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

Infection of Medicago truncatula by the Root-Knot Nematode Meloidogyne javanica Does Not Require Early Nodulation Genes

Because of the developmental similarities between root nodules induced by symbiotic rhizobia and root galls formed by parasitic nematodes, we investigated the involvement of nodulation genes in the infection of Medicago truncatula by the root knot nematode (RKN), Meloidogyne javanica. We found that gall formation, including giant cell formation, pericycle and cortical cell division, as well as egg laying, occurred successfully in the non-nodulating mutants nfp1 (nod factor perception1), nin1 (nodule inception1) and nsp2 (nodulation signaling pathway2) and the cytokinin perception mutant cre1 (cytokinin receptor1). Gall and egg formation were significantly reduced in the ethylene insensitive, hypernodulating mutant skl (sickle), and to a lesser extent, in the low nodulation, abscisic acid insensitive mutant latd/nip (lateral root-organ defective/numerous infections and polyphenolics). Despite its supernodulation phenotype, the sunn4 (super numeric nodules4) mutant, which has lost the ability to autoregulate nodule numbers, did not form excessive numbers of galls. Co-inoculation of roots with nematodes and rhizobia significantly reduced nodule numbers compared to rhizobia-only inoculated roots, but only in the hypernodulation mutant skl. Thus, this effect is likely to be influenced by ethylene signaling, but is not likely explained by resource competition between galls and nodules. Co-inoculation with rhizobia also reduced gall numbers compared to nematode-only infected roots, but only in the wild type. Therefore, the protective effect of rhizobia on nematode infection does not clearly depend on nodule number or on Nod factor signaling. Our study demonstrates that early nodulation genes that are essential for successful nodule development are not necessary for nematode-induced gall formation, that gall formation is not under autoregulation of nodulation control, and that ethylene signaling plays a positive role in successful RKN parasitism in M. truncatula.

Local endoreduplication as a feature of intracellular fungal accommodation in arbuscular mycorrhizas

The intracellular accommodation of arbuscular mycorrhizal (AM) fungi is a paradigmatic feature of this plant symbiosis that depends on the activation of a dedicated signaling pathway and the extensive reprogramming of host cells, including striking changes in nuclear size and transcriptional activity. By combining targeted sampling of early root colonization sites, detailed confocal imaging, flow cytometry and gene expression analyses, we demonstrate that local, recursive events of endoreduplication are triggered in the Medicago truncatula root cortex during AM colonization. AM colonization induces an increase in ploidy levels and the activation of endocycle specific markers. This response anticipates the progression of fungal colonization and is limited to arbusculated and neighboring cells in the cortical tissue. Furthermore, endoreduplication is not induced in M. truncatula mutants for symbiotic signaling pathway genes. On this basis, we propose endoreduplication as part of the host cell prepenetration responses that anticipate AM fungal accommodation in the root cortex.

Pivotal role of LBD16 in root and root-like organ initiation

In the post-embryonic stage of Arabidopsis thaliana, roots can be initiated from the vascular region of the existing roots or non-root organs; they are designated as lateral roots (LRs) and adventitious roots (ARs), respectively. Some root-like organs can also be initiated from the vasculature. In tissue culture, auxin-induced callus, which is a group of pluripotent root-primordium-like cells, is formed via the rooting pathway. The formation of feeding structures from the vasculature induced by root-knot nematodes also borrows the rooting pathway. In this review, we summarize and discuss recent progress on the role of LATERAL ORGAN BOUNDARIES DOMAIN16 (LBD16; also known as ASYMMETRIC LEAVES2-LIKE18, ASL18), a member of the LBD/ASL gene family encoding plant-specific transcription factors, in roots and root-like organ initiation. Different root and root-like organ initiation processes have distinct priming mechanisms to specify founder cells. All these priming mechanisms converge to activate LBD16 expression in the primed founder cells. The activation of LBD16 expression leads to organ initiation via promotion of cell division and establishment of root-primordium identity. Therefore, LBD16 might play a common and pivotal role in root and root-like organ initiation.

Multiple Nodulation Genes Are Up-Regulated During Establishment of Reniform Nematode Feeding Sites in Soybean

The semi-endoparastic reniform nematode (Rotylenchulus reniformis) infects over 300 plant species. Females penetrate host roots and induce formation of complex, multinucleate feeding sites called syncytia. While anatomical changes associated with reniform nematode infection are well documented, little is known about their molecular basis. We grew soybean (Glycine max) in a split-root growth system, inoculated half of each root system with R. reniformis, and quantified gene expression in infected and control root tissue at four dates after inoculation. Over 6,000 genes were differentially expressed between inoculated and control roots on at least one date (false discovery rate [FDR] = 0.01, |log2FC| ≥ 1), and 507 gene sets were significantly enriched or depleted in inoculated roots (FDR = 0.05). Numerous genes up-regulated during syncytium formation had previously been associated with rhizobia nodulation. These included the nodule-initiating transcription factors CYCLOPS, NSP1, NSP2, and NIN, as well as multiple nodulins associated with the plant-derived peribacteroid membrane. Nodulation-related NIP aquaporins and SWEET sugar transporters were induced, as were plant CLAVATA3/ESR-related (CLE) signaling proteins and cell cycle regulators such as CCS52A and E2F. Nodulins and nodule-associated genes may have ancestral functions in normal root development and mycorrhization that have been co-opted by both parasitic nematodes and rhizobial bacteria to promote feeding site and nodule formation.

Anatomical Alterations in Plant Tissues Induced by Plant-Parasitic Nematodes

Plant-parasitic nematodes (PPNs) interact with plants in different ways, for example, through subtle feeding behavior, migrating destructively through infected tissues, or acting as virus-vectors for nepoviruses. They are all obligate biotrophic parasites as they derive their nutrients from living cells which they modify using pharyngeal gland secretions prior to food ingestion. Some of them can also shield themselves against plant defenses to sustain a relatively long lasting interaction while feeding. This paper is centered on cell types or organs that are newly induced in plants during PPN parasitism, including recent approaches to their study based on molecular biology combined with cell biology-histopathology. This issue has already been reviewed extensively for major PPNs (i.e., root-knot or cyst nematodes), but not for other genera (viz. Nacobbus aberrans, Rotylenchulus spp.). PPNs have evolved with plants and this co-evolution process has allowed the induction of new types of plant cells necessary for their parasitism. There are four basic types of feeding cells: (i) non-hypertrophied nurse cells; (ii) single giant cells; (iii) syncytia; and (iv) coenocytes. Variations in the structure of these cells within each group are also present between some genera depending on the nematode species viz. Meloidogyne or Rotylenchulus. This variability of feeding sites may be related in some way to PPN life style (migratory ectoparasites, sedentary ectoparasites, migratory ecto-endoparasites, migratory endoparasites, or sedentary endoparasites). Apart from their co-evolution with plants, the response of plant cells and roots are closely related to feeding behavior, the anatomy of the nematode (mainly stylet size, which could reach different types of cells in the plant), and the secretory fluids produced in the pharyngeal glands. These secretory fluids are injected through the stylet into perforated cells where they modify plant cytoplasm prior to food removal. Some species do not produce specialized feeding sites (viz. Ditylenchus, Subanguina), but may develop a specialized modification of the root system (e.g., unspecialized root galls or a profusion of roots). This review introduces new data on cell types and plant organs stimulated by PPNs using sources varying from traditional histopathology to new holistic methodologies.

