Seeds of the root parasitic plants of the genus Orobanche germinate specifically in response to host-derived germination signals, which enables parasites to detect and attack preferred hosts. The best characterized class of germination stimulants is the strigolactones (SLs), although some species respond to non-SL compounds, such as dehydrocostus lactone (DCL). Recent work indicates that SLs are perceived by members of the KARRIKIN-INSENSITIVE2 (KAI2) gene family, and suggests that within parasitic Orobanchaceae the KAI2 genes have undergone duplication and specialization. The "diverged" clade of these genes, termed KAI2d, has been shown to bind SL germination stimulants in model system assays, but the precise role for KAI2d in regulating germination specificity in a parasitic plant has not been demonstrated. To address this issue, we used genetic and genomic approaches involving two closely related species, Orobanche cernua and O. cumana, which differ primarily in host range and stimulant preference. Orobanche cernua parasitizes tomato (and other Solanaceous crops) and responds to orobanchol, the major SL from tomato roots, whereas O. cumana specifically parasitizes sunflower and responds to DCL. Crosses between O. cernua and O. cumana produced hybrid populations that segregate for stimulant specificity, creating a tractable genetic system. Orobanche cernua contains four KAI2d genes (numbered OrceKAI2d1-4), while O. cumana contains six genes (OrcuKAI2d1-6). The DNA from 94 F2 hybrids was genotyped to identify the KAI2d gene composition and these were correlated with germination phenotype. The pattern of segregation indicated that the KAI2d genes are linked, but pointed to OrceKAI2d2 as a likely orobanchol receptor. Response to DCL was associated with inheritance of all O. cumana KAI2d genes together. Each KAI2d gene was expressed in the Arabidopsis thaliana kai2 mutant background and tested for ability to recover the mutant phenotype when exposed to SLs (including orobanchol, 5-deoxystrigol and GR24) or DCL. One O. cernua gene, OrceKAI2d2, responded to all SLs, but not DCL in this system. No DCL-specific KAI2 genes were identified. In summary, we have identified the likely SL receptor in O. cernua, and show evidence that the DCL receptor is either not a KAI2d protein, or uses KAI2d in combination with other signaling pathway components. / Ph. D. / The mechanisms by which parasitic plants of the family Orobanchaceae detect their hosts is a long-standing mystery in plant science. For over half a century it has been known that seeds of parasitic plants will lie dormant until they detect a host-derived germination stimulant. Upon perception of an appropriate germination stimulant, the parasite seeds will send out a radical that has approximately 72 hours to reach a host root before the limited nutrients within the seed are exhausted. The practical impact of this plant signaling regulation is profound, as the parasites in this family include some of the most destructive weeds in the world, including broomrapes (Orobanche and Phelipanche species) and witchweeds (Striga species). Scientists have sought to understand the signaling mechanisms in order to produce crop plants that don’t produce/exude the signal or to create chemicals that can mimic stimulants and artificially trigger parasite seed germination. Our goal was to further the understanding of the parasite germination mechanism by determining the genes involved in parasite host specificity in Orobanche, of which most members germinate in response to strigolactones (SLs).
Recent work indicates that SLs are perceived by members of the KARRIKIN-INSENSITIVE2 (KAI2) gene family and suggests that within parasitic Orobanchaceae the KAI2 genes have undergone duplication and specialization. The “diverged” clade of these genes, termed KAI2d, has been shown to bind SL germination stimulants in model system assays, but the precise role for KAI2d in regulating germination specificity in a parasitic plant has not been demonstrated. To this end we used two closely related species that differ in their germination stimulant and host preferences. Orobanche cernua which like most members of Orobanchaceae responds to a SL, and O. cumana which has switched to responding to a novel germination stimulant, dehydrocostus lactone (DCL). Through genetic and genomic studies of these two species, we demonstrated that one O. cernua gene, OrceKAI2d2, responded to all SLs, but not DCL in this system. No DCL-specific KAI2 genes were identified. In summary, we have identified the likely SL receptor in O. cernua, and show evidence that the DCL receptor is either not a KAI2d protein, or uses KAI2d in combination with other signaling pathway components.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/94423 |
| Date | 17 April 2018 |
| Creators | Larose, Hailey Lee Ann |
| Contributors | Plant Pathology, Physiology and Weed Science, Westwood, James H., Haak, David C., Brunner, Amy Marie, Tholl, Dorothea |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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