Abstract

SUMMARY Brassinosteroids (BRs) are essential phytohormones that regulate plant growth and development, including hypocotyl elongation in Arabidopsis thaliana . Although the core BR signaling components have been well characterized, the mechanisms that control the nuclear localization and stability of key transcription factors such as BZR1 and BES1 remain less understood. Here, we identify IMPORTIN BETA 4 (IMB4) as a critical regulator of BR-mediated hypocotyl elongation. Loss-of-function imb4-1 mutants exhibit reduced hypocotyl elongation in the dark and display reduced sensitivity to exogenous brassinolide and enhanced sensitivity to the BR biosynthesis inhibitor brassinazole. IMB4 physically interacts with BZR1 and BES1, promotes their nuclear accumulation, and enhances their stability by protecting them from 26S proteasome-mediated degradation. Genetic analysis shows that the bzr1-1D and bes1-1D alleles partially rescue the imb4-1 hypocotyl phenotype, and expression of BR-responsive genes is reduced in imb4-1 but restored by bzr1-1D . Mechanistically, IMB4 interferes with the interaction between BIN2 and BZR1/BES1, thereby inhibiting their degradation. Moreover, IMB4 expression is induced by BR treatment, suggesting a positive feedback loop. Our findings uncover a previously unrecognized role for IMB4 in regulating BR signaling and highlight the importance of nuclear transport machinery in hormone-mediated developmental programs.