Active community structure of microeukaryotes in a rice (Oryza sativa L.) rhizosphere revealed by RNA-based PCR-DGGE

The rhizosphere is one of the hot spots in soil ecosystems for a variety of microorganisms. In this study, we explored the seasonal change of the microeukaryotic community of a rice rhizosphere focusing on the active members through an RNA-based molecular approach. Rice plants (Oryza sativa L.) were grown in a pot where the rhizosphere was compartmented from bulk soil with a nylon gauze. The Eh in the rhizosphere compartment indicated that the rhizosphere was under oxic conditions in the initial stage of plant growth and then suddenly became anoxic or suboxic. Denaturing gradient gel electrophoresis targeting 18S rRNA-transcribed cDNA demonstrated that the active community of microeukaryotes in the rice rhizosphere was different from that in the bulk soil. The rhizosphere community showed a temporal shift in accordance with the shift of the redox conditions having three stages: the oxic before maximum tillering stage, anoxic/suboxic stage before maximum tillering stage, and anoxic/suboxic stage in the panicle initiation stage and thereafter. Active members specific to the rhizosphere at either the oxic or anoxic/suboxic stage were found: Heterolobosea amoeba, ciliates, and Chytridiomycota fungi for the oxic stage and oomycetes, ciliates and Ascomycota fungi in the anoxic/suboxic stage. The present results demonstrate that a specific group of microeukaryotes inhabit the rice rhizosphere even under anoxic/suboxic conditions and play various ecological roles as plant parasites, microbial grazers and organic decomposers.