responses. Furthermore, genes encoding proteins involved in chaperoning, protein interactions and protein degradation were differentially expressed as part of the metabolic reprogramming of cellular activities in support of immunity and defense.

Effects of galU mutation on Pseudomonas syringae-plant interactions

Bacterial galU coding for a uridine diphosphate-glucose pyrophosphorylase plays an important role in carbohydrates biosynthesis, including synthesis of lipopolysaccharides (LPS), membrane-derived oligosaccharides, and capsular polysaccharides. In this study, we characterized the galU mutant of Pseudomonas syringae pv. syringae 61 (Psy61), a necrotizing plant pathogen whose pathogenicity depends on a functional type III secretion system (T3SS), and showed that the Psy61 galU mutant had reduced biofilm formation ability, was nonmotile, and had an assembled T3SS structure but failed to elicit hypersensitive response in resistant plants and necrotic lesions in susceptible plants. Moreover, the defective LPS and other pathogen-associated molecular patterns (PAMPs) on the surface of the Psy61 galU mutant were capable of inducing PAMP-triggered immunity, which severely compromised the ability of the Psy61 galU mutant to survive in planta. Our results demonstrated that the complete LPS protected plant-pathogenic bacteria from host innate immunity, similar to what was found in animal pathogens, prior to the translocation of T3S effectors and bacterial multiplication.

Genome-wide sequencing data reveals virulence factors implicated in banana Xanthomonas wilt

Banana Xanthomonas wilt is a newly emerging disease that is currently threatening the livelihoods of millions of farmers in East Africa. The causative agent is Xanthomonas campestris pathovar musacearum (Xcm), but previous work suggests that this pathogen is much more closely related to species Xanthomonas vasicola than to X. campestris. We have generated draft genome sequences for a banana-pathogenic strain of Xcm isolated in Uganda and for a very closely related strain of X. vasicola pathovar vasculorum, originally isolated from sugarcane, that is nonpathogenic on banana. The draft sequences revealed overlapping but distinct repertoires of candidate virulence effectors in the two strains. Both strains encode homologues of the Pseudomonas syringae effectors HopW, HopAF1 and RipT from Ralstonia solanacearum. The banana-pathogenic and non-banana-pathogenic strains also differed with respect to lipopolysaccharide synthesis and type-IV pili, and in at least several thousand single-nucleotide polymorphisms in the core conserved genome. We found evidence of horizontal transfer between X. vasicola and very distantly related bacteria, including members of other divisions of the Proteobacteria. The availability of these draft genomes will be an invaluable tool for further studies aimed at understanding and combating this important disease.

Lipopolysaccharides and plant innate immunity

Plants posses an innate immune system that has many parallels with those found in mammals and insects. A range of molecules of microbial origin called Microbe Associated Molecular Patterns (MAMPs) act to trigger basal defense responses in plants. These elicitors include lipopolysaccharides (LPS) from diverse Gram-negative bacteria. Both core oligosaccharide and the lipid A moieties of LPS as well as synthetic O-antigen oligosaccharides have activity in inducing defense responses in the model plant Arabidopsis thaliana. Very little is known of the mechanism of LPS perception by plants, although plant receptors for other MAMPs such as flagellin have been described. Recent work has implicated the Arabidopsis syntaxin PEN1 as a potential actor in LPS induction of plant defenses, which may suggest a role for vesicle trafficking in the signalling process.

Lipopolysaccharides in Rhizobium-legume symbioses

The establishment of nitrogen-fixing symbiosis between a legume plant and its rhizobial symbiont requires that the bacterium adapt to changing conditions that occur with the host plant that both promotes and allows infection of the host root nodule cell, regulates and resists the host defense response, permits the exchange of metabolites, and contributes to the overall health of the host. This adaptive process involves changes to the bacterial cell surface and, therefore, structural modifications to the lipopolysaccharide (LPS). In this chapter, we describe the structures of the LPSs from symbiont members of the Rhizobiales, the genetics and mechanism of their biosynthesis, the modifications that occur during symbiosis, and their possible functions.

