Light perception in aerial tissues enhances DWF4 accumulation in root tips and induces root growth

Many attempts have been made to characterize the activities of brassinosteroids (BRs), which are important plant hormones. The crosstalk between light perception and the BR signalling pathway has been extensively studied regarding its effects on photomorphogenesis, especially in elongating etiolated hypocotyls. In contrast, how and where the light induces BR biosynthesis remain uncharacterized. DWF4 is one of the main enzymes involved in the BR biosynthesis pathway in Arabidopsis thaliana. We established DWF4-GUS A. thaliana lines in a homozygous dwf4-102 genetic background, but functionally complemented with a genomic DWF4 sequence fused in-frame with a β-glucuronidase (GUS) marker gene. The DWF4-GUS plants enabled the visualization of the accumulation of DWF4 under different conditions. We investigated the effects of aboveground light on root and hypocotyl growth. We observed that root length increased when shoots were maintained under light irrespective of whether roots were exposed to light. We also determined that light perception in aerial tissues enhanced DWF4 accumulation in the root tips. Overall, our data indicate that BR biosynthesis is promoted in the root tip regions by an unknown mechanism in distantly located shoot tissues exposed to light, leading to increased root growth.