Applying Reversible Mutations of Nodulation and Nitrogen-Fixation Genes to Study Social Cheating in Rhizobium etli-Legume Interaction

Abstract

Mutualisms are common in nature, though these symbioses can be quite permeable to cheaters in situations where one individual parasitizes the other by discontinuing cooperation yet still exploits the benefits of the partnership. In the Rhizobium-legume system, there are two separate contexts, namely nodulation and nitrogen fixation processes, by which resident Rhizobium individuals can benefit by cheating. Here, we constructed reversible and irreversible mutations in key nodulation and nitrogen-fixation pathways of Rhizobium etli and compared their interaction with plant hosts Phaseolus vulgaris to that of wild type. We show that R. etli reversible mutants deficient in nodulation factor production are capable of intra-specific cheating, wherein mutants exploit other Rhizobium individuals capable of producing these factors. Similarly, we show that R. etli mutants are also capable of cheating inter-specifically, colonizing the host legume yet contributing nothing to the partnership in terms of nitrogen fixation. Our findings indicate that cheating is possible in both of these frameworks, seemingly without damaging the stability of the mutualism itself. These results may potentially help explain observations suggesting that legume plants are commonly infected by multiple bacterial lineages during the nodulation process.

Test of Host Sanction Hypothesis in Soybean Plants Co-inoculated with Nitrogen Fixing and Non-fixing Bradyrhizobium japonicum

“…On the other hand, and using an experimental method avoiding potentially confounding effects like the Ar: O 2 treatment, our present results are in agreement with our previous finding of no sanctions towards a cheating B. japonicum strain in the soybean split-root experiment mentioned earlier [26]. Other recent works on legume-rhizobia mutualisms using co-inoculated plants are given evidence that plants do not sanction cheating strains (in determinate nodules: Medicago truncatula-Sinorhizobium meliloti system: Gubry et al [31]; Phaseolus vulgaris-Rhizobium etli system: Ling et al [32]; in indeterminate nodules: M. truncatula and S. meliloti system: Heath and Tiffin [33]. On the other hand, works from Sachs et al [34] and Regus et al [35] on plants of Lotus strigosus (determinate nodules) co-inoculated with Bradyrhizobium strains with different fixation abilities, reported plant-host sanctions on the inefficient strains.…”

“…To our knowledge, there is only one published work with experimental conditions comparable to our present work, i.e., specifically testing plant-host sanctions in plants with determinate nodules and co-inoculating strains with equivalent nodulation rates but differing in fixation ability. Ling et al [32] constructed Rhizobium etli mutants in nifA genes responsible for nitrogen fixation and coinoculated plants of Phaseolus vulgaris (determinate nodules) with these deficient mutants and a normally fixing wild type strain. Ling et al [32] found a high number of cooccupied nodules and thus were able to confidently calculate the CFU number of nifA mutant and wild type strain in those nodules.…”

Changes in soluble amino acid composition during Canavalia ensiformis development: responses to nitrogen deficiency

“…Some prokaryotic microorganisms or soil bacteria have developed a unique biochemical mechanism that allow them to reduce N 2 to ammonia. Many of these microorganisms are able to associate symbiotically with other organisms, but leguminous is the only group of higher plants that have developed a wide symbiotic association (Fabra et al 2010;Ling et al 2013). The direct uptake of ammonium from the soil by the roots, as final product of nitrate assimilation or nitrogenase activity, is rapidly assimilated by the GS/GOGAT cycle, glutamine synthetase (GS, EC 6.3.1.2), glutamine oxoglutarate aminotransferase or simply glutamate synthase (GOGAT, EC 1.4.7.1) .…”

O nitrato influencia o metabolismo de compostos nitrogenados em calopogônio (Calopogonium mucunoides) ao longo do ciclo de vida

“…Alguns destes organismos (Rhizobium) têm a capacidade de associarem-se às raízes de leguminosas, formando nódulos ixadores de N 2 (LING et al, 2013).…”

“…De acordo com revisão realizada por Liu et al (2011), a presença de N mineral na rizosfera reduz a nodulação, o estabelecimento do nódulo e a atividade da enzima nitrogenase. Os danos ao aparato de ixação dependem da concentração de N no solo e, aparentemente, concentrações menores do que 2 mM de N combinado parecem não prejudicar as taxas de FBN.…”