The multifunctional leucine-rich repeat receptor kinase BAK1 is implicated in Arabidopsis development and immunity

Plant receptor-like kinases (RLKs) are transmembrane proteins with putative N-terminal extracellular ligand-binding domains and C-terminal intracellular protein kinase domains. RLKs have been implicated in multiple physiological programs including plant development and immunity to microbial infection. Arabidopsis thaliana gene expression patterns support an important role of this class of proteins in biotic stress adaptation. Here, we provide a comprehensive survey of plant immunity-related RLK gene expression. We further document the role of the Arabidopsis Brassinosteroid Insensitive 1 (BRI1)-associated receptor kinase 1 (BAK1) in seemingly unrelated biological processes, such as plant development and immunity, and propose a role of this protein as an adaptor molecule that is required for proper functionality of numerous RLKs. This view is supported by the identification of an additional RLK, PEPR1, and its closest homolog, PEPR2 as BAK1-interacting RLKs.

A novel regulatory system in plants involving medium-chain fatty acids

Polyethylene glycol sorbitan monoacylates (Tween) are detergents of widespread use in plant sciences. However, little is known about the plant response to these compounds. Interestingly, the structure of Tweens' detergents (especially from Tween 20) resembles the lipid A structure from gram-negative bacteria polysaccharides (a backbone with short saturated fatty acids). Thus, different assays (microarray, GC-MS, RT-PCR, Northern blots, alkalinization and mutant analyses) were conducted in order to elucidate physiological changes in the plant response to Tween 20 detergent. Tween 20 causes a rapid and complex change in transcript abundance which bears all characteristics of a pathogenesis-associated molecular pattern (PAMP)/elicitor-induced defense response, and they do so at concentrations which cause no detectable deleterious effects on plant cellular integrity. In the present work, it is shown that the PAMP/elicitor-induced defense responses are caused by medium-chain fatty acids which are efficiently released from the Tween backbone by the plant, notably lauric acid (12:0) and methyl lauric acid. These compounds induce the production of ethylene, medium alkalinization and gene activation in a jasmonate-independent manner. Medium-chain fatty acids are thus novel elicitors/regulators of plant pathogen defense as they have being proved in animals.

Regulation and secretion of Xanthomonas virulence factors

Plant pathogenic bacteria of the genus Xanthomonas cause a variety of diseases in economically important monocotyledonous and dicotyledonous crop plants worldwide. Successful infection and bacterial multiplication in the host tissue often depend on the virulence factors secreted including adhesins, polysaccharides, LPS and degradative enzymes. One of the key pathogenicity factors is the type III secretion system, which injects effector proteins into the host cell cytosol to manipulate plant cellular processes such as basal defense to the benefit of the pathogen. The coordinated expression of bacterial virulence factors is orchestrated by quorum-sensing pathways, multiple two-component systems and transcriptional regulators such as Clp, Zur, FhrR, HrpX and HpaR. Furthermore, virulence gene expression is post-transcriptionally controlled by the RNA-binding protein RsmA. In this review, we summarize the current knowledge on the infection strategies and regulatory networks controlling secreted virulence factors from Xanthomonas species.

Transcriptome analysis of a bacterially induced basal and hypersensitive response of Medicago truncatula

Research using the well-studied model legume Medicago truncatula has largely focused on rhizobium symbiosis, while little information is currently available for this species on pathogen-induced transcriptome changes. We have performed a transcriptome analysis of this species with the objective of studying the basal (BR, no visible symptoms) and hypersensitive response (HR, plant cell death) in its leaves at 6 and at 24 h after infection by HR-negative (hrcC mutant) and HR-inducing Pseudomonas syringae pv. syringae strains, respectively. Although there were no visible symptoms at the BR, the alterations in gene expression were comparable to those found with the HR. Both responses resulted in the transcriptional alteration of hundreds of plant genes; however, the responses in the HR were usually more intense. The reactions to HR-inducing and HR-negative bacterial strains were significantly overlapping. Parallel up- or down-regulation of genes with the same function occurred frequently. However, some plant processes were regulated in one direction; for example, most of the protein synthesis-related genes were activated and all of the photosynthetic/chloroplast genes were suppressed during BR. The possible roles of several functional classes (e.g., cell rescue, signaling, defense, cell death, etc.) of transcriptionally altered genes are discussed. The results of the comparison with available mycorrhizal and nodule expression data show that there is a significant overlap between nodulation and the leaf defense response and that during the early stage of the nodulation in roots, Sinorhizobium meliloti induces a fluctuation in the transcription of BR- and HR-responsive genes.

