ELUCIDATING THE ROLE OF THE FLOWERING REGULATOR FLK IN PATHOGEN DEFENSE IN ARABIDOPSIS THALIANA

Abstract

Plant pathogens pose a considerable risk to global agriculture, threatening global food supply and causing hundreds of billions of dollars in economic losses each year. To protect our agriculture, plant scientists seek to further characterize the diverse mechanisms of pathogen defense via the model plant Arabidopsis thaliana. In a screen for novel defense genes, the Lu Lab revealed a putative role of the flowering activator FLK as a positive regulator of pathogen defense. FLK encodes a highly conserved, putative KH-repeat RNA binding protein that localizes to the nucleus, and that is characterized for its roles in flowering time regulation and flower morphogenesis. From these preliminary findings, we sought to elucidate the role of FLK in pathogen defense, in order to better understand the pathways governing pathogen defense in Arabidopsis, and by extension, commercially important crop plants. We developed flk complementation lines expressing GFP-tagged FLK, which exhibit high levels of FLK-GFP expression, yield a nuclear FLK-GFP signal, and rescue defense and flowering time phenotypes in the flk-1 mutant background. I found that FLK positively regulates defense, including salicylic acid (SA)-mediated signaling, to the biotrophic pathogen Pseudomonas syringae. Via plants with mutations at flk and SA regulatory genes, we found that FLK interacts with SA genes to coordinately regulate flowering time and SA-mediated defense. Using conserved bacterial peptides that elicit defense responses, I illustrated a role for FLK in the positive regulation of basal defense, including some of the earliest immune responses. RNA-seq analysis supported these findings, with diverse defense genes regulated by FLK. I also found that FLK negatively regulates defense against the necrotrophic Botrytis cinerea. Because reactive oxygen species (ROS) underlie myriad defense activities, I incorporated assays to induce ROS in flk plants, which conferred a role for FLK as a negative regulator of ROS scavenging, likely providing a basis for the role of FLK in responses to both abiotic and biotic stress. Together, these findings affirm a role for FLK at the intersection of development and biotic and abiotic stress, including pathogen defense, which lays the groundwork for future studies that aim to bolster the robustness of crop plants.