Adapted Biotroph Manipulation of Plant Cell Ploidy

Diverse plant biotrophs that establish a sustained site of nutrient acquisition induce localized host endoreduplication. Endoreduplication is a process by which cells successively replicate their genomes without mitosis, resulting in an increase in nuclear DNA ploidy. Elevated ploidy is associated with enhanced cell size, metabolic capacity, and the capacity to differentiate. Localized host endoreduplication induced by adapted plant biotrophs promotes biotroph colonization, development, and/or proliferation. When induced host endoreduplication is limited, biotroph growth and/or development are compromised. Herein, we examine a diverse set of plant-biotroph interactions to identify (a) common host components manipulated to promote induced host endoreduplication and (b) biotroph effectors that facilitate this induced host process. Shared mechanisms to promote host endoreduplication and development of nutrient exchange/feeding sites include manipulation centered on endocycle entry at the G2-M transition as well as yet undefined roles for differentiation regulators (e.g., CLE peptides) and pectin/cell wall modification.

Molecular Transducers from Roots Are Triggered in Arabidopsis Leaves by Root-Knot Nematodes for Successful Feeding Site Formation: A Conserved Post-Embryogenic De novo Organogenesis Program?

Root-knot nematodes (RKNs; Meloidogyne spp.) induce feeding cells (giant cells; GCs) inside a pseudo-organ (gall) from still unknown root cells. Understanding GCs ontogeny is essential to the basic knowledge of RKN-plant interaction and to discover novel and effective control strategies. Hence, we report for the first time in a model plant, Arabidopsis, molecular, and cellular features concerning ectopic de novo organogenesis of RKNs GCs in leaves. RKNs induce GCs in leaves with irregular shape, a reticulated cytosol, and fragmented vacuoles as GCs from roots. Leaf cells around the nematode enter G2-M shown by ProCycB1;1:CycB1;1(NT)-GUS expression, consistent to multinucleated GCs. In addition, GCs nuclei present irregular and varied sizes. All these characteristics mentioned, being equivalent to GCs in root-galls. RKNs complete their life cycle forming a gall/callus-like structure in the leaf vascular tissues resembling auxin-induced callus with an auxin-response maxima, indicated by high expression of DR5::GUS that is dependent on leaf auxin-transport. Notably, induction of leaves calli/GCs requires molecular components from roots crucial for lateral roots (LRs), auxin-induced callus and root-gall formation, i.e., LBD16. Hence, LBD16 is a xylem pole pericycle specific and local marker in LR primordia unexpectedly induced locally in the vascular tissue of leaves after RKN infection. LBD16 is also fundamental for feeding site formation as RKNs could not stablish in 35S::LBD16-SRDX leaves, and likely it is also a conserved molecular hub between biotic and developmental signals in Arabidopsis either in roots or leaves. Moreover, RKNs induce the ectopic development of roots from leaf and root-galls, also formed in mutants compromised in LR formation, arf7/arf19, slr, and alf4. Therefore, nematodes must target molecular signatures to induce post-embryogenic de novo organogenesis through the LBD16 callus formation pathway partially different from those prevalent during normal LR development.

Gall-forming root-knot nematodes hijack key plant cellular functions to induce multinucleate and hypertrophied feeding cells

Among plant-parasitic nematodes, the root-knot nematodes (RKNs) of the Meloidogyne spp. are the most economically important genus. RKN are root parasitic worms able to infect nearly all crop species and have a wide geographic distribution. During infection, RKNs establish and maintain an intimate relationship with the host plant. This includes the creation of a specialized nutritional structure composed of multinucleate and hypertrophied giant cells, which result from the redifferentiation of vascular root cells. Giant cells constitute the sole source of nutrients for the nematode and are essential for growth and reproduction. Hyperplasia of surrounding root cells leads to the formation of the gall or root-knot, an easily recognized symptom of plant infection by RKNs. Secreted effectors produced in nematode salivary glands and injected into plant cells through a specialized feeding structure called the stylet play a critical role in the formation of giant cells. Here, we describe the complex interactions between RKNs and their host plants. We highlight progress in understanding host plant responses, focusing on how RKNs manipulate key plant processes and functions, including cell cycle, defence, hormones, cellular scaffold, metabolism and transport.

Combined genetic and transcriptomic analysis reveals three major signalling pathways activated by Myc-LCOs in Medicago truncatula

Myc-LCOs are newly identified symbiotic signals produced by arbuscular mycorrhizal (AM) fungi. Like rhizobial Nod factors, they are lipo-chitooligosaccharides that activate the common symbiotic signalling pathway (CSSP) in plants. To increase our limited understanding of the roles of Myc-LCOs we aimed to analyse Myc-LCO-induced transcriptional changes and their genetic control. Whole genome RNA sequencing (RNA-seq) was performed on roots of Medicago truncatula wild-type plants, and dmi3 and nsp1 symbiotic mutants affected in nodulation and mycorrhizal signalling. Plants were treated separately with the two major types of Myc-LCOs, sulphated and nonsulphated. Generalized linear model analysis identified 2201 differentially expressed genes and classified them according to genotype and/or treatment effects. Three genetic pathways for Myc-LCO-regulation of transcriptomic reprogramming were highlighted: DMI3- and NSP1-dependent; DMI3-dependent and NSP1-independent; and DMI3- and NSP1-independent. Comprehensive analysis revealed overlaps with previous AM studies, and highlighted certain functions, especially signalling components and transcription factors. These data provide new insights into mycorrhizal signalling mechanisms, supporting a role for NSP1, and specialisation for NSP1-dependent and -independent pathways downstream of DMI3. Our data also indicate significant Myc-LCO-activated signalling upstream of DMI3 and/or parallel to the CSSP and some constitutive activity of the CSSP.

