Auxin interacts with kynurenine metabolism during organogenesis in Hypericum perforatum L

Recent studies have demonstrated the presence of kynurenine (KYN) and kynurenic acid (KYNA) in several plant species, but their function in plant growth and metabolism remains undefined. We hypothesized that KYN and KYNA are metabolites of auxin and play a role in plant morphogenesis. To test our hypothesis, we developed a plant tissue-culture-based bioassay using Hypericum perforatum (St. John's wort; SJW), a model system for auxin and indoleamine metabolism which uses exogenous treatment with IAA, KYN or pharmacological inhibitors of KYN metabolism in humans (PF- 04859989, PF; RO -61-8048, RO or JM6) to investigate both morphogenetic and metabolic responses of kynurenine metabolism in planta. 10μM KYN supplementation increased root number and internodal length relative to control, and increased concentrations of downstream metabolites KYN, KYNA, and 3-hydroxyanthranilic acid (3-HAA) in both roots and shoots. Application of the inhibitors JM6 and PF which targets KYN monooxygenase (KMO) and KYN aminotransferase, respectively, lead to an increase in the pool of KYN present in tissues, but did not significantly reduce KYNA or 3-HAA. To investigate interactions between auxin and KYN, treatment with exogenously applied IAA with or without the inhibitors was also evaluated. IAA alone, significantly increased KYN and 3-HAA in explants. Co-application of JM6 and IAA partially recovered root organogenesis in explants. IAA + JM6 co-treatment also lead to the highest KYN accumulation in tissues, supporting an interaction between the pathways. Together, these data propose a role for KYN in IAA metabolism, shoot and root organogenesis.