Flavonols regulate root hair development by modulating accumulation of reactive oxygen species in the root epidermis

Reactive oxygen species (ROS) are signaling molecules produced by tissue-specific respiratory burst oxidase homolog (RBOH) enzymes to drive development. In Arabidopsis thaliana , ROS produced by RBOHC was previously reported to drive root hair elongation. We identified a specific role for one ROS, H 2 O 2 , in driving root hair initiation and demonstrated that localized synthesis of flavonol antioxidants control the level of H 2 O 2 and root hair formation. Root hairs form from trichoblast cells that express RBOHC and have elevated H 2 O 2 compared with adjacent atrichoblast cells that do not form root hairs. The flavonol-deficien 4 mutant has elevated ROS in trichoblasts and elevated frequency of root hair formation compared with the wild type. The increases in ROS and root hairs in 4 are reversed by genetic or chemical complementation. Auxin-induced root hair initiation and ROS accumulation were reduced in an rbohc mutant and increased in 4 , consistent with flavonols modulating ROS and auxin transport. These results support a model in which localized synthesis of RBOHC and flavonol antioxidants establish patterns of ROS accumulation that drive root hair formation.