Intracellular accommodation of microbes by plants: a common developmental program for symbiosis and disease?

Expression profiling in Medicago truncatula identifies more than 750 genes differentially expressed during nodulation, including many potential regulators of the symbiotic program

In this study, we describe a large-scale expression-profiling approach to identify genes differentially regulated during the symbiotic interaction between the model legume Medicago truncatula and the nitrogen-fixing bacterium Sinorhizobium meliloti. Macro- and microarrays containing about 6,000 probes were generated on the basis of three cDNA libraries dedicated to the study of root symbiotic interactions. The experiments performed on wild-type and symbiotic mutant material led us to identify a set of 756 genes either up- or down-regulated at different stages of the nodulation process. Among these, 41 known nodulation marker genes were up-regulated as expected, suggesting that we have identified hundreds of new nodulation marker genes. We discuss the possible involvement of this wide range of genes in various aspects of the symbiotic interaction, such as bacterial infection, nodule formation and functioning, and defense responses. Importantly, we found at least 13 genes that are good candidates to play a role in the regulation of the symbiotic program. This represents substantial progress toward a better understanding of this complex developmental program.

Functional genomics of arbuscular mycorrhiza: decoding the symbiotic cell programme

More extensive insight into plant genes involved in the symbiotic programme of arbuscular mycorrhiza is presently being achieved by global approaches that aim to discover novel genes or subsets of genes that are essential to cell programmes in the different steps of plant–fungal interactions. The strategy of functional genomics based on large-scale differential RNA expression analyses (differential-display reverse transcriptase - PCR), electronic Northerns, suppressive subtractive hybridization, DNA chips) is presented, with a focus on arbuscular mycorrhiza in Pisum sativum and Medicago truncatula. The most recent knowledge about gene networks that are modulated in roots during arbuscular establishment and functioning is discussed.Key words: arbuscular mycorrhiza, symbiotic programme, gene expression, pea, annual alfalfa.

Cytological, genetic, and molecular analysis to characterize compatible and incompatible interactions between Medicago truncatula and Colletotrichum trifolii

In this study, a new pathosystem was established using the model plant Medicago truncatula and Colletotrichum trifolii, the causal agent of anthracnose on Medicago sativa. Screening of a few M. truncatula lines identified Jemalong and F83005.5 as resistant and susceptible to Colletotrichum trifolii race 1, respectively. Symptom analysis and cytological studies indicated that resistance of Jemalong was associated with a hypersensitive response of the plant. The two selected lines were crossed, and inoculations with C. trifolii were performed on the resulting F1 and F2 progenies. Examination of the disease phenotypes indicated that resistance was dominant and was probably due to a major resistance gene. Molecular components of the resistance were analyzed through macroarray experiments. Expression profiling of 126 expressed sequence tags corresponding to 92 genes, which were selected for their putative functions in plant defense or signal transduction, were compared in Jemalong and F83005.5 lines. A strong correlation was observed between the number of up-regulated genes and the resistance phenotype. Large differences appeared at 48 h postinoculation; more than 40% of the tested genes were up-regulated in the Jemalong line compared with only 10% in the susceptible line. Interestingly, some nodulin genes were also induced in the resistant line upon inoculation with C. trifolii.

Recognition events in AM symbiosis: analysis of fungal gene expression at the early appressorium stage

Arbuscular mycorrhizal symbiosis is induced upon a series of recognition events involving the reorganization of both plant and fungal cellular programs culminating in the formation of appressoria on the epidermal root cells. In this work we monitored for the first time the genetic changes occurring in the fungal partner during early appressorium development. We established an in vitro system of Glomus mosseae and Petroselinum crispum for studying appressorium formation and found that after 120 h first appressoria developed in the root epidermis. We have constructed a fungal subtractive suppressive library enriched in genes up-regulated at this stage. Our aim was to identify early signaling events during plant recognition leading to appressoria formation. The library contains 375 clones with an average size of 500 bp. From these, 200 clones were sequenced and most of them represent gene fragments with no known homologues (63%) and therefore putative new genes specific to the mycorrhiza symbiosis. Reverse-Northern blot and RT-PCR analyses confirmed that ca. 30% of the genes present in the library were up-regulated upon plant induction after 120 h. Among the genes with homologues in other organisms we found several genes common to other plant-microbe interactions including some genes related to Ca2+-dependent signaling. The up-regulation of these genes opens the possibility that Ca2+ plays a role in the early stages of mycorrhiza formation as it has been found in other plant-microbe interactions such as the Rhizobium symbiosis or the Magnaporthe grisea/rice pathogenic interaction.

