Current Research on Parasitic Flowering Plants
Approximately 1% (4000 species) of all angiosperms are parasitic, that is they attach to other vascular plants by means of haustoria. These plants have continued to be the focus of multidisciplinary research owing to their importance as agronomically significant weeds and as models for studying developmental, physiological and molecular processes. Because of their intimate and complex interrelationships with host plants, parasitic angiosperms display evolutionary modifications at the biochemical, cellular, anatomical, and ecological levels that are novel among angiosperms.
Parasitic plants have been the specific focus of six international symposia; the most recent meeting in Cordoba, Spain (see Cubero and Moreno, 1996). Although all parasitic plants have received some attention at these symposia, the major emphasis has been on devastating crop pathogens such as Striga and Orobanche. The recently published "Parasitic Plants" (Press and Graves 1995) is a welcomed and needed resource that deals with modern topics such as the physiology of seed germination and haustorial initiation; mineral, carbon and nitrogen relations; and genome organization. Just as this book has attempted to "fill the gap" since the publication of Kuijt (1969), a need exists to provide a forum to more fully explore the cellular, biochemical and structural aspects of all parasitic plants. The goal of the proposed symposium is to bring together those researchers conducting state-of-the-art research on parasitic angiosperms. Unlike the past International Parasitic Weed Symposia where the main focus has been on applied aspects (such as control), the proposed symposium will be organized around basic biological research, especially research at the cellular, biochemical and molecular level. We anticipate that the range of topics presented will be of broad interest in the botanical community and will provide a vehicle for communication among specialists working on different aspects of parasitic plant biology.
Speakers (in order of their presentations):
Abstracts Printed Below:
CURRENT UNDERSTANDING OF PARASITIC PLANT RELATIONSHIPS AS INFLUENCED BY MOLECULAR PHYLOGENETICS D. L. Nickrent (Department of Plant Biology and the Center for Systematic Biology, Southern Illinois University, Carbondale, IL 62901-6509 USA)
The higher-level classification of flowering plants is currently undergoing a revolution given the influx of DNA sequence data derived from the three subcellular genomes. Separate and combined analyses of nuclear, plastid, and mitochondrial genes are providing resolution of many long-standing questions regarding the placement of parasitic plants within the angiosperms. The hemiparasitic order Santalales will be used as an example to demonstrate how multiple DNA sequences, analyzed separately and in combination, can provide a biologically meaningful phylogeny. In contrast, a number of holoparasitic (nonphotosynthetic) plant families have proven intractable to phylogenetic analyses owing to gene losses and high substitution rates in typically conserved genes. Results derived from analyses of nuclear and mitochondrial small-subunit rDNA will be presented that provide compelling evidence for the placement of three of these families (in the broad sense) within angiosperms: Balanophoraceae, Hydnoraceae, and Rafflesiaceae.
IMPACTS OF PARASITIC ANGIOSPERMS ON THEIR HOSTS: ARE THEY DRIVEN BY SOURCE-SINK RELATIONS? J. R. Watling, A. L. Gurney, J. D. Scholes and M. C. Press (Department of Animal & Plant Sciences, University of Sheffield, S10 2TN, UK)
Two models are proposed that account for the impact of parasitic angiosperms on their hosts. The first is driven by host-parasite source-sink relations and is illustrated by the impact of Orobanche infecting tobacco and tomato. In these systems, differences in productivity between uninfected and infected plants can be accounted for by the carbon allocated to the parasite. In addition to this model, in some systems, most notably interactions between Striga and cereals, such source-sink interactions are subjugated by additional effects. The mechanistic bases of such responses are still unknown and hypotheses are presented on the basis of the impact of Striga on the metabolism of its host.
TRANSCRIPTIONAL RESPONSES TO HOST ROOT SIGNALS IN PARASITIC AND NON-PARASITIC SCROPHULARIACEAE M. Matvienko and J. I. Yoder (Department of Vegetable Crops, University of California, Davis, CA 95616 USA).
We are interested in understanding how parasitic Scrophulariaceae recognize and respond to host root signals in the rhizosphere. When root tips of the hemiparasite Triphysaria versicolor are exposed to host root exudates in vitro, secondary haustoria develop within hours (see http://veghome.ucdavis.edu/yoder/yoderlab/ for timelapse animation of haustorium development). We isolated several hundred cDNAs differentially abundant in T. versicolor root tips shortly after exposure to the haustorial inducing factor 2,6 dimethoxybenzoquinone (DMBQ). Most of these genes are also induced in T. versicolor by maize root exudates. In contrast, when DMBQ is applied to the roots of the closely related non-parasite Lindenbergia muriana, most of these genes are either not induced or suppressed, consistent with the phytotoxicity of DMBQ. The expression patterns of a few genes likely involved in host signal recognition and transduction will be discussed in detail.
