Martin Mascher

The restriction of plant‐symbiont dinitrogen fixation by an insect semiochemical had not been previously described. Here we report on a glycosylated triketide δ‐lactone from Nephrotoma cornicina crane flies, cornicinine, that causes chlorosis in the floating‐fern symbioses from the genus Azolla . Only the glycosylated trans‐A form of chemically synthesized cornicinine was active: 500 nM cornicinine in the growth medium turned all cyanobacterial filaments from Nostoc azollae inside the host leaf‐cavities into akinetes typically secreting CTB‐bacteriocins. Cornicinine further inhibited akinete germination in Azolla sporelings, precluding re‐establishment of the symbiosis during sexual reproduction. It did not impact development of the plant Arabidopsis thaliana or several free‐living cyanobacteria from the genera Anabaena or Nostoc but affected the fern host without cyanobiont. Fern‐host mRNA sequencing from isolated leaf cavities confirmed high NH 4 ‐assimilation and proanthocyanidin biosynthesis in this trichome‐rich tissue. After cornicinine treatment, it revealed activation of Cullin‐RING ubiquitin‐ligase‐pathways, known to mediate metabolite signaling and plant elicitation consistent with the chlorosis phenotype, and increased JA‐oxidase, sulfate transport and exosome formation. The work begins to uncover molecular mechanisms of cyanobiont differentiation in a seed‐free plant symbiosis important for wetland ecology or circular crop‐production today, that once caused massive CO 2 draw‐down during the Eocene geological past.