Complex interactions of temperature, light and tissue damage on seagrass wasting disease in Zostera marina

<b>Abstract</b>The temperate seagrass species eelgrass <i>Zostera marina</i> can be infected by the wasting disease pathogen <i>Labyrinthula zosterae, </i>which is believed to have killed about 90% of the seagrass in the Atlantic Ocean in the 1930s. It is not known why this opportunistic pathogen sometimes becomes virulent, but the recurrent outbreaks may be due to a weakening of the <i>Z. marina</i> plants from adverse environmental changes.This study investigated the individual and interactive effects of multiple extrinsic factors (temperature, light, and tissue damage) on the host-pathogen interaction between <i>Z. marina</i> and <i>L. zosterae</i> in a fully crossed infection experiment. The degree of infection was measured as both lesion coverage and<i> L. zosterae</i> cell concentration. We also investigated if the treatment factors affect the chemical defense of the host, measured as the inhibitory capacity of seagrass extracts in bioassays with <i>L. zosterae</i>. Finally, gene expression of a set of targeted genes was quantified in order to investigate how the treatments change <i>Z. marina</i>’s response to infection.Light had a pronounced effect on <i>L. zosterae</i> infection measured as lesion coverage, where reduced light conditions increased lesions by 35%. The response to light on <i>L. zosterae</i> cell concentration was more complex and showed significant interaction with the temperature treatment. Cell concentration was also significantly affected by physical damage, where damage surprisingly resulted in a reduced cell concentration of the pathogen. No treatment factor caused detectable decrease in the inhibitory capacity of the seagrass extracts. There were several interactive effects between <i>L. zosterae</i> infection and the treatment factors on <i>Z. marina</i> growth, and on the expression of genes associated with immune defense, phenol synthesis and primary metabolism, showing that the molecular reaction towards <i>L. zosterae</i> infection depends on prevailing environmental conditions. Our study shows that individual or interactive effects of light, temperature and tissue damage can affect multiple aspects of host-pathogen interactions in seagrass. These results highlights the complexity of marine host-pathogen systems, showing that more multi-factorial investigations are needed to gain a better understanding of disease in marine plants under different environmental conditions.