MEPS 488:35-49 (2013)  -  DOI: https://doi.org/10.3354/meps10415

ABSTRACT: Plants are exposed to a variety of ecological threats from herbivores, pathogens, and parasites. In cases in which chemical defenses play a role in plant resistance, plants may produce a single molecule that inhibits a diverse array of enemies, or they may invest in a suite of deterrent compounds that each protect against specific threats. The snail Littoraria irrorata exerts substantial top-down control over smooth cordgrass Spartina alterniflora by culturing and grazing fungi on plant tissues. To combat fungal farming, S. alterniflora produces chemical defenses that inhibit fungal growth and reduce L. irrorata grazing. Guided by ecological assays, we isolated a fatty acid (α-dimorphecolic acid) from S. alterniflora that inhibited growth of Mycosphaerella sp., a marsh fungus commonly farmed by L. irrorata. Mycosphaerella sp. was more susceptible to the inhibitory effects of α-dimorphecolic acid than another farmed fungus, Phaeosphaeria spartinicola. Several phenolic compounds isolated from S. alterniflora deterred grazing by L. irrorata, of which one, the flavonoid glycoside orientin, was fully characterized. These defenses are not potent enough to completely deter fungi and snails but may slow down the negative effects caused by fungal farming. In a heavily grazed marsh, chemical defenses were constitutively expressed in S. alterniflora even after a month-long experiment in which exposure to fungi and herbivores was manipulated. Thus, S. alterniflora relies on multiple types of secondary metabolites instead of a single class of molecule to combat associated herbivores and fungi. Although α-dimorphecolic acid was not expressed in sufficient concentration on plant surfaces to prevent fungal establishment, this chemical defense may reduce fungal growth in plant tissues and increase the resistance of S. alterniflora to fungal farming.

KEY WORDS: Littoraria irrorata · Antiherbivore · Antifungal · Chemical defense · Induction · Herbivore · Pathogen