Distinct roles of Lotus japonicus SYMRK and SYM15 in root colonization and arbuscule formation

•  The colonization of Lotus japonicus roots by the arbuscular mycorrhizal fungus Glomus intraradices was analysed in plant mutants affected in the symbiosis genes, SYM15 or SYMRK. SYMRK encodes an LRR receptor-like kinase that is, like the SYM15 gene, essential for both mycorrhizal and rhizobial symbioses. •  Different colonization patterns were observed in growing vs meristematically arrested roots. •  Three steps in the interaction were differentially impaired in the mutants: surface opening, where the anticlinal cell walls of two adjacent epidermal cells separate from each other in the vicinity of fungal hyphae; intracellular passage of hyphae through an exodermal cell and an adjacent cell of the outermost cortical layer; and arbuscule formation in cells of the two innermost cortical layers. •  The combined results indicate that LjSYMRK is required for the intracellular passage through exodermis and outermost cortical cell layer whereas LjSYM15 is required for surface opening and arbuscule formation.

Cell surface interactions of Rhizobium bacteroids and other bacterial strains with symbiosomal and peribacteroid membrane components from pea nodules

Samples of Rhizobium bacteroids isolated from pea nodule symbiosomes reacted positively with a monoclonal antibody recognizing N-linked glycan epitopes on plant glycoproteins associated with the peribacteroid membrane and peribacteroid fluid. An antiserum recognizing the symbiosomal lectin-like glycoprotein PsNLEC-1 also reacted positively. Samples of isolated bacteroids also reacted with an antibody recognizing a glycolipid component of the peribacteroid membrane and plasma membrane. Bacterial cells derived from free-living cultures then were immobilized on nitrocellulose sheets and tested for their ability to associate with components of plant extracts derived from nodule fractionation. A positive antibody-staining reaction indicated that both PsNLEC-1 and membrane glycolipid had become associated with the bacterial surface. A range of rhizobial strains with mutants affecting cell surface polysaccharides all showed similar interactions with PsNLEC-1 and associated plant membranes, with the exception of strain B659 (a deep-rough lipopolysaccharide mutant of Rhizobium leguminosarum). However, the presence of a capsule of extracellular polysaccharide apparently prevented interactions between rhizobial cells and these plant components. The importance of a close association between peribacteroid membranes, PsNLEC-1, and the bacterial surface is discussed in the context of symbiosome development.

Targeted gene disruption of glycerol-3-phosphate dehydrogenase in Colletotrichum gloeosporioides reveals evidence that glycerol is a significant transferred nutrient from host plant to fungal pathogen

Unidirectional transfer of nutrients from plant host to pathogen represents a most revealing aspect of the parasitic lifestyle of plant pathogens. Whereas much effort has been focused on sugars and amino acids, the identification of other significant metabolites is equally important for comprehensive characterization of metabolic interactions between plants and biotrophic fungal pathogens. Employing a strategy of targeted gene disruption, we generated a mutant strain (gpdhDelta) defective in glycerol-3-phosphate dehydrogenase in a hemibiotrophic plant pathogen, Colletotrichum gloeosporioides f.sp. malvae. The gpdhDelta strain had severe defects in carbon utilization as it could use neither glucose nor amino acids for sustained growth. Although the mutant mycelia were able to grow on potato dextrose agar medium, they displayed arrhythmicity in growth and failure to conidiate. The metabolic defect of gpdhDelta could be entirely ameliorated by glycerol in chemically defined minimal medium. Furthermore, glycerol was the one and only metabolite that could restore rhythmic growth and conidiation of gpdhDelta. Despite the profound defects in carbon source utilization, in planta the gpdhDelta strain exhibited normal pathogenicity, proceeded normally in its life cycle, and produced abundant conidia. Analysis of plant tissues at the peripheral zone of fungal infection sites revealed a time-dependent reduction in glycerol content. This study provides strong evidence for a role of glycerol as a significant transferred metabolite from plant to fungal pathogen.

