An evolutionarily ancient Fatty Acid Desaturase is required for the synthesis of hexadecatrienoic acid, which is the main source of the bioactive jasmonate in Marchantia polymorpha

Jasmonates are fatty acid derived hormones that regulate multiple aspects of plant development, growth and stress responses. Bioactive jasmonates differ between vascular plants and bryophytes (using jasmonoyl-L-isoleucine; JA-Ile and dinor-12-oxo-10,15(Z)-phytodienoic acid; dn-OPDA, respectively), but bind an evolutionarily conserved COI1 receptor. Whilst the biosynthetic pathways of JA-Ile in the model vascular plant Arabidopsis thaliana have been elucidated, the details of dn-OPDA biosynthesis in bryophytes are still unclear. Here, we identify an ortholog of Arabidopsis Fatty Acid Desaturase 5 (AtFAD5) in the model liverwort Marchantia polymorpha and show that FAD5 function is ancient and conserved between species separated by more than 450 million years of independent evolution. Similar to AtFAD5, MpFAD5 is required for the synthesis of 7Z-hexadecenoic acid. Consequently, in Mpfad5 mutants the hexadecanoic pathway is blocked, the dn-OPDA levels almost completely depleted and normal chloroplast development is impaired. Our results demonstrate that the main source of dn-OPDA in Marchantia is the hexadecanoic pathway and the contribution of the octadecanoid pathway, i.e. from OPDA, is minimal. Remarkably, despite extremely low levels of the bioactive hormone (dn-OPDA), MpCOI1-mediated responses to wounding and insect feeding can still be activated in Mpfad5 mutants, suggesting that dn-OPDA is not the only bioactive jasmonate and COI1 ligand in Marchantia. Transcriptomic analysis of Mpfad5 mutants 4 replicates Mpfad5 (Experiment) vs Tak-1 (Control)