ON BECOMING A PARASITIC PLANT: STRATEGIES AND MECHANISMS IN HAUSTORIAL DEVELOPMENT R. C. O'Malley, W. J. Keyes, D. Kim and D. G. Lynn (Searle Chemistry Laboratory, The University of Chicago, Chicago, IL 60637 USA)
The temporal and spatial control of the transition from vegetative to parasitic mode is critical to any parasite, but essential to the sessile parasitic plants. This transition in Striga spp. has been proposed to be controlled by constitutive production of activated oxygen species (AOS) at the Striga root meristem. This strategy allows a parasite to exploit abundant host enzymes to oxidatively generate a diffusible xenognostic quinone signal from host cell wall phenols. The quinone signals are recognized via a parasite redox receptor, possibly via existing defense pathways, to regulate the temporal and spatial commitment of haustorial development. Critical downstream genes, most notably the cell enlarging enzyme expansin, have now been identified and correlated with the commitment to haustorial development. Analysis of these molecular markers has made it possible to develop a complete model for the signaling event initiating haustorial development. The characterization of these events offers insight into the functional evolution of the parasitic plants.
PHYLOGENETIC AND MOLECULAR ANALYSIS OF PARASITIC SCROPHULARIACEAE AND OROBANCHACEAE. C. W. dePamphilis*, N. D. Young1, A. D. Wolfe2, and T. J. Barkman* (*Pennsylvania State Univ., University Park, PA 16802, 1Trinity Univ., San Antonio TX 78212, 2Ohio State Univ.,Columbus, OH 43210, USA)
Parasitism has evolved many times in angiosperms, but only Scrophulariaceae (including Orobanchaceae) has members representing the full range of parasitic ability from facultative hemiparasites to nonphotosynthetic holoparasites. Phylogenetic analysis of multiple plastid genes gives strong evidence for the monophyly of parasitic Scrophulariaceae and Orobanchaceae, and a detailed hypothesis for the origin and evolution of parasitism in this group. All these parasites plus Lindenbergia (nonparasitic sister to the parasite clade) may be classified in an expanded Orobanchaceae. One origin of parasitism is strongly implied, but holoparasitism has evolved at least 5 times in this group. Plastid gene and genome evolution is remarkably diverse, including rapid structural evolution and large shifts in substitution rates, and retention or modification of genes for alternative function in nonphotosynthetic plants.
HOST DEFENSE-GENE REGULATION IN RESPONSE TO PARASITISM BY OROBANCHE J. H. Westwood and C. L. Cramer (Department of Plant Pathology, Physiology, and Weed Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA)
Little is known about molecular-level responses of plants to attack by parasitic angiosperms. The influence of Orobanche parasitization on regulation of host defense-related genes is being studied using transgenic plants containing promoter: GUS fusions and by northern hybridization analysis. Parasitized tobacco roots show increased expression of genes known to be induced by wounding alone (hmg1) or in association with local response to ingress by specific pathogens (hmg2, PAL, CHS). By contrast, a gene (PR1a) expressed in associated with systemic induction following virus or elicitor treatment, but not wounding, was not induced by Orobanche. These results suggest that host defense responses to the parasite may be mediated primarily by physical damage to root tissue caused by penetration of the Orobanche haustorium.
CARBON AND NITROGEN RELATIONS OF PARASITIC PLANTS: RESPONSES TO ELEVATED CO2 AND COMMUNITY LEVEL IMPACTS M. C. Press and J. R. Watling (Department of Animal & Plant Sciences, University of Sheffield, S10 2TN, UK)
Elevated concentrations of CO2 may exert important effects on parasitic angiosperm-host associations through impacts on host and parasite gas exchange. The degree of these impacts is a function of the extent to which host responses to infection are source-sink driven, the degree of parasitic autotrophy for carbon, host photosynthetic type (C3 vs C4) and the extent to which nutrients and water limit growth and photosynthesis. Community-level impacts (under both ambient and elevated CO2) are likely to be driven by competitive interactions. Such interactions will operate both between host and parasite for resources within the vascular system and between host and non-host species for resources in the atmospheric and edaphic environment. In addition, parasitic angiosperms may impact on nutrient cycling, thus providing a second mechanism controlling their impact at the community level.
SIUC / College of Science / Parasitic Plant Connection
Last updated: 20-Aug-99 / dln