Maturation of nematode-induced galls in Medicago truncatula is related to water status and primary metabolism modifications

Root-knot nematodes are obligatory plant parasitic worms that establish and maintain an intimate relationship with their host plants. During a compatible interaction, these nematodes induce the redifferentiation of root cells into multinucleate and hypertrophied giant cells (GCs). These metabolically active feeding cells constitute the exclusive source of nutrients for the nematode. We analyzed the modifications of water status, ionic content and accumulation of metabolites in development and mature galls induced by Meloidogyne incognita and in uninfected roots of Medicago truncatula plants. Water potential and osmotic pressure are significantly modified in mature galls compared to developing galls and control roots. Ionic content is significantly modified in galls compared to roots. Principal component analyses of metabolite content showed that mature gall metabolism is significantly modified compared to developing gall metabolism. The most striking differences were the three-fold increase of trehalose content associated to the five-fold diminution in glucose concentration in mature galls. Gene expression analysis showed that trehalose accumulation was, at least, partially linked to a significantly lower expression of the trehalase gene in mature galls. Our results point to significant modifications of gall physiology during maturation.

Phytohormone regulation of legume-rhizobia interactions

The symbiosis between legumes and nitrogen fixing bacteria called rhizobia leads to the formation of root nodules. Nodules are highly organized root organs that form in response to Nod factors produced by rhizobia, and they provide rhizobia with a specialized niche to optimize nutrient exchange and nitrogen fixation. Nodule development and invasion by rhizobia is locally controlled by feedback between rhizobia and the plant host. In addition, the total number of nodules on a root system is controlled by a systemic mechanism termed 'autoregulation of nodulation'. Both the local and the systemic control of nodulation are regulated by phytohormones. There are two mechanisms by which phytohormone signalling is altered during nodulation: through direct synthesis by rhizobia and through indirect manipulation of the phytohormone balance in the plant, triggered by bacterial Nod factors. Recent genetic and physiological evidence points to a crucial role of Nod factor-induced changes in the host phytohormone balance as a prerequisite for successful nodule formation. Phytohormones synthesized by rhizobia enhance symbiosis effectiveness but do not appear to be necessary for nodule formation. This review provides an overview of recent advances in our understanding of the roles and interactions of phytohormones and signalling peptides in the regulation of nodule infection, initiation, positioning, development, and autoregulation. Future challenges remain to unify hormone-related findings across different legumes and to test whether hormone perception, response, or transport differences among different legumes could explain the variety of nodules types and the predisposition for nodule formation in this plant family. In addition, the molecular studies carried out under controlled conditions will need to be extended into the field to test whether and how phytohormone contributions by host and rhizobial partners affect the long term fitness of the host and the survival and competition of rhizobia in the soil. It also will be interesting to explore the interaction of hormonal signalling pathways between rhizobia and plant pathogens.

Nitrogen-fixing Rhizobium-legume symbiosis: are polyploidy and host peptide-governed symbiont differentiation general principles of endosymbiosis?

The symbiosis between rhizobia soil bacteria and legumes is facultative and initiated by nitrogen starvation of the host plant. Exchange of signal molecules between the partners leads to the formation of root nodules where bacteria are converted to nitrogen-fixing bacteroids. In this mutualistic symbiosis, the bacteria provide nitrogen sources for plant growth in return for photosynthates from the host. Depending on the host plant the symbiotic fate of bacteria can either be reversible or irreversible. In Medicago plants the bacteria undergo a host-directed multistep differentiation process culminating in the formation of elongated and branched polyploid bacteria with definitive loss of cell division ability. The plant factors are nodule-specific symbiotic peptides. About 500 of them are cysteine-rich NCR peptides produced in the infected plant cells. NCRs are targeted to the endosymbionts and the concerted action of different sets of peptides governs different stages of endosymbiont maturation. This review focuses on symbiotic plant cell development and terminal bacteroid differentiation and demonstrates the crucial roles of symbiotic peptides by showing an example of multi-target mechanism exerted by one of these symbiotic peptides.

Involvement of papain and legumain proteinase in the senescence process of Medicago truncatula nodules

The symbiotic interaction between legumes and Rhizobiaceae leads to the formation of new root organs called nodules. Within the nodule, Rhizobiaceae differentiate into nitrogen-fixing bacteroids. However, this symbiotic interaction is time-limited as a result of the initiation of a senescence process, leading to a complete degradation of bacteroids and host plant cells. The increase in proteolytic activity is one of the key features of this process. In this study, we analysed the involvement of two different classes of cysteine proteinases, MtCP6 and MtVPE, in the senescence process of Medicago truncatula nodules. Spatiotemporal expression of MtCP6 and MtVPE was investigated using promoter- β-glucuronidase fusions. Corresponding gene inductions were observed during both developmental and stress-induced nodule senescence. Both MtCP6 and MtVPE proteolytic activities were increased during stress-induced senescence. Down-regulation of both proteinases mediated by RNAi in the senescence zone delayed nodule senescence and increased nitrogen fixation, while their early expression promoted nodule senescence. Using green fluorescent protein fusions, in vivo confocal imaging showed that both proteinases accumulated in the vacuole of uninfected cells or the symbiosomes of infected cells. These data enlighten the crucial role of MtCP6 and MtVPE in the onset of nodule senescence.

MtZR1, a PRAF protein, is involved in the development of roots and symbiotic root nodules in Medicago truncatula

PRAF proteins are present in all plants, but their functions remain unclear. We investigated the role of one member of the PRAF family, MtZR1, on the development of roots and nitrogen-fixing nodules in Medicago truncatula. We found that MtZR1 was expressed in all M. truncatula organs. Spatiotemporal analysis showed that MtZR1 expression in M. truncatula roots was mostly limited to the root meristem and the vascular bundles of mature nodules. MtZR1 expression in root nodules was down-regulated in response to various abiotic stresses known to affect nitrogen fixation efficiency. The down-regulation of MtZR1 expression by RNA interference in transgenic roots decreased root growth and impaired nodule development and function. MtZR1 overexpression resulted in longer roots and significant changes to nodule development. Our data thus indicate that MtZR1 is involved in the development of roots and nodules. To our knowledge, this work provides the first in vivo experimental evidence of a biological role for a typical PRAF protein in plants.

