Plant species differ in their ability to reduce allocation to non‐beneficial arbuscular mycorrhizal fungi

Abstract

Abstract. Theory suggests that cheaters threaten the persistence of mutualisms, but that sanctions to prevent cheating can stabilize mutualisms. In the arbuscular mycorrhizal symbiosis, reports of parasitism suggest that reductions in plant carbon allocation are not universally effective. I asked whether plant species differences in mycorrhizal responsiveness would affect both their susceptibility to parasitism and their reduction in allocation to nonbeneficial arbuscular mycorrhizal fungi (AMF) in high-phosphorus soils. In a greenhouse experiment, I found that two C 3 grasses, Bromus inermis and Elymus repens, effectively suppressed root colonization and AMF hyphal abundance. Increases in soil phosphorus did not reduce the degree to which AMF increased plant biomass. In contrast, two C 4 grasses, Andropogon gerardii and Schizachyrium scoparium, more weakly reduced root colonization and failed to suppress AMF hyphal abundance. Consequently, they experienced strong declines in their response to AMF, and one species suffered parasitism. Thus, species differ in susceptibility to parasitism and their reduction in allocation to non-beneficial AMF. These differences may affect the distribution and abundance of plants and AMF, as well as the stability of the mutualism.

The Coexistence of Hosts with Different Abilities to Discriminate against Cheater Partners: An Evolutionary Game-Theory Approach

“…While Foster and Kokko (2006) have demonstrated the importance of variability in partner cooperation to maintain host discrimination, our model proposes that cheaters can facilitate host species coexistence. The existence of host species with different capacities to discriminate against cheater partners has recently been demonstrated to occur in a wide variety of mutualisms (Pellmyr 1994;Keirs et al 2007;Bever et al 2009;Heil et al 2009;Jandér and Herre 2010;Kiers et al 2011;Grman 2012), suggesting these population dynamics may be a general feature of mutualisms. In this context, giving and discriminating hosts act as reservoirs of the cheater and mutualist partners each requires to coexist.…”

“…Recent work has demonstrated that when fertilization reduces the benefit of mycotrophy, C 3 plants have reduced levels of mycorrhizal colonization, while C 4 grasses are nonresponsive (Grman 2012;Grman and Robinson 2013). Thus, one could hypothesize that C 3 grasses are candidate discriminators, while C 4 grasses are candidate givers.…”

“…Thus, while C 3 grasses are less responsive to mycorrhizal fungi than C 4 grasses, they also tend to develop finer and more branched root systems that could increase their ability to acquire soil nutrients in the absence of fungal partners (Grman 2012). Similarly, plants growing on the most P-impoverished soils in the world develop specialized cluster-roots that are generally nonmycotrophic and adapted to acquire nutrients in recalcitrant chemical forms (Lambers 2008).…”

“…An alternative mechanism is one that involves negative feedbacks among coexisting species, such that giver hosts act as a susceptible reservoir for the production of cheater partners, while discriminator hosts sustain sufficient mutualists to prevent cheaters from driving the givers extinct. Indeed, the requisite cheaters and variability in host species' ability to discriminate is documented in the mutualisms between figs and pollinator wasps (Jandér and Herre 2010), Acacia and herbivore defense ants (Heil et al 2009), and plants and mycorrhiza that enhance nutrient acquisition Kiers et al 2011;Grman 2012).…”

“…Acacia species that are better at promoting vigilant defensive ant species invest more resources into costly extrafloral nectaries than species that tolerate cheater, nondefensive ants (Heil et al 2009). Plant hosts capable of reducing carbon allocation to mycorrhiza in conditions where the mycorrhiza do not promote growth and nutrition derive less benefit from mycorrhiza in conditions when the fungi cooperate (Grman 2012;Grman and Robinson 2013). In these examples, discrimination is a costly bet-hedging strategy that reduces both the harm of associating with a cheater partner and the benefit of associating with a cooperative partner.…”