Multiple chemical defenses produced by Spartina alterniflora deter farming snails and their fungal crop

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 Resale or republication not permitted without written consent of the publisher

Negative outcomes of novel trophic interactions along mangrove range edges

“…Consumer feeding behavior and physiological condition are complex dynamics that are affected not just by exposure to new food sources (such as the appearance of a novel plant in a system undergoing tropicalization) but also by the presence of phytochemicals (i.e., fatty acids, tannins, phenolics) (Barbehenn & Constabel, 2011; Sieg et al, 2013), pollutants, heavy metals, and diseases (Callahan & Weis, 1983; De Wolf & Rashid, 2008), the fluctuating relationships between diet assimilation efficiency and organism nutritional requirements (Cox & Murray, 2006; Morton, 2018), and environmental considerations including sediment dynamics, light/shade levels, and the presence of other plant species (e.g., Iacarella & Helmuth, 2011; Stagg & Mendelssohn, 2012; Thurman, 1984). Identifying the drivers of food preference was beyond the scope of this study, but the knowledge that common basal consumers avoid consuming Avicennia has important implications for energy flow in encroached wetlands where Spartina is disappearing.…”