Modelling the effect of horizontal transfer of sym genes, strain competition and nodule co-occupation on nitrogen fixing and non-fixing rhizobia coexistence in Rhizobium-legume symbiosis

MOYANO GABRIEL; MARCO DIANA; KNOPOFF DAMIAN; TURNER CRISTINA; TORRES GERMAN

Granada

Congreso; 20th International Congress of Nitrogen Fixation; 2017

In the Rhizobium-legume mutualism, partner benefits are clear: plant receives nitrogen from Rhizobium fixing bacteroids inside the nodules that in turn receive carbon compounds from the plant. However, strains of nodulating rhizobia that do not fix (or fix low) nitrogen are common in the soil and even coexist in the same plant (1), competing for nodulation and causing low crop yields (2). Here, we extend a previously developed mathematical model to explain fixing and non-fixing rhizobia coexistence (3) to include other factors conferring more realistic conditions, like horizontal transmission of symbiotic (sym) genes, turning non-nodulating strains into nodulating rhizobia, and competition between fixing and non-fixing strains for nodulation and nodule co-occupation. The model consists of a system of population equations representing the plant population, the populations of saprophytic bacteria living in the soil (fixing and non-fixing rhizobia, and rhizobia without sym genes), and bacteroids inside nodules. Parameter values were taken from ad-hoc experiments using with two strains of B. japonicum, a highly efficient nitrogen fixing wild-type strain, and non-fixing, nifH mutant derivatives with equal nodulation abilities but lacking nitrogenase activity (3, 4). We explored a range of competition coefficient values, changing competitive advantage from fixing to non-fixing strains. Model results showed that plant populations are able to maintain a stable equilibrium by getting fixing rhizobia needed to provide a minimum N2 amount, despite of the presence of non-fixing rhizobia in the soil and inside single- and co-occupied nodules. Horizontal transfer of the sym genes exerts a quantitative effect on fixing and non-fixing rhizobia densities in the soil. Competition for nodulation changes the proportion of single- and co-occupied nodules and fixing and non-fixing rhizobia densities in the soil. Our modelling results are important to theoretical understanding of the Rhizobium-legume symbiotic persistence in the presence of rhizobial strains benefiting but not contributing to the mutualistic relationship. Also, our results are highly relevant to improve agricultural inoculation practices, providing the needed practical knowledge to design more competitive and efficient inoculants.  (1) Singleton, P. and Tavares, J. (1986) Applied Environ. Microbiol. 51:1013-1018.(2) Amarger, N. (1981) Soil Biol. Biochem. 13: 475-480.(3) Marco, D. et al. (2009) J. Theor. Biol. 259: 423-433.(4) Marco, D. et al. (2015) J. Sci. Res. Rep. 6: 99-108