Plant recognition of symbiotic bacteria requires two LysM receptor-like kinases

Although most higher plants establish a symbiosis with arbuscular mycorrhizal fungi, symbiotic nitrogen fixation with rhizobia is a salient feature of legumes. Despite this host range difference, mycorrhizal and rhizobial invasion shares a common plant-specified genetic programme controlling the early host interaction. One feature distinguishing legumes is their ability to perceive rhizobial-specific signal molecules. We describe here two LysM-type serine/threonine receptor kinase genes, NFR1 and NFR5, enabling the model legume Lotus japonicus to recognize its bacterial microsymbiont Mesorhizobium loti. The extracellular domains of the two transmembrane kinases resemble LysM domains of peptidoglycan- and chitin-binding proteins, suggesting that they may be involved directly in perception of the rhizobial lipochitin-oligosaccharide signal. We show that NFR1 and NFR5 are required for the earliest physiological and cellular responses to this lipochitin-oligosaccharide signal, and demonstrate their role in the mechanism establishing susceptibility of the legume root for bacterial infection.

Reactive oxygen species and ethylene play a positive role in lateral root base nodulation of a semiaquatic legume

Lateral root base nodulation on the tropical, semiaquatic legume Sesbania rostrata results from two coordinated, Nod factor-dependent processes: formation of intercellular infection pockets and induction of cell division. Infection pocket formation is associated with cell death and production of hydrogen peroxide. Pharmacological experiments showed that ethylene and reactive oxygen species mediate Nod factor responses and are required for nodule initiation, whereby induction of division and infection could not be uncoupled. Application of purified Nod factors triggered cell division, and both Nod factors and ethylene induced cavities and cell death features in the root cortex. Thus, in S. rostrata, ethylene and reactive oxygen species act downstream from the Nod factors in pathways that lead to formation of infection pockets and initiation of nodule primordia.

EST mining and functional expression assays identify extracellular effector proteins from the plant pathogen Phytophthora

Plant pathogenic microbes have the remarkable ability to manipulate biochemical, physiological, and morphological processes in their host plants. These manipulations are achieved through a diverse array of effector molecules that can either promote infection or trigger defense responses. We describe a general functional genomics approach aimed at identifying extracellular effector proteins from plant pathogenic microorganisms by combining data mining of expressed sequence tags (ESTs) with virus-based high-throughput functional expression assays in plants. PexFinder, an algorithm for automated identification of extracellular proteins from EST data sets, was developed and applied to 2147 ESTs from the oomycete plant pathogen Phytophthora infestans. The program identified 261 ESTs (12.2%) corresponding to a set of 142 nonredundant Pex (Phytophthora extracellular protein) cDNAs. Of these, 78 (55%) Pex cDNAs were novel with no significant matches in public databases. Validation of PexFinder was performed using proteomic analysis of secreted protein of P. infestans. To identify which of the Pex cDNAs encode effector proteins that manipulate plant processes, high-throughput functional expression assays in plants were performed on 63 of the identified cDNAs using an Agrobacterium tumefaciens binary vector carrying the potato virus X (PVX) genome. This led to the discovery of two novel necrosis-inducing cDNAs, crn1 and crn2, encoding extracellular proteins that belong to a large and complex protein family in Phytophthora. Further characterization of the crn genes indicated that they are both expressed in P. infestans during colonization of the host plant tomato and that crn2 induced defense-response genes in tomato. Our results indicate that combining data mining using PexFinder with PVX-based functional assays can facilitate the discovery of novel pathogen effector proteins. In principle, this strategy can be applied to a variety of eukaryotic plant pathogens, including oomycetes, fungi, and nematodes.

Overexpression of a Brassica nigra cDNA gives enhanced resistance to Leptosphaeria maculans in B. napus

Using a polymerase chain reaction-based strategy, we have isolated a cDNA sequence from Brassica nigra, denoted Lm1, which significantly improves blackleg resistance when expressed in transgenic oilseed rape, B. napus. Lm1 was shown to map to locations on linkage groups 3 and 8 in the Brassica B-genome previously linked to both cotyledon, adult leaf, and stem resistance. B. napus plants transformed with Lm1 under the control of a constitutive promoter exhibited broad spectrum resistance to all L. maculans isolates tested, but enhanced resistance to Alternaria brassicae and Verticillium longisporum was not observed. A transcript corresponding to the cDNA size was induced in B. nigra 12 h after fungal challenge from a level of weak constitutive expression previous to inoculation. The Lm1 sequence bears no resemblance to previously characterized plant resistance genes but has two predicted transmembrane motifs. Several sequences with high homology to Lm1 were found in the databases. Lm1 appears to be a member of a larger group of related genes present in a variety of plant species. Most of them have unknown functions, but homology between Lm1 and the nodule inception gene of Lotus japonicus suggests an intriguing link between defense-related and symbiotic pathways.

