The role of host plant chemistry and light-spectral traits in host-finding and self-defense by parasitic plants in the genus Cuscuta (Convolvulaceae)

Parasitic plants are notorious for their devastating effects on global food production. They are also gaining recognition for playing keystone roles in ecosystem composition and functioning. Despite their significance in agriculture and nature, many aspects of their basic ecology are not well characterized, including two topics addressed in this dissertation: host-finding and subsequent three-way host-parasite-herbivore interactions.;To address these topics we worked with obligately parasitic vines in the genus Cuscuta (Convolvulaceae), also referred to as dodder. Dodder seedlings are extremely limited in their foraging range and will therefore perish if they do not locate a nearby suitable host plant. Prior work revealed that Cuscuta campestris seedlings orient toward hosts by responding to at least two classes of cues---host volatiles and host light-spectral cues. It is not known whether Cuscuta species that are more host-specific than the cosmopolitan C. campestris incorporate these classes of host cues in their foraging behaviors. In a series of biological preference assays with seedlings of Cuscuta epilinum, a putative specialist of flax plants, we found no evidence that these parasites use volatile cues to select their preferred host plants (flax). Rather, C. epilinum seedlings manifested positive growth responses to light cues common to all chlorophyllous plants, and they were surprisingly more attracted to tomato plants, which are inferior hosts, than to flax plants. In subsequent choice tests we found that seedlings prefer objects (whether plants or model plants) characterized by low red to far-red wavelengths (R:FR) over objects characterized by higher R:FR. R:FR varies as a function of host architecture and of host location. We conclude that seedling responses to R:FR cues represent a strategic foraging strategy to locate the nearest host plants of suitable quality.;Our second line of research examined three-way chemical interactions that occur between host plants, parasitic plants and insect herbivores. Many records in the scientific literature document the fact that secondary metabolites from host plants can translocate into parasitic plants, but little is known concerning the ecological implications of this phenomenon, despite well characterized roles for many secondary metabolites in plant defense. We present the novel finding that glucosinolates, secondary metabolites found in Brassicas and their relatives, transfer readily into dodder parasites. To examine the importance of this transfer in vivo, we grew multiple genotypes of Arabidopsis thaliana Col-0 that vary in their ability to produce aliphatic and indole glucosinolates and infested them with the parasite Cuscuta gronovii. Here we report our findings that host-derived glucosinolates or related metabolites can protect dodder against Acrythosiphon pisum aphids (Hemiptera: Homoptera: Aphidae), which refuse to settle on dodder that contains host glucosinolates. In contrast, Myzus persicae aphids are not at all deterred by the presence of glucosinolates in dodder vines, which indicates that protective benefits of glucosinolate uptake are contingent upon the susceptibility of the herbivore species. We similarly investigated the importance of glucosinolate transfer for deterring oviposition by Lygus rubrosignatus plant bugs (Hemiptera: Miridae) and find tentative evidence for a deterrent effect of these compounds.;During the course of these studies we observed that dodder growth is negatively correlated with the presence of indole glucosinolates, camalexin and auxin in Arabidopsis, which presents the intriguing possibility that well-characterized anti-insect and anti-microbial defense compounds in Arabidopsis might also contribute to defense against plant parasites.