TCP13 promotes shade avoidance syndrome-like responses by directly targeting a subset of shade-responsive gene promoters.

TCP13 belongs to a TCP subgroup implicated in the shade avoidance syndrome (SAS), but its exact role remains unclear. Here, we show TCP13 promotes the SAS-like response by enhancing hypocotyl elongation and suppressing flavonoid biosynthesis as a part of the incoherent feed-forward loop in light signaling. Shade is known to promote the SAS by activating PIF-auxin signaling in plants, but we found no evidence in a transcriptome analysis that TCP13 activates PIF-auxin signaling. Instead, we found TCP13 mimics shade by activating the expression of a subset of shade-inducible and cell elongation-promoting SAUR genes including SAUR19, via direct targeting of their promoters. We also found TCP13 and PIF4, a molecular proxy for shade, repress the expression of flavonoid biosynthetic genes by directly targeting both shared and distinct sets of biosynthetic gene promoters. Together, our results indicate TCP13 promotes the SAS-like response by directly targeting a subset of shade-responsive genes without activating the PIF-auxin signaling pathway.