Alkaloids produced by a defensive symbiont may have evolved from a plant stress metabolite
J. PAN (1), M. Bhardwaj (1), R. B. Grossman (1), C. L. Schardl (1). (1) University of Kentucky, Lexington, KY, U.S.A.
Grasses endophyte (<i>Epichloë</i> and <i>Neotyphodium</i> spp.) contributes to host defense through the production of a suite of alkaloids. The loline alkaloids are broad-spectrum insecticidal agents that accumulate abundantly in the symbiotic host. Their biosynthesis is directed by <i>LOL</i> cluster of up to 11 genes. A proposed biosynthetic pathway starts from condensation of proline and homoserine. After decarboxylation, PLP-enzymes, LolD and LolT, catalyze ring closure reactions to give the pyrrolizidine ring of lolines. Elimination of the <i>lolD</i> gene supported this hypothesis. The mutant failed to produce any detectible lolines when it was introduced back into the host. When the grass harboring the mutant endophyte was fed with 1,5-diazabicyclo[4.3.0]nonane (DABN), a small amount of <i>N</i>-formylloline, the typical pathway end product, was consistently observed. This indicated that the endophyte could take up DABN imbibed by the grass and the other loline biosynthetic enzymes could accomplish the subsequent steps. Interestingly, DABN is also a plant metabolite produced by the action of an apoplastic polyamine oxidase known only in grasses. This interesting coincidence and the observation that DABN chemically complements the fungal <i>lolD</i> mutant suggested an intriguing hypothesis: lolines may have originated by fungal transformation of a grass apoplast metabolite into potent plant-defense compounds. This hypothesis also has great implications for the evolution of the <i>LOL</i> cluster.
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