Peribacteroid space acidification: a marker of mature bacteroid functioning in Medicago truncatula nodules

Legumes form a symbiotic interaction with Rhizobiaceae bacteria, which differentiate into nitrogen-fixing bacteroids within nodules. Here, we investigated in vivo the pH of the peribacteroid space (PBS) surrounding the bacteroid and pH variation throughout symbiosis. In vivo confocal microscopy investigations, using acidotropic probes, demonstrated the acidic state of the PBS. In planta analysis of nodule senescence induced by distinct biological processes drastically increased PBS pH in the N2 -fixing zone (zone III). Therefore, the PBS acidification observed in mature bacteroids can be considered as a marker of bacteroid N2 fixation. Using a pH-sensitive ratiometric probe, PBS pH was measured in vivo during the whole symbiotic process. We showed a progressive acidification of the PBS from the bacteroid release up to the onset of N2 fixation. Genetic and pharmacological approaches were conducted and led to disruption of the PBS acidification. Altogether, our findings shed light on the role of PBS pH of mature bacteroids in nodule functioning, providing new tools to monitor in vivo bacteroid physiology.

Nematode feeding sites: unique organs in plant roots

Although generally unnoticed, nearly all crop plants have one or more species of nematodes that feed on their roots, frequently causing tremendous yield losses. The group of sedentary nematodes, which are among the most damaging plant-parasitic nematodes, cause the formation of special organs called nematode feeding sites (NFS) in the root tissue. In this review we discuss key metabolic and cellular changes correlated with NFS development, and similarities and discrepancies between different types of NFS are highlighted.

Complementary and dose-dependent action of AtCCS52A isoforms in endoreduplication and plant size control

· The dimension of organs depends on the number and the size of their component cells. Formation of polyploid cells by endoreduplication cycles is predominantly associated with increases in the cell size and implicated in organ growth. In plants, the CCS52A proteins play a major role in the switch from mitotic to endoreduplication cycles controlling thus the number of mitotic cells and the endoreduplication events in the differentiating cells. · Arabidopsis has two CCS52A isoforms; AtCCS52A1 and AtCCS52A2. Here we focused on their roles in endoreduplication and cell size control during plant development. We demonstrate their complementary and dose-dependent actions that are dependent on their expression patterns. Moreover, the impact of CCS52A overexpression on organ size in transgenic plants was dependent on the expression level; while enhanced expression of the CCS52A genes positively correlated with the ploidy levels, organ sizes were negatively affected by strong overexpression whereas milder overexpression resulted in a significant increase in the organ sizes. · Taken together, these finding support both complementary and dose-dependent actions for the Arabidopsis CCS52A isoforms in plant development and demonstrate that elevated ectopic CCS52A expression positively correlates with organ size, opening a route to higher biomass production.

miR396 affects mycorrhization and root meristem activity in the legume Medicago truncatula

The root system is crucial for acquisition of resources from the soil. In legumes, the efficiency of mineral and water uptake by the roots may be reinforced due to establishment of symbiotic relationships with mycorrhizal fungi and interactions with soil rhizobia. Here, we investigated the role of miR396 in regulating the architecture of the root system and in symbiotic interactions in the model legume Medicago truncatula. Analyses with promoter-GUS fusions suggested that the mtr-miR396a and miR396b genes are highly expressed in root tips, preferentially in the transition zone, and display distinct expression profiles during lateral root and nodule development. Transgenic roots of composite plants that over-express the miR396b precursor showed lower expression of six growth-regulating factor genes (MtGRF) and two bHLH79-like target genes, as well as reduced growth and mycorrhizal associations. miR396 inactivation by mimicry caused contrasting tendencies, with increased target expression, higher root biomass and more efficient colonization by arbuscular mycorrhizal fungi. In contrast to MtbHLH79, repression of three GRF targets by RNA interference severely impaired root growth. Early activation of mtr-miR396b, concomitant with post-transcriptional repression of MtGRF5 expression, was also observed in response to exogenous brassinosteroids. Growth limitation in miR396 over-expressing roots correlated with a reduction in cell-cycle gene expression and the number of dividing cells in the root apical meristem. These results link the miR396 network to the regulation of root growth and mycorrhizal associations in plants.

Nematode-induced endoreduplication in plant host cells: why and how?

Plant-parasitic root-knot and cyst nematodes have acquired the ability to induce remarkable changes in host cells during the formation of feeding sites. Root-knot nematodes induce several multinucleate giant cells inside a gall whereas cyst nematodes provoke the formation of a multinucleate syncytium. Both strategies impinge on the deregulation of the cell cycle, involving a major role for endoreduplication. This review will first describe the current knowledge on the control of normal and aberrant cell cycles. Thereafter, we will focus on the role of both cell-cycle routes in the transformation process of root cells into large and highly differentiated feeding sites as induced by the phytoparasitic root-knot and cyst nematodes.

The anaphase-promoting complex/cyclosome in control of plant development

Temporal controlled degradation of key cell division proteins ensures a correct onset of the different cell cycle phases and exit from the cell division program. In light of the cell cycle, the Anaphase-Promoting Complex/Cyclosome (APC/C) is an important conserved multi-subunit ubiquitin ligase, marking targets for degradation by the 26S proteasome. However, whereas the APC/C has been studied extensively in yeast and mammals, only in the last decade has the plant APC/C started to unveil its secrets. Research results have shown the importance of the APC/C core complex and its activators during gametogenesis, growth, hormone signaling, symbiotic interactions, and endoreduplication onset. In addition, recently, the first plant APC/C inhibitors have been reported, allowing a fine-tuning of APC/C activity during the cell cycle. Together with the identification of the first APC/C targets, a picture emerges of APC/C activity being essential for many different developmental processes.

CCS52 and DEL1 genes are key components of the endocycle in nematode-induced feeding sites

The establishment of galls and syncytia as feeding sites induced by root-knot and cyst nematodes, respectively, involves a progressive increase in nuclear and cellular size. Here we describe the functional characterization of endocycle activators CCS52A, CCS52B and a repressor of the endocycle, DEL1, during two types of nematode feeding site development in Arabidopsis thaliana. In situ hybridization analysis showed that expression of CCS52A1 and CCS52B was strongly induced in galls and syncytia and DEL1 was stably but weakly expressed throughout feeding site development. Down-regulation and over-expression of CCS52 and DEL1 in Arabidopsis drastically affected giant cell and syncytium growth, resulting in restrained nematode development, illustrating the need for mitotic activity and endo-reduplication for feeding site maturation. Exploiting the mechanism of endo-reduplication may be envisaged as a strategy to control plant-parasitic nematodes.

