Effector Repertoire and Host Transcriptomic Responses in the Tripartite Interaction Between the bloom-forming diatom Coscinodiscus granii , the oomycete Lagenisma coscinodisci and Co-occurring Pedinellale Pteridomonas

Parasitic interactions play a central role in shaping phytoplankton community dynamics. Understanding the host response is crucial to fully comprehend these interactions. In this article, we investigate the tripartite interaction between the bloom-forming centric diatom Coscinodiscus granii , its oomycete parasite Lagenisma coscinodisci , and a co-occurring heterotrophic flagellate ( Pteridomonas sp.).

A major challenge in infection transcriptomics is that cells are often at different infection stages. To overcome this, we manually sorted C. granii cells at comparable infection stages to perform transcriptomic analyses. We assembled the first transcriptomes of both C. granii and L. coscinodisci . Our analysis reveals a sophisticated effector repertoire in L. coscinodisci , including canonical oomycete virulence factors such as Crinklers, RxLR effectors, cystatins, and transposon-associated proteins, as well as components of the RNAi machinery (Argonaute, Dicer, RdRP) and cyclophilins.

Notably, C. granii exhibits a transcriptional response involving proteases and exosome-related pathways, suggesting a deeply conserved, possibly ancient, mechanism of defense. In parallel, we analyzed the differential expression of Pteridomonas , which consistently co-occurred in culture, and identified a distinct transcriptional profile characterized by the upregulation of motility genes, highlighting a striking mobility strategy.

Thanks to the exceptionally large host size, together with available transcriptomic and metabolomic data, this tripartite system provides a unique marine model to study oomycete-diatoms interactions.