Parasitism is a highly successful life strategy and a theme that cuts across plant and animal kingdoms. Parasitic plants are those that directly invade the tissues of other plants to fulfill at least some of their nutritional requirements. In plants, parasitic species are found in at least 13 angiosperm families, indicating that parasitic lineages have evolved from nonparasitic progenitors multiple independent times in the course of evolution. The result is a diverse assemblage of plants, ranging from mistletoes that grow in the tops of trees to root parasites whose existence is primarily subterranean.
Our goals are to understand the mechanisms through which plants acquire competence for parasitism and to learn how the parasite genome changes in response to acquiring heterotrophic capacities. Because some parasitic plants also inflict considerable damage to crop plants, especially in the developing world, their genomic characterization sets the groundwork for developing novel control strategies.
The origin of parasitism in plants represents a transition from an autotrophic to heterotrophic lifestyle. This transition is marked by the plant's ability to locate and parasitize host plants. These processes are mediated through haustoria, parasitic organs that invade host tissues and establish a physiological continuum for the translocation of host resources. The ability to form haustoria is the defining character of parasitic plants and it is of interest to learn how the genes for these processes evolved.
Once a plant acquires a capacity for parasitism, it begins to respond to a set of evolutionary selective forces that differ markedly from those of autotrophic plants. Specifically, a parasite must detect neighboring plants, connect to them via the haustorium, and integrate its physiology with that of the host. As a consequence of host-dependence, the parasite is relieved of many functions required by autotrophic plants. The most striking function lost is photosynthesis, as evidenced by reduced leaf size, lack of chlorophyll, and disappearance of major sections of the chloroplast genome.
Parasitic Orobanchaceae are devastating agricultural pests, with particular impact in developing nations. At present, over two thirds of the 73 million hectares of farmland cultivated for cereal grains and legumes in Africa are infested with one or more Striga species, affecting the livelihoods of 100 million farmers in 25 countries. Orobanche spp. attack many important dicotyledonous crop plants, affecting host growth and resource allocation such that yield losses may reach 100%.
Striga and Orobanche infestations already exist in the US where they have been controlled only by costly treatments. Controlling parasitic plants is particularly difficult using conventional mechanical or chemical approaches because the parasites are underground and inaccessible even as they reduce crop yields. Efforts to breed crops with resistance to parasitic plants have met with little success. Information on the genes that are important in parasitism will facilitate this process, and may lead to novel strategies for developing genetic resistance against these pests.