Nitrogen fixation

About: Nitrogen fixation is a research topic. Over the lifetime, 7940 publications have been published within this topic receiving 232921 citations. The topic is also known as: GO:0009399.

A Rhizobium meliloti lipopolysaccharide mutant altered in competitiveness for nodulation of alfalfa.

Abstract: A transposon Tn5-induced mutant of Rhizobium meliloti Rm2011, designated Rm6963, showed a rough colony morphology on rich and minimal media and an altered lipopolysaccharide (LPS). Major differences from the wild-type LPS were observed in (i) hexose and 2-keto-3-deoxyoctonate elution profiles of crude phenol extracts chromatographed in Sepharose CL-4B, (ii) silver-stained sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis patterns of crude and purified LPS fractions, and (iii) immunoreactivities otherwise present in purified LPS of the parental strain Rm2011. In addition, Rm6963 lost the ability to grow in Luria-Bertani medium containing the hydrophobic compounds sodium deoxycholate or SDS and showed a decrease in survival in TY medium supplemented with high calcium concentrations. The mutant also had altered symbiotic properties. Rm6963 formed nodules that fixed nitrogen but showed a delayed or even reduced ability to nodulate the primary root of alfalfa without showing changes in the position of nodule distribution profiles along the roots. Furthermore, 2 to 3 weeks after inoculation, plants nodulated by Rm6963 were smaller than control plants inoculated with wild-type bacteria in correlation with a transient decrease in nitrogen fixation. In most experiments, the plants recovered later by expressing a full nitrogen-fixing phenotype and developing an abnormally high number of small nodules in lateral roots after 1 month. Rm6963 was also deficient in the ability to compete for nodulation. In coinoculation experiments with equal bacterial numbers of both mutant and wild-type rhizobia, only the parent was recovered from the uppermost root nodules. A strain ratio of approximately 100 to 1 favoring the mutant was necessary to obtain an equal ratio (1:1) of nodule occupancy. These results show that alterations in Rm6963 which include LPS changes lead to an altered symbiotic phenotype during the association with alfalfa that affects the timing of nodule emergence, the progress of nitrogen fixation, and the strain competitiveness for nodulation.

Isotopic discriminations during the accumulation of nitrogen by soybeans

Abstract: Soybeans were grown in a glasshouse in sand-vermiculite medium supplied daily with a mineral nutrient solution essentially free of combined N or containing 5 mM nitrate of known 15N abundance. The natural abundance of 15N in parts of plants and in nitrogen remaining in the medium was determined from 15 days after planting until fruiting. In nodulated plants completely dependent on N2 fixation for growth, the δ15N of plant nitrogen was uniformly negative at 56 days (overall mean: -0.90± 0.17) after adjustment for the effect of seed nitrogen. The δ15N of root nodules increased with time (max. 9.6‰), as that of shoots declined (min. - 1.3 ‰). The δ15N of every mainstem trifoliolate leaf and of the first (unifoliolate) leaf declined from initially positive values (0.5 to 2 ‰) to about - 2‰ with similar time courses, irrespective of the time of initiation. There were no significant losses of N from the plants during growth. There were differences between the δ15N of the total N of root-bleeding xylem sap and of sap extracted by vacuum treatment of stems. These were due to differences between the proportions of ureide-N and amino-N and between the δ15N values of these components. When nodulated plants were supplied daily with 5 mM nitrate (δ15N = 7.68‰) between 21 and 35 days, N2 fixation was reduced to 63% of N assimilated but growth and accumulation of nitrogen were affected little. Following removal of nitrate, there were changes in growth which led to enhanced nodulation and N2 fixation. The δ15N of the total N of trifoliolate leaves which were initiated or expanded before or during the period of nitrate treatment remained positive; those expanded or initiated after the treatment became negative in δ15N, as in the corresponding leaves of untreated nodulated plants. The δ15N of nodules was unaffected by the nitrate treatment. In plants (non-nod. Clark '63) supplied continuously with nitrate, the δ15N of the total N of entire plants rose quickly from values for seeds, but to values significantly higher than in the nitrate. These results are discussed in relation to the effects on the use of 15N natural abundance data for estimating utilisation of atmospheric N2 by nodulated plants.

New sources of high-temperature tolerant rhizobia for Phaseolus vulgaris L.

Abstract: Common bean (Phaseolus vulgaris L.) represents an important crop in tropics, but previous screenings of Rhizobium leguminosarum bv. phaseoli did not show strains that could fix N2 in symbiosis with bean at temperatures higher than 35°C (Hungria and Franco, 1993). However, there are other rhizobia and bradyrhizobia species that nodulate some tropical leguminous trees and can fix N2 at high temperatures. In a trial of rhizobial strains isolated from leguminous trees, we found that 14 out of 21 isolates from Gliricidia, Lonchocarpus and Leucaena were also able to nodulate common beans at optimal temperatures (28/23°C, day/night). When we exposed beans inoculated with these strains to high temperature conditions, 40°C/8 h/day, some of them accumulated at flowering time as much or more N as bean plants receiving mineral N. These broad host-range sources of rhizobia capable of fixing nitrogen with bean at high temperature seem to have the potential to improve yields in tropical soils.

Flux capacities and acclimation costs in Trichodesmium from the Gulf of Mexico

Abstract: Phytoplankton function and acclimation are driven by catalytic protein complexes that mediate key physiological transformations, including generation of photosynthetic ATP and reductant, and carbon and nitrogen fixation. Quantitation of capacities for these processes allows estimation of rates for key ecosystem processes, and identification of factors limiting primary productivity. We herein present molar quantitations of PSI, PSII, ATP synthase, RuBisCO and the Fe protein of nitrogenase of Trichodesmium collected from the Gulf of Mexico, in comparison to determinations for a range of cyanobacteria growing in culture. Using these measurements, estimates were generated for Trichodesmium capacities for carbon fixation of 1–3.4 g C g chl a −1 h−1 and nitrogen fixation of 0.06–0.17 g N g chl a −1 h−1, with diel variations in capacities. ATP synthase levels show that ATP synthesis capacity is sufficient to support these levels of carbon and nitrogen fixation, and that ATP synthase levels change over the day in accordance with the ATP demands of nitrogenase and RuBisCO activity. Levels of measured complexes indicate that Trichodesmium manifests n-type diel light acclimation through rapid changes in RuBisCO:PSII, supported by significant investment of cellular nitrogen. The plasticity in the levels and stoichiometry of these core complexes show that changes in the abundance of core protein complexes are an important component of acclimation and regulation of metabolic function by Trichodesmium populations.

Oxygen and the regulation of nitrogen fixation in legume nodules

Abstract: In N2-fixing legume nodules, O2 is required in large amounts for aerobic respiration, yet nitrogenase, the bacterial enzyme that fixes N2, is O2 labile. A high rate of O2 consumptition and a cortical barrier to gas diffusion work together to maintain a low, non-inhibitory O2 concentration in the central, infected zone of the nodule. At this low O2 concentration, cytosolic leghemoglobin is required to facilitate the diffusion of O2 through the infected cell to the bacteria. The resistance of the cortical diffusion barrier is variable and is used by legume nodules to regulate the O2 concentration in the infected cells such that it limits aerobic respiration and N2 fixation at all times. The resistance of the diffusion barrier and therefore the degree of O2 limitation seems to be regulated in response to changes in the O2 concentration of the central infected zone, the supply of phloem sap to the nodule, and the rate of N assimilation into the end products of fixation.