Molecular diversity and phylogeny of rhizobia associated with Lablab purpureus (Linn.) grown in Southern China

Sustainable Applications of Endophytic Bacteria and Their Physiological/Biochemical Roles on Medicinal and Herbal Plants: Review

Bacterial endophytes reside within the tissues of living plant species without causing any harm or disease to their hosts. These endophytes can be isolated, identified, characterized, and used as biofertilizers. Moreover, bacterial endophytes increase the plants' resistance against diseases, pests, and parasites, and are a promising source of pharmaceutically important bioactives. For instance, the production of antibiotics, auxins, biosurfactants, cytokinin's, ethylene, enzymes, gibberellins, nitric oxide organic acids, osmolytes, and siderophores is accredited to the existence of various bacterial strains. Thus, this manuscript intends to review the sustainable applications of endophytic bacteria to promote the growth, development, and chemical integrity of medicinal and herbal plants, as well as their role in plant physiology. The study of the importance of bacterial endophytes in the suppression of diseases in medicinal and herbal plants is crucial and a promising area of future investigation.

Phylogenetic diversity analysis reveals Bradyrhizobium yuanmingense and Ensifer aridi as major symbionts of mung bean (Vigna radiata L.) in Pakistan

The present study was carried out to evaluate the diversity of rhizobia associated with nodules of mung bean in Pakistan, because this information is necessary for inoculum development. Based on sequence analysis of 16S rRNA gene of thirty-one bacteria, 11 were assigned to genus Bradyrhizobium, 17 to Ensifer, and 3 to Rhizobium. Phylogenetic analyses on the basis of 16S-23S ITS region, atpD, recA, nifH, and nodA of representative strains revealed that B. yuanmingense is the predominant species distributed throughout different mung bean-growing areas. Among the fast-growing rhizobia, Ensifer aridi was predominant in Faisalabad, Layyah, and Rawalpindi, while E. meliloti in Thal desert. Sequence variations and phylogeny of nifH and nodA genes suggested that these genes might have been co-evolved with the housekeeping genes and maintained by vertical gene transfer in rhizobia detected in the present study. Host infectivity assay revealed the successful nodulation of host by rhizobia related to genera Bradyrhizobium, Ensifer and Rhizobium. Among all, Bradyrhizobium and Ensifer spp. inoculation exhibited a significantly higher number of nodules (11-34 nodules plant-1) and nitrogenase activity (nodule ARA 60-110 μmol g-1 h-1). Contrary to the previous studies, our data reveal that B. yuanmingense and E. aridi are predominant species forming effective nodules in mung bean in Pakistan. Furthermore, to the best of our knowledge, this is the first report showing the effective symbiosis of E. aridi, E. meliloti, and Rhizobium pusense with mung bean. The diversity of rhizobia in different habitats revealed in the present study will contribute towards designing site-specific inocula for mung bean.

Competition for electrons favours N2 O reduction in denitrifying Bradyrhizobium isolates

Bradyrhizobia are common members of soil microbiomes and known as N₂ -fixing symbionts of economically important legumes. Many are also denitrifiers, which can act as sinks or sources for N₂ O. Inoculation with compatible rhizobia is often needed for optimal N₂ -fixation, but the choice of inoculant may have consequences for N₂ O emission. Here, we determined the phylogeny and denitrification capacity of Bradyrhizobium strains, most of them isolated from peanut-nodules. Analyses of genomes and denitrification end-points showed that all were denitrifiers, but only ~1/3 could reduce N₂ O. The N₂ O-reducing isolates had strong preference for N₂ O- over NO₃ ^- -reduction. Such preference was also observed in a study of other bradyrhizobia and tentatively ascribed to competition between the electron pathways to Nap (periplasmic NO₃ ^- reductase) and Nos (N₂ O reductase). Another possible explanation is lower abundance of Nap than Nos. Here, proteomics revealed that Nap was instead more abundant than Nos, supporting the hypothesis that the electron pathway to Nos outcompetes that to Nap. In contrast, Paracoccus denitrificans, which has membrane-bond NO₃ ^- reductase (Nar), reduced N₂ O and NO₃ ^- simultaneously. We propose that the control at the metabolic level, favouring N₂ O reduction over NO₃ ^- reduction, applies also to other denitrifiers carrying Nos and Nap but lacking Nar.

