Zinc Finger of Arabidopsis Thaliana 6 is Involved in Melatonin-mediated Auxin Signaling Through Interacting INDETERMINATE DOMAIN15 and INDOLE-3-ACETIC ACID 17

Although accumulating evidence demonstrates the cross talk between melatonin and auxin as derivatives of tryptophan, the underlying signaling events remain unclear. In this study, we found that melatonin and auxin mediated the transcriptional levels of zinc finger of Arabidopsis thaliana (ZAT6) in a mutually antagonistic manner. ZAT6 negatively modulated the endogenous auxin level, and ZAT6 knockdown plants were less sensitive to melatonin-regulated auxin biosynthesis, indicating its involvement in melatonin-mediated auxin accumulation. Additionally, the identification of INDETERMINATE DOMAIN15 (IDD15) and INDOLE-3-ACETIC ACID 17 (IAA17) in Arabidopsis that interacted with ZAT6 in vivo provided new insight of ZAT6-mediated auxin signaling. Further investigation showed that ZAT6 repressed the transcription activation of IDD15 on the YUC2 promoter, while ZAT6 inhibited the interaction of TRANSPORT INHIBITOR RESPONSE 1 (TIR1) and IAA17 through competitively binding to IAA17. Thus, both auxin synthesis and the auxin response were negatively modulated by ZAT6. Taken together, ZAT6 is involved in melatonin-mediated auxin signaling through forming an interacting complex of auxin signaling pathway in Arabidopsis.