Overexpression of the bacterial tryptophan oxidase RebO affects auxin biosynthesis and Arabidopsis development

Both tryptophan (Trp) and auxin are essential for plant growth and Trp is a precursor for auxin biosynthesis. Concentrations of Trp and auxin need to be tightly controlled to ensure optimal growth and development. It has been very difficult to study the homeostasis of these two essential and inter-dependent compounds. Auxin is mainly synthesized from Trp via a two-step pathway using indole-3-pyruvate (IPA) as the intermediate. Here we used a bacterial Trp oxidase RebO, which does not exist in Arabidopsis and which converts Trp to the imine form of IPA, to modulate IPA levels in Arabidopsis. Our results demonstrate that Arabidopsis plants use two strategies to ensure that no excess IPA is made from Trp. IPA is made from Trp by the TAA family of aminotransferases, which we show catalyzes the reverse reaction when IPA level is high. Moreover, excess IPA is converted back to Trp by the VAS1 aminotransferase. We show that the VAS1-catalyzed reaction is very important for Trp homeostasis. This work not only elucidates the intricate biochemical mechanisms that control the homeostasis of Trp, IPA, and auxin, but also provides novel tools for further biochemical studies on Trp metabolism and auxin biosynthesis in plants.