CaPrx, a Coffea arabica gene encoding a putative class III peroxidase induced by root-knot nematode infection

Class III peroxidases (Prxs) are enzymes involved in a multitude of physiological and stress-related processes in plants. Here, we report on the characterization of a putative peroxidase-encoding gene from Coffea arabica (CaPrx) that is expressed in early stages of root-knot nematode (RKN) infection. CaPrx showed enhanced expression in coffee roots inoculated with RKN (at 12 h post-inoculation), but no significant difference in expression was observed between susceptible and resistant plants. Assays using transgenic tobacco plants harboring a promoter-β-glucuronidase (GUS) fusion revealed that the CaPrx promoter was exclusively active in the galls induced by RKN. In cross sections of galls, GUS staining was predominantly localized in giant cells. Up-regulation of GUS expression in roots of transgenic plants following RKN inoculation was observed within 16 h. Moreover, no increase in GUS expression after treatment with jasmonic acid was detected. Altogether, these results point to a putative role of this peroxidase in the general coffee response to RKN infection.

Plant genes involved in harbouring symbiotic rhizobia or pathogenic nematodes

The establishment and development of plant-microorganism interactions involve impressive transcriptomic reprogramming of target plant genes. The symbiont (Sinorhizobium meliloti) and the root knot-nematode pathogen (Meloidogyne incognita) induce the formation of new root organs, the nodule and the gall, respectively. Using laser-assisted microdissection, we specifically monitored, at the cell level, Medicago gene expression in nodule zone II cells, which are preparing to receive rhizobia, and in gall giant and surrounding cells, which play an essential role in nematode feeding and constitute the typical root swollen structure, respectively. We revealed an important reprogramming of hormone pathways and C1 metabolism in both interactions, which may play key roles in nodule and gall neoformation, rhizobia endocytosis and nematode feeding. Common functions targeted by rhizobia and nematodes were mainly down-regulated, whereas the specificity of the interaction appeared to involve up-regulated genes. Our transcriptomic results provide powerful datasets to unravel the mechanisms involved in the accommodation of rhizobia and root-knot nematodes. Moreover, they raise the question of host specificity and the evolution of plant infection mechanisms by a symbiont and a pathogen.

(Homo)glutathione deficiency impairs root-knot nematode development in Medicago truncatula

Root-knot nematodes (RKN) are obligatory plant parasitic worms that establish and maintain an intimate relationship with their host plants. During a compatible interaction, RKN induce the redifferentiation of root cells into multinucleate and hypertrophied giant cells essential for nematode growth and reproduction. These metabolically active feeding cells constitute the exclusive source of nutrients for the nematode. Detailed analysis of glutathione (GSH) and homoglutathione (hGSH) metabolism demonstrated the importance of these compounds for the success of nematode infection in Medicago truncatula. We reported quantification of GSH and hGSH and gene expression analysis showing that (h)GSH metabolism in neoformed gall organs differs from that in uninfected roots. Depletion of (h)GSH content impaired nematode egg mass formation and modified the sex ratio. In addition, gene expression and metabolomic analyses showed a substantial modification of starch and γ-aminobutyrate metabolism and of malate and glucose content in (h)GSH-depleted galls. Interestingly, these modifications did not occur in (h)GSH-depleted roots. These various results suggest that (h)GSH have a key role in the regulation of giant cell metabolism. The discovery of these specific plant regulatory elements could lead to the development of new pest management strategies against nematodes.

Down-regulation of SymRK correlates with a deficiency in vascular bundle development in Phaseolus vulgaris nodules

The symbiotic interaction of legumes and rhizobia results in the formation of nitrogen-fixing nodules. Nodulation depends on the finely coordinated expression of a battery of genes involved in the infection and the organogenesis processes. After Nod factor perception, symbiosis receptor kinase (SymRK) receptor triggers a signal transduction cascade essential for nodulation leading to cortical cell divisions, infection thread (IT) formation and final release of rhizobia to the intracellular space, forming the symbiosome. Herein, the participation of SymRK receptor during the nodule organogenesis in Phaseolus vulgaris is addressed. Our findings indicate that besides its expression in the nodule epidermis, in IT, and in uninfected cells of the infection zone, PvSymRK immunolocalizes in the root and nodule vascular system. On the other hand, knockdown expression of PvSymRK led to the formation of scarce and defective nodules, which presented alterations in both IT/symbiosome formation and vascular system.

Functional roles of effectors of plant-parasitic nematodes

Plant pathogens have evolved a variety of different strategies that allow them to successfully infect their hosts. Plant-parasitic nematodes secrete numerous proteins into their hosts. These proteins, called effectors, have various functions in the plant cell. The most studied effectors to date are the plant cell wall degrading enzymes, which have an interesting evolutionary history since they are believed to have been acquired from bacteria or fungi by horizontal gene transfer. Extensive genome, transcriptome and proteome studies have shown that plant-parasitic nematodes secrete many additional effectors. The function of many of these is less clear although during the last decade, several research groups have determined the function of some of these effectors. Even though many effectors remain to be investigated, it has already become clear that they can have very diverse functions. Some are involved in suppression of plant defences, while others can specifically interact with plant signalling or hormone pathways to promote the formation of nematode feeding sites. In this review, the most recent progress in the understanding of the function of plant-parasitic nematode effectors is discussed.

Horizontal gene transfer in nematodes: a catalyst for plant parasitism?

The origin of plant parasitism within the phylum Nematoda is intriguing. The ability to parasitize plants has originated independently at least three times during nematode evolution and, as more molecular data has emerged, it has become clear that multiple instances of horizontal gene transfer (HGT) from bacteria and fungi have played a crucial role in the nematode's adaptation to this new lifestyle. The first reported HGT cases in plant-parasitic nematodes were genes encoding plant cell wall-degrading enzymes. Other putative examples of HGT were subsequently described, including genes that may be involved in the modulation of the plant's defense system, the establishment of a nematode feeding site, and the synthesis or processing of nutrients. Although, in many cases, it is difficult to pinpoint the donor organism, candidate donors are usually soil dwelling and are either plant-pathogenic or plant-associated microorganisms, hence occupying the same ecological niche as the nematodes. The exact mechanisms of transfer are unknown, although close contacts with donor microorganisms, such as symbiotic or trophic interactions, are a possibility. The widespread occurrence of horizontally transferred genes in evolutionarily independent plant-parasitic nematode lineages suggests that HGT may be a prerequisite for successful plant parasitism in nematodes.

