R. Miethling et al., Variation of microbial rhizosphere communities in response to crop species, soil origin, and inoculation with Sinorhizobium meliloti L33, MICROB ECOL, 40(1), 2000, pp. 43-56
A greenhouse study with soil-plant microcosms was conducted in order to com
pare the effect of crop species, soil origin, and a bacterial inoculant on
the establishment of microbial communities colonizing plant roots. Two crop
species, alfalfa (Medicago sativa) and rye (Secale cereale), were grown se
parately in two soils collected from agricultural fields at different locat
ions and with differing histories of leguminous crop rotation. A subset of
microcosms was inoculated at 10(6) cfu g(-1) soil with the luciferase marke
r gene-lagged Sinorhizobium meliloti strain L33, a symbiotic partner of M.
sativa. Microbial consortia were collected from the rhizospheres of alfalfa
after 10 weeks of incubation and from rye after 11 weeks. S. meliloti L33
populations were one to two orders of magnitude higher in the rhizospheres
of alfalfa than of rye. In soil with previous alfalfa cultivation, 80% of t
he alfalfa nodules were colonized by indigenous bacteria, while in the othe
r soil alfalfa was colonized almost exclusively (>90%) with S. meliloti L33
. Three community-level targeting approaches were used to characterize the
variation of the extracted microbial rhizosphere consortia: (1) Community l
evel physiological profiles (CLPP), (2) fatty acid methyl ester analysis (F
AME), and (3) diversity of PCR amplified 16S rRNA target sequences from dir
ectly extracted ribosomes, determined by temperature gradient gel electroph
oresis (TGGE). All approaches identified the crop species as the major dete
rminant of microbial community characteristics. Consistently, the influence
of soil was of minor importance, while a modification of the alfalfa-assoc
iated microbial community structure after inoculation with S. meliloti L33
was only consistently observed by using TGGE.