Response of the Arabidopsis indolic secondary metabolite network to infection with Colletotrichum higginsianum

In plants, the indolic amino acid tryptophan (Trp) serves as the precursor for the synthesis of a plethora of indolic secondary metabolites with allelopathic activity against microbes or toxicity against herbivores. In the cruciferous model plant Aradidopsis, indolic glucosinolates, the major phytoalexin camalexin, carbonyl nitriles and indole carboxylic acids are abundant products of the branched biosynthetic pathway that originates from the Trp oxidation product indole-acetaldoxime (IAOx). To date, it has not been intensely investigated, (i) how the Arabidopsis indolic metabolic network responds to fungal infection and (ii) which indolic metabolites play a role for compatibility upon fungal attack. To provide a systematic case study, we have employed a combination of single, double, triple and quadruple Arabidopsis mutants lacking selected combinations of indolic metabolites for leaf infections with the hemibiotrophic ascomycete <i>Colletotrichum higginsianum</i>. Our study revealed that only camalexin, but neither indolic glucosinolates nor the recently described phytoalexin 4-hydroxy-carbonyl nitrile (4-OH-ICN) had a significant role for the resistance towards <i>C. higginsianum</i>. Besides its known relevance during the late necrotrophic phase, our data suggest a role of camalexin in early post-penetration defense. Our study also indicates that, indole acetonitrile (IAN) can be produced upon cleavage of indolic glucosinolate by myrosinases in pathogen challenged leaves and feed into camalexin biosynthesis in case further IAN conversion by CYP71B6 is blocked. Downstream of IAN, we found that AAO1 and CYP71B6 act redundantly in the accumulation of indole carboxylic acid (ICOOH). We also revealed that <i>CYP71A12</i> has a stronger contribution to camalexin biosynthesis in <i>C. higginsianum</i> infected leaves than in previously investigated abiotic stress models. While our dataset suggests clear, but subtle differences in the response of the indole metabolic network in pathogen and abiotic challenge, we can rule out a contribution of the fungal pathogen to the observed differences due to our study design.