Early transcriptomic events in microdissected Arabidopsis nematode-induced giant cells

Root-knot nematodes differentiate highly specialized feeding cells in roots (giant cells, GCs), through poorly characterized mechanisms that include extensive transcriptional changes. While global transcriptome analyses have used galls, which are complex root structures that include GCs and surrounding tissues, no global gene expression changes specific to GCs have been described. We report on the differential transcriptome of GCs versus root vascular cells, induced in Arabidopsis by Meloidogyne javanica at a very early stage of their development, 3 days after infection (d.p.i.). Laser microdissection was used to capture GCs and root vascular cells for microarray analysis, which was validated through qPCR and by a promoter-GUS fusion study. Results show that by 3 d.p.i., GCs exhibit major gene repression. Although some genes showed similar regulation in both galls and GCs, the majority had different expression patterns, confirming the molecular distinctiveness of the GCs within the gall. Most of the differentially regulated genes in GCs have no previously assigned function. Comparisons with other transcriptome analyses revealed similarities between GCs and cell suspensions differentiating into xylem cells. This suggests a molecular link between GCs and developing vascular cells, which represent putative GC stem cells. Gene expression in GCs at 3 d.p.i. was also found to be similar to crown galls induced by Agrobacterium tumefaciens, a specialized root biotroph.

Differing requirements for flavonoids during the formation of lateral roots, nodules and root knot nematode galls in Medicago truncatula

* In this study, we tested whether the organogenesis of symbiotic root nodules, lateral roots and root galls induced by parasitic root knot nematodes (Meloidogyne javanica) was regulated by the presence of flavonoids in the roots of Medicago truncatula. Flavonoids accumulate in all three types of root organ, and have been hypothesized previously to be required for secondary root organogenesis because of their potential role as auxin transport regulators. * Using RNA interference to silence the flavonoid biosynthetic pathway in M. truncatula, we generated transformed flavonoid-deficient hairy roots which were used to study flavonoid accumulation, cell division and organogenesis of nodules, lateral roots and root galls. * Flavonoid-deficient roots did not form nodules, as demonstrated previously, but showed altered root growth in response to rhizobia. By contrast, flavonoid-deficient roots showed no difference in the number of lateral roots and root galls. Galls on flavonoid-deficient roots formed normal giant cells, but were shorter, and were characterized by reduced numbers of dividing pericycle cells. * We rejected the hypothesis that flavonoids are required as general regulators of the organogenesis of secondary root organs, but flavonoids appear to be necessary for nodulation. Possible reasons for this difference in the requirement for flavonoids are discussed.

Comprehensive transcriptome profiling in tomato reveals a role for glycosyltransferase in Mi-mediated nematode resistance

Root-knot nematode (RKN; Meloidogyne spp.) is a major crop pathogen worldwide. Effective resistance exists for a few plant species, including that conditioned by Mi in tomato (Solanum lycopersicum). We interrogated the root transcriptome of the resistant (Mi+) and susceptible (Mi-) cultivars 'Motelle' and 'Moneymaker,' respectively, during a time-course infection by the Mi-susceptible RKN species Meloidogyne incognita and the Mi-resistant species Meloidogyne hapla. In the absence of RKN infection, only a single significantly regulated gene, encoding a glycosyltransferase, was detected. However, RKN infection influenced the expression of broad suites of genes; more than half of the probes on the array identified differential gene regulation between infected and uninfected root tissue at some stage of RKN infection. We discovered 217 genes regulated during the time of RKN infection corresponding to establishment of feeding sites, and 58 genes that exhibited differential regulation in resistant roots compared to uninfected roots, including the glycosyltransferase. Using virus-induced gene silencing to silence the expression of this gene restored susceptibility to M. incognita in 'Motelle,' indicating that this gene is necessary for resistance to RKN. Collectively, our data provide a picture of global gene expression changes in roots during compatible and incompatible associations with RKN, and point to candidates for further investigation.

Nuclear DNA endoreduplication and expression of the mitotic inhibitor Ccs52 associated to determinate and lupinoid nodule organogenesis

Lotus japonicus determinate nodules differ greatly from indeterminate nodules in their organogenesis and morphological characteristics, whereas Lupinus albus lupinoid nodules share features of determinate and indeterminate nodules. The mitotic inhibitor Ccs52A is essential for endoreduplication and ploidy-dependent cell enlargement during symbiotic cell differentiation in Medicago truncatula indeterminate nodules. ccs52A homolog genes were isolated from lupin and lotus nodules; the deduced Ccs52A proteins showed high sequence similarity with other Cdh-1-type activators of the anaphase-promoting complex and were grouped with A-type Ccs52 proteins from different plants. In lupin, ccs52A expression was restricted to the earlier stages of nodule development, whereas ccs52A transcripts accumulated in lotus nodule primordia and, to a lesser extent, in mature nodules. Nodule development in Lupinus albus involved a progressive increase in nuclear and cellular size and ploidy level; similarly, Lotus japonicus nodules contained polyploid nuclei and enlarged cells in the infected zone. Nevertheless, in situ hybridization experiments showed the highest ccs52A expression in the inner cortex cells of the lupin nodule primordium, probably associated to the increased size of these cells in mature nodules. In view of our results, Ccs52A-mediated endoreduplication appears to be a universal mechanism required for nodule cell differentiation during the establishment of nitrogen-fixing symbioses.

MtENOD11 gene activation during rhizobial infection and mycorrhizal arbuscule development requires a common AT-rich-containing regulatory sequence

The MtENOD11 gene from the model legume Medicago truncatula is transcriptionally activated both in response to Sinorhizobium meliloti Nod factors and throughout infection of root tissues by the nitrogen-fixing microsymbiont. To identify the regulatory sequences involved in symbiosis-related MtENOD11 expression, a series of promoter deletions driving the beta-glucuronidase reporter gene were analyzed in transgenic M. truncatula roots. These studies have revealed that distinct regulatory regions are involved in infection-related MtENOD11 expression compared with preinfection (Nod factor-mediated) expression. In particular, the 257-bp promoter sequence immediately upstream from the start codon is sufficient for infection-related expression, but is unable to drive gene transcription in response to the Nod factor elicitor. This truncated promoter is also sufficient to confer MtENOD11 expression during both the arbuscular mycorrhizal (AM) association and the parasitic interaction with root-knot nematodes. Site-directed mutagenesis further showed that a previously identified nodule-specific AT-rich motif is required for high-level MtENOD11 expression during S. meliloti infection as well as during AM colonization. However, mutation of this motif does not affect gene expression associated with nematode-feeding sites. Taken together, these results suggest a close link between regulatory mechanisms controlling transcriptional early nodulin gene activation during both rhizobial and AM root endosymbioses.

