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.