Characterizing host-pathogen interactions between Zostera marina and Labyrinthula zosterae

Code, output files, figures, and supplementary information associated with the manuscript, "Characterizing host-pathogen interactions between <em>Zostera marina</em> and <em>Labyrinthula zosterae</em>." <br> Seagrass meadows serve as an integral component of coastal ecosystems but are declining rapidly due to numerous anthropogenic stressors including climate change. Eelgrass wasting disease, caused by opportunistic <em>Labyrinthula </em>spp., is an increasing concern with rising seawater temperature. To better understand the host-pathogen interaction, we paired whole organism physiological assays with dual transcriptomic analysis of the infected host and parasite. Eelgrass (<em>Zostera marina</em>) shoots were placed in one of two temperature treatments, 11° C or 18° C, acclimated for 10 days, and exposed to a waterborne inoculation containing infectious <em>Labyrinthula zosterae</em> (<em>Lz</em>) or sterile seawater. At two- and five-days post-exposure, pathogen load, visible disease signs, whole leaf phenolic content, and both host- and pathogen- transcriptomes were characterized. Two days after exposure, more than 90% of plants had visible lesions and <em>Lz</em> DNA was detectable in 100% percent of sampled plants in the <em>Lz </em>exposed treatment. Concentrations of total phenolic compounds were lower after 5 days of combined exposure to warmer temperatures and <em>Lz</em>, but were unaffected in other treatments. Concentrations of condensed tannins were not affected by <em>Lz</em> or temperature, and did not change over time. Analysis of the eelgrass transcriptome revealed 540 differentially expressed genes in response to <em>Lz </em>exposure, but not temperature. <em>Lz</em>-exposed plants had gene expression patterns consistent with increased defense responses through altered regulation of phytohormone biosynthesis, stress response, and immune function pathways. Analysis of the pathogen transcriptome revealed up-regulation of genes potentially involved in breakdown of host defense, chemotaxis, phagocytosis, and metabolism. The lack of a significant temperature signal was unexpected, but suggests a more pronounced physiological response to <em>Lz</em> infection as compared to temperature. Pre-acclimation of eelgrass plants to the temperature treatments may have contributed to the limited physiological responses to temperature. Collectively, these data characterize a widespread physiological response to pathogen attack and demonstrate the value of paired transcriptomics to understand infections in a host-pathogen system.