Inferring Roles in Defense from Metabolic Allocation of Rice Diterpenoids

Abstract

<p>Among their responses to microbial infection, plants deploy an arsenal of natural antibiotic products. Historically these have been identified on the basis of their antibiotic activity in vitro, which leaves open the question of their relevance to defense in planta. The vast majority of such natural products from the important crop plant rice (<em>Oryza sativa</em>) are diterpenoids whose biosynthesis proceeds via either <em>ent</em>- or <em>syn</em>-copalyl diphosphate (CPP) intermediates, which were isolated on the basis of their antibiotic activity against the fungal blast pathogen <em>Magnaporthe oryzae</em>. However, rice plants in which the gene for the <em>syn</em>-CPP synthase <em>Os-CPS4</em> is knocked out do not exhibit increased susceptibility to <em>M. oryzae</em>. Here, we show that knocking out or knocking down <em>Os-CPS4</em> actually decreases susceptibility to the bacterial leaf blight pathogen <em>Xanthomonas oryzae</em>. By contrast, genetic manipulation of the gene for the <em>ent</em>-CPP synthase <em>Os-CPS2</em> alters susceptibility to both <em>M. oryzae</em> and <em>X. oryzae</em>. Despite the secretion of diterpenoids dependent on <em>Os-CPS2</em> or <em>Os-CPS4</em>from roots, neither knockout exhibited significant changes in the composition of their rhizosphere bacterial communities. Nevertheless, rice plants allocate substantial metabolic resources toward <em>syn</em>- as well as <em>ent</em>-CPP derived diterpenoids upon infection/induction. Further investigation revealed that <em>Os-CPS4</em> plays a role in fungal non-host disease resistance. Thus, examination of metabolic allocation provides important clues into physiological function.</p>