Genetic analysis of a pH-regulated operon from Rhizobium tropici CIAT899 involved in acid tolerance and nodulation competitiveness

Rhizobium tropici CIAT899 is highly acid tolerant and a good competitor for Phaseolus vulgaris nodule occupancy at low pH values. Using Tn5 mutagenesis, we identified an operon required for acid tolerance and nodulation competitiveness. The insertion was mapped to the 5' end of atvA, encoding a product with high sequence identity to the agro-bacterial AcvB virulence protein. Complementation analyses indicated that atvA is an ortholog of acvB, both genes being required for acid tolerance. A Ser/Ala substitution in the LIPASE_SER motif of AtvA resulted in an acid sensitive Fix+ but very poorly competing strain, demonstrating that Ser-313 is essential for AtvA function. atvA is the second gene in an operon that is transcriptionally upregulated by acid shock. The acid-responsive promoter was mapped to a 469-bp intergenic region located upstream of lpiA, the first gene in the operon. lpiA-like genes are found in several alpha, beta, and gamma Proteobacteria that interact with eukaryotic host cells, and they are predicted to encode membrane proteins related to the FmtC/MprF family from low G+C Firmicutes. The latter proteins are involved in resistance to cationic antimicrobial peptides. A nonpolar deletion in lpiA caused a sevenfold decrease in relative nodulation competitiveness.

Exploring root symbiotic programs in the model legume Medicago truncatula using EST analysis

We report on a large-scale expressed sequence tag (EST) sequencing and analysis program aimed at characterizing the sets of genes expressed in roots of the model legume Medicago truncatula during interactions with either of two microsymbionts, the nitrogen-fixing bacterium Sinorhizobium meliloti or the arbuscular mycorrhizal fungus Glomus intraradices. We have designed specific tools for in silico analysis of EST data, in relation to chimeric cDNA detection, EST clustering, encoded protein prediction, and detection of differential expression. Our 21 473 5'- and 3'-ESTs could be grouped into 6359 EST clusters, corresponding to distinct virtual genes, along with 52 498 other M.truncatula ESTs available in the dbEST (NCBI) database that were recruited in the process. These clusters were manually annotated, using a specifically developed annotation interface. Analysis of EST cluster distribution in various M.truncatula cDNA libraries, supported by a refined R test to evaluate statistical significance and by 'electronic northern' representation, enabled us to identify a large number of novel genes predicted to be up- or down-regulated during either symbiotic root interaction. These in silico analyses provide a first global view of the genetic programs for root symbioses in M.truncatula. A searchable database has been built and can be accessed through a public interface.

Evolution of signal transduction in intracellular symbiosis

Plant roots form intracellular symbioses with fungi and bacteria resulting in arbuscular mycorrhiza and nitrogen-fixing root nodules, respectively. A novel receptor like-kinase has been discovered that is required for the transduction of both bacterial and fungal symbiotic signals. This kinase defines an ancient signalling pathway that probably evolved in the context of arbuscular mycorrhiza and has been recruited subsequently for endosymbiosis with bacteria. An ancestral symbiotic interaction of roots with intracellular bacteria might have emerged from such a recruitment, in the progenitor of the nodulating clade of plants. Analysis of symbiotic mutants of host plants and bacterial microsymbionts has revealed that present-day endosymbioses require the coordinated induction of more than one signalling pathway for development.

