Prone to fix: Resilience of the active nitrogen-fixing rice root microbiome
B. REINHOLD-HUREK (1), M. Sabale (2), A. Sarkar (2), T. Hurek (1) (1) University of Bremen, Germany; (2) University of Bremen, Germany

Shifting from flooded paddy systems to non-flooded cropping is likely to affect microbial nitrogen cycling. We combined methods of microbial ecology (NG sequencing of amplicons), and a reductionist approach with pure cultures of the endophytic diazotroph Azoarus sp.. Root-associated activity of nitrogenase gene expression was assessed by quantitative RT-PCR of nifH. For rice, expression levels were surprisingly stable under non-flooded, flooded conditions, or in response to conventional nitrogen fertilizer applications. In contrast, the active diazotrophic population of maize roots was not resistant to N-fertilization, nifH expression strongly decreased. Concordant changes in the diazotrophic resident or active communities were detected by nifH amplicon sequence analysis, based on bacterial DNA or mRNA, respectively. For high-resolution analyses of the endobiome in gnotobiotic culture, we developed a dual fluorescence reporter system for Azoarcus sp. BH72 which allows to quantify and visualize gene expression simultaneously. This allowed us to demonstrate sites of active nitrogen fixation (gene expression) in association with rice roots. We confirmed that at low nitrogen fertilizer levels, endophytic nifH gene expression persisted in rice roots, while it was repressed in maize roots. This supports our observation of remarkable stability of nitrogen fixation in association with rice roots in contrast to maize and demonstrates fundamental plant difference in interaction.

Abstract Number: P4-115
Session Type: Poster