Phytochrome B inhibits darkness-induced hypocotyl adventitious root formation by stabilizing IAA14 and suppressing ARF7 and ARF19.

Adventitious roots (ARs) are an important root type for plants and display a high phenotypic plasticity in response to different environmental stimuli. Previous studies found that dark-light transition can trigger AR formation from the hypocotyl of etiolated Arabidopsis thaliana, which was used as a model for the identification of regulators of AR biogenesis. However, the central regulatory machinery for darkness-induced hypocotyl AR (HAR) remains elusive. Here, we report that photoreceptors suppress HAR biogenesis through regulating the molecular module essential for lateral roots. We found that hypocotyls embedded in soil or in continuous darkness are able to develop HARs, wherein photoreceptors act as negative regulators. Distinct from wound-induced ARs that require WOX11 and WOX12, darkness-induced HARs are fully dependent on ARF7, ARF19, WOX5/7, and LBD16. Further studies established that PHYB interacts with IAA14, ARF7, and ARF9. The interactions stabilize IAA14 and inhibit the transcriptional activities of ARF7 and ARF19 and thus suppress biogenesis of darkness-induced HARs. This finding not only revealed the central machinery controlling HAR biogenesis but also illustrated that AR formation could be initiated by multiple pathways.