Nitrogen-Fixing Pseudomonads Isolated from Roots of Plants Grown in the Canadian High Arctic

Abstract

Root-associated bacteria capable of reducing acetylene to ethylene (biological nitrogen fixation) were isolated from various native plants grown in the Canadian High Arctic. All the strains belonged to the genus Pseudomonas but varied in several physiological characteristics. The rates of acetylene reduction at 14 or 20°C were higher than at 25 or 9°C. Six strains reduced acetylene at 4°C. All the strains exhibited chemotaxis to L-asparagine in semisolid agar at 4 to 25°C. Eleven strains colonized roots of canola (Brassica campestris cv. Tobin) in field soil at population densities of log 4.3 to log 5.1 CFU/g of fresh root at 14°C and log 4.0 to log 5.2 CFU/g of fresh root at 25°C. Some of these nitrogen-fixing pseudomonad strains demonstrated a competitive advantage for root colonization over other rhizosphere bacteria at low temperatures. The combined capabilities of nitrogen fixation and root colonization by diazotrophic pseudomonads may be useful for the development of a biofertilizer inoculant for temperate and cold regions.

Colonization and growth promotion characteristics ofEnterobactersp. andHerbaspirillumsp. onBrassica oleracea

“…However, nitrogen-fixing Azospirillum strains have been isolated from the roots of other members of the Brassicaceae, namely Brassica juncea (mustard) (Saha et al 1985), Brassica chinensis (Chinese cabbage) and Brassica rapa (Chinese mustard) (Gamo and Ahn 1991). Similarly, nitrogen-fixing pseudomonads have been reported and isolated from the roots of canola (Brassica campestris) (Lifshitz et al 1986).…”

N2-fixing pseudomonads and related soil bacteria

“…Putative Ne-fixing pseudomonad strains have been identified as belonging to eight acknowledged Pseudomonas species (P. diminuta, P. fluorescens, P. paucimobilis (proposed new name Sphingornonas paucimobilis [32]), P. pseudoflat;a, P. putida, P. saccharophila, P. stutzeri and P. t:esicularis) listed in Bergey's Manual of Systematic Bacteriology and fall into the rRNA groups I, III 98 and IV [7] (Table 2). Of these, P. fluorescens [33], P. putida [34,35], and P. stutzeri strains JM300 [36,37] and CMT.9.A [36,38] belong to approved species in rRNA homology group I, which is kept in the genuine Pseudomonas genus classified under the Proteobacteria gamma subdivision (rRNA superfamily II) according to molecular phylogeny [6,14,23]. Further taxonomic studies of these strains, especially those including DNA-rRNA hybridisation, are obviously needed to ascertain their classification.…”

“…soil, water, humans, animals, plants, and foods) where they can be saprophytic and may become beneficial or pathogenic to their hosts [7,12,41,42]. They are one of the dominant groups of bacteria in marine [43] and soil [44] systems including the rhizosphere where they can I (1(I fix N 2 [35,38,[45][46][47][48][49][50][51][52][53], produce plant growth-promoting substances [34,54,55] or excrete siderophores that exert antagonistic activity against soil-borne pathogens [56,57]. Hence, they have been considered as potential biological fertilizers and pesticides [34,[58][59][60][61].…”

“…This may be due to their indirect nitrogen contribution which occurs mainly after death and mineralisation of their cellular nitrogenous components. Alternatively, the plant growth-stimulating effect may be more pronounced than N 2fixing activity in pseudomonads that have both properties, e.g.P, putida GRI2-2 [34,35]. Geo-graphically, pseudomonads can be found in the tropical [7], temperate [47,48], arctic [35] and antarctic [62] regions.…”

“…However, no true psychrophiles have been discovered. A few rhizobia [145] and diazotrophic pseudomonads [33,35,114] have been reported to be psychrotrophic. The N2-fixing activities of these or-ganisms are not only limited by low temperature itself but also by the increasing dissolved O, with decreasing temperature.…”