The endosymbiosis-induced genes ENOD40 and CCS52a are involved in endoparasitic-nematode interactions in Medicago truncatula

Plants associate with a wide range of mutualistic and parasitic biotrophic organisms. Here, we investigated whether beneficial plant symbionts and biotrophic pathogens induce distinct or overlapping regulatory pathways in Medicago truncatula. The symbiosis between Sinorhizobium meliloti and this plant results in the formation of nitrogen-fixing root nodules requiring the activation of specific genes in the host plant. We studied expression patterns of nodule-expressed genes after infection with the root-knot nematode Meloidogyne incognita. Two regulators induced during nodule organogenesis, the early nodulin gene ENOD40 involved in primordium formation and the cell cycle gene CCS52a required for cell differentiation and endoreduplication, are expressed in galls of the host plant. Expression analysis of promoter-uidA fusions indicates an accumulation of CCS52a transcripts in giant cells undergoing endoreduplication, while ENOD40 expression is localized in surrounding cell layers. Transgenic plants overexpressing ENOD40 show a significantly higher number of galls. In addition, out of the 192 nodule-expressed genes tested, 38 genes were upregulated in nodules at least threefold compared with control roots, but only two genes, nodulin 26 and cyclin D3, were found to be induced in galls. Taken together, these results suggest that certain events, such as endoreduplication, cell-to-cell communication with vascular tissues, or water transport, might be common between giant cell formation and nodule development.

A model for the development of the rhizobial and arbuscular mycorrhizal symbioses in legumes and its use to understand the roles of ethylene in the establishment of these two symbioses

We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encounter a regulatory level of ethylene at the epidermis – outermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants.Key words: AM, epidermis, evolution, pea, rhizobia, sym mutant.

Dual requirement of the LjSym4 gene for mycorrhizal development in epidermal and cortical cells of Lotus japonicus roots

• The LjSym4 mutation leads to Lotus japonicus plants that are defective in arbuscular mycorrhiza (AM) development. • Two alleles of LjSym4 with different phenotypic strength are compared here. The development of AM was assessed by considering five parameters related to fungal structures present in root segments from wild-type and mutant plants. The distribution of intercellular hyphae was determined using semithin sections from resin-embedded roots. Cellular interactions were investigated ultrastructurally, whereas cell wall components from the host plant were identified using immunogold labeling. • In roots of Ljsym4-1 mutant, fungal hyphae were mostly restricted to the intercellular spaces of the cortex, indicating a block to infection by mutant cortical cells, which resulted in a very low number of arbuscules. • This observation suggests the presence of an additional, genetically defined 'checkpoint' for mycorrhizal development, located at the wall of cortical cells. The LjSym4 gene is therefore required for infection of both epidermal and cortical cells by AM fungi.

ActP controls copper homeostasis in Rhizobium leguminosarum bv. viciae and Sinorhizobium meliloti preventing low pH-induced copper toxicity

Two 'calcium-irreparable' acid-sensitive mutants were identified after mutagenizing Rhizobium leguminosarum bv. viciae and Sinorhizobium meliloti with Tn5. Each mutant contains a single copy of the transposon which, inserted within the actP gene, prevents expression of a P-type ATPase that belongs to the CPx heavy metal-transporting subfamily. Here, we show that both actP-knockout mutants show sensitivity to copper; omission of this heavy metal from low pH-buffered media restores acid tolerance to these strains. Furthermore, complementation of the mutant phenotype requires only the actPgene. An actP-gusA fusion in R. leguminosarum was transcriptionally regulated by copper in a pH-dependent manner.Downstream to actP in both organisms is the hmrR gene that encodes a heavy metal-responsive regulator (HmrR) that belongs to the merR class of regulatory genes. Insertional Inactivation of hmrR abolished transcriptional activation of actP by copper ions and increased the basal level of its expression in their absence. These observations suggest that HmrR can regulate actP transcription positively and negatively. We show that copper homeostasis is an essential mechanism for the acid tolerance of these root nodule bacteria since it prevents this heavy metal from becoming overtly toxic in acidic conditions.

Developmental genetics and evolution of symbiotic structures in nitrogen-fixing nodules and arbuscular mycorrhiza

