Cooperative functioning between phenylalanine ammonia lyase and isochorismate synthase activities contributes to salicylic acid biosynthesis in soybean

<jats:title>Summary</jats:title><jats:p> <jats:list list-type="bullet"> <jats:list-item><jats:p>Salicylic acid (<jats:styled-content style="fixed-case">SA</jats:styled-content>), an essential regulator of plant defense, is derived from chorismate via either the phenylalanine ammonia lyase (<jats:styled-content style="fixed-case">PAL</jats:styled-content>) or the isochorismate synthase (<jats:styled-content style="fixed-case">ICS</jats:styled-content>) catalyzed steps. The <jats:styled-content style="fixed-case">ICS</jats:styled-content> pathway is thought to be the primary contributor of defense‐related <jats:styled-content style="fixed-case">SA</jats:styled-content>, at least in Arabidopsis. We investigated the relative contributions of <jats:styled-content style="fixed-case">PAL</jats:styled-content> and <jats:styled-content style="fixed-case">ICS</jats:styled-content> to defense‐related <jats:styled-content style="fixed-case">SA</jats:styled-content> accumulation in soybean (<jats:italic>Glycine max</jats:italic>).</jats:p></jats:list-item> <jats:list-item><jats:p>Soybean plants silenced for five <jats:styled-content style="fixed-case">PAL</jats:styled-content> isoforms or two <jats:styled-content style="fixed-case">ICS</jats:styled-content> isoforms were analyzed for <jats:styled-content style="fixed-case">SA</jats:styled-content> concentrations and <jats:styled-content style="fixed-case">SA</jats:styled-content>‐derived defense responses to the hemibiotrophic pathogens <jats:italic>Pseudomonas syringae</jats:italic> and <jats:italic>Phytophthora sojae</jats:italic>.</jats:p></jats:list-item> <jats:list-item><jats:p>We show that, unlike in Arabidopsis, <jats:styled-content style="fixed-case">PAL</jats:styled-content> and <jats:styled-content style="fixed-case">ICS</jats:styled-content> pathways are equally important for pathogen‐induced <jats:styled-content style="fixed-case">SA</jats:styled-content> biosynthesis in soybean. Knock‐down of either pathway shuts down <jats:styled-content style="fixed-case">SA</jats:styled-content> biosynthesis and abrogates pathogen resistance. Moreover, unlike in Arabidopsis, pathogen infection is associated with the suppression of <jats:italic><jats:styled-content style="fixed-case">ICS</jats:styled-content></jats:italic> gene expression. Pathogen‐induced biosynthesis of <jats:styled-content style="fixed-case">SA</jats:styled-content> via the <jats:styled-content style="fixed-case">PAL</jats:styled-content> pathway correlates inversely with phenylalanine concentrations. Although infections with either virulent or avirulent strains of the pathogens increase <jats:styled-content style="fixed-case">SA</jats:styled-content> concentrations, resistance protein‐mediated response to avirulent <jats:italic>P. sojae</jats:italic> strains may function in an <jats:styled-content style="fixed-case">SA</jats:styled-content>‐independent manner.</jats:p></jats:list-item> <jats:list-item><jats:p>These results show that <jats:styled-content style="fixed-case">PAL</jats:styled-content>‐ and <jats:styled-content style="fixed-case">ICS</jats:styled-content>‐catalyzed reactions function cooperatively in soybean defense and highlight the importance of <jats:styled-content style="fixed-case">PAL</jats:styled-content> in pathogen‐induced <jats:styled-content style="fixed-case">SA</jats:styled-content> biosynthesis.</jats:p></jats:list-item> </jats:list> </jats:p>