Evolving ideas of legume evolution and diversity: a taxonomic perspective on the occurrence of nodulation

A gene-based map of the Nod factor-independent Aeschynomene evenia sheds new light on the evolution of nodulation and legume genomes

Aeschynomene evenia has emerged as a new model legume for the deciphering of the molecular mechanisms of an alternative symbiotic process that is independent of the Nod factors. Whereas most of the research on nitrogen-fixing symbiosis, legume genetics and genomics has so far focused on Galegoid and Phaseolid legumes, A. evenia falls in the more basal and understudied Dalbergioid clade along with peanut (Arachis hypogaea). To provide insights into the symbiotic genes content and the structure of the A. evenia genome, we established a gene-based genetic map for this species. Firstly, an RNAseq analysis was performed on the two parental lines selected to generate a F₂ mapping population. The transcriptomic data were used to develop molecular markers and they allowed the identification of most symbiotic genes. The resulting map comprised 364 markers arranged in 10 linkage groups (2n = 20). A comparative analysis with the sequenced genomes of Arachis duranensis and A. ipaensis, the diploid ancestors of peanut, indicated blocks of conserved macrosynteny. Altogether, these results provided important clues regarding the evolution of symbiotic genes in a Nod factor-independent context. They provide a basis for a genome sequencing project and pave the way for forward genetic analysis of symbiosis in A. evenia.

Segregation of nod-containing and nod-deficient bradyrhizobia as endosymbionts of Arachis hypogaea and as endophytes of Oryza sativa in intercropped fields of Bengal Basin, India

Bradyrhizobial invasion in dalbergoid legumes like Arachis hypogaea and endophytic bacterial invasions in non-legumes like Oryza sativa occur through epidermal cracks. Here, we show that there is no overlap between the bradyrhizobial consortia that endosymbiotically and endophytically colonise these plants. To minimise contrast due to phylogeographic isolation, strains were collected from Arachis/Oryza intercropped fields and a total of 17 bradyrhizobia from Arachis (WBAH) and 13 from Oryza (WBOS) were investigated. 16SrRNA and concatenated dnaK-glnII-recA phylogeny clustered the nodABC-positive WBAH and nodABC-deficient WBOS strains in two distinct clades. The in-field segregation is reproducible under controlled conditions which limits the factors that influence their competitive exclusion. While WBAH renodulated Arachis successfully, WBOS nodulated in an inefficient manner. Thus, Arachis, like other Aeschynomene legumes support nod-independent symbiosis that was ineffectual in natural fields. In Oryza, WBOS recolonised endophytically and promoted its growth. WBAH however caused severe chlorosis that was completely overcome when coinfected with WBOS. This explains the exclusive recovery of WBOS in Oryza in natural fields and suggests Nod-factors to have a role in counterselection of WBAH. Finally, canonical soxY1 and thiosulphate oxidation could only be detected in WBOS indicating loss of metabolic traits in WBAH with adaptation of symbiotic lifestyle.

Early signaling, synthesis, transport and metabolism of ureides

The symbiosis between α nitrogen (N2)-fixing Proteobacteria (family Rhizobiaceae) and legumes belonging to the Fabaceae (a single phylogenetic group comprising three subfamilies: Caesalpinioideae, Mimosoideae and Papilionoideae) results in the formation of a novel root structure called a nodule, where atmospheric N2 is fixed into NH3(+). In the determinate type of nodules harbored by Rhizobium-nodulated Fabaceae species, newly synthesized NH3(+) is finally converted into allantoin (C4H6N4O3) and allantoic acid (C4H8N4O4) (ureides) through complex pathways involving at least 20 different enzymes that act synchronously in two types of nodule cells with contrasting ultrastructure, including the tree nodule cell organelles. Newly synthesized ureides are loaded into the network of nodule-root xylem vessels and transported to aerial organs by the transpirational water current. Once inside the leaves, ureides undergo an enzymatically driven reverse process to yield NH4(+) that is used for growth. This supports the role of ureides as key nitrogen (N)-compounds for the growth and yield of legumes nodulated by Rhizobium that grow in soils with a low N content. Thus, a concrete understanding of the mechanisms underlying ureide biogenesis and catabolism in legumes may help agrobiologists to achieve greater agricultural discoveries. In this review we focus on the transmembranal and transorganellar symplastic and apoplastic movement of N-precursors within the nodules, as well as on the occurrence, localization and properties of enzymes and genes involved in the biogenesis and catabolism of ureides. The synthesis and transport of ureides are not unique events in Rhizobium-nodulated N2-fixing legumes. Thus, a brief description of the synthesis and catabolism of ureides in non-legumes was included for comparison. The establishment of the symbiosis, nodule organogenesis and the plant's control of nodule number, synthesis and translocation of ureides via feed-back inhibition mechanisms are also reviewed.

Contribution of Individual Chemoreceptors to Sinorhizobium meliloti Chemotaxis Towards Amino Acids of Host and Nonhost Seed Exudates

Plant seeds and roots exude a spectrum of molecules into the soil that attract bacteria to the spermosphere and rhizosphere, respectively. The alfalfa symbiont Sinorhizobium meliloti utilizes eight chemoreceptors (McpT to McpZ and IcpA) to mediate chemotaxis. Using a modified hydrogel capillary chemotaxis assay that allows data quantification and larger throughput screening, we defined the role of S. meliloti chemoreceptors in sensing its host, Medicago sativa, and a closely related nonhost, Medicago arabica. S. meliloti wild type and most single-deletion strains displayed comparable chemotaxis responses to host or nonhost seed exudate. However, while the mcpZ mutant responded like wild type to M. sativa exudate, its reaction to M. arabica exudate was reduced by 80%. Even though the amino acid (AA) amounts released by both plant species were similar, synthetic AA mixtures that matched exudate profiles contributed differentially to the S. meliloti wild-type response to M. sativa (23%) and M. arabica (37%) exudates, with McpU identified as the most important chemoreceptor for AA. Our results show that S. meliloti is equally attracted to host and nonhost legumes; however, AA play a greater role in attraction to M. arabica than to M. sativa, with McpZ being specifically important in sensing M. arabica.

