Parasitic angiosperms attach to and penetrate their host vascular system by the development of a highly specialized organ termed a haustorium. The haustorium serves as a physical bridge that allows the parasite to obtain the water and nutrients necessary for its survival. The development of this unique structure is controlled by a series of chemical signals from the host plant that condition the germination, attachment and penetration stages of growth. Using a combination of in vivo and in vitro approaches Drs. Timko and Riopel have begun to examine the biochemical and genetic mechanisms involved in haustorial induction and development in Striga asiatica (witchweed), a major parasite of agronomically important grasses. They have standardized methods to synchronize growth of a large number of individual plant in order to look at sequential developmental stages (germination, radical extension, pre- attachment haustorial development, attachment, and host penetration) in axenic culture in 3-4 days. The overall goal of the present experiments is to define the cellular and molecular genetic events that constitute the developmental pathway in Striga leading from haustorial initiation to successful attachment and penetration of a susceptible host plant species. They propose to complete the characterization of the putative root and haustorial specific cDNA clones identified in the previous studies and initiate additional rounds of differential screening to identify gene products expressed during the attachment and penetration phases of haustorial development. The goal of these studies is to establish subsets of clones representing gene products temporally and spatially distinct in their expression during the pre-attachment, attachment, and penetration stages of Striga haustorial differentiation. The patterns of gene expression activated during attachment and differentiation of the penetration peg will be compared in Striga haustoria developed in contact with host versus non-host tissues. Such information will assist in the subsequent attempts to define the signal transduction pathway controlling overall haustorial development. A greater understanding of the biochemical and cellular factors governing haustorial differentiation may contribute to future manipulation of host resistance. When completed these studies will provide substantial new information on a unique developmental process in plants.
