N ₂ Fixation by Azospirillum brasilense and Its Incorporation into Host Setaria italica

Abstract

Growth and nitrogen fixation were followed during the life cycle of Setaria italica (foxtail millet) inoculated with Azospirillum brasilense in controlledenvironment growth chambers. The plants were fertilized at seeding with a limiting amount of combined nitrogen and maintained with an N-free mineral solution. During maturation of the plants, substantial nitrogenase activity, measured by acetylene reduction, developed in the rhizosphere, with total fixation estimated to be equivalent to 20% of the N in the inoculated plants. The peak of this activity coincided with depletion of soluble nitrogen from the system, which in turn was reflected by a sharp decrease in the nitrate reductase activity of the leaves. A. brasilense was found in association with the root populations at 8 x 107

Colonization and growth promotion characteristics ofEnterobactersp. andHerbaspirillumsp. onBrassica oleracea

“…Gyaneshwar et al (2002) detailed the acetylene reduction activity of surface-colonizing bacteria; most of the bacteria in their study, particularly the bacteria expressing the nitrogenase Fe-protein, were observed only on the root surfaces. The transfer of nitrogen from the root surface to plants in cases where large bacterial populations are present on the root surface or in the rhizosphere presumably occurs through bacterial decomposition (Okon et al 1983). In rice inoculated with Herbaspirillum seropedicae, the significant increase in plant growth observed by Gyaneshwar et al (2002) and James et al (2002) in parallel with a significant increase in the quantity of fixed N suggests that endophytic bacteria contribute to plant growth by providing fixed N. However, endophytic bacteria are also reported to promote plant growth without enhancing N content, implying that they influence plant growth by other means (Gyaneshwar et al 2001), for example, by the excretion of phytohormones (Chi et al 2005).…”

Robust biological nitrogen fixation in a model grass–bacterial association

“…strain BH72 and kallar grass (Hurek et al ., ), Klebsiella spp. and wheat (Iniguez et al ., ) or H. seropedicae Z67 and rice (Boddey et al ., ; Gyaneshwar et al ., ) and Azospirillum brasilense in Setaria italica (Okon et al ., ). However, the levels of nitrogen fixation reported would provide little or no contribution to the plant's overall nitrogen demand.…”

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

“…which are transferred to their host plants appear to be small (3,21,29). Investigations in which the 15N isotope dilution technique was used indicated that most of the fixed nitrogen remained below ground, probably still bound to bacterial cells, and contributed only very little to the upper plant parts (7,34,38). In experiments performed with wheat plants that were monoxenically associated with Azospirillum brasilense, only 1 to 2% of the shoot nitrogen originated from atmospheric sources (6a).…”