Formation and exudation of non-volatile products of the arabidiol triterpenoid degradation pathway in <i>Arabidopsis</i> roots

Triterpenoids produced by plants play important roles in the protection against biotic stress. Roots of <i>Arabidopsis thaliana</i> produce different triterpenoids, which include the tricyclic triterpene diol, arabidiol. In a degradation reaction induced by infection with the oomycete pathogen, <i>Pythium irregulare</i>, arabidiol is cleaved to the 11-carbon volatile homoterpene, (<i>E</i>)-4,8-dimethyl-1,3,7-nonatriene (DMNT), and the 19-carbon ketone, apo-arabidiol. The arabidiol pathway and its volatile breakdown product DMNT have been implicated in the defense against <i>P. irregulare</i> infection. Here we show that the non-volatile breakdown product apo-arabidiol is further converted to the acetylated derivative α−14-acetyl-apo-arabidiol via a presumed epimerization and subsequent acetylation reaction. α−14-acetyl-apo-arabidiol and the detected intermediates in the derivatization pathway are partially exuded from the root indicating possible defensive activities of these molecules in the rhizosphere. The conversion steps of apo-arabidiol vary among different <i>Arabidopsis</i> accessions and are present in only rudimentary form in the close relative <i>Arabidopsis lyrata</i>, which supports an intra- and inter-specific modularity in triterpenoid metabolism.