N-fixation in legumes--An assessment of the potential threat posed by ozone pollution

The growth, development and functioning of legumes are often significantly affected by exposure to tropospheric ozone (O3) pollution. However, surprisingly little is known about how leguminous Nitrogen (N) fixation responds to ozone, with a scarcity of studies addressing this question in detail. In the last decade, ozone impacts on N-fixation in soybean, cowpea, mung bean, peanut and clover have been shown for concentrations which are now commonly recorded in ambient air or are likely to occur in the near future. We provide a synthesis of the existing literature addressing this issue, and also explore the effects that may occur on an agroecosystem scale by predicting reductions in Trifolium (clovers) root nodule biomass in United Kingdom (UK) pasture based on ozone concentration data for a "high" (2006) and "average" ozone year (2008). Median 8% and 5% reductions in clover root nodule biomass in pasture across the UK were predicted for 2006 and 2008 respectively. Seasonal exposure to elevated ozone, or short-term acute concentrations >100 ppb, are sufficient to reduce N-fixation and/or impact nodulation, in a range of globally-important legumes. However, an increasing global burden of CO2, the use of artificial fertiliser, and reactive N-pollution may partially mitigate impacts of ozone on N-fixation.

Sinorhizobium fredii HH103 bacteroids are not terminally differentiated and show altered O-antigen in nodules of the Inverted Repeat-Lacking Clade legume Glycyrrhiza uralensis

In rhizobial species that nodulate inverted repeat-lacking clade (IRLC) legumes, such as the interaction between Sinorhizobium meliloti and Medicago, bacteroid differentiation is driven by an endoreduplication event that is induced by host nodule-specific cysteine rich (NCR) antimicrobial peptides and requires the participation of the bacterial protein BacA. We have studied bacteroid differentiation of Sinorhizobium fredii HH103 in three host plants: Glycine max, Cajanus cajan and the IRLC legume Glycyrrhiza uralensis. Flow cytometry, microscopy analyses and viability studies of bacteroids as well as confocal microscopy studies carried out in nodules showed that S. fredii HH103 bacteroids, regardless of the host plant, had deoxyribonucleic acid (DNA) contents, cellular sizes and survival rates similar to those of free-living bacteria. Contrary to S. meliloti, S. fredii HH103 showed little or no sensitivity to Medicago NCR247 and NCR335 peptides. Inactivation of S. fredii HH103 bacA neither affected symbiosis with Glycyrrhiza nor increased bacterial sensitivity to Medicago NCRs. Finally, HH103 bacteroids isolated from Glycyrrhiza, but not those isolated from Cajanus or Glycine, showed an altered lipopolysaccharide. Our studies indicate that, in contrast to the S. meliloti-Medicago model symbiosis, bacteroids in the S. fredii HH103-Glycyrrhiza symbiosis do not undergo NCR-induced and bacA-dependent terminal differentiation.

Phylogeny of Symbiotic Genes and the Symbiotic Properties of Rhizobia Specific to Astragalus glycyphyllos L

The phylogeny of symbiotic genes of Astragalus glycyphyllos L. (liquorice milkvetch) nodule isolates was studied by comparative sequence analysis of nodA, nodC, nodH and nifH loci. In all these genes phylograms, liquorice milkvetch rhizobia (closely related to bacteria of three species, i.e. Mesorhizobium amorphae, Mesorhizobium septentrionale and Mesorhizobium ciceri) formed one clearly separate cluster suggesting the horizontal transfer of symbiotic genes from a single ancestor to the bacteria being studied. The high sequence similarity of the symbiotic genes of A. glycyphyllos rhizobia (99-100% in the case of nodAC and nifH genes, and 98-99% in the case of nodH one) points to the relatively recent (in evolutionary scale) lateral transfer of these genes. In the nodACH and nifH phylograms, A. glycyphyllos nodule isolates were grouped together with the genus Mesorhizobium species in one monophyletic clade, close to M. ciceri, Mesorhizobium opportunistum and Mesorhizobium australicum symbiovar biserrulae bacteria, which correlates with the close relationship of these rhizobia host plants. Plant tests revealed the narrow host range of A. glycyphyllos rhizobia. They formed effective symbiotic interactions with their native host (A. glycyphyllos) and Amorpha fruticosa but not with 11 other fabacean species. The nodules induced on A. glycyphyllos roots were indeterminate with apical, persistent meristem, an age gradient of nodule tissues and cortical vascular bundles. To reflect the symbiosis-adaptive phenotype of rhizobia, specific for A. glycyphyllos, we propose for these bacteria the new symbiovar "glycyphyllae", based on nodA and nodC genes sequences.

microRNA160 dictates stage-specific auxin and cytokinin sensitivities and directs soybean nodule development

Legume nodules result from coordinated interactions between the plant and nitrogen-fixing rhizobia. The phytohormone cytokinin promotes nodule formation, and recent findings suggest that the phytohormone auxin inhibits nodule formation. Here we show that microRNA160 (miR160) is a key signaling element that determines the auxin/cytokinin balance during nodule development in soybean (Glycine max). miR160 appears to promote auxin activity by suppressing the levels of the ARF10/16/17 family of repressor ARF transcription factors. Using quantitative PCR assays and a fluorescence miRNA sensor, we show that miR160 levels are relatively low early during nodule formation and high in mature nodules. We had previously shown that ectopic expression of miR160 in soybean roots led to a severe reduction in nodule formation, coupled with enhanced sensitivity to auxin and reduced sensitivity to cytokinin. Here we show that exogenous cytokinin restores nodule formation in miR160 over-expressing roots. Therefore, low miR160 levels early during nodule development favor cytokinin activity required for nodule formation. Suppression of miR160 levels using a short tandem target mimic (STTM160) resulted in reduced sensitivity to auxin and enhanced sensitivity to cytokinin. In contrast to miR160 over-expressing roots, STTM160 roots had increased nodule formation, but nodule maturation was significantly delayed. Exogenous auxin partially restored proper nodule formation and maturation in STTM160 roots, suggesting that high miR160 activity later during nodule development favors auxin activity and promotes nodule maturation. Therefore, miR160 dictates developmental stage-specific sensitivities to auxin and cytokinin to direct proper nodule formation and maturation in soybean.

Medicago truncatula natural resistance-associated macrophage Protein1 is required for iron uptake by rhizobia-infected nodule cells

Iron is critical for symbiotic nitrogen fixation (SNF) as a key component of multiple ferroproteins involved in this biological process. In the model legume Medicago truncatula, iron is delivered by the vasculature to the infection/maturation zone (zone II) of the nodule, where it is released to the apoplast. From there, plasma membrane iron transporters move it into rhizobia-containing cells, where iron is used as the cofactor of multiple plant and rhizobial proteins (e.g. plant leghemoglobin and bacterial nitrogenase). MtNramp1 (Medtr3g088460) is the M. truncatula Natural Resistance-Associated Macrophage Protein family member, with the highest expression levels in roots and nodules. Immunolocalization studies indicate that MtNramp1 is mainly targeted to the plasma membrane. A loss-of-function nramp1 mutant exhibited reduced growth compared with the wild type under symbiotic conditions, but not when fertilized with mineral nitrogen. Nitrogenase activity was low in the mutant, whereas exogenous iron and expression of wild-type MtNramp1 in mutant nodules increased nitrogen fixation to normal levels. These data are consistent with a model in which MtNramp1 is the main transporter responsible for apoplastic iron uptake by rhizobia-infected cells in zone II.

