Seagrasses are susceptible to attack by net-forming protists within the genus Labyrinthula (Phylum Labyrinthulomycota). These ubiquitous, largely saprophytic or weakly parasitic organisms can periodically become virulent pathogens, causing the seagrass wasting disease and contributing to sudden population declines. These scientists suggest that differences in susceptibility of seagrass populations to Labyrinthula attack arise as changing environmental conditions alter the ability of seagrasses to maintain or develop disease resistance by the production of phenolic compounds via the shikimic acid /phenylpropenoid (SA/PP) pathway. The concentrations of phenolic compounds in seagrass leaves and shoots are the only characteristics that have been linked with resistance to the wasting disease. Previous studies indicate that, for certain species, disease resistance is correlated with the content of phenolic acids and other polymeric phenolics, that phenolic production can be induced by Labyrinthula attack and/or wounding, and that seagrass phenolics inhibit the growth of this pathogen in culture. While the evidence for a link between seagrass phenolics and disease resistance is correlative and comes from relatively few studies on a limited number of species, it indicates that seagrasses, like all terrestrial plants examined to date, exhibit induced defenses against pathogen attack. The evidence also suggests that seagrass responses involve activation of the SA/PP pathway leading to the production of plant phenolics. However, the regulation of the SA/PP pathway has not yet been examined in seagrasses, and we do not fully understand how environmental factors may affect disease resistance in seagrasses. Drs. Arnold, Boettcher and Tanner will investigate the regulation of the SA/PP pathway and the resistance of seagrasses to infection by Labyrinthula. Specifically, the goals of the research are: (1) to test the hypothesis that key SA/PP pathway enzymes are induced by Labyrinthula attack, (2) to determine which phenolics accumulate in response to SA/PP pathway induction, (3) to test, in vivo, the link between phenolic acid / condensed tannin levels and seagrass resistance, (4) to determine whether SA/PP pathway induction and the accumulation of phenolic compounds are affected by specific environmental conditions, and (5) to test the proposed link between these conditions, altered SA/PP metabolism and changes in seagrass resistance to Labyrinthula attack. These questions will be addressed in mesocosm populations of two species, the tropical/subtropical seagrass Thalassia testudinum and the temperate seagrass Zostera marina, both of which are subject to periodic wasting disease outbreaks and exhibit induced production of phenolic compounds in response to pathogen attack and/or wounding. Activities of SA/PP pathway enzymes, concentrations of phenolic acids and polymeric phenols, and indicators of physiological health (growth, photosynthesis, concentrations of carbohydrates and proteins) will be examined in infected and pathogen-free plants. They will also compare the responses of these seagrasses to infection under different environmental conditions. Finally, artificial inhibitors of specific SA/PP pathway enzymes will be used to determine the ability of plants to resist Labyrinthula attack when they are unable to produce SA/PP phenolics.
