Defects In Nitrogen Fixation Research Articles

DBD plasma assisted synthesis of MO@Fe/Ce-MOFs with rich oxygen defects as efficient photocatalysts for nitrogen fixation

A series of metal oxide/ metal-organic frameworks (MOFs) composites (MO@Fe/Ce-MOFs) were synthesized by partial thermal decomposition of Fe/Ce-MOFs which were synthesized with dielectric barrier discharge (DBD) plasma. The heterojunction interface between MOFs and metal oxides can effectively promote the electron interface transfer. When Fe was introduced, the energy band of the catalysts became narrower, and the increase of oxygen defects concentration slowed down the recombination rate of photogenerated electrons and holes, which greatly improved the photocatalytic performance. MO@Fe/Ce-MOFs exhibited more efficient visible light photocatalytic performance than that of Fe/Ce-MOFs. MO@Fe/Ce-MOFs-b (the molar ratio of Fe to Ce is 1:3) formed an abundant oxygen defect that facilitated the photocatalytic nitrogen fixation reaction. For MO@Fe/Ce-MOFs-b, the NH4+ generation rate reached 299 μmol·L−1·h−1 after 2 h photoreaction under visible light with N2 and deionized water as solvent. Also, MO@Fe/Ce-MOFs-b showed high cyclic stability. After three repeated cycles, the NH4+ generation rate maintained about 95%. This work provided a new method for the preparation of MO@MOFs and emphasized the importance of oxygen defects in nitrogen fixation.

The nitrate-reduction gene cluster components exert lineage-dependent contributions to optimization of Sinorhizobium symbiosis with soybeans.

Receiving nodulation and nitrogen fixation genes does not guarantee rhizobia an effective symbiosis with legumes. Here, variations in gene content were determined for three Sinorhizobium species showing contrasting symbiotic efficiency on soybeans. A nitrate-reduction gene cluster absent in S. sojae was found to be essential for symbiotic adaptations of S. fredii and S. sp. III. In S. fredii, the deletion mutation of the nap (nitrate reductase), instead of nir (nitrite reductase) and nor (nitric oxide reductase), led to defects in nitrogen-fixation (Fix- ). By contrast, none of these core nitrate-reduction genes were required for the symbiosis of S. sp. III. However, within the same gene cluster, the deletion of hemN1 (encoding oxygen-independent coproporphyrinogen III oxidase) in both S. fredii and S. sp. III led to the formation of nitrogen-fixing (Fix+ ) but ineffective (Eff- ) nodules. These Fix+ /Eff- nodules were characterized by significantly lower enzyme activity of glutamine synthetase indicating rhizobial modulation of nitrogen-assimilation by plants. A distant homologue of HemN1 from S. sojae can complement this defect in S. fredii and S. sp. III, but exhibited a more pleotropic role in symbiosis establishment. These findings highlighted the lineage-dependent optimization of symbiotic functions in different rhizobial species associated with the same host.

Functional characterisation of brassinosteroid receptor MtBRI1 in Medicago truncatula

Brassinosteroids are phytohormones involved in plant development and physiological processes. Brassinosteroids Insensitive 1 (BRI1) is required for BR perception and initiation of subsequent signal transduction in Arabidopsis. In this study, the orthologue of BRI1 in the model legume species Medicago truncatula, MtBRI1, was identified and characterised. Three allelic Tnt1 insertion mutants, mtbri1-1, mtbri1-2, and mtbri1-3, were obtained from the M. truncatula Tnt1 insertion population. mtbri1 mutants displayed characteristic bri1 mutant phenotypes: extreme dwarfness, dark green curled leaves, short primary roots, less lateral roots, and insensitive to exogenous brassinolide (BL). Moreover, mtbri1 mutants show decreased total nodule number and defects in nitrogen fixation. MtBRI1 is able to complement an Arabidopsis BRI1 mutant, bri1-5. Similar to the interaction of BRI1 and BAK1 in Arabidopsis, MtBRI1 interacts with MtSERK1 in vivo. Global gene expression profiling revealed that the expression of BR biosynthesis genes and SAUR genes are significantly altered in mtbri1 mutants. MapMan analysis indicated that genes involved in signaling, hormone, cell wall, and biotic stress responses are over-represented in differentially expressed genes. Taken together, the results indicate that MtBRI1 is the BR receptor in M. truncatula and that BR signaling may play a conserved role in balancing plant growth and defenses.

Transcriptomic Analysis of Sinorhizobium meliloti and Medicago truncatula Symbiosis Using Nitrogen Fixation-Deficient Nodules.

The bacterium Sinorhizobium meliloti interacts symbiotically with legume plant hosts such as Medicago truncatula to form nitrogen-fixing root nodules. During symbiosis, plant and bacterial cells differentiate in a coordinated manner, resulting in specialized plant cells that contain nitrogen-fixing bacteroids. Both plant and bacterial genes are required at each developmental stage of symbiosis. We analyzed gene expression in nodules formed by wild-type bacteria on six plant mutants with defects in nitrogen fixation. We observed differential expression of 482 S. meliloti genes with functions in cell envelope homeostasis, cell division, stress response, energy metabolism, and nitrogen fixation. We simultaneously analyzed gene expression in M. truncatula and observed differential regulation of host processes that may trigger bacteroid differentiation and control bacterial infection. Our analyses of developmentally arrested plant mutants indicate that plants use distinct means to control bacterial infection during early and late symbiotic stages.

Plant and bacterial symbiotic mutants define three transcriptionally distinct stages in the development of the Medicago truncatula/Sinorhizobium meliloti symbiosis.

