Extracellular Compounds from Pathogenic Bacterium Pseudoalteromonas piscicida X-8 Cause Bleaching Disease, Triggering Active Defense Responses in Commercially Farmed Saccharina japonica

SIMPLE SUMMARY: Bleaching disease frequently occurs at the late nursery stage in the farmed brown alga Saccharina japonica with Pseudoalteromonas piscicida X-8 (PpX-8) having been identified as a pathogenic bacterium. However, whether PpX-8 can trigger active defense responses in S. japonica via programmed cell death still remains unknown. In this study, PpX-8 extracellular compounds were more pathogenic (p < 0.05) before being heated, indicating that the virulence factors of PpX-8 may exist in its extracellular compounds and that they are heat-sensitive. Based on the typical morphological changes and biochemical characteristics of 3′-OH ends resulting from DNA cleavage, as well as the enzymatic activity of caspase-3-like protein, PpX-8 extracellular compounds induced programmed cell death and thus triggered active defense responses in farmed S. japonica. The results of this work indicate that seaweeds and higher plants are conservative in their active defense responses when facing pathogenic bacterial infection, and lay the foundation for further investigation of the virulence mechanisms of PpX-8. ABSTRACT: Pathogenic bacteria can trigger active defense responses in higher plants, leading to hypersensitive programmed cell death (PCD) to against those bacteria. However, related research on seaweeds is very limited. Pseudoalteromonas piscicida X-8 (PpX-8) has been identified as the pathogen that causes bleaching disease in commercially farmed Saccharina japonica. In this study, using an inoculation assay and microscopic observations, we found that the proportion of bleaching tissue pieces inoculated with PpX-8 extracellular compounds was significantly higher (p < 0.05) than that inoculated with heated extracellular compounds, indicating that the virulence factors of PpX-8 exist in extracellular compounds and they are heat-sensitive. Using TEM, we observed typical morphological characteristics of PCD after inoculation with extracellular compounds, including chloroplast shrinkage, cytoplasmic vacuolation, and intact mitochondrial structures. Moreover, we detected biochemical characteristics of PCD, such as 3′-OH ends resulting from DNA cleavage and caspase-3-like enzymatic activity, using a TUNEL assay and fluorescence staining. Therefore, PpX-8 extracellular compounds can induce PCD, thus triggering active defense responses in S. japonica. These results indicate that seaweeds and higher plants are conservative in their active defense responses against pathogenic bacteria. The results of this study lay the foundation for further investigation of the virulence mechanisms of PpX-8.