Preferential allocation, physio‐evolutionary feedbacks, and the stability and environmental patterns of mutualism between plants and their root symbionts

Abstract

SummaryThe common occurrence of mutualistic interactions between plants and root symbionts is problematic. As the delivery of benefit to hosts involves costs to symbionts, symbionts that provide reduced benefit to their host are expected to increase in frequency. Plants have been shown to allocate preferentially to the most efficient symbiont and this preferential allocation may stabilize the mutualism.I construct a general model of the interactive feedbacks of host preferential allocation and the dynamics of root symbiont populations to evaluate the stability of nutritional mutualisms.Preferential allocation can promote the evolution of mutualism even when the cost to the symbiont is very large. Moreover, the physiological plasticity of preferential allocation likely leads to coexistence of beneficial and nonbeneficial symbionts. For arbuscular mycorrhizal fungi, which facilitate plant uptake of phosphorus (P), the model predicts greater P transfer from these fungi per unit carbon invested with decreasing concentrations of soil P and with increasing concentrations of atmospheric CO 2 , patterns that have been observed in laboratory and field studies.This framework connects physiological plasticity in plant allocation to population processes that determine mutualism stability and, as such, represents a significant step in understanding the stability and environmental patterns in mutualism.

Arbuscular Mycorrhiza Stimulates Biological Nitrogen Fixation in Two Medicago spp. through Improved Phosphorus Acquisition

“…Under high P availability or low light conditions, the coexistence of two root symbionts becomes a burden for the plant host (Ballhorn et al, 2016). Indirect support for this theory can be observed in the suppression of root colonization by AM fungus in most of the plant–substrate treatments with increasing P-fertilization ( Figure 2 ), which is consistent with the preferential allocation hypothesis (Bever, 2015). …”

Phylogenetic structure of arbuscular mycorrhizal community shifts in response to increasing soil fertility

“…However, whether Glomus species in our study site are weak competitors for carbohydrates or whether they just have very strong host specificity remain to be determined through further investigation. Although competition can sometimes produce a phylogenetically clustered pattern even when functional traits are phylogenetically conserved (Mayfield and Levine, 2010), in our case, a clustered pattern in unfertilized soil can be largely attributed to environmental filtering, because plants under extremely low P availability (<3 mg kg À1 soil in our study site) can be expected to actively select AM fungi with high P-uptake ability (Kiers et al, 2011;Bever, 2015), which has been shown to be phylogenetically conserved (Powell et al, 2009). , the significant differences between columns are indicated with dissimilar letters using Tukey's HSD test (P 0.05) after fitting linear mixed effects models.…”

“…Second, because species of AM fungi have different niches and are known to prefer to inhabit different soil conditions (Schechter and Bruns, 2008;Alguacil et al, 2010;Dumbrell et al, 2010), fertilization may directly select taxa that grow best in the enriched conditions. Third, there is good evidence that plants actively select AM fungal taxa that best provision nutrients (Parniske, 2008;Kiers et al, 2011;Bever, 2015) and many field studies have shown that certain plants select particular AM fungi (e.g. Vandenkoornhuyse et al, 2003;Veresoglou and Rillig, 2014); thus, the loss of plant species caused by fertilization can lead to significant changes in the AM fungal community (Liu et al, 2012).…”

Host discrimination in modular mutualisms: a theoretical framework for meta-populations of mutualists and exploiters

“…[31,37]). Our model also does not consider the impact of imperfect mixing in the partner community-a process that could lead to positive associations and repeat encounters among the offspring of individual modules.…”