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.

<p dir="ltr">Plants emit an amazing diversity of volatile organic compounds (VOCs) that in addition to being utilized by humans for a multitude of applications, allow plants to communicate with their environment, and play numerous roles in plant growth and development. Plants must be able to perceive and distinguish between VOC cues mediating plant-plant, plant-insect, and plant-microbe interactions to appropriately respond to stimuli. Due to the plethora of biological processes dependent on VOCs, significant progress has been made towards understanding the biosynthesis of plant VOCs and their regulation, and, in recent years, the molecular mechanisms involved in VOC emission. However, to date, little is known about how VOCs are perceived by plants and trigger cellular response(s). In animals, VOCs are recognized by odorant receptors known as G-protein-coupled receptor (GPCR) proteins. However, the few GPCR genes identified in plants appear to have different functions and the lack of a reliable marker for VOC perception has hampered research in this field.</p><p dir="ltr">The discovery of natural fumigation of terpenoids in petunias provides a means of studying VOC perception and the downstream signaling pathways by providing a visual indicator of perception. Transcriptomic analysis of wild-type and transgenic petunias deficient in terpenoid synthesis revealed a link between terpene perception in pistils with the karrikin-like signaling pathway. By utilizing biochemical, computational, and in planta experiments, we demonstrate that of the four petunia karrikin-insensitive receptors (PhKAI2), one of the Lamiid-specific KAI2 intermediate clade receptors, PhKAI2ia, can stereo-specifically perceive the (−)-germacrene D signal emitted from the floral tubes, triggering a KAI2-mediated signaling cascade and affecting plant fitness. Downregulation of PhKAI2ia results in significantly smaller stigmas compared to wild-type, and the phenotype cannot be complemented by the treatment of pistils with (−)-germacrene D, indicating that PhKAI2ia transgenic plants are acting as deaf receptors. We also show that the binding of (−)-germacrene D to PhKAI2ia is sufficient to induce complex formation with more axillary growth 2 (PhMAX2) and the subsequent degradation of suppressor of MAX2 (PhSMAX1a).</p><p dir="ltr">Altogether, our research uncovers the role(s) of the intermediate clade of KAI2 receptors, illuminates the involvement of a KAI2ia-dependent signaling pathway in volatile communication, and provides new insights into plant olfaction and the long-standing question about the nature of potential endogenous KAI2 ligand(s).</p>