Low soil phosphorus conditioning enhances mutualism in Rhizophagus irregularis regardless of nuclear organization, even when phosphorus conditions shift.

Strains of the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis belong to one of two nuclear organizations: homokaryons, which carry genetically similar nuclei, and dikaryons, which harbour two genetically distinct nuclear populations. Although traits tend to be conserved within each group, it is unclear whether their capacity to adapt or exhibit phenotypic plasticity in response to environmental change differs. Here, we tested whether dikaryotic strains have greater potential than homokaryotic strains for rapid adaptation or plasticity in response to shifts in soil phosphorus (P) availability. The experiment first included a conditioning phase in which we measured the growth responses of Allium ampeloprasum L. inoculated with four homokaryotic or four dikaryotic strains under contrasting soil P levels. Next, in the adaptive potential phase, we grew the conditioned strains under both low and high P to assess whether prior P exposure influenced subsequent mutualistic functioning. During conditioning, host biomass was greater under high than low P for both nuclear groups, but increased roughly 3.2-fold for dikaryons and 1.9-fold for homokaryons. In the adaptive potential phase, strains conditioned under low P enhanced host biomass significantly more than those conditioned under high P, and by a similar amount in both P environments. This effect did not differ between dikaryons and homokaryons, but its magnitude varied markedly among individual strains, from no benefit to a roughly two-fold increase in host biomass. Together, these results provide the first experimental evidence that conditioning AM fungi under low soil P can enhance their subsequent benefits to host plants, even when P availability increases. Soil P conditioning strategies for AM fungi, therefore, merit consideration in agriculture and other managed ecosystems, although the strong strain dependence of the effect implies that strain choice and conditioning regime would need to be optimized together.