Identification of specific and universal virulence factors in Burkholderia cenocepacia strains by using multiple infection hosts

Over the past few decades, strains of the Burkholderia cepacia complex have emerged as important pathogens for patients suffering from cystic fibrosis. Identification of virulence factors and assessment of the pathogenic potential of Burkholderia strains have increased the need for appropriate infection models. In previous studies, different infection hosts, including mammals, nematodes, insects, and plants, have been used. At present, however, the extent to which the virulence factors required to infect different hosts overlap is not known. The aim of this study was to analyze the roles of various virulence factors of two closely related Burkholderia cenocepacia strains, H111 and the epidemic strain K56-2, in a multihost pathogenesis system using four different model organisms, namely, Caenorhabditis elegans, Galleria mellonella, the alfalfa plant, and mice or rats. We demonstrate that most of the identified virulence factors are specific for one of the infection models, and only three factors were found to be essential for full pathogenicity in several hosts: mutants defective in (i) quorum sensing, (ii) siderophore production, and (iii) lipopolysaccharide biosynthesis were attenuated in at least three of the infection models and thus may represent promising targets for the development of novel anti-infectives.

Rhizosphere Bacterial Signalling: A Love Parade Beneath Our Feet

Plant roots support the growth and activities of a wide variety of microorganisms that may have a profound effect on the growth and/or health of plants. Among these microorganisms, a high diversity of bacteria have been identified and categorized as deleterious, beneficial, or neutral with respect to the plant. The beneficial bacteria, termed plant growth-promoting rhizobacteria (PGPR), are widely studied by microbiologists and agronomists because of their potential in plant production. Azospirillum, a genus of versatile PGPR, is able to enhance the plant growth and yield of a wide range of economically important crops in different soils and climatic regions. Plant beneficial effects of Azospirillum have mainly been attributed to the production of phytohormones, nitrate reduction, and nitrogen fixation, which have been subject of extensive research throughout the years. These elaborate studies made Azospirillum one of the best-characterized genera of PGPR. However, the genetic and molecular determinants involved in the initial interaction between Azospirillum and plant roots are not yet fully understood. This review will mainly highlight the current knowledge on Azospirillum plant root interactions, in the context of preceding and ongoing research on the association between plants and plant growth-promoting rhizobacteria.

BENEFICIAL RHIZOSPHERE PSEUDOMONADS

Among the many bacteria present on and around the root, Pseudomonas bacteria are (among) the best root colonizers and therefore very suitable to apply for beneficial purposes. In this chapter, we discuss the possibilities to use such bacteria for the following purposes: fertilization of the plant, stimulation of plant growth and yield, reduction of plant stress, and reduction of plant diseases. This research was supported by numerous grants, especially from the Dutch Organization for scientific research (NWO), EET, the European Commission and INTAS.

Bioactive and Structural Metabolites of Pseudomonas and Burkholderia Species Causal Agents of Cultivated Mushrooms Diseases

Pseudomonas tolaasii, P. reactans and Burkholderia gladioli pv. agaricicola, are responsible of diseases on some species of cultivated mushrooms. The main bioactive metabolites produced by both Pseudomonas strains are the lipodepsipeptides (LDPs) tolaasin I and II and the so called White Line Inducing Principle (WLIP), respectively, LDPs which have been extensively studied for their role in the disease process and for their biological properties. In particular, their antimicrobial activity and the alteration of biological and model membranes (red blood cell and liposomes) was established. In the case of tolaasin I interaction with membranes was also related to the tridimensional structure in solution as determined by NMR combined with molecular dynamic calculation techniques. Recently, five news minor tolaasins, tolaasins A-E, were isolated from the culture filtrates of P. tolaasii and their chemical structure was determined by extensive use of NMR and MS spectroscopy. Furthermore, their antimicrobial activity was evaluated on target micro-organisms (fungi–-including the cultivated mushrooms Agaricus bisporus, Lentinus edodes, and Pleurotus spp.–-chromista, yeast and bacteria). The Gram positive bacteria resulted the most sensible and a significant structure-activity relationships was apparent. The isolation and structure determination of bioactive metabolites produced by B. gladioli pv. agaricicola are still in progress but preliminary results indicate their peptide nature. Furthermore, the exopolysaccharide (EPS) from the culture filtrates of B. gladioli pv. agaricicola, as well as the O-chain and lipid A, from the lipo-polysaccharide (LPS) of the three bacteria, were isolated and the structures determined.