Housing helpful invaders: the evolutionary and molecular architecture underlying plant root-mutualist microbe interactions

Plant root rhizosphere interactions with mutualistic microbes are diverse and numerous, having evolved over time in response to selective pressures on plants to attain anchorage and nutrients. These relationships can be considered to be formed through a combination of architectural connections: molecular architecture interactions that control root-microbe perception and regulate the balance between host and symbiont and developmental architecture interactions that enable the microbes to be 'housed' in the root and enable the exchange of compounds. Recent findings that help to understand the common architecture that exists between nodulation and mycorrhizal interactions, and how this architecture could be re-tuned to develop new symbioses, are discussed here.

Leguminous plants: inventors of root nodules to accommodate symbiotic bacteria

Legumes and a few other plant species can establish a symbiotic relationship with nitrogen-fixing rhizobia, which enables them to survive in a nitrogen-deficient environment. During the course of nodulation, infection with rhizobia induces the dedifferentiation of host cells to form primordia of a symbiotic organ, the nodule, which prepares plants to accommodate rhizobia in host cells. While these nodulation processes are known to be genetically controlled by both plants and rhizobia, recent advances in studies on two model legumes, Lotus japonicus and Medicago truncatula, have provided great insight into the underlying plant-side molecular mechanism. In this chapter, we review such knowledge, with particular emphasis on two key processes of nodulation, nodule development and rhizobial invasion.

Eocene Podocarpium (Leguminosae) from South China and its biogeographic implications

Podocarpium A. Braun ex Stizenberger is one of the most common legumes in the Neogene of Eurasia, including fossil fruits, seeds, leaves, and possible flower and pollen grains. This genus is not completely consistent with any extant genera according to gross morphological characters and poorly preserved cuticular structures reported in previous studies. The fossil pods collected from the coal-bearing series of the Changchang Basin of Hainan Island and Maoming Basin of Guangdong, South China, are examined by morphologically comparative work, with special reference to venation patterns and placental position. These distinctive features, as well as the ovule development of pods from different developmental stages and the epidermal structure of the pods, as distinguished from previous records lead to the conclusion that these fossils can be recognized as a new species of Podocarpium, P. eocenicum sp. nov. This new discovery indicates that Podocarpium had arrived in South China by the Eocene. Investigation on the fossil records of this extinct genus shows that P. eocenicum is the earliest and lowest latitude fossil data. The possible occurrence pattern of this genus is revealed as follows: Podocarpium had distributed in the South China at least in the middle Eocene, and then migrated to Europe during the Oligocene; in the Miocene this genus reached its peak in Eurasia, spreading extensively across subtropical areas to warm temperate areas; finally, Podocarpium shrank rapidly and became extinct in Eurasia during the Pliocene.

Lipochitooligosaccharides modulate plant host immunity to enable endosymbioses

Symbiotic nitrogen-fixing rhizobium bacteria and arbuscular mycorrhizal fungi use lipochitooligosaccharide (LCO) signals to communicate with potential host plants. Upon a compatible match, an intimate relation is established during which the microsymbiont is allowed to enter root (-derived) cells. Plants perceive microbial LCO molecules by specific LysM-domain-containing receptor-like kinases. These do not only activate a common symbiosis signaling pathway that is shared in both symbioses but also modulate innate immune responses. Recent studies revealed that symbiotic LCO receptors are closely related to chitin innate immune receptors, and some of these receptors even function in symbiosis as well as immunity. This raises questions about how plants manage to translate structurally very similar microbial signals into different outputs. Here, we describe the current view on chitin and LCO perception in innate immunity and endosymbiosis and question how LCOs might modulate the immune system. Furthermore, we discuss what it takes to become an endosymbiont.

Going underground: root traits as drivers of ecosystem processes

Ecologists are increasingly adopting trait-based approaches to understand how community change influences ecosystem processes. However, most of this research has focussed on aboveground plant traits, whereas it is becoming clear that root traits are important drivers of many ecosystem processes, such as carbon (C) and nutrient cycling, and the formation and structural stability of soil. Here, we synthesise emerging evidence that illustrates how root traits impact ecosystem processes, and propose a pathway to unravel the complex roles of root traits in driving ecosystem processes and their response to global change. Finally, we identify research challenges and novel technologies to address them.

Divergent nod-containing Bradyrhizobium sp. DOA9 with a megaplasmid and its host range

Bradyrhizobium sp. DOA9, a non-photosynthetic bacterial strain originally isolated from the root nodules of the legume Aeschynomene americana, is a divergent nod-containing strain. It exhibits a broad host range, being able to colonize and efficiently nodulate the roots of most plants from the Dalbergioid, Millettioid, and Robinioid tribes (7 species of Papilionoideae). In all cases, nodulation was determinate. The morphology and size of DOA9 bacteroids isolated from the nodules of various species of Papilionoideae were indistinguishable from the free-living form. However, they were spherical in Arachis hypogaea nodules. GusA-tagged DOA9 also colonized rice roots as endophytes. Since broad-host-range legume symbionts often carry multiple replicons in their genome, we analyzed the replicons for symbiosis genes by electrophoresis. DOA9 carried two replicons, a chromosome (cDOA9) and single megaplasmid (pDOA9) larger than 352 kb. The genes for nodulation (nodA, B, C) and nitrogen fixation (nifH) were localized on the megaplasmid. Southern blot hybridization revealed two copies of nodA on the megaplasmid, single copies of nodB and C on the megaplasmid, and one copy each of nifH on the chromosome and megaplasmid. These results suggested that Bradyrhizobium sp. DOA9 may have the unusual combination of a broad host range, bacteroid differentiation, and symbiosis-mediating replicons.

