Plants simultaneously interact with beneficial and antagonistic organisms such as mycorrhizal fungi and herbivores, respectively. Both mycorrhizae and herbivore damage cause rapid changes in source–sink dynamics within a plant. Mycorrhizae create long-term sinks for carbon within the roots while damage by leaf-chewing herbivores causes temporary whole-plant shifts in carbon and nitrogen allocation. Thus, induced responses to herbivory might depend on the presence or absence of mycorrhizae.
In a study published in AoBP, Orians et al. studied the net effects of such interactions on plant internal resources and growth. They examined the effects of mycorrhizal presence on induced resource partitioning in tomato (Solanum lycopersicon) in response to cues from a specialist herbivore, the tomato hornworm (Manduca sexta). Differences in plant size, growth and in the concentrations of carbon-based and nitrogen-based resources in three tissue types (apex, stem and roots) were quantified. The presence of mycorrhizae promoted plant growth, and altered sugar and starch levels. Simulated herbivory resulted in lower concentrations of most resources (sugar, starch and protein) in the rapidly growing apex tissue, while causing an increase in stem protein. Although they found mycorrhizae and herbivory had effects on the plant individually, the magnitude or direction of the individual effects generally did not differ when both organisms were present at the same time. These results suggest that herbivore cues, regardless of plant mycorrhizal status, reduce allocation of resources to the growing apex while promoting protein storage in the stem, a possible mechanism that could increase the tolerance of plants to damage.