Characterization of the Bradyrhizobium japonicum galE gene: its impact on lipopolysaccharide profile and nodulation of soybean

The galE gene from Bradyrhizobium japonicum 61A101C, a soybean endosymbiont, was cloned and characterized. Its deduced amino-acid sequence showed a high similarity with that of other rhizobia. Functional identification of the galE gene was achieved by complementation of a galE mutant strain, PL2, with a series of pKM subclones. Disruption of the B. japonicum galE gene affects the lipopolysaccharide profile compared with that of the wild type, suggesting that galE is responsible for alteration of lipopolysaccharide structure. Examination of nodule formation by the wild-type and galE mutant revealed that the former displayed normal nodule development on soybean roots, whereas the latter showed no nodule formation at all time points examined except for 20 days after inoculation when <10% of soybean formed pseudo-nodules.

Breaking the barriers: microbial effector molecules subvert plant immunity

Adaptation to specialized environments allows microorganisms to inhabit an enormous variety of ecological niches. Growth inside plant tissues is a niche offering a constant nutrient supply, but to access this niche, plant defense mechanisms ranging from passive barriers to induced defense reactions have to be overcome. Pathogens have to break several, if not all, of these barriers. For this purpose, they secrete effector molecules into plant cells to interfere with individual defense responses. Plant defense is organized in multiple layers, and therefore the action of effectors likely follows this same order, leading to a hierarchy in effector orchestration. In this review we summarize the latest findings regarding the level at which effectors manipulate plant immunity. Particular attention is given to those effectors whose mechanism of action is known. Additionally, we compare methods to identify and characterize effector molecules.

The role of horizontal transfer in the evolution of a highly variable lipopolysaccharide biosynthesis locus in xanthomonads that infect rice, citrus and crucifers

Background

Lipopolysaccharide (LPS) is a pathogen associated molecular pattern (PAMP) of animal and plant pathogenic bacteria. Variation at the interstrain level is common in LPS biosynthetic gene clusters of animal pathogenic bacteria. This variation has been proposed to play a role in evading the host immune system. Even though LPS is a modulator of plant defense responses, reports of interstrain variation in LPS gene clusters of plant pathogenic bacteria are rare.

Results

In this study we report the complete sequence of a variant 19.9 kb LPS locus present in the BXO8 strain of Xanthomonas oryzae pv. oryzae (Xoo), the bacterial blight pathogen of rice. This region is completely different in size, number and organization of genes from the LPS locus present in most other strains of Xoo from India and Asia. Surprisingly, except for one ORF, all the other ORFs at the BXO8 LPS locus are orthologous to the genes present at this locus in a sequenced strain of X. axonopodis pv. citri (Xac; a pathogen of citrus plants). One end of the BXO8 LPS gene cluster, comprised of ten genes, is also present in the related rice pathogen, X. oryzae pv. oryzicola (Xoc). In Xoc, the remainder of the LPS gene cluster, consisting of seven genes, is novel and unrelated to LPS gene clusters of any of the sequenced xanthomonads. We also report substantial interstrain variation suggestive of very recent horizontal gene transfer (HGT) at the LPS biosynthetic locus of Xanthomonas campestris pv. campestris (Xcc), the black rot pathogen of crucifers.

Conclusion

Our analyses indicate that HGT has altered the LPS locus during the evolution of Xanthomonas oryzae pathovars and suggest that the ancestor of all Xanthomonas oryzae pathovars had an Xac type of LPS gene cluster. Our finding of interstrain variation in two major xanthomonad pathogens infecting different hosts suggests that the LPS locus in plant pathogenic bacteria, as in animal pathogens, is under intense diversifying selection.

How rhizobial symbionts invade plants: the Sinorhizobium-Medicago model

Nitrogen-fixing rhizobial bacteria and leguminous plants have evolved complex signal exchange mechanisms that allow a specific bacterial species to induce its host plant to form invasion structures through which the bacteria can enter the plant root. Once the bacteria have been endocytosed within a host-membrane-bound compartment by root cells, the bacteria differentiate into a new form that can convert atmospheric nitrogen into ammonia. Bacterial differentiation and nitrogen fixation are dependent on the microaerobic environment and other support factors provided by the plant. In return, the plant receives nitrogen from the bacteria, which allows it to grow in the absence of an external nitrogen source. Here, we review recent discoveries about the mutual recognition process that allows the model rhizobial symbiont Sinorhizobium meliloti to invade and differentiate inside its host plant alfalfa (Medicago sativa) and the model host plant barrel medic (Medicago truncatula).