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Background Nitrogen fixing bacteria isolated from hot arid areas in Asia, Africa and America but from diverse leguminous plants have been recently identified as belonging to a possible new species of Ensifer ( Sinorhizobium ). In this study, 6 strains belonging to this new clade were compared with Ensifer species at the genome-wide level. Their capacities to utilize various carbon sources and to establish a symbiotic interaction with several leguminous plants were examined. Results Draft genomes of selected strains isolated from Morocco (Merzouga desert), Mexico (Baja California) as well as from India (Thar desert) were produced. Genome based species delineation tools demonstrated that they belong to a new species of Ensifer . Comparison of its core genome with those of E. meliloti , E. medicae and E. fredii enabled the identification of a species conserved gene set. Predicted functions of associated proteins and pathway reconstruction revealed notably the presence of transport systems for octopine/nopaline and inositol phosphates. Phenotypic characterization of this new desert rhizobium species showed that it was capable to utilize malonate, to grow at 48 °C or under high pH while NaCl tolerance levels were comparable to other Ensifer species. Analysis of accessory genomes and plasmid profiling demonstrated the presence of large plasmids that varied in size from strain to strain. As symbiotic functions were found in the accessory genomes, the differences in symbiotic interactions between strains may be well related to the difference in plasmid content that could explain the different legumes with which they can develop the symbiosis. Conclusions The genomic analysis performed here confirms that the selected rhizobial strains isolated from desert regions in three continents belong to a new species. As until now only recovered from such harsh environment, we propose to name it Ensifer aridi . The presented genomic data offers a good basis to explore adaptations and functionalities that enable them to adapt to alkalinity, low water potential, salt and high temperature stresses. Finally, given the original phylogeographic distribution and the different hosts with which it can develop a beneficial symbiotic interaction, Ensifer aridi may provide new biotechnological opportunities for degraded land restoration initiatives in the future.

Ensifer sp. PC2 is an aerobic, motile, Gram-negative, non-spore-forming rod that was isolated from a nitrogen-fixing nodule of the tree legume P. cineraria (L.) Druce (Khejri), which is a keystone species that grows in arid and semi-arid regions of the Indian Thar desert. Strain PC2 exists as a dominant saprophyte in alkaline soils of Western Rajasthan. It is fast growing, well-adapted to arid conditions and is able to form an effective symbiosis with several annual crop legumes as well as species of mimosoid trees and shrubs. Here we describe the features of Ensifer sp. PC2, together with genome sequence information and its annotation. The 8,458,965 bp high-quality permanent draft genome is arranged into 171 scaffolds of 171 contigs containing 8,344 protein-coding genes and 139 RNA-only encoding genes, and is one of the rhizobial genomes sequenced as part of the DOE Joint Genome Institute 2010 Genomic Encyclopedia for Bacteria and Archaea-Root Nodule Bacteria (GEBA-RNB) project proposal.

Aims To describe the structure of nodules of Vachellia (Acacia ) jacquemontii, and to characterise the rhizobia that occupy them. Methods Light and electron microscopy were used to analyse nodules. Rhizobia were characterised using their 16S rRNA, housekeeping and symbiosis-related gene sequences. Results Nodules of V. jacquemontii were typical of all other described mimosoid legumes. All 73 of the isolates were strains of Ensifer, and concatenated phylogenetic analysis of their housekeeping genes (rrs, recA, atpD, glnII and dnaK) suggested that they are novel, forming separate lineages close to E. saheli. The phytogenies of the symbiosis-essential genes nodA and nifH were inconsistent with the housekeeping phylogenies. The nodA sequences of most isolates were close to that of E. arboris HAMBI 1552T, but the nifH gene was found to be related to that of E. kostiensis HAMBI 1489T. All the tested Ensifer strains, except for AJ24, were found to be capable of nodulating other species of Vachellia as well as native Indian Mimosa and Prosopis spp. Conclusions Stressful conditions caused by the alkaline soil of the Thar Desert have resulted in V. jacquemontii being nodulated by diverse and promiscuous Ensifer species that are capable of nodulating other native members of the tribe Mimoseae.

