Brassinosteroids promote AUX/IAA transcriptional repressors and inhibit adventitious root formation in mung bean

Abstract

Adventitious root (AR) formation is important in vegetative propagation of horticultural and forestry crops. It is induced by wounding and controlled by several phytohormones, including auxin. Brassinosteroids (BR) are plant steroid hormones that regulate several aspects of plant growth and development and interact with auxin. Here, we investigated roles of BR in AR formation in mung bean hypocotyl cuttings using explants treated with exogenous BR. Our results demonstrated that BR treatments suppressed AR formation in a dose-dependent manner. Expression analysis of genes involved in auxin signaling and transport showed that BR up-regulated several genes encoding AUX/IAA transcriptional repressors, but not an auxin transporter PIN1. Analyses of activities of polyphenol oxidase (PPO), which plays roles in root primordia formation, suggested that BR may inhibit AR formation partly by reducing PPO activity in the hypocotyl cuttings. Our findings revealed roles of BR in promoting transcription of negative regulators of the auxin signaling pathway, thus blocking auxin-mediated AR formation.