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Legumes versus rhizobia: a model for ongoing conflict in symbiosis 
Joel L. Sachs Kenjiro W. Quides Camille E. Wendlandt

The legume–rhizobia association is a powerful model of the limits of host control over microbes. Legumes regulate the formation of root nodules that house nitrogen‐fixing rhizobia and adjust investment into nodule development and growth. However, the range of fitness outcomes in these traits reveals intense conflicts of interest between the partners. New work that we review and synthesize here shows that legumes have evolved varied mechanisms of control over symbionts, but that host control is often subverted by rhizobia. An outcome of this conflict is that both legumes and rhizobia have evolved numerous traits that can improve their own short‐term fitness in this interaction, but little evidence exists for any net improvement in the joint trait of nitrogen fixation.

Legume and rhizobia conflict over nodule number. (a) Costs vs benefits of nodulation are modeled. Plant costs to nodulation (carbon, C) are predicted to be a linear function of the number of nodules formed (Nod#) with a slope of m (cost per nodule): f{C} = m × Nod#. Plant benefits from nodulation (nitrogen, N) are predicted to be a negative exponential function, f{N} = α(1 − e^(B×Nod#)), with diminishing returns that reach an asymptote at α and diminish at a rate corresponding to B. (b) Net benefits of nodulation can be calculated by subtracting the cost from the benefit functions. The net benefit function for nodulation is unimodal, increasing with the formation of nodules (zone of cooperation) until the optimal number of nodules is reached (N–C^(max)), and above which additional nodules reduce the host benefit (zone of conflict). If too few or too many nodules are formed, the host does not acquire the net minimal benefit to set seed (i.e. < N–C^(min)). (c) Host fitness (i.e. growth, seed set) varies with the number of nodules formed. Lotus japonicus mutants have been generated that form too many nodules compared with wild‐type and thus experience reduced fitness.
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https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.15222
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