In the Medicago truncatula/Sinorhizobium meliloti symbiosis, the plant undergoes a series of developmental changes simultaneously, creating a root nodule and allowing bacterial entry and differentiation. Our studies of plant genes reveal novel transcriptional regulation during the establishment of the symbiosis and identify molecular markers that distinguish classes of plant and bacterial symbiotic mutants. We have identified three symbiotically regulated plant genes encoding a beta,1-3 endoglucanase (MtBGLU1), a lectin (MtLEC4), and a cysteine-containing protein (MtN31). MtBGLU1 is down-regulated in the plant 24 h after exposure to the bacterial signal, Nod factor. The non-nodulating plant mutant dmi1 is defective in the ability to down-regulate MtBGLU1. MtLEC4 and MtN31 are induced 1 and 2 weeks after bacterial inoculation, respectively. We examined the regulation of these two genes and three previously identified genes (MtCAM1, ENOD2, and MtLB1) in plant symbiotic mutants and wild-type plants inoculated with bacterial symbiotic mutants. Plant (bit1, rit1, and Mtsym1) and bacterial (exoA and exoH) mutants with defects in the initial stages of invasion are unable to induce MtLEC4, MtN31, MtCAM1, ENOD2, and MtLB1. Bacterial mutants (fixJ and nifD) and a subset of plant mutants (dnf2, dnf3, dnf4, dnf6, and dnf7) defective for nitrogen fixation induce the above genes. The bacA bacterial mutant, which senesces upon deposition into plant cells, and two plant mutants with defects in nitrogen fixation (dnf1 and dnf5) induce MtLEC4 and ENOD2 but not MtN31, MtCAM1, or MtLB1. These data suggest the presence of at least three transcriptionally distinct developmental stages during invasion of M. truncatula by S. meliloti.

Investigation of myo-inositol catabolism in Rhizobium leguminosarum bv. viciae and its effect on nodulation competitiveness.

Three discrete loci required for growth on myo-inositol in Rhizobium leguminosarum bv. viciae have been characterized. Two of these are catabolic loci that code for malonate semialdehyde dehydrogenase (iolA) and malonate semialdehyde dehydrogenase (iolD). IolD is part of a possible operon, iolDEB, although the functions of IolE and IolB are unknown. The third locus, int, codes for an ABC transport system that is highly specific for myo-inositol. LacZ analysis showed that mutation of iolD, which codes for one of the last steps in the catabolic pathway, prevents increased transcription of the entire pathway. It is likely that the pathway is induced by an end product of catabolism rather than myo-inositol itself. Mutants in any of the loci nodulated peas (Pisum sativum) and vetch (Vicia sativa) at the same rate as the wild type. Acetylene reduction rates and plant dry weights also were the same in the mutants and wild type, indicating no defects in nitrogen fixation. When wild-type 3841 was coinoculated onto vetch plants with either catabolic mutant iolD (RU360) or iolA (RU361), however, >95% of the nodules were solely infected with the wild type. The competitive advantage of the wild type was unaffected, even when the mutants were at 100-fold excess. The myo-inositol transport mutant (RU1487), which grows slowly on myo-inositol, was only slightly less competitive than the wild type. The nodulation advantage of the wild type was not the result of superior growth in the rhizosphere. Instead, it appears that the wild type may displace the mutants early on in the infection and nodulation process, suggesting an important role for myo-inositol catabolism.

Isolation and characterization of symbiotic mutants of bradyrhizobium sp. (Arachis) strain NC92: mutants with host-specific defects in nodulation and nitrogen fixation.

Random transposon Tn5 mutagenesis of Bradyrhizobium sp. (Arachis) strain NC92, a member of the cowpea cross-inoculation group, was carried out, and kanamycin-resistant transconjugants were tested for their symbiotic phenotype on three host plants: groundnut, siratro, and pigeonpea. Two nodulation (Nod- phenotype) mutants were isolated. One is unable to nodulate all three hosts and appears to contain an insertion in one of the common nodulation genes (nodABCD); the other is a host-specific nodulation mutant that fails to nodulate pigeonpea, elicits uninvaded nodules on siratro, and elicits normal, nitrogen-fixing nodules on groundnut. In addition, nine mutants defective in nitrogen fixation (Fix- phenotype) were isolated. Three fail to supply symbiotically fixed nitrogen to all three host plants. Surprisingly, nodules elicited by one of these mutants exhibit high levels of acetylene reduction activity, demonstrating the presence of the enzyme nitrogenase. Three more mutants have partially effective phenotypes (Fix +/-) in symbiosis with all three host plants. The remaining three mutants fail to supply fixed nitrogen to one of the host plants tested while remaining partially or fully effective on the other two hosts; two of these mutants are Fix- in pigeonpea and Fix +/- on groundnut and on siratro, whereas the other one is Fix- on groundnut but Fix+ on siratro and on pigeonpea. These latter mutants also retain significant nodule acetylene reduction activity, even in the ineffective symbioses. Such bacterial host-specific fixation (Hsf) mutants have not previously been reported.

Genetic analysis of a pleiotropic mutant of Klebsiella pneumoniae affected in nitrogen metabolism.

Genetic and reversion analyses of a thermosensitive pleiotropic mutant strain of Klebsiella pneumoniae with defects in nitrogen fixation and nitrogen metabolism have shown that the pleiotropie behaviour of mutant is due to a single mutation in a gene designated nim. This gene is contransducible with trp at a frequency of about 30% (using bacteriophage P1) and with cys at a frequency of about 14%. The gene order is cys, trp, nim. The defect in the nim mutant is complemented by the E. coli F' element, F'148.