LBD18 uses a dual mode of a positive feedback loop to regulate ARF expression and transcriptional activity in Arabidopsis

Summary A hierarchy of transcriptional regulators controlling lateral root formation in Arabidopsis thaliana has been identified, including the AUXIN RESPONSE FACTOR 7 ( ARF 7 )/ ARF 19‐ LATERAL ORGAN BOUNDARIES DOMAIN 16 ( LBD 16 )/ LBD 18 transcriptional network; however, their feedback regulation mechanisms are not known. Here we show that LBD 18 controls ARF activity using the dual mode of a positive feedback loop. We showed that ARF 7 and ARF 19 directly bind Aux RE in the LBD 18 promoter. A variety of molecular and biochemical experiments demonstrated that LBD 18 binds a specific DNA motif in the ARF 19 promoter to regulate its expression in vivo as well as in vitro . LBD 18 interacts with ARF s including ARF 7 and ARF 19 via the Phox and Bem1 domain of ARF to enhance the transcriptional activity of ARF 7 on Aux RE , and competes with auxin/indole‐3‐acetic acid ( IAA ) repressors for ARF binding, overriding the negative feedback loop exerted by Aux/ IAA repressors. Taken together, these results show that LBD 18 and ARF s form a double positive feedback loop, and that LBD 18 uses the dual mode of a positive feedback loop by binding directly to the ARF 19 promoter and through the protein–protein interactions with ARF 7 and ARF 19. This novel mechanism of feedback loops may constitute a robust feedback mechanism that ensures continued lateral root growth in response to auxin in Arabidopsis.