Background and AimsThe large monophyletic genus Mimosa comprises approx. 500 species, most of which are native to the New World, with Central Brazil being the main centre of radiation. All Brazilian Mimosa spp. so far examined are nodulated by rhizobia in the betaproteobacterial genus Burkholderia. Approximately 10 Mya, transoceanic dispersal resulted in the Indian subcontinent hosting up to six endemic Mimosa spp. The nodulation ability and rhizobial symbionts of two of these, M. hamata and M. himalayana, both from north-west India, are here examined, and compared with those of M. pudica, an invasive species.MethodsNodules were collected from several locations, and examined by light and electron microscopy. Rhizobia isolated from them were characterized in terms of their abilities to nodulate the three Mimosa hosts. The molecular phylogenetic relationships of the rhizobia were determined by analysis of 16S rRNA, nifH and nodA gene sequences.Key ResultsBoth native Indian Mimosa spp. nodulated effectively in their respective rhizosphere soils. Based on 16S rRNA, nifH and nodA sequences, their symbionts were identified as belonging to the alphaproteobacterial genus Ensifer, and were closest to the ‘Old World’ Ensifer saheli, E. kostiensis and E. arboris. In contrast, the invasive M. pudica was predominantly nodulated by Betaproteobacteria in the genera Cupriavidus and Burkholderia. All rhizobial strains tested effectively nodulated their original hosts, but the symbionts of the native species could not nodulate M. pudica.ConclusionsThe native Mimosa spp. in India are not nodulated by the Burkholderia symbionts of their South American relatives, but by a unique group of alpha-rhizobial microsymbionts that are closely related to the ‘local’ Old World Ensifer symbionts of other mimosoid legumes in north-west India. They appear not to share symbionts with the invasive M. pudica, symbionts of which are mostly beta-rhizobial.

To describe the structure of nodules of Vachellia (Acacia) jacquemontii, and to characterise the rhizobia that occupy them. Light and electron microscopy were used to analyse nodules. Rhizobia were characterised using their 16S rRNA, housekeeping and symbiosis-related gene sequences. Nodules of V. jacquemontii were typical of all other described mimosoid legumes. All 73 of the isolates were strains of Ensifer, and concatenated phylogenetic analysis of their housekeeping genes (rrs, recA, atpD, glnII and dnaK) suggested that they are novel, forming separate lineages close to E. saheli. The phylogenies of the symbiosis-essential genes nodA and nifH were inconsistent with the housekeeping phylogenies. The nodA sequences of most isolates were close to that of E. arboris HAMBI 1552.sup.T, but the nifH gene was found to be related to that of E. kostiensis HAMBI 1489.sup.T. All the tested Ensifer strains, except for AJ24, were found to be capable of nodulating other species of Vachellia as well as native Indian Mimosa and Prosopis spp. Stressful conditions caused by the alkaline soil of the Thar Desert have resulted in V. jacquemontii being nodulated by diverse and promiscuous Ensifer species that are capable of nodulating other native members of the tribe Mimoseae.

Nitrogen-fixing symbiosis is globally important in ecosystem functioning and agriculture, yet the evolutionary history of nodulation remains the focus of considerable debate. Recent evidence suggesting a single origin of nodulation followed by massive parallel evolutionary losses raises questions about why a few lineages in the N2-fixing clade retained nodulation and diversified as stable nodulators while most did not. Within legumes, nodulation is restricted to the two most diverse subfamilies, Papilionoideae and Caesalpinioideae, which show stable retention of nodulation across their core clades. We characterize two nodule anatomy types across 128 species in 56 of the 150 genera of the legume subfamily Caesalpinioideae: 1) fixation thread nodules (FTs), where nitrogen-fixing bacteroids are retained within the apoplast in modified infection threads and 2) symbiosomes, where rhizobia are symplastically internalized in the host cell cytoplasm within membrane-bound symbiosomes. Using a robust phylogenomic tree based on 997 genes from 146 caesalpinioid genera, we show that losses of nodulation are more prevalent in lineages with FTs. We propose that evolution of the symbiosome allows for a more intimate and enduring symbiosis through greater compartmentalisation of their rhizobial microsymbionts, resulting in greater evolutionary stability of nodulation across this species-rich pantropical clade of legumes. Competing Interest Statement The authors have declared no competing interest.

Global changes are responsible for climate change that ultimately affects plants and soil health that directly or indirectly influence the community and functional diversity of soil microbial populations. Plants secrete diverse compounds to interact with an array of microorganism present in the soil; therefore, vegetation type modifies rhizospheric microbial communities in addition to affecting native populations of microbes mainly by root exudates. Microorganisms living inside plants intimately interact with cells of the host releasing plant‐growth‐promoting compounds and taking up secreted metabolites. This synergistic interaction exemplifies double‐fitness trait that is active in plant‐endophyte partnership. The availability of nutrient elements is a major constraint to plant growth in most of the environments of the world. For sustainable and secure food production, new drives to find scientific and technological solutions are needed that can promote production in cereal crops. Understanding the role of nonsymbionts in this area would be helpful in development of bioinoculum, which is appealing as the fertility of the soil may be enhanced by inoculating the best suited bacterial strain that will give rise to a better plant population without any application of fertilizer. Inoculation of ornamental, forestry, agricultural, and desert crops with plant‐growth‐promoting endophytic bacteria can improve biomass production and can result in multiple effects plant vigor, plant height, early bloom, and chlorophyll content. The potential of nonsymbiotic nitrogen fixation can provide an improved solution to promote plant development. The understanding of mechanistic aspects of endophytic bacteria has great potential to aid in designing strategies to substantially improve the growth and health of host plants. Thus, associations of plants with beneficial endophytic strains can be a valuable addition to toolbox of sustainable agriculture.