CRISPR-Cas9 knockout of IBH1 fine-tunes brassinosteroid metabolism and modulates plant architecture and grain traits in durum wheat.

Brassinosteroids (BRs) are essential steroidal plant hormones that regulate cell elongation, vascular differentiation and reproductive development, thereby shaping plant architecture and grain yield. Fine-tuning BR signalling, rather than altering hormone accumulation, has been proposed as a potential strategy to regulate plant development. Here we investigated the role of IBH1 transcription factor, a negative regulator within the ILI1/PRE1-IBH1 module controlling BR-mediated cell elongation in durum wheat (Triticum turgidum ssp. durum), using a CRISPR-Cas9-mediated knockout approach. IBH1 loss of function was associated with transcriptional changes consistent with enhanced BR signalling, including the coordinated upregulation of key signalling components (BRI1 and BZR1) and downstream regulators (ILI1 and PRE1). Metabolic profiling revealed selective remodelling of BR biosynthesis, characterized by differential accumulation of specific pathway intermediates rather than a general increase in BR content. Phenotypic analyses showed that TdIBH1 mutants exhibited increased plant height, internode elongation, leaf inclination, grain size, and thousand grain weight, while no significant differences in fertility were detected. These findings support a role for TdIBH1 as a conserved negative regulator of BR-mediated growth in durum wheat and suggest that the manipulation of the ILI1/PRE1-IBH1 module may provide a potential strategy to modulate plant architecture and grain-related traits in cereal crops.