Genetic and molecular mechanisms of development are compared for two major plant-microbe endosymbioses: N(2)-fixing nodules (with rhizobia or actinomycetes Frankia) and arbuscular mycorrhiza (with Glomales fungi). Development from the primordia formed de novo in root tissues is common for all known types of N(2)-fixing nodules. However, their structure varies greatly with respect to: (i) tissue topology (location of vascular bundles is peripherical in legumes or central in non-legumes); (ii) position of nodule primordium (inner or outer cortex in legumes, pericycle in non-legumes); (iii) stability of apical meristem (persistent in the indeterminate nodules, transient in the determinate ones). In addition, legumes vary in ability to form compartments harboring endosymbiotic rhizobia and located intercellularly (infection threads) and intracellularly (symbiosomes). Using pea (Pisum sativum) symbiotic mutants, the nodule developmental program is dissected into a range of spatially and temporarily differentiated steps comprising four sub-programs (development of endosymbiotic compartments; nodule histogenesis; autoregulation of nodulation; bacteroid differentiation). The developmental mutations are suggested in some cases to reverse the endosymbiotic system into the morphologically simpler forms some of which may correspond to the ancestral stages of nodule evolution. The origin of legume-rhizobial and actinorhizal symbioses is suggested to be based on a set of preadaptations many of which had been evolved in angiosperms during coevolution with arbuscular mycorrhizal fungi (e.g., inter- and intracellular maintenance of symbionts, their control via defence-like reactions and recognition of chitin-like molecules). An analysis of parallel morphological variation in symbiotic mutants and wild-growing legume species enables us to reconstruct the major stages of evolution for N(2)-fixing symbioses.

Overlapping plant signal transduction pathways induced by a parasitic nematode and a rhizobial endosymbiont

Root-knot nematodes and rhizobia establish interactions with roots characterized by the de novo induction of host structures, termed giant cells and nodules, respectively. Two transcription regulators, PHAN and KNOX, required for the establishment of meristems were previously shown to be expressed in tomato giant cells. We isolated the orthologues of PHAN and KNOX (Mt-phan and Mt-knox-1) from the model legume Medicago truncatula, and established the spatial distribution of their expression in situ. We confirmed that Mt-phan and Mt-knox-1 are expressed in lateral root initials and in nematode-induced giant cells and showed that they are expressed in nodules induced by Sinorhizobium meliloti. Expression of both genes becomes spatially restricted as the nodules develop. We further examined nematode feeding sites for the expression of two genes involved in nodule formation, ccs52 (encodes a mitotic inhibitor) and ENOD40 (encodes an early, nodulation mitogen), and found transcripts of both genes to be present in and around giant cells induced in Medicago. Collectively, these results reveal common elements of host responses to mutualistic and parasitic plant endosymbionts and imply that overlapping regulatory pathways lead to giant cells and nodules. We discuss these pathways in the context of phytohormones and parallels between beneficial symbiosis and disease.

Genetics and genomics of root symbiosis

Model genetics and genomics have been developed as tools for studying the third largest family of flowering plants, the Leguminosae, which includes important crop plants. Functional genomics strategies for the global analysis of gene expression, the elucidation of pathways and reverse genetics are established. These approaches provide new possibilities for investigating rhizobial as well as mycorrhizal endosymbiosis. Plant genes with central functions in these mutualistic interactions have been identified by positional cloning and gene tagging. With progress in Lotus japonicus genome sequencing, which was recently initiated by Japanese researchers, comparative genomics will contribute to our understanding of symbiosis, pathogenesis and the evolution of plant genomes.

The Lotus japonicus LjSym4 gene is required for the successful symbiotic infection of root epidermal cells

The role of the Lotus japonicus LjSym4 gene during the symbiotic interaction with Mesorhizobium loti and arbuscular mycorrhizal (AM) fungi was analyzed with two mutant alleles conferring phenotypes of different strength. Ljsym4-1 and Ljsym4-2 mutants do not form nodules with M. loti. Normal root hair curling and infection threads are not observed, while a nodC-dependent deformation of root hair tips indicates that nodulation factors are still perceived by Ljsym4 mutants. Fungal infection attempts on the mutants generally abort within the epidermis, but Ljsym4-1 mutants allow rare, successful, infection events, leading to delayed arbuscule formation. On roots of mutants homozygous for the Ljsym4-2 allele, arbuscule formation was never observed upon inoculation with either of the two AM fungi, Glomus intraradices or Gigaspora margarita. The strategy of epidermal penetration by G. margarita was identical for Ljsym4-2 mutants and the parental line, with appressoria, hyphae growing between two epidermal cells, penetration of epidermal cells through their anticlinal wall. These observations define a novel, genetically controlled step in AM colonization. Although rhizobia penetrate the tip of root hairs and AM fungi access an entry site near the base of epidermal cells, the LjSym4 gene is necessary for the appropriate response of this cell type to both microsymbionts. We propose that LjSym4 is required for the initiation or coordinated expression of the host plant cell's accommodation program, allowing the passage of both microsymbionts through the epidermis layer.