Lignoid chemical defenses in the freshwater macrophyte Saururus cernuus

Abstract

Chemical defense against herbivores has rarely been investigated for freshwater plants, possibly due to the common misconception that herbivory on aquatic macrophytes is low and would not select for chemical defenses. In previous work, the freshwater angiosperm Saururus cernuus was shown to be a low preference food for omnivorous crayfish despite its high nutrient value and relatively soft texture. We used feeding by the crayfish Procambarus clarkii to guide fractionation of the deterrent lipid-soluble extract of this plant, leading to the identification of seven deterrent lignoid metabolites, (− )-licarin A, (+ )-saucernetin, (−)-dihydroguaiaretic acid, (− )-sauriols A and B, ( − )-saucerneol, and (− )-saucerneol methyl ether. Lignans have been implicated in terrestrial plant chemical defenses as insect growth inhibitors, insect toxins, nematocides, antibacterial, and antifungal agents. However, these activities have rarely been demonstrated using ecologically relevant methodologies in terrestrial systems, and never before in freshwater systems. The widespread nature of lignans amongst very distantly related plants, along with their rich diversity of molecular structure, suggests that they could play a large role in mediating plant-herbivore interactions. In addition to the lignoid compounds we identified, there were other compounds present in low concentration or unstable compounds that were deterrent, that did not appear to be lignans, but that we were unable to identify. This plant thus appears to be defended by a complex mixture of natural products.

Chemical Defense in Elodea nuttallii Reduces Feeding and Growth of Aquatic Herbivorous Lepidoptera

“…Not many studies have yet investigated chemical defenses in submerged freshwater angiosperms. Many studies have focused on marine macroalgae (Hay 1996;Paul et al 2006) and emergent or floating-leaved freshwater angiosperms Dorn et al 2001;Kubanek et al 2001;Prusak et al 2005). These studies often used omnivorous crustaceans as grazers, against which extracts of submersed species have turned out to be inactive Prusak et al 2005).…”

“…So far, only a few studies have addressed feeding deterrents in submersed freshwater plants, and studies with herbivorous freshwater insects are especially rare (see Dorn et al 2001). Mostly crayfish have been used in bioassays, as these omnivores feed in a wide variety of laboratory assays (Lodge 1991;Kubanek et al 2001). Thus far, nobody has succeeded in rearing Acentria or other aquatic lepidoptera on food other than submersed macrophytes.…”

Larvicidal Activity of (−)-Dihydroguaiaretic Acid Derivatives againstCulex pipiens

“…4,5) The antifeedant activity of (À)-DGA in crayfish has also been reported. 6) However, whether DGA and SECO demonstrate insecticidal activity and whether this activity varies in relation to their structure has remained unknown. The insecticidal activity of lignans against larvae of the Culex pipiens mosquito was therefore measured to extend our knowledge of the structureactivity relationships of lignans.…”

Induced chemical defenses in a freshwater macrophyte suppress herbivore fitness and the growth of associated microbes

“…To date, 22 species of freshwater macrophytes have been demonstrated to produce chemical defenses against herbivores (Newman et al 1996;Cronin 1998;Kubanek et al 2001;Cronin et al 2002;Prusak et al 2005;Parker et al 2006Parker et al , 2007Miller and Provenza 2007;Erhard et al 2007), and 3 species have been shown to induce either structural defenses (Center and Van 1989;Kouki 1993) or an unidentiWed defense (JeVries 1990), but only 1 species (Nuphar luteum) is known to induce a chemical defense . The chemicals responsible for this induced defense were not identiWed and the physiological eVects on herbivores were not investigated .…”