Aeschynomene indica-Nodulating Rhizobia Lacking Nod Factor Synthesis Genes: Diversity and Evolution in Shandong Peninsula, China

Aeschynomene indica is a semiaquatic legume that forms both stem and root nodules with rhizobia. Some A. indica rhizobia (AIRs) have been reported to nodulate the host using a Nod factor-independent pathway and possess photosynthetic abilities. To investigate the diversity and community structure of AIRs in China, a total of 300 rhizobial isolates were acquired from the root and stem nodules of A. indica grown at 4 sites in Shandong Peninsula, China. Nineteen representative strains were selected according to their recA phylogeny. With further classification in comparison with reference strains, 10 Bradyrhizobium genospecies were defined based on the 16S rRNA gene phylogeny and multilocus sequence analysis (MLSA) of housekeeping genes (HKGs) recA, atpD, glnII, dnaK, gyrB, and rpoB In addition, 6 genospecies were found only in China. No nodulation gene (nodA, nodB, nodC, or nodZ) was detected in the AIRs isolates by PCR amplification and Southern blotting. Phylogenetic analysis of nifH and the photosynthesis-related gene pufLM revealed their common origins. All representative strains formed root nodules, but only 9 representative strains for 4 genospecies formed stem nodules on A. indica, indicating that the stem nodulation process of A. indica is limited to some strains. The nucleotide diversity and recombination events of the HKGs, as well as nifH and pufLM genes, showed that mutation contributes more than recombination in evolution. The distribution of dominant AIR genospecies was mainly affected by available nitrogen, organic carbon, total nitrogen, and pH. Our study helps to characterize the diversity and evolution of AIRs.IMPORTANCE Aeschynomene indica rhizobia (AIRs) can form both root and stem nodules via Nod factor-independent processes, which distinguishes them from other rhizobia. This study systematically uncovered the diversity and community composition of A. indica rhizobia distributed in eastern China. Our results reclassified all the A. indica rhizobia across the world and represent a useful contribution to evaluating the diversity and distribution of the symbiont. The presence of novel genospecies specifically distributed in China enriched the A. indica rhizobia resources and provided insight into the geographic distribution of rhizobia. The phylogenetic relationship between nifH and pufLM of A. indica rhizobia across the world provides insight into the evolution of their nitrogen fixation and photosynthetic abilities.

Genetic diversity and evolution of Bradyrhizobium populations nodulating Erythrophleum fordii, an evergreen tree indigenous to the southern subtropical region of China

The nodulation of Erythrophleum fordii has been recorded recently, but its microsymbionts have never been studied. To investigate the diversity and biogeography of rhizobia associated with this leguminous evergreen tree, root nodules were collected from the southern subtropical region of China. A total of 166 bacterial isolates were obtained from the nodules and characterized. In a PCR-based restriction fragment length polymorphism (RFLP) analysis of ribosomal intergenic sequences, the isolates were classified into 22 types within the genus Bradyrhizobium. Sequence analysis of 16S rRNA, ribosomal intergenic spacer (IGS), and the housekeeping genes recA and glnII classified the isolates into four groups: the Bradyrhizobium elkanii and Bradyrhizobium pachyrhizi groups, comprising the dominant symbionts, Bradyrhizobium yuanmingense, and an unclassified group comprising the minor symbionts. The nodC and nifH phylogenetic trees defined five or six lineages among the isolates, which was largely consistent with the definition of genomic species. The phylogenetic results and evolutionary analysis demonstrated that mutation and vertical transmission of genes were the principal processes for the divergent evolution of Bradyrhizobium species associated with E. fordii, while lateral transfer and recombination of housekeeping and symbiotic genes were rare. The distribution of the dominant rhizobial populations was affected by soil pH and effective phosphorus. This is the first report to characterize E. fordii rhizobia.