Characterization of a symbiotically defective serine auxotroph ofMesorhizobium ciceri

xMAP Technology: Applications in Detection of Pathogens

xMAP technology is applicable for high-throughput, multiplex and simultaneous detection of different analytes within a single complex sample. xMAP multiplex assays are currently available in various nucleic acid and immunoassay formats, enabling simultaneous detection and typing of pathogenic viruses, bacteria, parasites and fungi and also antigen or antibody interception. As an open architecture platform, the xMAP technology is beneficial to end users and therefore it is used in various pharmaceutical, clinical and research laboratories. The main aim of this review is to summarize the latest findings and applications in the field of pathogen detection using microsphere-based multiplex assays.

Key roles of microsymbiont amino acid metabolism in rhizobia-legume interactions

Rhizobia are bacteria in the α-proteobacterial genera Rhizobium, Sinorhizobium, Mesorhizobium, Azorhizobium and Bradyrhizobium that reduce (fix) atmospheric nitrogen in symbiotic association with a compatible host plant. In free-living and/or symbiotically associated rhizobia, amino acids may, in addition to their incorporation into proteins, serve as carbon, nitrogen or sulfur sources, signals of cellular nitrogen status and precursors of important metabolites. Depending on the rhizobia-host plant combination, microsymbiont amino acid metabolism (biosynthesis, transport and/or degradation) is often crucial to the establishment and maintenance of an effective nitrogen-fixing symbiosis and is intimately interconnected with the metabolism of the plant. This review summarizes past findings and current research directions in rhizobial amino acid metabolism and evaluates the genetic, biochemical and genome expression studies from which these are derived. Specific sections deal with the regulation of rhizobial amino acid metabolism, amino acid transport, and finally the symbiotic roles of individual amino acids in different plant-rhizobia combinations.

Rhizobium pusense sp. nov., isolated from the rhizosphere of chickpea (Cicer arietinum L.)

A novel bacterial strain, designated NRCPB10T, was isolated from rhizosphere soil of chickpea (Cicer arietinum L.) in Pusa, New Delhi, India. The 16S rRNA gene sequence of strain NRCPB10T showed highest similarity (98.9 %) to that of Rhizobium radiobacter NCPPB 2437T, followed by Rhizobium larrymoorei AF3-10T (97.7 %) and Rhizobium rubi IFO 13261T (97.4 %). Phylogenetic analysis of strain NRCPB10T based on the housekeeping genes recA and atpD confirmed its position as distinct from recognized Rhizobium species. Levels of DNA–DNA relatedness between strain NRCPB10T and R. radiobacter ICMP 5785T, R. larrymoorei LMG 21410T and R. rubi ICMP 6428T were 51.0, 32.6 and 27.3 %, respectively. Cellular fatty acids of strain NRCPB10T were C18 : 1ω7c (58.9 %), C16 : 0 (15.5 %), C19 : 0 cyclo ω8c (11.5 %), iso-C16 : 1 (5.8 %), C16 : 0 3-OH (4.5 %), C16 : 1ω7c (2.1 %) and C18 : 0 (1.3 %). The G+C content of the genomic DNA of strain NRCPB10T was 59.0 mol%. Strain NRCPB10T did not nodulate chickpea plants or induce tumours in tobacco plants. Phenotypic and physiological properties along with SDS-PAGE of whole-cell soluble proteins differentiated strain NRCPB10T from its closest phylogenetic neighbours. On the basis of data from the present polyphasic taxonomic study, strain NRCPB10T is considered to represent a novel species of the genus Rhizobium, for which the name Rhizobium pusense sp. nov. is proposed. The type strain is NRCPB10T ( = LMG 25623T = JCM 16209T = NCIMB 14639T).

Mutation in the lysA gene impairs the symbiotic properties of Mesorhizobium ciceri

A Tn5-induced mutant of Mesorhizobium ciceri, TL28, requiring the amino acid lysine for growth on minimal medium was isolated and characterized. The Tn5 insertion in the mutant strain TL28 was located on a 6.8-kb EcoRI fragment of the chromosomal DNA. Complementation analysis with cloned DNA indicated that 1.269 kb of DNA of the 6.8-kb EcoRI fragment restored the wild-type phenotype of the lysine-requiring mutant. This region was further characterized by DNA sequence analysis and was shown to contain a coding sequence homologous to lysA gene of different bacteria. The lys (-) mutant TL28 was unable to elicit development of effective nodules on the roots of Cicer arietinum L. There was no detectable level of lysine in the root exudates of chickpea. However, addition of lysine to the plant growth medium restored the ability of the mutant to produce effective nodules with nitrogen fixation ability on the roots of C. arietinum.

Characterization of an rpoN mutant of Mesorhizobium ciceri

AIMS

To study the genetic basis of C(4)-dicarboxylate transport (Dct) in relation to symbiotic nitrogen fixation in Mesorhizobium ciceri.

METHODS AND RESULTS

A Tn5-induced mutant strain (TL16) of M. ciceri, unable to grow on C(4)-dicarboxylates, was isolated from the wild-type strain TAL 620. The mutant lacked activities of the enzymes, which use C(4)-dicarboxylates as substrate. The sequencing of the 3.2kb EcoRI fragment, which was the site of Tn5 insertion, revealed three complete and two partial open reading frames. In the mutant, Tn5 interrupted the rpoN gene, of which only one copy was there. Complementation and biochemical studies suggest that the M. ciceri rpoN activity is required for C(4)-Dct, maturation of bacteroids and symbiotic nitrogen fixation. The fine structure of the ineffective nodules produced by TL16 on Cicer arietinum L changed in comparison with those produced by the wild type.

CONCLUSIONS

The mutant strain TL16 suffered a disruption in the rpoN gene. Only one copy of rpoN gene is present in M. ciceri. The mutation abolishes Dct activity. It additionally abolishes the symbiotic nitrogen fixation activity of the bacteroids in the nodules.

SIGNIFICANCE AND IMPACT OF THE STUDY

This first document in M. ciceri shows that a functional rpoN gene is essential for the transport of dicarboxylic acids and symbiotic nitrogen fixation.

Auxotrophy in rhizobia revisited

Among the various types of mutations studied in rhizobia, the auxotrophic mutations (which confer on the mutants the inability to synthesize certain essential substances such as amino acids, vitamins and nucleic acids), are the most favoured ones as these can be used as suitable markers for genetic analysis. An important property of rhizobia is their effectiveness i.e. their ability to fix atmospheric nitrogen into ammonia within the nodule. Special interest in this category of mutations by rhizobial geneticists is due to the fact that there is a strong correlation between the metabolic defects and the ineffectiveness (Nod(-) and/or Fix(-)) of the rhizobial strains. Auxotrophic mutants of various species of rhizobia with defects in the synthesis of nucleic bases, vitamins and amino acids have been obtained by mutagenising with physical, chemical and Tn5 mutagens. These mutants have been used in mapping studies as well as in establishing a correlation between its metabolic requirement and symbiotic relationship with the host plant. The present review deals with the isolation of auxotrophs, and their genetic, biochemical and symbiotic characterization. The review also encompasses the studies on the elucidation of biosynthetic pathways of nutritional substances in rhizobia.