Shaping bacterial symbiosis with legumes by experimental evolution

Nitrogen-fixing symbionts of legumes have appeared after the emergence of legumes on earth, approximately 70 to 130 million years ago. Since then, symbiotic proficiency has spread to distant genera of α- and β-proteobacteria, via horizontal transfer of essential symbiotic genes and subsequent recipient genome remodeling under plant selection pressure. To tentatively replay rhizobium evolution in laboratory conditions, we previously transferred the symbiotic plasmid of the Mimosa symbiont Cupriavidus taiwanensis in the plant pathogen Ralstonia solanacearum, and selected spontaneous nodulating variants of the chimeric Ralstonia sp. using Mimosa pudica as a trap. Here, we pursued the evolution experiment by submitting two of the rhizobial drafts to serial ex planta-in planta (M. pudica) passages that may mimic alternating of saprophytic and symbiotic lives of rhizobia. Phenotyping 16 cycle-evolved clones showed strong and parallel evolution of several symbiotic traits (i.e., nodulation competitiveness, intracellular infection, and bacteroid persistence). Simultaneously, plant defense reactions decreased within nodules, suggesting that the expression of symbiotic competence requires the capacity to limit plant immunity. Nitrogen fixation was not acquired in the frame of this evolutionarily short experiment, likely due to the still poor persistence of final clones within nodules compared with the reference rhizobium C. taiwanensis. Our results highlight the potential of experimental evolution in improving symbiotic proficiency and for the elucidation of relationship between symbiotic capacities and elicitation of immune responses.

Role of auxin during intercellular infection of Discaria trinervis by Frankia

Nitrogen-fixing nodules induced by Frankia in the actinorhizal plant Discaria trinervis result from a primitive intercellular root invasion pathway that does not involve root hair deformation and infection threads. Here, we analyzed the role of auxin in this intercellular infection pathway at the molecular level and compared it with our previous work in the intracellular infected actinorhizal plant Casuarina glauca. Immunolocalisation experiments showed that auxin accumulated in Frankia-infected cells in both systems. We then characterized the expression of auxin transporters in D. trinervis nodules. No activation of the heterologous CgAUX1 promoter was detected in infected cells in D. trinervis. These results were confirmed with the endogenous D. trinervis gene, DtAUX1. However, DtAUX1 was expressed in the nodule meristem. Consistently, transgenic D. trinervis plants containing the auxin response marker DR5:VENUS showed expression of the reporter gene in the meristem. Immunolocalisation experiments using an antibody against the auxin efflux carrier PIN1, revealed the presence of this transporter in the plasma membrane of infected cells. Finally, we used in silico cellular models to analyse auxin fluxes in D. trinervis nodules. Our results point to the existence of divergent roles of auxin in intercellularly- and intracellularly-infected actinorhizal plants, an ancestral infection pathways leading to root nodule symbioses.

Actinorhizal root nodule symbioses: what is signalling telling on the origins of nodulation?

Two groups of bacteria are able to induce the formation of nitrogen-fixing nodules: proteobacteria called rhizobia, which associate with Legumes or Parasponia and actinobateria from the genus Frankia which are able to interact with ∼220 species belonging to eight families called actinorhizal plants. Legumes and different lineages of actinorhizal plants differ in bacterial partners, nodule organogenesis and infection patterns and have independent evolutionary origins. However, recent technical achievements are revealing a variety of conserved signalling molecules and gene networks. Actinorhizal interactions display several primitive features and thus provide the ideal opportunity to determine the minimal molecular toolkit needed to build a nodule and to understand the evolution of root nodule symbioses.

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.

Rhizobial infection in Adesmia bicolor (Fabaceae) roots

The native legume Adesmia bicolor shows nitrogen fixation efficiency via symbiosis with soil rhizobia. The infection mechanism by means of which rhizobia infect their roots has not been fully elucidated to date. Therefore, the purpose of the present study was to identify the infection mechanism in Adesmia bicolor roots. To this end, inoculated roots were processed following conventional methods as part of our root anatomy study, and the shape and distribution of root nodules were analyzed as well. Neither root hairs nor infection threads were observed in the root system, whereas infection sites-later forming nodules-were observed in the longitudinal sections. Nodules were found to form between the main root and the lateral roots. It can be concluded that in Adesmia bicolor, a bacterial crack entry infection mechanism prevails and that such mechanism could be an adaptive strategy of this species which is typical of arid environments.

The first complete chloroplast genome of the Genistoid legume Lupinus luteus: evidence for a novel major lineage-specific rearrangement and new insights regarding plastome evolution in the legume family

BACKGROUND AND AIMS

To date chloroplast genomes are available only for members of the non-protein amino acid-accumulating clade (NPAAA) Papilionoid lineages in the legume family (i.e. Millettioids, Robinoids and the 'inverted repeat-lacking clade', IRLC). It is thus very important to sequence plastomes from other lineages in order to better understand the unusual evolution observed in this model flowering plant family. To this end, the plastome of a lupine species, Lupinus luteus, was sequenced to represent the Genistoid lineage, a noteworthy but poorly studied legume group.

METHODS

The plastome of L. luteus was reconstructed using Roche-454 and Illumina next-generation sequencing. Its structure, repetitive sequences, gene content and sequence divergence were compared with those of other Fabaceae plastomes. PCR screening and sequencing were performed in other allied legumes in order to determine the origin of a large inversion identified in L. luteus.

KEY RESULTS

The first sequenced Genistoid plastome (L. luteus: 155 894 bp) resulted in the discovery of a 36-kb inversion, embedded within the already known 50-kb inversion in the large single-copy (LSC) region of the Papilionoideae. This inversion occurs at the base or soon after the Genistoid emergence, and most probably resulted from a flip-flop recombination between identical 29-bp inverted repeats within two trnS genes. Comparative analyses of the chloroplast gene content of L. luteus vs. Fabaceae and extra-Fabales plastomes revealed the loss of the plastid rpl22 gene, and its functional relocation to the nucleus was verified using lupine transcriptomic data. An investigation into the evolutionary rate of coding and non-coding sequences among legume plastomes resulted in the identification of remarkably variable regions.

CONCLUSIONS

This study resulted in the discovery of a novel, major 36-kb inversion, specific to the Genistoids. Chloroplast mutational hotspots were also identified, which contain novel and potentially informative regions for molecular evolutionary studies at various taxonomic levels in the legumes. Taken together, the results provide new insights into the evolutionary landscape of the legume plastome.

