Ii WRKY34 Regulates Plant Root Growth Through a Direct Interaction With NAC1 and Its Interactor NAC18 in Isatis indigotica

ABSTRACT WRKY34 transcription factor has a substantial role in inducing the accumulation of bioactive lignan compounds, improving the biomass of hairy roots, and increasing stress tolerance in Isatis indigotica . However, the precise role of the Ii WRKY34 transcriptional network in modulating root growth remains to be elucidated. Here, we observed that heterogeneous expression of Ii WRKY34 increased the primary root length in Arabidopsis thaliana . Ii WRKY34 bound directly to the promoter of Ii NAC1, which itself bound to the promoter of an auxin biosynthesis gene, Ii YUCCA6. Molecular docking displayed the complex of Ii WRKY34‐DBD and the DNA duplex of W‐box on the promoter. Additionally, an interaction occurred between Ii NAC1 and Ii NAC18, and heterogeneous expression of either transcription factor resulted in the same phenotype as Ii WRKY34‐expressing lines of A. thaliana . Thus, we believe that Ii WRKY34 functions as an upstream trigger of auxin signaling during root growth. The resulting model clarifies the molecular mechanism of Ii WRKY34‐regulated root growth, which may have applications in enhancing agricultural yields of I. indigotica and possibly other plants in the family Brassicaceae.