UC Davis

Vector-borne bacterial plant diseases have a tremendous impact on agriculture around the world. These pathogens infect a myriad of crops, from solanaceous crops such as potato and tomato to tree crops such as citrus. These pathogens are unique because they are heavily reliant on their plant and insect hosts and as a result, have lost many essential biosynthetic pathways that free-living bacteria require. Because of the loss of these pathways, these bacteria are incredibly fastidious and many are still unculturable, making them difficult organisms to study in the laboratory. Our approach to studying these organisms is by examining the perception of microbial features by plants, which is one of the key mechanisms of plant immunity. In this dissertation, I examine two different sides of microbial feature perception in vector-borne bacterial plant pathosystems: a) the ability of plants to perceive a variety of microbial features from pathogens, and b) the perception of microbial features from vector-borne pathogens versus their free-living relatives. My results show that in Rutaceae, a large family of plants containing citrus Huanglongbing-susceptible citrus and their relatives, variation in the perception of microbial features occurs both within and between Rutaceae sub-tribes. Additionally, we were able to discover ten Rutaceae genotypes that could perceive a microbial feature from Candidatus Liberibacter asiaticus, the causative agent of citrus Huanglongbing. These ten genotypes may be useful to study further for resistance mechanisms that could be transferred to susceptible citrus. In another study, I also demonstrate that vector-borne bacterial plant pathogens tend to carry less copies of immunogenic microbial features in their genomes. The microbial features from vector-borne bacterial plant pathogens also tend to be non-immunogenic in plants. Interestingly, a microbial feature present in a citrus-Huanglongbing pathogen is perceived in solanaceous plants, while a feature present in a solanaceous plant-infecting pathogen is not perceived. I have also co-authored research on two different studies of Candidatus Liberibacter pathogens, which are vector-borne bacteria that cause devastating diseases on several plant hosts. One study is dedicated to understanding the evolution of bacteria in the Liberibacter genus, and another is dedicated to understanding the mechanisms behind SDE1, an effector from Candidatus Liberibacter asiaticus. Collectively, this dissertation work highlights a landscape of immune responses across vector-susceptible plants as well as characterizes microbial features from vector-borne pathogens to gain a broader understanding of how plants may or may not be able to defend themselves from these pathogens.