A number of studies have shown that an association with mycorrhizal fungi can alter the outcome of interactions between plants and their enemies. While the directions of these effects vary, their strength suggests the need for greater attention to multispecies interactions among plant enemies, plants, and mycorrhizal fungi. We recognize that mycorrhizal fungi could effect plant enemies by improving plant nutrition, modifying plant tolerance, or modifying plant defenses. In addition, mycorrhizal fungi could directly interfere with pathogen infection, herbivory, or parasitism by occupying root space. We formalize these alternative outcomes of multispecies interactions and explore the long-term dynamics of the plant-enemy interactions based on these different scenarios using a general model of interactions between plants and plant enemies. We then review the literature in terms of the assumptions of the alternative mechanisms and the predictions of these models. Through this effort, we identify new directions in the study of tritrophic interactions between enemies, plants, and soil mutualists.

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Parasitic plants utilize cell wall degrading enzymes (CWDEs) to penetrate their host plant and attach to host vasculature. Inhibition of CWDEs may confer resistance on susceptible hosts, offering a strategy for parasitic plant control. Here, exogenous application of green tea catechins, which inhibit pectin methylesterase (a CWDE), was used to delay parasitic plant attachment. Texas bluebonnet (Lupinus texensis Hook.) was grown with its naturally occurring root hemiparasite, Texas paintbrush (Castilleja indivisa Engelm.). Catechins, fertilizer (ammonium nitrate), or water were added to soil twice weekly. Parasite attachment was detected using Dragendorff reagent. Effects on growth and germination were also assessed. Parasite attachment to the host was prevented by 5 g L−1 of catechins through 20 d and reduced through 36 d after parasite seedling addition. Five grams per liter of catechins delayed germination in Texas bluebonnet but not in Texas paintbrush. To test the effects of CWDE inhibitors on aboveground parasitism, Arabidopsis thaliana (L.) Heynh. seedlings were grown with field dodder (Cuscuta pentagona Engelm.), a promiscuous shoot holoparasite. Ten grams per liter of catechins applied to the A. thaliana shoot surface inhibited field dodder attachment, although prehaustorial development was not affected. Our findings corroborate existing evidence that CWDE inhibition is a critical component of natural resistance to parasitic plants and open a new area of research into the use of exogenous CWDE inhibitors to control parasitic plant attachment.

Environmental programs in the UnitedStates face technical challenges that inhibit the ability to use diatoms in water quality monitoring and assessment projects. Specifically, inconsistent taxonomy can obscure diatom responses to environmental variables. Problems are the result of (1) limited access to a common set of taxonomic references, especially those that are geographically relevant, (2) inefficient enumeration protocols, (3) lack of complete and transparent documentation of taxa, and (4) limited opportunities for continued education, training, and knowledge sharing. However, robust resources and practices are available to improve diatom data quality and interpretation. Several resources improve diatom data quality, including a publicly accessible taxonomic reference (diatoms.org) and recommended practices. These practices include adoption of the voucher floras, random sample assignment, replicate microscope slides, and improved quality control. Finally, the Society for Freshwater Science Diatom Taxonomic Certification Committee is developing educational materials and certification exams to support practitioner training and to increase the diatom research knowledge base. The resources and practices in this article are broadly applicable to improving basic and applied research on diatoms worldwide.