Mediating mutualisms: farm management practices and evolutionary changes in symbiont co‐operation

Abstract

Summary1. Root symbionts (rhizobia and arbuscular mycorrhizae) are often assumed to increase agricultural productivity consistently. However, rhizobial and mycorrhizal strains vary in effectiveness, resulting in symbiotic associations that range from parasitic to mutualistic. 2. The extent to which different farming practices mediate evolutionary changes along this continuum of symbiont effectiveness is rarely discussed. However, evolutionary theory suggests that (i) fertilizer use will favour parasitism unless host-plants impose sanctions against less-effective mutualists; (ii) tillage will have contrasting effects because it decreases within-plant symbiont relatedness but also decreases the risk that mutualism will benefit future competitors; (iii) crop rotation can act as a selective agent against dominating symbiont genotypes; and (iv) rhizobial inoculation adds beneficial strains to the soil but may increase the frequency of mixed nodules that allow parasitic strains to escape host sanctions. 3. However, the existing empirical data are inadequate to test our predictions thoroughly. Changes in species composition have been documented as a result of management practices, but evolutionary changes in symbiont effectiveness are difficult to detect. Therefore, a major aim of this study was to stimulate research that will assesses directly changes in symbiotic effectiveness as a function of management practices.

Negotiation, Sanctions, and Context Dependency in the Legume-Rhizobium Mutualism

“…Empirical research on mycorrhizae supports this prediction (Johnson 1993): when big bluestem plants were fertilized, less beneficial fungal species took over their roots. Kiers et al (2002) note that no conclusive data exist for the evolutionary response of rhizobia to nitrogen fertilization and go on to argue that the effect of increased nitrogen fertilization on the evolution of rhizobium effectiveness is ambiguous, as plant sanctions might counteract the effect of reduced plant dependence on nodules. Our model suggests otherwise.…”

Evolution of microbial markets

“…This is of particular relevance for supply and demand variations induced by climate change and the response of mutualisms to these changes (112). Another example is in agriculture: the application of fertilizer can reduce the market value of microbial mutualists providing nutrients to plant host and thus drive a reduction in their reliance on mutualistic microbial partners (113,114).…”

Human selection and the relaxation of legume defences against ineffective rhizobia

“…It has been proposed that breeding for yield under high fertilizer regimes could relax sanctions or other losslimiting traits (Kiers et al 2002). Alternatively, increased fertilizer regimes may favour genotypes that are less tolerant of marginally effective strains (stricter sanctions) because the plant only needs a few (top-performing) nodules to make up the small deficit in soil N. We therefore ask whether there have been any trends in the sanctioning response over decades.…”