T. Heulin et al., BACILLUS-POLYMYXA AND RAHNELLA-AQUATILIS, THE DOMINANT N2-FIXING BACTERIA ASSOCIATED WITH WHEAT RHIZOSPHERE IN FRENCH SOILS, European journal of soil biology, 30(1), 1994, pp. 35-42
In a study of adaptive traits of rhizosphere inhabiting N2-fixing bact
eria, 56 strains representative for the diazotrophic population were i
solated from the rhizosphere of three week old spring wheat plants (cv
. Castan). For the isolation procedure, a medium was used which contai
ned the exudates of sterile plantlets of wheat (the ''spermosphere mod
el''). N2-fixing Bacillus polymyxa and Bacillus circulans were dominan
t in three out of four French soils studied. The dominant N2-fixing mi
croflora of the fourth soil was a population of Rahnella aquatilis, an
enteric bacterium closely related to Enterobacter agglomerans and Erw
inia herbicola. The sizes of these N2-fixing populations were between
1 and 5 x 10(5) cfu g-1 dry weight of rhizosphere soil. These two rhiz
osphere populations differed markedly by two important characters: (1)
when associated with plants under gnotobiotic conditions, the average
acetylene reduction activity was much higher in Bacillus than in R. a
quatilis strains; (2) in vitro, 90% of Bacillus isolates exhibited a c
lear antagonistic activity against Gaeumannomyces graminis var. tritic
i (Ggt), the causative agent of the take-all disease of wheat, whereas
none of the 25 strains of R. aquatilis tested showed any antogonistic
activity.