Cytokinin-activated DIRIGENT13 promotes lignan synthesis to regulate root growth and abiotic stress tolerance by modulating ROS homeostasis in Arabidopsis

Dirigent (DIR) proteins mediate regio- and stereoselectivity of phenoxy radical coupling of monolignols for (neo)lignan synthesis. However, the role of DIR proteins and lignans in plant development and stress responses remains elusive. Here, we show that Arabidopsis DIRIGENT13 (DIR13) is a direct target of cytokinin signaling, being strongly upregulated via binding of cytokinin-responsive ARR1, ARR10 and ARR12 to the DIR13 promoter. DIR13 was localized to the root endodermis and at the margins of lateral root primordia and lateral roots. While not necessary for Casparian strip formation, DIR13 promoted both main and lateral root growth. Untargeted metabolomics and imaging mass spectrometry analyses unveiled the role of DIR13 in facilitating (neo)lignan synthesis in primary and lateral roots. DIR13 attenuated salt stress-mediated inhibition of germination and root growth. Moreover, DIR13 activated reactive oxygen species (ROS) accumulation both under control and salt stress conditions, and cytokinin further enhanced the salt-induced ROS production, possibly via salt- and cytokinin-mediated synergistic effects on transcriptional activation of DIR13. Our results suggest the ability of lignans to affect root growth, possibly via disturbing ROS homeostasis through their ROS-scavenging activity. Overall, we uncover a role for DIR13-produced lignans in the ROS-regulated abiotic stress tolerance and the control of root architecture.