Arbuscular mycorrhizal fungi affect plant tolerance and chemical defences to herbivory through different mechanisms

Abstract

1. Arbuscular mycorrhizal fungi (AMF) are ubiquitous plant symbionts that affect plant nutrient status, patterns of resource allocation and rates of plant growth. In addition, AMF influence the expression of plant defence, which may affect subsequent interactions between plants and higher trophic levels. Tolerance to herbivory and chemical defence represent two distinct defence strategies of plants, and there is accumulating evidence that AMF can influence each strategy separately. However, investigations on the simultaneous effects of AMF on tolerance and chemical defence mechanisms are lacking. 2. Using six milkweed (Asclepias) species and three levels of AMF inoculum, we show that plant tolerance to herbivory increases with foliar P concentrations, while both foliar cardenolide concentration and latex exudation (major chemical defences in milkweeds) increase with foliar N concentrations, but decrease with plant growth rate. Additionally, foliar cardenolide concentration increases with foliar P concentration, and latex exudation increases with root biomass. Because AMF have significant effects on all of these traits (foliar N and P concentrations, root biomass and growth rate), AMF can change plant tolerance and chemical defence simultaneously, albeit through different mechanisms. 3. Synthesis. Overall, plant tolerance and chemical defence to herbivores are affected by AMF through changes in basic plant physiology such as nutrient status, allocation patterns and growth rate, and our study suggests that it may be possible to predict effects of AMF on plant defences through these simple traits.

Effects of phenological synchronization on caterpillar early-instar survival under a changing climate

“…Phenology can drive these relationships in similar ways as with multicellular natural enemies, as insect immunity can vary with host plant synchrony (Martemyanov et al 2015). The study of mycorrhizae (e.g., Tao et al 2016), phylloplane flora (e.g., Leong et al 1997), midgut biota (e.g., Mason and Raffa 2014;Martemyanov et al 2016), and pathogens (e.g., Hajek et al 1990;Leong et al 1997;van Frankenhuyzen et al 2007;Gowler et al 2015) in plant-herbivore systems is suggesting potentially important roles for multitrophic interactions with these poorly understood organisms. These relationships are only beginning to be identified and their phenologies or responses to climate have yet to be examined.…”

“…Cardenolide consumption thus appears to mediate a trade-off between toxicity and protection. The production of these compounds is further influenced by yet another trophic level: arbuscular mycorrhizal fungi increase both cardenolides and latex production in infected plants, but the effect on monarch populations is not clear (Tao et al 2016).…”

Mycorrhizae Alter Toxin Sequestration and Performance of Two Specialist Herbivores

“…We used four North American milkweed species (Asclepias curassavica, A. latifolia, A. syriaca, and A. incarnata) that show constitutive and AMF-mediated variation in milkweed defenses and nutritive quality Tao et al, 2016a). Asclepias incarnata and A. syriaca seeds were collected from naturally occurring populations in Livingston County, MI, and A. latifolia and A. curassavica seeds were purchased from commercial sources (Alplains and Butterfly Encounters Inc., respectively).…”

“…Some of this variation may be explained by the stage of the association between the plant and AMF; aphids, for example, tend to benefit only after at least a month of AMF establishment (Tomczak and Müller, 2017). This variation in herbivore responses to AMF may also be a consequence of plant species-specific responses of plant traits to the presence of AMF (e.g., Grman, 2012;Barber et al, 2013;Anacker et al, 2014;Tao et al, 2016a) and the density or identity of AMF inoculum available to the plant (Garrido et al, 2010;Hunter, 2011, 2013;Barber et al, 2013).…”

“…Furthermore, we expected that AMF colonization would improve the performance of aphids and caterpillars by increasing plant nutritive quality and biomass, outweighing the negative effects of increased cardenolide concentrations on the herbivores. Because the outcomes of many AMF-plant associations are specific to the AMF and plant species (e.g., Grman, 2012;Barber et al, 2013;Anacker et al, 2014;Tao et al, 2016a), we expected the magnitude of the effects of AMF on herbivore sequestration and performance to vary among plant species and with the level of AMF inoculum available to the plant.…”