Iron distribution through the developmental stages of Medicago truncatula nodules

Paramount to symbiotic nitrogen fixation (SNF) is the synthesis of a number of metalloenzymes that use iron as a critical component of their catalytical core. Since this process is carried out by endosymbiotic rhizobia living in legume root nodules, the mechanisms involved in iron delivery to the rhizobia-containing cells are critical for SNF. In order to gain insight into iron transport to the nodule, we have used synchrotron-based X-ray fluorescence to determine the spatio-temporal distribution of this metal in nodules of the legume Medicago truncatula with hitherto unattained sensitivity and resolution. The data support a model in which iron is released from the vasculature into the apoplast of the infection/differentiation zone of the nodule (zone II). The infected cell subsequently takes up this apoplastic iron and delivers it to the symbiosome and the secretory system to synthesize ferroproteins. Upon senescence, iron is relocated to the vasculature to be reused by the shoot. These observations highlight the important role of yet to be discovered metal transporters in iron compartmentalization in the nodule and in the recovery of an essential and scarce nutrient for flowering and seed production.

A paradigm for endosymbiotic life: cell differentiation of Rhizobium bacteria provoked by host plant factors

Symbiosis between Rhizobium bacteria and legumes leads to the formation of the root nodule. The endosymbiotic bacteria reside in polyploid host cells as membrane-surrounded vesicles where they reduce atmospheric nitrogen to support plant growth by supplying ammonia in exchange for carbon sources and energy. The morphology and physiology of endosymbionts, despite their common function, are highly divergent in different hosts. In galegoid plants, the endosymbionts are terminally differentiated, uncultivable polyploid cells, with remarkably elongated and even branched Y-shaped cells. Bacteroid differentiation is controlled by host peptides, many of which have antibacterial activity and require the bacterial function of BacA. Although the precise and combined action of several hundred host peptides and BacA has yet to be discovered, similarities, especially to certain insect-bacterium symbioses involving likewise host peptides for manipulation of endosymbionts, suggest convergent evolution. Rhizobium-legume symbiosis provides a rich source of information for understanding host-controlled endosymbiotic life in eukaryotic cells.

Rethinking mutualism stability: cheaters and the evolution of sanctions

How cooperation originates and persists in diverse species, from bacteria to multicellular organisms to human societies, is a major question in evolutionary biology. A large literature asks: what prevents selection for cheating within cooperative lineages? In mutualisms, or cooperative interactions between species, feedback between partners often aligns their fitness interests, such that cooperative symbionts receive more benefits from their hosts than uncooperative symbionts. But how do these feedbacks evolve? Cheaters might invade symbiont populations and select for hosts that preferentially reward or associate with cooperators (often termed sanctions or partner choice); hosts might adapt to variation in symbiont quality that does not amount to cheating (e.g., environmental variation); or conditional host responses might exist before cheaters do, making mutualisms stable from the outset. I review evidence from yucca-yucca moth, fig-fig wasp, and legume-rhizobium mutualisms, which are commonly cited as mutualisms stabilized by sanctions. Based on the empirical evidence, it is doubtful that cheaters select for host sanctions in these systems; cheaters are too uncommon. Recognizing that sanctions likely evolved for functions other than retaliation against cheaters offers many insights about mutualism coevolution, and about why mutualism evolves in only some lineages of potential hosts.

Radiation of the Nod-independent Aeschynomene relies on multiple allopolyploid speciation events

• The semi-aquatic legumes belonging to the genus Aeschynomene constitute a premium system for investigating the origin and evolution of unusual symbiotic features such as stem nodulation and the presence of a Nod-independent infection process. This latter apparently arose in a single Aeschynomene lineage. But how this unique Nod-independent group then radiated is not yet known. • We have investigated the role of polyploidy in Aeschynomene speciation via a case study of the pantropical A. indica and then extended the analysis to the other Nod-independent species. For this, we combined SSR genotyping, genome characterization through flow cytometry, chromosome counting, FISH and GISH experiments, molecular phylogenies using ITS and single nuclear gene sequences, and artificial hybridizations. • These analyses demonstrate the existence of an A. indica polyploid species complex comprising A. evenia (C. Wright) (2n = 2x = 20), A. indica L. s.s. (2n = 4x = 40) and a new hexaploid form (2n = 6x = 60). This latter contains the two genomes present in the tetraploid (A. evenia and A. scabra) and another unidentified genome. Two other species, A. pratensis and A. virginica, are also shown to be of allopolyploid origin. • This work reveals multiple hybridization/polyploidization events, thus highlighting a prominent role of allopolyploidy in the radiation of the Nod-independent Aeschynomene.

Evolution of a symbiotic receptor through gene duplications in the legume-rhizobium mutualism

The symbiosis between legumes and nitrogen-fixing rhizobia co-opted pre-existing endomycorrhizal features. In particular, both symbionts release lipo-chitooligosaccharides (LCOs) that are recognized by LysM-type receptor kinases. We investigated the evolutionary history of rhizobial LCO receptor genes MtLYK3-LjNFR1 to gain insight into the evolutionary origin of the rhizobial symbiosis. We performed a phylogenetic analysis integrating gene copies from nonlegumes and legumes, including the non-nodulating, phylogenetically basal legume Cercis chinensis. Signatures of differentiation between copies were investigated through patterns of molecular evolution. We show that two rounds of duplication preceded the evolution of the rhizobial symbiosis in legumes. Molecular evolution patterns indicate that the resulting three paralogous gene copies experienced different selective constraints. In particular, one copy maintained the ancestral function, and another specialized into perception of rhizobial LCOs. It has been suggested that legume LCO receptors evolved from a putative ancestral defense-related chitin receptor through the acquisition of two kinase motifs. However, the phylogenetic analysis shows that these domains are actually ancestral, suggesting that this scenario is unlikely. Our study underlines the evolutionary significance of gene duplication and subsequent neofunctionalization in MtLYK3-LjNFR1 genes. We hypothesize that their ancestor was more likely a mycorrhizal LCO receptor, than a defense-related receptor kinase.

Common and divergent roles of plant hormones in nodulation and arbuscular mycorrhizal symbioses

All of the classical plant hormones have been suggested to influence nodulation, including some that interact with the Autoregulation of Nodulation (AON) pathway. Leguminous plants strictly regulate the number of nodules formed through this AON pathway via a root-shoot-root loop that acts to suppress excessive nodulation. A related pathway, the Autoregulation of Mycorrhization (AOM) pathway controls the more ancient, arbuscular mycorrhizal (AM) symbiosis. A comparison of the published responses to the classical hormones in these 2 symbioses shows that most influence the symbioses in the same direction. This may be expected if they affect the symbioses via common components of these symbiotic regulatory pathways. However, some hormones influence these symbioses in opposite directions, suggesting a more complex relationship, and probably one that is not via the common components of these pathways. In a recent paper we showed, using a genetic approach, that strigolactones and brassinosteroids do not act downstream of the AON genes examined and argued that they probably act independently to promote nodule formation. Recently it has been shown that the control of nodulation via the AON pathway involves mobile CLE peptide signals. It is therefore suggested that a more direct avenue to determine if the classical hormones play a direct role in the autoregulatory pathways is to further examine whether CLE peptides and other components of these processes can influence, or be influenced by, the classical hormones. Such studies and other comparisons between the nodulation and mycorrhizal symbioses should allow the role of the classical hormones in these critical symbioses to be rapidly advanced.

