Legume plants host nitrogen-fixing endosymbiotic Rhizobium bacteria in root nodules. In Medicago truncatula, the bacteria undergo an irreversible (terminal) differentiation mediated by hitherto unidentified plant factors. We demonstrated that these factors are nodule-specific cysteine-rich (NCR) peptides that are targeted to the bacteria and enter the bacterial membrane and cytosol. Obstruction of NCR transport in the dnf1-1 signal peptidase mutant correlated with the absence of terminal bacterial differentiation. On the contrary, ectopic expression of NCRs in legumes devoid of NCRs or challenge of cultured rhizobia with peptides provoked symptoms of terminal differentiation. Because NCRs resemble antimicrobial peptides, our findings reveal a previously unknown innovation of the host plant, which adopts effectors of the innate immune system for symbiosis to manipulate the cell fate of endosymbiotic bacteria.

Symbiosis between legumes and Rhizobium bacteria leads to the formation of root nodules where bacteria in the infected plant cells are converted into nitrogen-fixing bacteroids. Nodules with a persistent meristem are indeterminate, whereas nodules without meristem are determinate. The symbiotic plant cells in both nodule types are polyploid because of several cycles of endoreduplication (genome replication without mitosis and cytokinesis) and grow consequently to extreme sizes. Here we demonstrate that differentiation of bacteroids in indeterminate nodules of Medicago and related legumes from the galegoid clade shows remarkable similarity to host cell differentiation. During bacteroid maturation, repeated DNA replication without cytokinesis results in extensive amplification of the entire bacterial genome and elongation of bacteria. This finding reveals a positive correlation in prokaryotes between DNA content and cell size, similar to that in eukaryotes. These polyploid bacteroids are metabolically functional but display increased membrane permeability and are nonviable, because they lose their ability to resume growth. In contrast, bacteroids in determinate nodules of the nongalegoid legumes lotus and bean are comparable to free-living bacteria in their genomic DNA content, cell size, and viability. Using recombinant Rhizobium strains nodulating both legume types, we show that bacteroid differentiation is controlled by the host plant. Plant factors present in nodules of galegoid legumes but absent from nodules of nongalegoid legumes block bacterial cell division and trigger endoreduplication cycles, thereby forcing the endosymbionts toward a terminally differentiated state. Hence, Medicago and related legumes have evolved a mechanism to dominate the symbiosis.antimicrobial activity ͉ bacteroid ͉ endoreduplication ͉ Medicago ͉ nitrogen fixation

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.Plants have evolved symbiotic associations with soil microorganisms to facilitate their mineral nutrition. An example is the specific interaction of different species of the Leguminosae (legumes) with the nitrogen-fixing soil bacteria from the Rhizobiaceae family (rhizobia). This symbiosis leads to the de novo formation of a root organ, the nodule, hosting nitrogen-fixing rhizobia that feed the host plant with ammonium. Another example is the widespread association of plants with fungi from the order of Glomales leading to the formation of arbuscular endomycorrhiza that extends the plant root system and facilitates nutrient uptake. The initial stages of rhizobial and mycorrhizal interactions share certain common molecular mechanisms (Albrecht et al., 1999;Kistner and Parniske, 2002). Because mycorrhizas are more common and ancient, the rhizobial symbiosis might have acquired existing mechanisms from them.Two major types of legume nodules are distinguished (Crespi and Gálvez, 2000): the indeterminate type, formed by e.g. Medicago truncatula, pea (Pisum sativum), broad bean (Vicia faba), white clover (Trifolium repens), or Galega orientalis, and the determinate type, formed by e.g. Lotus japonicus or soybean (Glycine max). Indeterminate nodules have a complex structure composed of different central tissues surrounded by a cortex (Vasse et al., 1990). The persistent apical meristem is zone I. In zone II, postmeristematic cells gradually differentiate and become infected with rhizobia, encapsulated in a membrane envelope. Interzone II-III is characterized by amyloplast accumulation and major transcriptional changes in both plant and bacterial cells. The proximal zone III is compos...

Rhizobia reside as symbiosomes in the infected cells of legume nodules to fix atmospheric nitrogen. The symbiotic relation is strictly controlled, lasts for some time, but eventually leads to nodule senescence. We present a comprehensive transcriptomics study to understand the onset of nodule senescence in the legume Medicago truncatula. Distinct developmental stages with characteristic gene expression were delineated during which the two symbiotic partners were degraded consecutively, marking the switch in nodule tissue status from carbon sink to general nutrient source. Cluster analysis discriminated an early expression group that harbored regulatory genes that might be primary tools to interfere with pod filling-related or stress-induced nodule senescence, ultimately causing prolonged nitrogen fixation. Interestingly, the transcriptomes of nodule and leaf senescence had a high degree of overlap, arguing for the recruitment of similar pathways.

A bacterial membrane protein, BacA, protects Sinorhizobium meliloti against the antimicrobial activity of host peptides, enabling the peptides to induce bacterial persistence rather than bacterial death.

In legume nodules, rhizobia differentiate into nitrogen-fixing forms called bacteroids, which are enclosed by a plant membrane in an organelle-like structure called the symbiosome. In the Inverted Repeat-Lacking Clade (IRLC) of legumes, this differentiation is terminal due to irreversible loss of cell division ability and is associated with genome amplification and different morphologies of the bacteroids that can be swollen, elongated, spherical, and elongated-branched, depending on the host plant. In , this process is orchestrated by nodule-specific cysteine-rich peptides (NCRs) delivered into developing bacteroids. Here, we identified the predicted NCR proteins in 10 legumes representing different subclades of the IRLC with distinct bacteroid morphotypes. Analysis of their expression and predicted sequences establishes correlations between the composition of the NCR family and the morphotypes of bacteroids. Although NCRs have a single origin, their evolution has followed different routes in individual lineages, and enrichment and diversification of cationic peptides has resulted in the ability to impose major morphological changes on the endosymbionts. The wide range of effects provoked by NCRs such as cell enlargement, membrane alterations and permeabilization, and biofilm and vesicle formation is dependent on the amino acid composition and charge of the peptides. These effects are strongly influenced by the rhizobial surface polysaccharides that affect NCR-induced differentiation and survival of rhizobia in nodule cells.

