Brassinosteroid (BR) alleviates Al toxicity in rice (Oryza sativa) through a gibberellic acid (GA) dependent pathway

Aluminum (Al) toxicity in acidic soils significantly limits crop productivity by inhibiting root growth and nutrient uptake. Brassinosteroid (BR), a key plant hormone, has emerged as a crucial regulator of stress tolerance. This study investigates the role of BR in mitigating Al toxicity in rice ( Oryza sativa ) and explores the interplay between BR and gibberellic acid (GA) under Al stress. Our results show that Al stress rapidly elevates endogenous BR level in rice roots, which alleviates Al-induced root growth inhibition. Specifically, 0.01 μM BR achieves a maximum remission rate of approximately 20% for Al-induced root growth inhibition. BR application significantly reduced Al accumulation in root tips (by approximately 25%) and cell walls, particularly by inhibiting the overexpression of XYLOGLUCAN ENDO TRANSHYDROLASE/GLUCOSYLASE 5/9/11/19 ( OsXTH5/9/11/19 ), which reduces Al binding in hemicellulose fractions and upregulating Cellulose synthase A catalytic subunit 4/7/9 ( OsCesA4/7/9 ) to enhance cell wall’s barrier function against Al. BR also modulates hemicellulose composition and suppresses the expression of the Al transporter gene Nramp aluminum transporter 1 ( OsNRAT1 ), thereby reducing Al uptake into root tip cells. Additionally, BR mitigates Al toxicity via the GSK3/SHAGGY-Like KINASE 2 ( OsGSK2 )-mediated BR signaling pathway: OsGSK2 overexpression lines ( OsGSK2-OE ) exhibit enhanced Al resistance, while RNAi lines ( OsGSK2-RNAi ) show increased Al sensitivity. Furthermore, BR upregulates key expansin genes ALPHA-EXPANSIN 1/4/7 ( OsEXPA1/4/7 ) and BETA-EXPANSIN 2 ( OsEXPB2 ), which promotes root growth under Al stress. Notably, under Al stress, BR enhances endogenous GA levels through the transcription factor BRASSINAZOLE RESISTANT 1 ( OsBZR1 ): Al represses OsBZR1 expression, which is rescued by BR treatment. This process involves upregulating GA biosynthetic genes such as OsGA20OX , restoring Al-suppressed gene expression, and mitigating Al-induced OsGA2OX overexpression to maintain GA homeostasis. Exogenous GA treatment mimics BR’s alleviatory effects, highlighting critical crosstalk between the BR and GA pathways. This study provides novel insights into BR-mediated Al detoxification, revealing a key BR-GA interaction that enhances Al tolerance in rice and offers potential for improving crop resilience in acidic soils. • BR alleviates Al-induced root inhibition and reduces Al accumulation in rice roots and cell walls. • BR modifies cell wall composition via OsXTHs , OsCesAs , and OsNRAT1 to restrict Al uptake and binding. • BR signals through OsGSK2 and upregulates expansins to promote root growth under Al stress. • BR maintains GA homeostasis via OsBZR1 , revealing a key BR-GA crosstalk for rice Al tolerance.