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

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.

Adverse Environmental Perturbations May Threaten Kelp Farming Sustainability by Exacerbating Enterobacterales Diseases

“…68,69 Laboratory experiments have demonstrated that extracellular compounds from the Pseudoalteromonas strain induce similar defense responses leading to programmed cell death, and disease development in S. japonica. 70 On the dominance of Enterobacterales in diseased kelp, Enterobacterales, particularly the MAGs of Pseudoalteromonas, were found to harbor a repertoire of genotypic determinants encoding pathogen-associated factors such as chemotaxis, motility, secretion systems, and pathogenic biofilm formation (Supporting Information). Although these traits are not directly related to bacterial pathogenesis, they are known to aid pathogenic bacteria in manipulating host cell signaling, evading the host immune system, and strengthening their adherence to host cells and tissues, which is a critical step in establishing an infection.…”

“…However, the outcome and induction of the disease depend on the strength of the interactions between the host, pathogen, and environmental stressors. , Laboratory experiments have demonstrated that extracellular compounds from the Pseudoalteromonas strain induce similar defense responses leading to programmed cell death, and disease development in S. japonica …”

Assessing the Plastisphere from Floating Plastics in the Northwestern Mediterranean Sea, with Emphasis on Viruses

“…Pseudoalteromonas piscicida , another highly abundant bacteria found in the sampled mesoplastics from this study, was previously identified as a pathogen that causes bleaching disease in commercially farmed brown algae Saccharina japonica [ 103 ]. In addition, Planctomycetes, traditionally recognized as an environmental bacteria, have recently been linked to human pathology as opportunistic pathogens, arousing a great interest for clinical microbiologists [ 104 ].…”