Recognition of host plants by parasitic angiosperms
We are interested in understanding the genetic mechanisms governing the interaction of parasitic angiosperms and their plant hosts. Parasitic plants in the Orobanchaceae are interesting because their growth, development, and physiological behaviors are modified in response to molecular and tactile stimuli provided by host roots. The most obvious phenotype of parasitic plant-host plant signaling is the initiation of haustorium development by host root factors. When a parasite root contacts a host, cortical cells localized in the parasite root at the point of contact,begin an isodiametric expansion that results in a swollen knob on the side of the parasite root closest to the host. Nearly simultaneously, epidermal cells overlaying the swollen area begin to develop into long haustorial hairs that function in host root attachment. The early development of haustoria can be monitored under amicroscope by applying host root exudates or purified chemical haustorium inducing factors to aseptically growing parasite seedlings. Once the haustorium is firmly attached to a host, cells within the haustorium develop which invade through the host root reaching the host vasculature. The parasite then establishes vascular connections that allow the transport of sugars, secondary metabolites and informational macromolecules between host and parasite.
Haustorium inducing chemicals include quinones, hydroquinones and polyphenols. The current model for host factor recognition, proposed originally by David Lynn and colleagues  proposes that radicals produced during enzymatic oxidoreduction of active quinones trigger a redox sensitive signaling pathway leading to haustorium development. We evaluated this hypothesis by miss-expressing oxidoreductase genes in parasite roots and monitoring their phenotypes in response to host contact. Using RNAi, we showed that expression of a single electron reducing quinone oxidoreductase gene in parasite roots is necessary for haustorium initiation [2-3]. This gene encodes the first step in the haustorium development signaling pathway.
The redox model of haustorium signaling. The haustoria inducing quinone (in this case DMBQ) enters the parasite cell where it is acted on by two enzymes; TvQR1 and TvQR2. TvQR1 catalyzes the single electron reduction of quinones (1) to radical semiquinones (2) which initiate haustorium development. The enzyme TvQR2 catalyses a two electron reduction converting the quinone to a hydroquinone (3). From 
Parasitism can be debilitating for the host and some of the world’s worst agricultural pests are parasitic weeds . The most notorious is Striga, a parasitic weed estimated to infect over 50 million hectares of arable farmland under cultivation with cereals and legumes in sub-Saharan Africa . The FAO estimates that the livelihoods of over 300 million Africans are negatively impacted by this single parasitic genus . Orobanche spp. can be similarly destructive, estimated to threaten 16 million hectares in the Mediterranean and West Asia .
We use Triphysaria as an experimentally tractable surrogate for the weedy Orobanchaceae. Triphysaria is a small genus of facultative parasites that grow as herbaceous annuals in grassland fields and coastal bluffs across Western Pacific coastal regions . Triphysaria are indigenous to North America and, unlike Striga or Orobanche, pose no environmental or agricultural concerns. Triphysaria can grow in the absence of host plants but will infect a broad spectrum of plants, including Arabidopsis, rice and maize, when available. We have developed a number of genetic resources for Triphysaria including seed collections representing extensive natural variation, sequence databanks, and gene transformation [9-12].
We are developing an RNAi mediated host resistance strategy based on the movement of RNAi silencing molecules from the host to parasite. We are designing hairpin plasmid constructions targeted against vital genes in the parasite. The constructions are transformed into host plants which do not display silencing phenotypes because the hairpin sequences are parasite specific. The silencing signals are translocated across the haustorium from host into the parasite where its silencing activity is expressed. We proved the concept of this approach by the silencing of a GUS gene in Triphysaria roots that had parasitized lettuce or Arabidopsis transgenic for a GUS silencing construct . Current experiments are directed towards understanding the molecular mechanisms governing the early steps in host root recognition and invasion. We have taken a transcript profiling strategy to distinguish genes specifically active in parasitic species exposed to host root factors. Transcripts expressed in Triphysaria root tips during haustorium development have been sequenced by Sanger sequencing and more recently by Illumina RNA seq. Transcript profiling using cDNA arrays and in silica gene annotation has been used to identify a set of genes transcriptionally regulated by contact with host roots. These are being transformed into Triphysaria roots to determine parasitic phenotypes when the genes are inhibited or ectopically expressed.
1. Keyes, W.J., et al., Semagenesis and the parasitic angiosperm Striga asiaticae. Plant J, 2007. 51: p. 707-716.
2. Bandaranayake, P.C.G., et al., A single-electron reducing quinone oxidoreductase is necessary to induce haustorium development in the root parasitic plant Triphysaria. The Plant Cell 2010. 22(4): p. 1404-1419.
3. Matvienko, M., et al., Quinone oxidoreductase message levels are differentially regulated in parasitic and non-parasitic plants exposed to allelopathic quinones. Plant Journal, 2001. 25: p. 375 - 387.
4. Parker, C. and C.R. Riches, Parasitic Weeds of the World: Biology and Control. 1993, Wallingford: CAB International. 332.
5. Ejeta, G., The Striga scourge in Africa: a growing problem, in Integrating New Technologies for Striga Control: Toward Ending the Witch-hunt, G. Ejeta and J. Gressel, Editors. 2007, World Scientific Publishing Co: Hackensack, NJ. p. 3-16
6. Mboob, S.S., A regional programme for West and Central Africa, in Striga- Improved management in Africa, T.O. Robson and H.R. Broad, Editors. 1988, Food and Agriculture Organization of the United Nations: Maroua, Cameroon. p. 190-194.
7. Parker, C., Observations on the current status of Orobanche and Striga problems worldwide. Pest Management Science, 2009. 65(5): p. 453-459.
8. Hickman, J.C., The Jepson Manual; Higher Plants of California. 1993, Berkeley, CA: University of California Press. pp 1400.