No abstract

The origin of novel traits is recognized as an important process underlying many major evolutionary radiations. We studied the genetic basis for the evolution of haustoria, the novel feeding organs of parasitic flowering plants, using comparative transcriptome sequencing in three species of Orobanchaceae. Around 180 genes are upregulated during haustorial development following host attachment in at least two species, and these are enriched in proteases, cell wall modifying enzymes, and extracellular secretion proteins. Additionally, about 100 shared genes are upregulated in response to haustorium inducing factors prior to host attachment. Collectively, we refer to these newly identified genes as putative “parasitism genes.” Most of these parasitism genes are derived from gene duplications in a common ancestor of Orobanchaceae and Mimulus guttatus, a related nonparasitic plant. Additionally, the signature of relaxed purifying selection and/or adaptive evolution at specific sites was detected in many haustorial genes, and may play an important role in parasite evolution. Comparative analysis of gene expression patterns in parasitic and nonparasitic angiosperms suggests that parasitism genes are derived primarily from root and floral tissues, but with some genes co-opted from other tissues. Gene duplication, often taking place in a nonparasitic ancestor of Orobanchaceae, followed by regulatory neofunctionalization, was an important process in the origin of parasitic haustoria.

The Parasitic Plant Genome Project has sequenced transcripts from three parasitic species and a nonparasitic relative in the Orobanchaceae with the goal of understanding genetic changes associated with parasitism. The species studied span the trophic spectrum from free-living nonparasite to obligate holoparasite. Parasitic species used wereTriphysaria versicolor, a photosynthetically competent species that opportunistically parasitizes roots of neighboring plants;Striga hermonthica, a hemiparasite that has an obligate need for a host; andOrobanche aegyptiaca, a holoparasite with absolute nutritional dependence on a host.Lindenbergia philippensisrepresents the closest nonparasite sister group to the parasitic Orobanchaceae and was included for comparative purposes. Tissues for transcriptome sequencing from each plant were gathered to identify expressed genes for key life stages from seed conditioning through anthesis. Two of the species studied,S. hermonthicaandO. aegyptiaca, are economically important weeds and the data generated by this project are expected to aid in research and control of these species and their relatives. The sequences generated through this project will provide an abundant resource of molecular markers for understanding population dynamics, as well as provide insight into the biology of parasitism and advance progress toward understanding parasite virulence and host resistance mechanisms. In addition, the sequences provide important information on target sites for herbicide action or other novel control strategies such as trans-specific gene silencing.

Species of Orobanchaceae parasitize the roots of nearby host plants to rob them of water and other nutrients. Parasitism can be debilitating to the host plant, and some of the world's most pernicious agricultural pests are parasitic weeds. We demonstrate here that interfering hairpin constructs transformed into host plants can silence expression of the targeted genes in the parasite. Transgenic roots of the hemi-parasitic plant Triphysaria versicolor expressing the GUS reporter gene were allowed to parasitize transgenic lettuce roots expressing a hairpin RNA containing a fragment of the GUS gene (hpGUS). When stained for GUS activity, Triphysaria roots attached to non-transgenic lettuce showed full GUS activity, but those parasitizing transgenic hpGUS lettuce lacked activity in root tissues distal to the haustorium. Transcript quantification indicated a reduction in the steady-state level of GUS mRNA in Triphysaria when they were attached to hpGUS lettuce. These results demonstrate that the GUS silencing signal generated by the host roots was translocated across the haustorium interface and was functional in the parasite. Movement across the haustorium was bi-directional, as demonstrated in double-junction experiments in which non-transgenic Triphysaria concomitantly parasitized two hosts, one transgenic for hpGUS and the other transgenic for a functional GUS gene. Observation of GUS silencing in the second host demonstrated that the silencing trigger could be moved from one host to another using the parasite as a physiological bridge. Silencing of parasite genes by generating siRNAs in the host provides a novel strategy for controlling parasitic weeds.