Post by Zoe Getman-Pickering
One of the first lessons you learn in ecology (be it a class or research) is that the natural world is infinitely complex with countless direct and indirect interactions, and it is one we ecologist repeatedly learn through our careers. I was reminded of this lesson reading the paper, “Mycorrhizal abundance affects the expression of plant resistance traits and herbivore performance” by Rachel Vannette and Mark Hunter.
Arbuscular mycorrhizae (AMF) are fungi that form symbiotic relationships with plants, commonly as a mutualism where the fungi get a cushy crib in the plant roots plus photosynthate, while the plant gets assistance in soil nutrient absorption. While this is one of the textbook examples of mutualism, the relationship between AMF and plants becomes more complex as we look through the lens of multiple trophic levels and indirect effects. While AMF may seem beneficial, Vannette and Hunter show that it can have negative effects as well by affecting plant herbivore interactions.
Vanette and Hunter explore AMF tri-trophic interactions by testing the effect of AMF abundance and genotype on milkweed’s (Asclepias syriaca) performance and the performance of its specialized herbivore the monarch butterfly (Danaus plexippus). Five genotypes of milkweed were grown with different amounts of AMF inoculation and different species of AMF. A single monarch butterfly egg was attached to each of the plants and allowed to feed for 5 days after hatching. They collected data on above ground biomass, damage, herbivore weight, and concentrations of toxic compounds.
As previous studies have shown, increased abundance of AMR increases plant phosphorus nutrition. This increased nutrition affects the expression of herbivore resistance traits. While plant response varied heavily depending on the species of mycorrhizae, trichome density, leaf mass, latex production, foliar P and cardenolide concentration (nasty bitter poisons produced by the milkweed) were all affected by AMR abundance.
The species of Arbuscular mycorrhizae fungi present also plays an important role in the interactions. While an increase in AMF lead to an increase in herbivore performance regardless of the AMF species, the level of increase is differentially dependent on AMF species. For instance, AMF species and abundances had a much greater impact on cardenolides variation compared to plant genotypes. AMF species also significantly affected biomass, percent phosphorus and leaf area. As logic dictates, the mycorrhizae species that are most efficient at sequestering most phosphorus have the greatest impact on traits that are P dependent. So abundance and species can strongly affect plant-insect interactions by affecting a number of plant traits. This, and other studies indicate that ABF may have pretty important impacts on plant fitness and community composition.
Amazingly enough, the abundance of AMF explained more variation than plant genotype did. Quite a number of studies show how important genotypic variation is in creating variation in responses. The fact that AMF explains even more variation is pretty cool. The physiological mechanisms are still unknown, so if anyone out there is looking for a thesis project, look into this!
The tri-trophic connections between plants, micorrhyzal symbionts and herbivores are still being elucidated. We need many more studies exploring mechanisms and variation between systems before these concepts can be applied to restoration or agriculture. For now, all that we have is the tantalizing glimmer of the tangled relationship between fungus, insects, and plants.
Vannette, R. L. & Hunter, M. D. Mycorrhizal abundance affects the expression of plant resistance traits and herbivore performance. J Ecol 101, 1019–1029 (2013).