IiWRKY34 Regulates Plant Root Growth Through a Direct Interaction With NAC1 and Its Interactor NAC18 in Isatis indigotica.

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 IiWRKY34 transcriptional network in modulating root growth remains to be elucidated. Here, we observed that heterogeneous expression of IiWRKY34 increased the primary root length in Arabidopsis thaliana. IiWRKY34 bound directly to the promoter of IiNAC1, which itself bound to the promoter of an auxin biosynthesis gene, IiYUCCA6. Molecular docking displayed the complex of IiWRKY34-DBD and the DNA duplex of W-box on the promoter. Additionally, an interaction occurred between IiNAC1 and IiNAC18, and heterogeneous expression of either transcription factor resulted in the same phenotype as IiWRKY34-expressing lines of A. thaliana. Thus, we believe that IiWRKY34 functions as an upstream trigger of auxin signaling during root growth. The resulting model clarifies the molecular mechanism of IiWRKY34-regulated root growth, which may have applications in enhancing agricultural yields of I. indigotica and possibly other plants in the family Brassicaceae.