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Dichrostachys cinerea (L.) Wight & Arn, which belongs to the Mimosoid clade of the legume subfamily Caesalpinioideae, was introduced into India and has since become invasive across wide areas of the country. It is nodulated, and like all other mimosoids it has indeterminate nodules with its microsymbionts housed in membrane-bound symbiosomes rather than within cell wall-enclosed fixation threads. Fifty-eight bacterial strains were isolated from root nodules on plants growing in soils from 13 sampling sites in India with various agroclimatic conditions. Genetic analysis of 36 strains resulted in diverse RAPD genotypes, with equal composition of Ensifer and Bradyrhizobium as its root nodule microsymbionts. Multi locus sequence analysis (MLSA) of 12 strains using the recA, glnII, atpD and 16S rRNA genes revealed significant genetic diversity forming novel clades and lineages and are potential new species. The D. cinerea strains were variants of local symbionts previously described as rhizobia associated with native and exotic mimosoid trees, as well as rhizobia associated with the non-mimosoid Caesalpinioid shrub Chamaecrista pumila and wild Papilionoid legumes from India. The symbiosis essential genes (nodA and nifH) of the D. cinerea strains were diverse and clustered according to geographical origin. Mosaic combinations of core and sym genes were harbored by both Ensifer and Bradyrhizobium suggesting gradual diversification and microevolution of rhizobia under pressure from the host in combination with edaphic and environmental factors. The dominant microsymbionts of native and invasive legumes, including D. cinerea, in alkaline soils of India are essentially of the 'E. aridi' and B. yuanmingense types. Dichrostachys cinerea rhizobia were symbiotically efficient on their homologous host, but also have ability to nodulate the crop Vigna radiata, and hence may be good candidates to be used for inoculants on legume crops as well as on Mimosoid trees (P. cineraria, V. nilotica, V. raddiana, S. senegal) used in sustainable agroforestry practices to enhance soil nitrogen content. [ABSTRACT FROM AUTHOR]

Vachellia (syn. Acacia) nilotica subsp. indica is a widespread legume tree in India of ecological and economic importance due to its ability to nodulate and fix nitrogen. The structure of V. nilotica nodules was examined and diversity of its rhizobial symbionts in various locations in India was assessed. More than a hundred strains were purified from indeterminate Vachellia-type root nodules on seedlings grown in alkaline soils from different regions of Thar Desert in Western Rajasthan and in neighbouring states (Gujarat, Haryana, Punjab). Plants grown in acidic soils of Meghalaya and Nagaland failed to nodulate. Twenty-one strains identified as species of Ensifer (Sinorhizobium) based on their 16S rRNA gene sequences and grouped into seven recA-types. Eight strains were characterized using two additional housekeeping (glnII, dnaK) and symbiotic (nodA, nifH) genes. A concatenated core gene phylogeny revealed that V. nilotica-Ensifer strains are novel and genetically diversified from E. saheli, E. arboris, E. terangae and E. americanus. Most are distinct from Ensifer strains previously reported in mimosoid, caesalpinioid and papilionoid legume species of W. Rajasthan. These Ensifer strains harboured typical mimosoid-type symbiotic genes that were diversified from E. arboris, E. kostiensis and E. terangae. They nodulated other mimosoids (Prosopis cineraria, Mimosa hamata, Senegalia senegal and Vachellia spp.), but failed to make symbiotic interactions with a papilionoid crop species, Vigna radiata, suggesting their specificity towards mimosoids. Vachellia nilotica is quite promiscuous, fixing nitrogen in symbiosis with diverse strains of Ensifer, and has successfully invaded various agro-climatic regions, thus indicating that it could be exploited for agroforestry projects. [ABSTRACT FROM AUTHOR]

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 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, but the nifH gene was found to be related to that of E. kostiensis HAMBI 1489. 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. [ABSTRACT FROM AUTHOR]

Aims: To survey the occurrence of nodulated legumes in the arid and semi-arid areas of Western Rajasthan and to characterize their associated symbiotic bacteria. Methods: Herbaceous annual species were excavated whole, while tree species were studied as seedlings in the field or as trap plants in pot experiments. Nodules were examined by microscopy to confirm their effectiveness and to determine their internal structure. Bacteria isolated from the nodules were authenticated on their original hosts and were identified on the basis of 16S rRNA sequencing. Phylogenetic trees were inferred using the neighbour-joining method. Results: We studied 35 of more than 50 species of native legume reported from these areas. Legumes are drought escaping (annual species), drought tolerant perennials or trees possessing deep root systems and other adaptations to arid conditions. Nodulation was recorded in all members of the Papilionoideae and Mimosoideae, but only one species of Caesalpinioideae. Internal structure of nodules varied within these groups, especially with respect to the presence or absence of uninfected cells in the infected region. Full 16S rRNA gene sequencing revealed that the nodules harboured a range of nodulating bacteria belonging to the genera Sinorhizobium, Rhizobium and Bradyrhizobium, within which they formed separate sub clades. Conclusions: This study extends the range of legumes known to grow and nodulate in semi-arid regions, and provides information about their endosymbionts. [ABSTRACT FROM AUTHOR]