Genome-wide expression profiling of the host response to root-knot nematode infection in Arabidopsis

During a compatible interaction, root-knot nematodes (Meloidogyne spp.) induce the redifferentiation of root cells into multinucleate nematode feeding cells (giant cells). Hyperplasia and hypertrophy of the surrounding cells leads to the formation of a root gall. We investigated the plant response to root-knot nematodes by carrying out a global analysis of gene expression during gall formation in Arabidopsis, using giant cell-enriched root tissues. Among 22 089 genes monitored with the complete Arabidopsis transcriptome microarray gene-specific tag, we identified 3373 genes that display significant differential expression between uninfected root tissues and galls at different developmental stages. Quantitative PCR analysis and the use of promoter GUS fusions confirmed the changes in mRNA levels observed in our microarray analysis. We showed that a comparable number of genes were found to be up- and downregulated, indicating that gene downregulation might be essential to allow proper gall formation. Moreover, many genes belonging to the same family are differently regulated in feeding cells. This genome-wide overview of gene expression during plant-nematode interaction provides new insights into nematode feeding-cell formation, and highlights that the suppression of plant defence is associated with nematode feeding-site development.

Loss of susceptibility as an alternative for nematode resistance

Among plant pathogens, sedentary endoparasitic nematodes are one of the most damaging pests in global agriculture. These obligate parasites interact with their hosts in a quite unique and intriguing way. They induce the redifferentiation of root cells into specialized feeding cells essential for nematode growth and reproduction; thus, nematodes have evolved the ability to exploit plant genes and hijack host functions for their own requirements. Various approaches to engineer plants with resistance to parasitic nematodes have been pursued, most focusing on the introduction of resistance genes. An alternative strategy to achieve resistance is to exploit the susceptibility of plant disease. Better knowledge of the plant response during the compatible interaction should allow the identification of targets to engineer resistance to parasitic nematodes in crop species.

Microarray analysis of the gene expression profile induced by the endophytic plant growth-promoting rhizobacteria, Pseudomonas fluorescens FPT9601-T5 in Arabidopsis

Pseudomonas fluorescens FPT9601-T5 was originally identified as an endophytic plant growth-promoting rhizobacteria (PGPR) on tomato. To perform a molecular dissecttion of physiological and biochemical changes occurring in the host triggered by P. fluorescens FPT9601-T5 colonization, the model plant Arabidopsis was used in this study. Root colonization of Arabidopsis with P. fluorescens FPT9601-T5 promoted plant growth later than three weeks after inoculation and partially suppressed disease symptoms caused by Pseudomonas syringae pv. tomato DC3000, indicating that P. fluorescens FPT9601-T5 acted as a PGPR on Arabidopsis. To obtain a global view on transcript modification during the Arabidopsis-FPT9601-T5 interaction, we performed microarray analysis using Affymetrix Genechip probe arrays representing approximately 22,800 genes. The results showed that 95 and 105 genes were up- or down-regulated, respectively, more than twofold in FPT9601-T5-treated Arabidopsis plants as compared with control plants. Those up-regulated included genes involved in metabolism, signal transduction, and stress response. Noteworthy, upon FPT9601-T5 colonization, putative auxin-regulated genes and nodulin-like genes were up-regulated, and some ethylene-responsive genes were down-regulated. Our results suggest that P. fluorescens FPT9601-T5 triggered plant responses in a manner similar to known PGPR and, at least in some aspects, to rhizobia.

Root-knot nematodes and bacterial Nod factors elicit common signal transduction events in Lotus japonicus

The symbiosis responsible for nitrogen fixation in legume root nodules is initiated by rhizobial signaling molecules [Nod factors (NF)]. Using transgenically tagged microtubules and actin, we dynamically profiled the spatiotemporal changes in the cytoskeleton of living Lotus japonicus root hairs, which precede root-hair deformation and reflect one of the earliest host responses to NF. Remarkably, plant-parasitic root-knot nematodes (RKN) invoke a cytoskeletal response identical to that seen in response to NF and induce root-hair waviness and branching in legume root hairs via a signal able to function at a distance. Azide-killed nematodes do not produce this signal. A similar response to RKN was seen in tomato. Aspects of the host responses to RKN were altered or abolished by mutations in the NF receptor genes nfr1, nfr5, and symRK, suggesting that RKN produce a molecule with functional equivalence to NF, which we name NemF. Because the ability of RKN to establish feeding sites and reproduce was markedly reduced in the mutant lines, we propose that RKN have adapted at least part of the symbiont-response pathway to enhance their parasitic ability.

Transcriptome profiling in root nodules and arbuscular mycorrhiza identifies a collection of novel genes induced during Medicago truncatula root endosymbioses

Transcriptome profiling based on cDNA array hybridizations and in silico screening was used to identify Medicago truncatula genes induced in both root nodules and arbuscular mycorrhiza (AM). By array hybridizations, we detected several hundred genes that were upregulated in the root nodule and the AM symbiosis, respectively, with a total of 75 genes being induced during both interactions. The second approach based on in silico data mining yielded several hundred additional candidate genes with a predicted symbiosis-enhanced expression. A subset of the genes identified by either expression profiling tool was subjected to quantitative real-time reverse-transcription polymerase chain reaction for a verification of their symbiosis-induced expression. That way, induction in root nodules and AM was confirmed for 26 genes, most of them being reported as symbiosis-induced for the first time. In addition to delivering a number of novel symbiosis-induced genes, our approach identified several genes that were induced in only one of the two root endosymbioses. The spatial expression patterns of two symbiosis-induced genes encoding an annexin and a beta-tubulin were characterized in transgenic roots using promoter-reporter gene fusions.

