NH ₄ ⁺ -Excreting Azospirillum brasilense Mutants Enhance the Nitrogen Supply of a Wheat Host

Abstract

Spontaneous ethylenediamine-resistant mutants of Azospirillum brasilense were selected on the basis of their excretion of NH4'. Two mutants exhibited no repression of their nitrogenase enzyme systems in the presence of high (20 mM) concentrations of NH4'. The nitrogenase activities of these mutants on nitrogen-free minimal medium were two to three times higher than the nitrogenase activity of the wild type. The mutants excreted substantial amounts of ammonia when they were grown either under oxygen-limiting conditions (1 kPa of 02) or aerobically on nitrate or glutamate. The mutants grew well on glutamate as a sole nitrogen source but only poorly on NH4Cl. Both mutants failed to incorporate ["4C]methylamine. We demonstrated that nitrite ammonification occurs in the mutants. Wild-type A. brasilense, as well as the mutants, became established in the rhizospheres of axenically grown wheat plants at levels of >107 cells per g of root. The rhizosphere acetylene reduction activity was highest in the preparations containing the mutants. When plants were grown on a nitrogen-free nutritional medium, both mutants were responsible for significant increases in root and shoot dry matter compared with wild-type-treated plants or with noninoculated controls. Total plant nitrogen accumulation increased as well. When they were exposed to a 15N2-enriched atmosphere, both A. brasilense mutants incorporated significantly higher amounts of '5N inside root and shoot material than the wild type did. The results of our nitrogen balance and '5N enrichment studies indicated that NH4+-excreting A. brasilense strains potentially support the nitrogen supply of the host plants.

During stationary phase, Beijerinckia derxii shows nitrogenase activity concomitant with the release and accumulation of nitrogenated substances

“…Considering the intracellular presence of this nitrogenated material, including a possible protein reserve, the present study supports the hypothesis proposed by Yahalom et al (1984) and by Christiansen-Weniger and Van Veen (1991). Accordingly, free-living, diazotrophic microorganisms may contribute nitrogen to the environment, in the form of decomposed bacterial structures.…”

Optimization of growth conditions For zinc Solubilizing Plant Growth associated Bacteria and Fungi

“…However, reports of other researchers showed that IAA production was not detected after 5 d. Though it is reported that there is continuous decrease in IAA production after reaching the peak production, this pattern was also followed by our isolates. IAA production curves of the isolates showed continuous increase and decrease up to 12 d. These types of curves are in agreement with the IAA production curves reported by Torres-Rubioet al [26,27]. The reason for such fluctuations could be the utilization of IAA by the cells as nutrient during late stationary phase or production of IAA degrading enzymes by the cells which are inducible enzymes in presence of IAA [26].…”

Excretion of ammonium by a nifL mutant of Azotobacter vinelandii fixing nitrogen

“…Anabaena mutants resistant to L-methionine-DL-sulfoximine or the NH4' analog ethylenediamine excreted up to 1.6 mM NH4' (35,43,45). Among eubacteria, ammonium excretion was reported to occur in mutants of Klebsiella pneumoniae defective in ammonium assimilation or transport (1,8,41), in mutants of Rhodobacter capsulatus and Azospirillum brasiliense altered in the production of GS (27,48), in methylamine-resistant mutants of Azotobacter vinelandii (17), and in ethylenediamine-resistant mutants of A. brasiliense (9,27).…”