PhD defence by Pernille Sølvhøj Roelsgaard

TIME: May 9th, 2014 at 13:00
PLACE: A1-04.01

Main supervisor: Birger Lindberg Møller
Co-supervisors: Kirsten Jørgensen and Michael Foged Lyngkjær

Abstract Plants produce an impressive variety of bioactive natural products involved in defense, insect attraction and signaling. These compounds enable the plant to defend itself, communicate with the surroundings and survive in an environment with constant challenges and attackers. This study has focused on barley (Hordeum vulgare). Barley accumulates five hydroxynitrile glucosides, including one cyanogenic glucoside, in the epidermal cell layer. Cyanogenic glucosides are classically known as hydrogen cyanide-releasing defense compounds which act against generalist insects and herbivores. However, the defense capability of these compounds requires the activity of a specific β-glucosidase, and this β-glucosidase is not found in barley leaf tissue. Therefore, the role of hydroxynitrile glucosides in barley leaves is unclear. In contrast to acting as defense compounds, it has been suggested that hydroxynitrile glucosides can act as carbohydrate and nitrogen storage compounds and as reactive oxygen species (ROS) quenching compounds. A positive correlation between the hydroxynitrile glucoside content in barley and susceptibility toward the barley powdery mildew fungus (Blumeria graminis f.sp. hordei, Bgh) has been reported in the literature. In this thesis, the role of hydroxynitrile glucosides in the interaction between barley and Bgh is investigated. It is shown that the hydroxynitrile glucoside levels increase over time in barley leaves upon Bgh infection. In addition, isolation of fungal hyphae indicate that the hydroxynitrile glucosides are taken up by the fungus. Interestingly, it is also shown that barley deposits hydroxynitrile glucosides in the epicuticular wax layer. The reason for this is deposition is not known, but apoplastic deposition of defense compounds is in the literature reported to regulate defense related genes in maize. Barley plants that are gene-silenced in the first step of hydroxynitrile glucoside biosynthesis have been generated to further investigate these results. It appears that hydroxynitrile glucosides in barley have a dual role; up to a certain level Bgh profits from the hydroxynitrile glucosides as recognition factors and nutrition compounds. Above this threshold hydroxynitrile glucosides or their breakdown or turn-over products reach toxic levels which lead to a stunted Bgh phenotype and abortion of colonies. A putative ROS quenching role of hydroxynitrile glucosides is proposed. The results obtained in this Ph.D. study provide a unique insight demonstrating that hydroxynitrile glucosides play a far more complex role in barley defense against and susceptibility to Bgh than previously described. Future studies can build on the platforms established in this study to provide further insight into this exciting new picture of the dynamic roles of hydroxynitrile glucosides in barley.