Molecular Genetic Dissection of Haustorium Development in Parasitic Angiosperms

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Studies examining the molecular mechanisms of plant-plant interactions have been impeded compared to those in plant- microbe interactions in large part because simple laboratory based model systems are lacking. The interaction of parasitic Scrophulariaceae with their host plants offers one of the best opportunities for integrating modern biochemical, physiological, and genetic methodologies to understand how plants communicate in the environment. The overall objectives of this research are to learn how plant parasitism originates and evolves, to understand how chemical signals initiate the development of parasitic organs and mediate their functions, and to discern how parasitic plants recognize and distinguish hosts, non-hosts, and self. These studies are focused on the defining organ of plant parasitism, namely the haustorium. This specialized multi-functional structure recognizes the distance and direction to the host, penetrates host vascular tissue through a combination of enzymatic and invasive growth, and forms a conductive bridge through which the parasite robs nutrients from the host. While well characterized at the morphological level, almost nothing is known about the genetic mechanisms controlling haustorium initiation, development, or function. The experiments in this proposal make use of parasitic members of the Scrophulariaceae family as model organisms to study haustorium development because: all stages in the evolution of plant parasitism, ranging from complete autotrophs to obligate non-photosynthetic heterotrophs, are represented by contemporary species; the biochemistry of haustorium induction and histology of haustorium ontogeny are most advanced for this family; facultative parasitic Scrophulariaceae are readily grown in the presence or absence of host in both soil and axenic culture; haustoria of these species can be synchronously induced by in vitro treatment of seedlings with purified, host encoded compounds; one of the best devel oped model systems for studying plant development (Antirrhinum) is a member of this family; and parasitic species of this family are among the world's most devastating weed pests. These experiments take two complementary approaches to examine haustorium development in parasitic Scrophulariaceae. (1) Haustorium specific genes are being isolated and analyzed to obtain clues about their origin, their function in parasitic plants, and their role in non-parasitic relatives. Expression studies will indicate their specificity for heterotrophic plants and in situ hybridizations will determine their specificity for haustoria. (2) A model system for defining the genetic determinants of haustorium development is being developed. In particular, different hemiparasitic Scrophulariaceae are being evaluated for their amenability to genetic analyses including ease of growth in the presence and absence of host, amenability of both parasite and host to genetic analysis, and the fidelity of haustorium induction in vitro. Genetic colinearity relationships between selected parasitic Scrophulariaceae and Antirrhinum will be examined as a shortcut to developing the genetics of the parasitic member. Finally, the genomes of parasitic species will be examined for containing endogenous Tam-like transposable elements that can be later incorporated into mutagenesis experiments.