Homologous soybean and Arabidopsis genes share responsiveness to cyst nematode infection

SUMMARY We previously isolated a partial soybean cDNA clone (D17.1) whose corresponding transcript increases in susceptible roots 1 day post inoculation (dpi) with the soybean cyst nematode, Heterodera glycines. Here we isolated the corresponding full-length cDNA from a soybean cDNA library and designated this gene of unknown function Gm17.1. Time course RNA gel blot analyses revealed that Gm17.1 mRNA steady-state levels were elevated in soybean roots following H. glycines infection up to at least 6 dpi. For further in-depth study we identified a homologous Arabidopsis thaliana gene and designated this gene At17.1. Arabidopsis is successfully infected by the sugar beet cyst nematode (H. schachtii), a close relative of H. glycines. We isolated the At17.1 promoter, fused it to the beta-glucuronidase (GUS) reporter gene, and transformed this construct into Arabidopsis plants as well as soybean hairy roots. Histochemical analysis of plant materials containing the At17.1::GUS construct revealed that the At17.1 promoter is functional in Arabidopsis as well as in soybean and that during normal plant development the At17.1 promoter directs GUS expression predominantly to the vascular tissues and root tips of both plant species. When At17.1::GUS Arabidopsis plants and soybean hairy roots were inoculated with cyst nematodes, strong GUS activity was detected within the cyst nematode-induced feeding structures. Further tests of At17.1 promoter activity in Arabidopsis revealed that this promoter was induced by auxin, jasmonic acid, mannitol and dehydration. Quantitative real-time reverse transcription-polymerase chain reaction assays of At17.1 expression confirmed the observed promoter characteristics. Based on our expression data and the observation that both the soybean and the Arabidopsis homologues behaved in a similar fashion following cyst nematode infection, it is likely that these genes are closely associated with cyst nematode parasitism of plants, potentially with hormone and osmotic changes occurring in the developing nematode feeding cells. Furthermore, these data provide additional insights into the strengths of the Arabidopsis-H. schachtii pathosystem to study cyst nematode-plant interactions in lieu of less tractable pathosystems. This finding is supported by the fact that the Arabidopsis promoter tested here produced similar results in Arabidopsis and soybean.

Signaling between nematodes and plants

After hatching in the soil, root-knot nematodes must locate and penetrate a root, migrate into the vascular cylinder, and establish a permanent feeding site. Presumably, these events are accompanied by extensive signaling between the nematode parasite and the host. Hence, much emphasis has been placed on identifying proteins that are secreted by the nematode during the migratory phase. Further progress in understanding the signaling events has been made recently by studying the host response. Striking parallels can be drawn between the nematode-plant interaction and plant symbioses with other microorganisms, and evidence is emerging to suggest that nematodes acquired components of their parasitic armory from those microbes.

Endoreduplication and activation of the anaphase-promoting complex during symbiotic cell development

Postembryonic development of plant organs requires a constant interplay between the cell cycle and the developmental programs. Upon endo- and exogenous signals, plant cells can enter, exit or modify the cell cycle. Alteration of mitotic cycles to endoreduplication cycles, where the genome is duplicated without mitosis, is common in plants and may play a role in cell differentiation. The switch from the mitotic to endocycles is regulated by Ccs52A, a plant orthologue of the yeast and animal Cdhl proteins, acting as substrate-specific activator of the anaphase-promoting complex E3 ubiquitin ligase. Here, several aspects of endoreduplication are discussed with special attention on nitrogen-fixing nodule development where endoreduplication is an integral part of symbiotic cell differentiation.

Secretions of plant-parasitic nematodes: a molecular update

The interaction between sedentary endoparasitic nematodes and plants is fascinating, because these animals have developed an ingenious way to manipulate the plant's gene regulation and metabolism to their own advantage. They are able to form highly specialized feeding structures in the plant root to satisfy their nutritional demands for development and reproduction. This ability makes them extremely successful parasites with severe consequences for agriculture. Triggered by these economical losses, detailed studies of the parasitic interaction have been performed, which resulted in an extensive descriptive knowledge. However, the underlying biochemical and molecular events of this intimate relationship have still not been elucidated. It is generally accepted that secretions produced by the nematode are responsible for the dramatic alteration of specific cells in the host plant. In the past few years, the identification of genes coding for secreted proteins was a breakthrough in plant nematode research. However, the available information is still too limited to allow the formulation of a comprehensive model, mainly because the sequences of many of these genes are novel with no similar sequence found in the existing databases. A new challenge in the coming years will be the functional analysis of these putative parasitism genes.

Plant-nematode interactions

Root-knot nematodes and cyst nematodes are obligate, biotrophic pathogens of numerous plant species. These organisms cause dramatic changes in the morphology and physiology of their hosts. The molecular characterization of induced plant genes has provided insight into the plant processes that are usurped by nematodes as they establish their specialized feeding cells. Recently, several gene products have been identified that are secreted by the nematode during parasitism. The corresponding genes have strong similarity to microbial genes or to genes that are found in nematodes that parasitize animals. New information on host resistance genes and nematode virulence genes provides additional insight into this complex interaction.

Root-knot nematode parasitism and host response: molecular basis of a sophisticated interaction

SUMMARY Taxonomy: Eukaryota; Metazoa; Nematoda; Chromadorea; order Tylenchida; Tylenchoidea; Heteroderidae; genus Meloidogyne. Physical properties: Microscopic-non-segmented worms. Meloidogyne species can reproduce by apomixis, facultative meiotic parthenogenesis or obligate mitotic parthenogenesis. Obligate biotrophic parasites inducing the re-differentiation of plant cells into specialized feeding cells. Hosts: Meloidogyne spp. can infest more than 3000 plant species including vegetables, fruit trees, cereals and ornamental flowers.

SYMPTOMS

Root swellings called galls. Alteration of the root vascular system. Disease control: Cultural control, chemical control, resistant cultivars. Agronomic importance: Major threat to agriculture in temperate and tropical regions.

A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs

Transcriptome analysis of Medicago truncatula nodules has led to the discovery of a gene family named NCR (nodule-specific cysteine rich) with more than 300 members. The encoded polypeptides were short (60-90 amino acids), carried a conserved signal peptide, and, except for a conserved cysteine motif, displayed otherwise extensive sequence divergence. Family members were found in pea (Pisum sativum), broad bean (Vicia faba), white clover (Trifolium repens), and Galega orientalis but not in other plants, including other legumes, suggesting that the family might be specific for galegoid legumes forming indeterminate nodules. Gene expression of all family members was restricted to nodules except for two, also expressed in mycorrhizal roots. NCR genes exhibited distinct temporal and spatial expression patterns in nodules and, thus, were coupled to different stages of development. The signal peptide targeted the polypeptides in the secretory pathway, as shown by green fluorescent protein fusions expressed in onion (Allium cepa) epidermal cells. Coregulation of certain NCR genes with genes coding for a potentially secreted calmodulin-like protein and for a signal peptide peptidase suggests a concerted action in nodule development. Potential functions of the NCR polypeptides in cell-to-cell signaling and creation of a defense system are discussed.