Multiple sensors provide spatiotemporal oxygen regulation of gene expression in a < i > Rhizobium < /i > -legume symbiosis

by Paul J. Rutten, Harrison Steel, Graham A. Hood, Vinoy K. Ramachandran, Lucie McMurtry, Barney Geddes, Antonis Papachristodoulou, Philip S. Poole

Regulation by oxygen (O2) in rhizobia is essential for their symbioses with plants and involves multiple O2 sensing proteins. Three sensors exist in the pea microsymbiontRhizobium leguminosarum Rlv3841: hFixL, FnrN and NifA. At low O2 concentrations (1%) hFixL signals via FxkR to induce expression of the FixK transcription factor, which activates transcription of downstream genes. These includefixNOQP, encoding the high-affinitycbb3-type terminal oxidase used in symbiosis. In free-living Rlv3841, the hFixL-FxkR-FixK pathway was active at 1% O2, and confocal microscopy showed hFixL-FxkR-FixK activity in the earliest stages of Rlv3841 differentiation in nodules (zones I and II). Work on Rlv3841 inside and outside nodules showed that the hFixL-FxkR-FixK pathway also induces transcription offnrN at 1% O2 and in the earliest stages of Rlv3841 differentiation in nodules. We confirmed past findings suggesting a role for FnrN infixNOQP expression. However, unlike hFixL-FxkR-FixK, Rlv3841 FnrN was only active in the near-anaerobic zones III and IV of pea nodules. Quantification offixNOQP expression in nodules showed this was driven primarily by FnrN, with minimal direct hFixL-FxkR-FixK induction. Thus, FnrN is key for full symbiotic expression offixNOQP. Without FnrN, nitrogen fixation was reduced by 85% in Rlv3841, while eliminating hFix...

Source: PLoS Genetics - February 4, 2021 Category: Genetics & Stem Cells Authors: Paul J. Rutten Source Type: research