Evolution of symbiosis in the legume genus Aeschynomene

Legumes in the genus Aeschynomene form nitrogen-fixing root nodules in association with Bradyrhizobium strains. Several aquatic and subaquatic species have the additional capacity to form stem nodules, and some of them can symbiotically interact with specific strains that do not produce the common Nod factors synthesized by all other rhizobia. The question of the emergence and evolution of these nodulation characters has been the subject of recent debate. We conducted a molecular phylogenetic analysis of 38 different Aeschynomene species. The phylogeny was reconstructed with both the chloroplast DNA trnL intron and the nuclear ribosomal DNA ITS/5.8S region. We also tested 28 Aeschynomene species for their capacity to form root and stem nodules by inoculating different rhizobial strains, including nodABC-containing strains (ORS285, USDA110) and a nodABC-lacking strain (ORS278). Maximum likelihood analyses resolved four distinct phylogenetic groups of Aeschynomene. We found that stem nodulation may have evolved several times in the genus, and that all Aeschynomene species using a Nod-independent symbiotic process clustered in the same clade. The phylogenetic approach suggested that Nod-independent nodulation has evolved once in this genus, and should be considered as a derived character, and this result is discussed with regard to previous experimental studies.

Diversification of the phaseoloid legumes: effects of climate change, range expansion and habit shift

Understanding which factors have driven the evolutionary success of a group is a fundamental question in biology. Angiosperms are the most successful group in plants and have radiated and adapted to various habitats. Among angiosperms, legumes are a good example for such successful radiation and adaptation. We here investigated how the interplay of past climate changes, geographical expansion and habit shifts has promoted diversification of the phaseoloid legumes, one of the largest clades in the Leguminosae. Using a comprehensive genus-level phylogeny from three plastid markers, we estimate divergence times, infer habit shifts, test the phylogenetic and temporal diversification heterogeneity, and reconstruct ancestral biogeographical ranges. We found that the phaseoloid lineages underwent twice dramatic accumulation. During the Late Oligocene, at least six woody clades rapidly diverged, perhaps in response to the Late Oligocene warming and aridity, and a result of rapidly exploiting new ecological opportunities in Asia, Africa and Australia. The most speciose lineage is herbaceous and began to rapidly diversify since the Early Miocene, which was likely ascribed to arid climates, along with the expansion of seasonally dry tropical forests in Africa, Asia, and America. The phaseoloid group provides an excellent case supporting the idea that the interplay of ecological opportunities and key innovations drives the evolutionary success.

miR393 and miR164 influence indeterminate but not determinate nodule development

The roles of auxin in the regulation of symbiotic legume nodule formation are unclear. We recently showed that enhanced sensitivity to auxin resulting from overexpression of miR160 inhibits determinate nodule formation in soybean. We examined the roles of miR393 and miR164 in soybean (that forms determinate nodules) and Medicago truncatula (that forms indeterminate nodules). Our results together with previous studies suggest that indeterminate nodule formation requires a higher, but narrow window of auxin sensitivity and that miR164 regulation is not crucial for determinate nodule formation.

From β- to α-proteobacteria: the origin and evolution of rhizobial nodulation genes nodIJ

Although many α- and some β-proteobacterial species are symbiotic with legumes, the evolutionary origin of nitrogen-fixing nodulation remains unclear. We examined α- and β-proteobacteria whose genomes were sequenced using large-scale phylogenetic profiling and revealed the evolutionary origin of two nodulation genes. These genes, nodI and nodJ (nodIJ), play key roles in the secretion of Nod factors, which are recognized by legumes during nodulation. We found that only the nodulating β-proteobacteria, including the novel strains isolated in this study, possess both nodIJ and their paralogous genes (DRA-ATPase/permease genes). Contrary to the widely accepted scenario of the α-proteobacterial origin of rhizobia, our exhaustive phylogenetic analysis showed that the entire nodIJ clade is included in the clade of Burkholderiaceae DRA-ATPase/permease genes, that is, the nodIJ genes originated from gene duplication in a lineage of the β-proteobacterial family. After duplication, the evolutionary rates of nodIJ were significantly accelerated relative to those of homologous genes, which is consistent with their novel function in nodulation. The likelihood analyses suggest that this accelerated evolution is not associated with changes in either nonsynonymous/synonymous substitution rates or transition/transversion rates, but rather, in the GC content. Although the low GC content of the nodulation genes has been assumed to reflect past horizontal transfer events from donor rhizobial genomes with low GC content, no rhizobial genome with such low GC content has yet been found. Our results encourage a reconsideration of the origin of nodulation and suggest new perspectives on the role of the GC content of bacterial genes in functional adaptation.

Ectopic expression of miR160 results in auxin hypersensitivity, cytokinin hyposensitivity, and inhibition of symbiotic nodule development in soybean

Symbiotic root nodules in leguminous plants result from interaction between the plant and nitrogen-fixing rhizobia bacteria. There are two major types of legume nodules, determinate and indeterminate. Determinate nodules do not have a persistent meristem, while indeterminate nodules have a persistent meristem. Auxin is thought to play a role in the development of both these types of nodules. However, inhibition of rootward auxin transport at the site of nodule initiation is crucial for the development of indeterminate nodules but not determinate nodules. Using the synthetic auxin-responsive DR5 promoter in soybean (Glycine max), we show that there is relatively low auxin activity during determinate nodule initiation and that it is restricted to the nodule periphery subsequently during development. To examine if and what role auxin plays in determinate nodule development, we generated soybean composite plants with altered sensitivity to auxin. We overexpressed microRNA393 to silence the auxin receptor gene family, and these roots were hyposensitive to auxin. These roots nodulated normally, suggesting that only minimal/reduced auxin signaling is required for determinate nodule development. We overexpressed microRNA160 to silence a set of repressor auxin response factor transcription factors, and these roots were hypersensitive to auxin. These roots were not impaired in epidermal responses to rhizobia but had significantly reduced nodule primordium formation, suggesting that auxin hypersensitivity inhibits nodule development. These roots were also hyposensitive to cytokinin and had attenuated expression of key nodulation-associated transcription factors known to be regulated by cytokinin. We propose a regulatory feedback loop involving auxin and cytokinin during nodulation.

