Transcriptional response of Nautella italica R11 towards its macroalgal host uncovers new mechanisms of host-pathogen interaction

Macroalgae (seaweeds) are essential for the functioning of temperate marine ecosystems, but there is increasing evidence to suggest that their survival is under threat from anthropogenic stressors and disease. Nautella italica R11 is recognized as an aetiological agent of bleaching disease in the red alga, Delisea pulchra. Yet, there is a lack of knowledge surrounding the molecular mechanisms involved in this model host-pathogen interaction. Here we report that mutations in the gene encoding for a LuxR-type quorum sensing transcriptional regulator, RaiR, render N.italica R11 avirulent, suggesting this gene is important for regulating the expression of virulence phenotypes. Using an RNA sequencing approach, we observed a strong transcriptional response of N.italica R11 towards the presence of D.pulchra. In particular, genes involved in oxidative stress resistance, carbohydrate and central metabolism were upregulated in the presence of the host, suggesting a role for these functions in the opportunistic pathogenicity of N.italica R11. Furthermore, we show that RaiR regulates a subset of genes in N.italica R11, including those involved in metabolism and the expression of phage-related proteins. The outcome of this research reveals new functions important for virulence of N.italica R11 and contributes to our greater understanding of the complex factors mitigating microbial diseases in macroalgae.