Comparison of aerobic heterotrophic taxa isolated from four root domains of mature sugar beet (Beta vulgaris)

The distribution of bacteria in the rhizosphere, rhizoplane, interior root tissues (core) and lower root (all tissues) of mature sugar beet roots (Beta vulgaris) was compared. Of 556 isolates, 102 species from 40 genera were identified by fatty acid methyl ester gas-chromatographic (FAME-GC) analysis. The ten most common genera(Bacillus, 14%; Arthrobacter, 12%; Pseudomonas, 11%; Aureobacterium, 9%; Micrococcus, 6%; Xanthomonas, 5%; Alcaligenes, 4%; Flavobacterium, 3%; Agrobacterium, 3%; Microbacterium, 3%) accounted for 70% of isolates, and were found in each of three root domains (rhizosphere, rhizoplane and interior root tissues) on the two principal sampling occasions. Gram-positive strains were more abundant in the rhizosphere than the rhizoplane. Compared to the rhizoplane, rhizosphere bacterial communities were represented by a less diverse, more hierarchical distribution of species where twice as many isolates formed late developing colonies on isolation plates. Between October and January, the bacteria isolated from root interior tissues acquired a distinct change in taxonomic pattern, with decreased diversity and increased hierarchy. A bacterial continuum of similar taxa was observed which extended from the rhizosphere to interior root tissues.