Nodule morphology, symbiotic specificity and association with unusual rhizobia are distinguishing features of the genus Listia within the Southern African crotalarioid clade Lotononis s.l

BACKGROUND AND AIMS

The legume clade Lotononis sensu lato (s.l.; tribe Crotalarieae) comprises three genera: Listia, Leobordea and Lotononis sensu stricto (s.s.). Listia species are symbiotically specific and form lupinoid nodules with rhizobial species of Methylobacterium and Microvirga. This work investigated whether these symbiotic traits were confined to Listia by determining the ability of rhizobial strains isolated from species of Lotononis s.l. to nodulate Listia, Leobordea and Lotononis s.s. hosts and by examining the morphology and structure of the resulting nodules.

METHODS

Rhizobia were characterized by sequencing their 16S rRNA and nodA genes. Nodulation and N2 fixation on eight taxonomically diverse Lotononis s.l. species were determined in glasshouse trials. Nodules of all hosts, and the process of infection and nodule initiation in Listia angolensis and Listia bainesii, were examined by light microscopy.

KEY RESULTS

Rhizobia associated with Lotononis s.l. were phylogenetically diverse. Leobordea and Lotononis s.s. isolates were most closely related to Bradyrhizobium spp., Ensifer meliloti, Mesorhizobium tianshanense and Methylobacterium nodulans. Listia angolensis formed effective nodules only with species of Microvirga. Listia bainesii nodulated only with pigmented Methylobacterium. Five lineages of nodA were found. Listia angolensis and L. bainesii formed lupinoid nodules, whereas nodules of Leobordea and Lotononis s.s. species were indeterminate. All effective nodules contained uniformly infected central tissue. Listia angolensis and L. bainesii nodule initials occurred on the border of the hypocotyl and along the tap root, and nodule primordia developed in the outer cortical layer. Neither root hair curling nor infection threads were seen.

CONCLUSIONS

Two specificity groups occur within Lotononis s.l.: Listia species are symbiotically specific, while species of Leobordea and Lotononis s.s. are generally promiscuous and interact with rhizobia of diverse chromosomal and symbiotic lineages. The seasonally waterlogged habitat of Listia species may favour the development of symbiotic specificity.

The impact of beneficial plant-associated microbes on plant phenotypic plasticity

Plants show phenotypic plasticity in response to changing or extreme abiotic environments; but over millions of years they also have co-evolved to respond to the presence of soil microbes. Studies on phenotypic plasticity in plants have focused mainly on the effects of the changing environments on plants' growth and survival. Evidence is now accumulating that the presence of microbes can alter plant phenotypic plasticity in a broad range of traits in response to a changing environment. In this review, we discuss the effects of microbes on plant phenotypic plasticity in response to changing environmental conditions, and how this may affect plant fitness. By using a range of specific plant-microbe interactions as examples, we demonstrate that one way that microbes can alleviate the effect of environmental stress on plants and thus increase plant fitness is to remove the stress, e.g., nutrient limitation, directly. Furthermore, microbes indirectly affect plant phenotypic plasticity and fitness through modulation of plant development and defense responses. In doing so, microbes affect fitness by both increasing or decreasing the degree of phenotypic plasticity, depending on the phenotype and the environmental stress studied, with no clear difference between the effect of prokaryotic and eukaryotic microbes in general. Additionally, plants have the ability to modulate microbial behaviors, suggesting that they manipulate bacteria, enhancing interactions that help them cope with stressful environments. Future challenges remain in the identification of the many microbial signals that modulate phenotypic plasticity, the characterization of plant genes, e.g. receptors, that mediate the microbial effects on plasticity, and the elucidation of the molecular mechanisms that link phenotypic plasticity with fitness. The characterization of plant and microbial mutants defective in signal synthesis or perception, together with carefully designed glasshouse or field experiments that test various environmental stresses will be necessary to understand the link between molecular mechanisms controlling plastic phenotypes with the resulting effects on plant fitness.

Biological nitrogen fixation: rates, patterns and ecological controls in terrestrial ecosystems

New techniques have identified a wide range of organisms with the capacity to carry out biological nitrogen fixation (BNF)-greatly expanding our appreciation of the diversity and ubiquity of N fixers-but our understanding of the rates and controls of BNF at ecosystem and global scales has not advanced at the same pace. Nevertheless, determining rates and controls of BNF is crucial to placing anthropogenic changes to the N cycle in context, and to understanding, predicting and managing many aspects of global environmental change. Here, we estimate terrestrial BNF for a pre-industrial world by combining information on N fluxes with (15)N relative abundance data for terrestrial ecosystems. Our estimate is that pre-industrial N fixation was 58 (range of 40-100) Tg N fixed yr(-1); adding conservative assumptions for geological N reduces our best estimate to 44 Tg N yr(-1). This approach yields substantially lower estimates than most recent calculations; it suggests that the magnitude of human alternation of the N cycle is substantially larger than has been assumed.

Genotype delimitation in the Nod-independent model legume Aeschynomene evenia

Research on the nitrogen-fixing symbiosis has been so far focused on two model legumes, Medicago truncatula and Lotus japonicus, which use a sophisticated infection process involving infection thread formation. However, in 25% of the legumes, the bacterial entry occurs more simply in an intercellular fashion. Among them, some semi-aquatic Aeschynomene species present the distinctive feature to form nitrogen-fixing nodules on both roots and stems following elicitation by photosynthetic bradyrhizobia that do not produce Nod factors. This interaction is believed to represent a living testimony of the ancestral state of the rhizobium-legume symbiosis. To decipher the molecular mechanisms of this unique Nod-independent nitrogen-fixing symbiosis, we previously identified A. evenia C. Wright as an appropriate model legume, because it displays all the requisites for molecular and genetic approaches. To advance the use of this new model legume species, here we characterized the intraspecific diversity found in A. evenia. For this, the accessions available in germplasm banks were collected and subjected to morphological investigations, genotyping with RAPD and SSR markers, molecular phylogenies using ITS and single nuclear gene sequences, and cross-compatibility tests. These combined analyses revealed an important intraspecific differentiation that led us to propose a new taxonomic classification for A. evenia comprising two subspecies and four varieties. The A. evenia ssp. evenia contains var. evenia and var. pauciciliata whereas A. evenia ssp. serrulata comprises var. serrulata and var. major. This study provides information to exploit efficiently the diversity encountered in A. evenia and proposes subsp. evenia as the most appropriate subspecies for future projects aimed at identifying plant determinants of the Nod-independent symbiotic process.

