Parasitic Plant Genes Necessary for Host Plant Recognition

Parasitic Orobanchaceae use chemical and tactile stimuli provided by host roots to initiate the development of invasive haustoria. We are interested in discovering the molecular events associated with host signal perception and transduction in order to identify potential parasite gene targets for engineering weed resistant crops. We have sequenced thousands of mRNA transcripts expressed in roots of the hemiparasite Triphysaria during host signal recognition and early haustorium development. Because of the potential role of redox signaling in rhizosphere interactions between plants, we initially focused on two genes that encode quinone oxidoreductases and which are transcriptionally regulated in parasite roots by host roots or host root factors. The enzymes encoded by both genes catalyze the NAD(P)H dependent reduction of haustorium inducing quinones but through different mechanisms. The TvQR1 enzyme catalyzes single electron quinone reductions that produce unstable, radical semiquinones. In contrast, TvQR2 catalyzes two electron reductions that bypass the potentially toxic semiquinone intermediates. RNAi silencing of TvQR1 in Triphysaria results in a dramatic decrease in the number of haustoria produced while silencing TvQR2 has no effect on haustorium development. We propose that TvQR1 catalyzes a univalent reduction of host molecules that activates a redox sensitive transduction pathway that signals haustorium development.  Our model proposes that the balance between TvQR1 catalyzed semiquinone production and TvQR2 mediated semiquinone elimination describes the cellular environment for haustorium initiation. Current work is directed towards understanding the transcriptional regulation of TvQR1 with the goal of identifying cis and trans acting factors that regulate parasite specific gene regulation.