Deletion of rRNA Operons of Sinorhizobium fredii Strain NGR234 and Impact on Symbiosis With Legumes.

Ala Eddine Cherni,Xavier Perret

doi:10.3389/fmicb.2019.00154

Abstract

During their lifecycle, from free-living soil bacteria to endosymbiotic nitrogen-fixing bacteroids of legumes, rhizobia must colonize, and cope with environments where nutrient concentrations and compositions vary greatly. Bacterial colonization of legume rhizospheres and of root surfaces is subject to a fierce competition for plant exudates. By contrast root nodules offer to rhizobia sheltered nutrient-rich environments within which the cells that successfully propagated via infection threads can rapidly multiply. To explore the effects on symbiosis of a slower rhizobia growth and metabolism, we deleted one or two copies of the three functional rRNA operons of the promiscuous Sinorhizobium fredii strain NGR234 and examined the impact of these mutations on free-living and symbiotic lifestyles. Strains with two functional rRNA operons (NGRΔrRNA1 and NGRΔrRNA3) grew almost as rapidly as NGR234, and NGRΔrRNA1 was as proficient as the parent strain on all of the five legume species tested. By contrast, the NGRΔrRNA1,3 double mutant, which carried a single rRNA operon and grew significantly slower than NGR234, had a reduced symbiotic proficiency on Cajanus cajan, Macroptilium atropurpureum, Tephrosia vogelii, and Vigna unguiculata. In addition, while NGRΔrRNA1 and NGR234 equally competed for nodulation of V. unguiculata, strain NGRΔrRNA1,3 was clearly outcompeted by wild-type. Surprisingly, on Leucaena leucocephala, NGRΔrRNA1,3 was the most proficient strain and competed equally NGR234 for nodule occupation. Together, these results indicate that for strains with otherwise identical repertoires of symbiotic genes, a faster growth on roots and/or inside plant tissues may contribute to secure access to nodules of some hosts. By contrast, other legumes such as L. leucocephala appear as less selective and capable of providing symbiotic environments susceptible to accommodate strains with a broader spectrum of competences.

Highlights

Nitrogen-fixing symbioses between legumes and soil bacteria, commonly known as rhizobia, are responsible for introducing a large fraction of fixed N into terrestrial ecosystems
Previous studies have addressed the impact of modifying the number or sequence of ribosomal RNA (rRNA) operons on various cell processes such as response of Synechococcus cyanobacteria to temperature changes (Monshupanee et al, 2006), resistance to clarithromycin and spectinomycin in Mycobacterium smegmatis (Sander et al, 1996), relative fitness and competition in E. coli batch and chemostat cultures (Stevenson and Schmidt, 2004), E. coli cell morphology (Asai et al, 1999) or sporulation of B. subtilis (Yano et al, 2013)
Genuine polymorphisms between NGR234 rRNA operons were confirmed for the promoter and terminator regions, with the rRNA3 copy differing most from consensus

Summary

Introduction

Nitrogen-fixing symbioses between legumes and soil bacteria, commonly known as rhizobia, are responsible for introducing a large fraction of fixed N into terrestrial ecosystems. These beneficial plant-microbe associations come in many forms and shapes (Masson-Boivin et al, 2009; Sprent et al, 2013), yet all involve the intracellular colonization by soil rhizobia of legume cells grouped. Growth conditions within developing ITs must be favorable to rhizobia with enough nutrients to sustain a rapid bacterial division when needed It is unknown whether some legumes, at one stage of the infection process, favor strains with either faster or slower generation times, . Reduced nitrogen is assimilated by host plants in exchange for amino acids and carbon sources derived from photosynthesis that fuel the intense bacteroid metabolism (Poole et al, 2018)

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Results

Conclusion