Biological nitrogen fixation in non-legume plants

BACKGROUND

Nitrogen is an essential nutrient in plant growth. The ability of a plant to supply all or part of its requirements from biological nitrogen fixation (BNF) thanks to interactions with endosymbiotic, associative and endophytic symbionts, confers a great competitive advantage over non-nitrogen-fixing plants.

SCOPE

Because BNF in legumes is well documented, this review focuses on BNF in non-legume plants. Despite the phylogenic and ecological diversity among diazotrophic bacteria and their hosts, tightly regulated communication is always necessary between the microorganisms and the host plant to achieve a successful interaction. Ongoing research efforts to improve knowledge of the molecular mechanisms underlying these original relationships and some common strategies leading to a successful relationship between the nitrogen-fixing microorganisms and their hosts are presented.

CONCLUSIONS

Understanding the molecular mechanism of BNF outside the legume-rhizobium symbiosis could have important agronomic implications and enable the use of N-fertilizers to be reduced or even avoided. Indeed, in the short term, improved understanding could lead to more sustainable exploitation of the biodiversity of nitrogen-fixing organisms and, in the longer term, to the transfer of endosymbiotic nitrogen-fixation capacities to major non-legume crops.

Hydrogen peroxide-regulated genes in the Medicago truncatula-Sinorhizobium meliloti symbiosis

Reactive oxygen species (ROS), particularly hydrogen peroxide (H(2)O(2)), play an important role in signalling in various cellular processes. The involvement of H(2)O(2) in the Medicago truncatula-Sinorhizobium meliloti symbiotic interaction raises questions about its effect on gene expression. A transcriptome analysis was performed on inoculated roots of M. truncatula in which ROS production was inhibited with diphenylene iodonium (DPI). In total, 301 genes potentially regulated by ROS content were identified 2 d after inoculation. These genes included MtSpk1, which encodes a putative protein kinase and is induced by exogenous H(2)O(2) treatment. MtSpk1 gene expression was also induced by nodulation factor treatment. MtSpk1 transcription was observed in infected root hair cells, nodule primordia and the infection zone of mature nodules. Analysis with a fluorescent protein probe specific for H(2)O(2) showed that MtSpk1 expression and H(2)O(2) were similarly distributed in the nodule infection zone. Finally, the establishment of symbiosis was impaired by MtSpk1 downregulation with an artificial micro-RNA. Several genes regulated by H(2)O(2) during the establishment of rhizobial symbiosis were identified. The involvement of MtSpk1 in the establishment of the symbiosis is proposed.

Experimental evolution of nodule intracellular infection in legume symbionts

Soil bacteria known as rhizobia are able to establish an endosymbiosis with legumes that takes place in neoformed nodules in which intracellularly hosted bacteria fix nitrogen. Intracellular accommodation that facilitates nutrient exchange between the two partners and protects bacteria from plant defense reactions has been a major evolutionary step towards mutualism. Yet the forces that drove the selection of the late event of intracellular infection during rhizobium evolution are unknown. To address this question, we took advantage of the previous conversion of the plant pathogen Ralstonia solanacearum into a legume-nodulating bacterium that infected nodules only extracellularly. We experimentally evolved this draft rhizobium into intracellular endosymbionts using serial cycles of legume-bacterium cocultures. The three derived lineages rapidly gained intracellular infection capacity, revealing that the legume is a highly selective environment for the evolution of this trait. From genome resequencing, we identified in each lineage a mutation responsible for the extracellular-intracellular transition. All three mutations target virulence regulators, strongly suggesting that several virulence-associated functions interfere with intracellular infection. We provide evidence that the adaptive mutations were selected for their positive effect on nodulation. Moreover, we showed that inactivation of the type three secretion system of R. solanacearum that initially allowed the ancestral draft rhizobium to nodulate, was also required to permit intracellular infection, suggesting a similar checkpoint for bacterial invasion at the early nodulation/root infection and late nodule cell entry levels. We discuss our findings with respect to the spread and maintenance of intracellular infection in rhizobial lineages during evolutionary times.

Queuosine biosynthesis is required for sinorhizobium meliloti-induced cytoskeletal modifications on HeLa Cells and symbiosis with Medicago truncatula

Rhizobia are symbiotic soil bacteria able to intracellularly colonize legume nodule cells and form nitrogen-fixing symbiosomes therein. How the plant cell cytoskeleton reorganizes in response to rhizobium colonization has remained poorly understood especially because of the lack of an in vitro infection assay. Here, we report on the use of the heterologous HeLa cell model to experimentally tackle this question. We observed that the model rhizobium Sinorhizobium meliloti, and other rhizobia as well, were able to trigger a major reorganization of actin cytoskeleton of cultured HeLa cells in vitro. Cell deformation was associated with an inhibition of the three major small RhoGTPases Cdc42, RhoA and Rac1. Bacterial entry, cytoskeleton rearrangements and modulation of RhoGTPase activity required an intact S. meliloti biosynthetic pathway for queuosine, a hypermodifed nucleoside regulating protein translation through tRNA, and possibly mRNA, modification. We showed that an intact bacterial queuosine biosynthetic pathway was also required for effective nitrogen-fixing symbiosis of S. meliloti with its host plant Medicago truncatula, thus indicating that one or several key symbiotic functions of S. meliloti are under queuosine control. We discuss whether the symbiotic defect of que mutants may originate, at least in part, from an altered capacity to modify plant cell actin cytoskeleton.

Nodule development on the tropical legume Sesbania virgata under flooded and non-flooded conditions

The interaction between the Brazilian pioneer legume Sesbania virgata and its microsymbiont Azorhizobium doebereinerae leads to the formation of nitrogen-fixing nodules on roots that grow either in well-aerated soils or in wetlands. We studied the initiation and development of nodules under these alternative conditions. To this end, light and fluorescence microscopy were used to follow the bacterial colonisation and invasion into the host and, by means of transmission electron microscopy, we could observe the intracellular entry. Under hydroponic conditions, intercellular invasion took place at lateral root bases and mature nodules were round and determinate. However, on roots grown in vermiculite that allows aerated growth, bacteria also entered via root hair invasion and nodules were both of the determinate and indeterminate type. Such versatility in entry and developmental plasticity, as previously described in Sesbania rostrata, enables efficient nodulation in both dry and wet environments and are an important adaptive feature of this group of semi-tropical plants that grow in temporarily flooded habitats.