Karrikin signaling pathway regulates shoot gravitropism and tiller angle in rice

The tiller angle, shaped by shoot gravitropism, is a key feature of plant architecture. Plant hormones control a range of physiological processes that collectively determine the tiller angle. Karrikins (KARs) are bioactive butenolide compounds identified in smoke that regulate various processes of plant development. KARs, which are analogs of the proposed KARRIKIN INSENSITIVE2 (KAI2) ligand (KL), initiate downstream signaling events that closely resemble the pathway activated by another class of butenolide plant hormones known as strigolactones (SL). In rice, KAR/KL signaling has been demonstrated to influence mesocotyl elongation and arbuscular mycorrhizal symbiosis; however, its role in regulating tiller angle remains to be elucidated. In this study, we demonstrate that the d14l ( d14 - like ) mutant exhibits a smaller tiller angle, while the smax1 ( suppressor of max2 1 ) mutant and the d14l/smax1 double mutants display a larger tiller angle. Consistently, the d14l mutant shows an enhanced shoot gravitropic response, whereas the smax1 mutant and the d14l/smax1 double mutant exhibit a reduced response. The activation of KAR/KL signaling attenuates the shoot gravitropic response primarily by suppressing gravity-responsive genes. Moreover, we found that the KAR/KL signaling pathway influences shoot gravitropism by regulating the expression of LAZY1 ( LA1 ), a key regulator of tiller angle and shoot gravitropism. Together, this study reveals a novel molecular mechanism through which KAR/KL signaling controls plant architecture.