Plant organs originate from meristems where stem cells are maintained to produce continuously daughter cells that are the source of different cell types. The cell cycle switch gene CCS52A, a substrate specific activator of the anaphase promoting complex/ cyclosome (APC/C), controls the mitotic arrest and the transition of mitotic cycles to endoreduplication (ER) cycles as part of cell differentiation. Arabidopsis, unlike other organisms, contains 2 CCS52A isoforms. Here, we show that both of them are active and regulate meristem maintenance in the root tip, although through different mechanisms. The CCS52A1 activity in the elongation zone of the root stimulates ER and mitotic exit, and contributes to the border delineation between dividing and expanding cells. In contrast, CCS52A2 acts directly in the distal region of the root meristem to control identity of the quiescent center (QC) cells and stem cell maintenance. Cell proliferation assays in roots suggest that this control involves CCS52A2 mediated repression of mitotic activity in the QC cells. The data indicate that the CCS52A genes favor a low mitotic state in different cell types of the root tip that is required for meristem maintenance, and reveal a previously undescribed mechanism for APC/C mediated control in plant development.CDH1 ͉ cell differentiation ͉ endoreduplication ͉ quiescent center ͉ stem cells P lant growth and development depend on the persistent activity of meristems, allowing continuous postembryonic organogenesis. In the root tip, meristem maintenance is controlled by different mechanisms that involve the maintenance of stem cells in the root meristem (RM) and spatial control over mitotic exit at the RM-elongation zone (EZ) border.In the distal RM, stem cells are maintained in an undifferentiated state by the quiescent center (QC) cells (1). The QC represents a center of mitotic inactive cells resting in an extended G 1 phase (2). The stem cells around the QC divide according to strict spatial rules, and provide cell progenies that detach from the QC and differentiate into different root cell types (3). The auxin-PLETHORA (PLT) pathway provides positional information to set up the QC and surrounding stem cells whose activities depend on WOX5 and SHORT ROOT (SHR)-SCARECROW (SCR) transcription factors (4-7).As cells reach the RM-EZ border, they start to expand and terminally differentiate. Recently, it has been demonstrated that the spatial boundary of the RM and EZ is controlled by the rate of meristematic cell differentiation at this border (8). The transition involves exit from the mitotic cycle to the endocycle (9). In eukaryotes, endoreduplication (ER) onset requires inhibition of mitotic cyclin-dependent kinase (cdk) activities (10-12). This inhibition can be achieved by multiple mechanisms, but mostly by the degradation of mitotic cyclins by the anaphase promoting complex/cyclosome (APC/C) (13-15). The APC/C is an ubiquitin ligase that regulates cell cycle progression from metaphase to S phase by targeted degradation of numerous ce...

Symbiosis between rhizobia soil bacteria and legume plants results in the formation of root nodules where plant cells are fully packed with nitrogen fixing bacteria. In the host cells, the bacteria adapt to the intracellular environment and gain the ability for nitrogen fixation. Depending on the host plants, the symbiotic fate of bacteria can be either reversible or irreversible. In Medicago and related legume species, 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. Approximately 600 of them are nodule-specific cysteine-rich (NCR) peptides produced in the rhizobium-infected plant cells. NCRs are targeted to the endosymbionts, and concerted action of different sets of peptides governs different stages of endosymbiont maturation, whereas the symbiotic function of individual NCRs is unknown. This study focused on NCR247, a cationic peptide exhibiting in vitro antimicrobial activities. We show that NCR247 acts in those nodule cells where bacterial cell division is arrested and cell elongation begins. NCR247 penetrates the bacteria and forms complexes with many bacterial proteins. Interaction with FtsZ required for septum formation is one of the host interventions for inhibiting bacterial cell division. Complex formation with the ribosomal proteins affects translation and contributes to altered proteome and physiology of the endosymbiont. Binding to the chaperone GroEL amplifies the NCR247-modulated biological processes. We show that GroEL1 of Sinorhizobium meliloti is required for efficient infection, terminal differentiation, and nitrogen fixation.bacterial targets | antimicrobial peptides | protein interactions | translation inhibition | host peptides S ymbiosis between microbes and hosts, like legumes and various insects, often results in the formation of symbiotic organs where host cells harbor thousands of endosymbiotic bacteria. In rhizobium-legume symbiosis, rhizobia in the soil interact with their host legume and induce the formation of root nodules where bacteria reside in the host cytosol in membrane-enclosed vesicles called symbiosomes. The bacteria adapt to the endosymbiotic lifestyle in the plant cells and gain the capacity for nitrogen fixation, thereby ensuring the nitrogen need of the host plant.In addition to the physiological changes required for nitrogen fixation, in certain rhizobium-legume interactions, like in the Medicago truncatula-Sinorhizobium meliloti symbiosis, the bacteria undergo an additional remarkable differentiation process (1). The endosymbiotic bacteria called bacteroids become polyploid, noncultivable elongated, and frequently branched cells (2). This terminal bacterial differentiation is provoked by hundreds of nodule-specific cysteine-rich (NCR) host peptides that are exclusively expressed in the rhizobium-infected symbiotic cells (3).Nodule development and bacterial infection a...