Diversity and N-acyl-homoserine lactone production by Gammaproteobacteria associated with Avicennia marina rhizosphere of South Indian mangroves.

The diversity of N-acyl-homoserine lactone (AHL)-producing rhizosphere bacterial community associated with Avicennia marina in the mangrove ecosystems of South India was investigated. Approximately 800 rhizobacteria were isolated from A. marina, and they were screened for the production of AHL using two biosensors, Chromobacterium violaceum CV026 and Agrobacterium tumefaciens NTL4 (pZLR4). Among the total isolates screened, 7% of the rhizobacteria showed positive induction for AHL signals. The BOX-PCR profile of 56 positive isolates represented 11 distinct genotypic groups. Phylogenetic analyses of the 16S rRNA sequences of 16 representatives showed that the isolates belonged to the class Gammaproteobacteria, which represented six different genera: Pseudomonas, Aeromonas, Vibrio, Photobacterium, Serratia and Halomonas. The study also identified three AHL-producing species, namely, Photobacterium halotolerans MSSRF QS48, Vibrio xiamenensis MSSRF QS47 and Pseudomonas sp. MSSRF QS1 that had not been reported previously. AHL profiling by TLC detected short chains C4, C6 and C8-HSL, and long chains C10 and C12-HSL with both unsubstituted and substituted side chains among the 16 representative AHL positives. This is the first report concerning the diversity of AHL-producing Gammaproteobacteria from mangrove ecosystems exhibiting diverse AHL profiles.

Novel Phl-producing genotypes of finger millet rhizosphere associated pseudomonads and assessment of their functional and genetic diversity.

Genetic diversity of phlD gene, an essential gene in the biosynthesis of 2,4-diacetylphloroglucinol, was studied by restriction fragment length polymorphism (RFLP) in 20 Phl-producing pseudomonads isolated from finger millet rhizosphere. RFLP analysis of phlD gene displayed three patterns with HaeIII and TaqI enzymes. phlD gene sequence closely correlated with RFLP results and revealed the existence of three new genotypes G, H and I. Further, the phylogenetic and concatenated sequence analysis of the 16S rRNA, rpoB, gyrB, rpoD genes supported the hypothesis that these genotypes G, H and I were different from reported genotypes A-F. In all phylogenetic studies, the genotype G formed a distant clade from the groups of Pseudomonas putida and P. aeruginosa (sensu strictu), but the groups H and I were closely related to P. aeruginosa/P. stutzeri group. The Phl-producing pseudomonads exhibited antagonistic activity against Pyricularia grisea (TN508), Gaeumannomyces graminis (DSM1463), Fusarium oxysporum (DSM62297), Xanthomonas campestris (DSM3586) and Erwinia persicina (HMGU155). In addition, these strains exhibited various plant growth-promoting traits. In conclusion, this study displays the existence of novel Phl-producing pseudomonads genotypes G, H and I from finger millet rhizosphere, which formed taxonomically outward phylogenetic lineage from the groups of P. putida and P. aeruginosa (sensu strictu).

Phylogenetic diversity of rhizobia associated with horsegram [Macrotyloma uniflorum (Lam.) Verdc.] grown in South India based on glnII, recA and 16S-23S intergenic sequence analyses.

Horsegram [Macrotyloma uniflorum (Lam.) Verdc.) is an important grain legume and fodder crop in India. Information on root nodule endosymbionts of this legume in India is limited. In the present study, 69 isolates from naturally occurring root nodules of horsegram collected from two agro-eco-climatic regions of South India was analyzed by generation rate, acid/alkali reaction on YMA medium, restriction fragment length polymorphism analysis of 16S-23S rDNA intergenic spacer region (IGS), and sequence analyses of IGS and housekeeping genes glnII and recA. Based on the rDNA IGS RFLP by means of three restriction enzymes rhizobia were grouped in five clusters (I-V). By sequence analysis of 16S-23S rDNA IGS identified genotypes of horsegram rhizobia were distributed into five divergent lineages of Bradyrhizobium genus which comprised (I) the IGS type IV rhizobia and valid species B. yuanmingense, (II) the strains of IGS type I and Bradyrhizobium sp. ORS 3257 isolated from Vigna sp., (III) the strains of the IGS type II and Bradyrhizobium sp. CIRADAc12 from Acacia sp., (IV) the IGS type V strains and Bradyrhizobium sp. genospecies IV, and (V) comprising genetically distinct IGS type III strains which probably represent an uncharacterized new genomic species. Nearly, 87% of indigenous horsegram isolates (IGS types I, II, III, and V) could not be related to any other species within the genus Bradyrhizobium. Phylogeny based on housekeeping glnII and recA genes confirmed those results found by the analysis of the IGS sequence. All the isolated rhizobia nodulated Macrotyloma sp. and Vigna spp., and only some of them formed nodules on Arachis hypogeae. The isolates within each IGS type varied in their ability to fix nitrogen. Selection for high symbiotic effective strains could reward horsegram production in poor soils of South India where this legume is largely cultivated.