ARF19 acts as a transient auxin response enhancer during root gravitropism

Root gravitropism is regulated by an auxin gradient, causing the root to reorient downwards with gravity. Transcriptional activator auxin response factors (ARFs), such as ARF5, ARF6, ARF7, ARF8, and ARF19, play key roles in auxin signaling in seedling root tissues. ARF7 and ARF19 are both required for root gravitropism, but their regulatory relationship with each other and other activating ARFs remains unknown. Here, we identify that rapid and transient upregulation of ARF19 during gravitropic stimulus is functionally important. ARF7 and ARF8 are required to activate ARF19 upregulation via auxin response elements (AuxREs) in its promoter, while our data indicate that ARF19 does not self-activate. ARF7 can replace ARF19 but not vice versa. Although ARF7 shares overlapping targets with ARF19, they also have distinct functionally important targets, such as ARF19. Our results show that ARF7 and ARF19 have sub-functionalized, with ARF19 acting as an auxin response enhancer for a subset of ARF7 target genes.