1. ¹Faculty of Agricultural, Food and Environmental Quality Sciences, Hebrew University of Jerusalem, Rehovot, Israel
2. ²Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization, The Volcani Center, Bet-Dagan, Israel
3. ³Department of Food, Agricultural, and Biological Engineering, Ohio Agricultural Research and Development Center, Ohio State University, Wooster, OH, USA

Correspondence: Dr D Minz, Institute of Soil, Water and Environmental Sciences, Agriculture Research Organization, The Volcani Center, PO Box 6, Bet-Dagan 50-250, Israel. E-mail: minz@volcani.agri.gov.il

⁴Current address: SETI Institute, Mountain View, CA 94043, USA.

Received 23 February 2007; Revised 18 April 2007; Accepted 18 April 2007; Published online 24 May 2007.

Abstract

Microbial colonization of plant seeds and roots is a highly complex process in which soil and plant type can influence the composition of the root-associated and rhizosphere microbial communities. Amendment of compost, a common agricultural technique, introduces exogenous nutrients and microorganisms to the soil–plant environment, and can further influence microbial community composition in the plant environment. Although compost amendments can strongly influence soil and rhizosphere microbial communities, there is evidence that with increasing proximity to the root, plant influences predominate over soil effects. We hypothesized that the 'rhizosphere effect' observed with proximity to plant surfaces does not act equally on all microorganisms. To explore this issue, we examined two bacterial taxa that reproducibly colonized seed and root surfaces in an experiment examining the influence of compost amendment on plant-associated bacterial communities. Population-specific analyses revealed striking differences in the ecology of bacteria from the genus Chryseobacterium and the family Oxalobacteraceae in potting mix and plant-associated environments. Seed- and root-colonizing Oxalobacteraceae populations were highly sensitive to plant effects, and phylogenetic analyses of root-colonizing Oxalobacteraceae revealed the presence of root-associated populations that were highly similar, regardless of treatment, and differed from the potting mix populations detected at the same sampling points. Conversely, Chryseobacterium community composition was found to be essentially invariant within treatments, but was strongly influenced by compost amendment. This persistence and stable nature of the Chryseobacterium community composition demonstrates that rhizosphere selection is not the exclusive factor involved in determining the composition of the cucumber spermosphere and rhizosphere communities.