Non-nodulating Paenirhizobium fix nitrogen in association with cereal roots

Journal article

Cereal root microbiomes harbour diverse diazotrophic bacteria, yet the taxa capable of sustained nitrogen fixation in association with cereal roots remain poorly characterised. Here, two high- performing nitrogen-fixing strains (B6 and J2) were isolated from barley roots and identified as belonging to the family Rhizobiacae in the genus Paenirhizobium. Both strains possess plasmid- encoded canonical rhizobial nif and fix genes for nitrogen fixation but lack nodulation genes. Their genomes have a 5.7 Mb chromosome and four repABC plasmids. Unlike most nodulating rhizobia, strains B6 and J2 fixed nitrogen in laboratory culture on a range of carbon sources, achieving maximal activity on organic acids at low ammonium (<0.5 mM) and oxygen concentrations (1–3%). Both strains colonised the total root systems of barley plants, with population densities of 10⁶ CFU g^-1 fresh root weight. Strains fixed high levels of nitrogen on barley plants, similar to or greater than other known free-living diazotrophs.

These findings expand the ecological context of rhizobial nitrogen fixation and identify cereal- associated Paenirhizobium as a previously unrecognised component of the diazotrophic cereal root microbiome.

Authors: Campbell, MJ; Jorrin, B; Katz, Z; Tkacz, A; Ledermann, R

• Publication status: Accepted
• Peer review status: Peer reviewed
• Publisher: Oxford University Press
• Journal: ISME Communications: New Developments in Microbial Ecology
• Acceptance date: 2026-06-03
• EISSN: 2730-6151
• Language: English
• Keywords: Biological Nitrogen Fixation (BNF); free-living diazotrophs; Paenirhizobium; nif-fix genes; acetylene reduction assays (ARAs)