A Loss of Robustness Mutant, drmy1, Reveals the Role for Brassinosteroids in Coordinating Sepal Growth. May 23, 2025 – 12:20pm – Plant Science 404. 

 Arabidopsis sepals must grow in a coordinated and robust fashion to a consistent size and shape to close and protect the developing flower bud. To understand how this robust coordination occurs, we use the loss of robustness mutant drmy1 which has variable sepal initiation and growth, causing failure of the sepals to close the flower bud. Early outgrowth defects in drmy1 lead to a large difference in the sizes of different sepals within individual flower buds. Using single cell RNA-seq, we found changes in expression of genes related to a class of plant hormones known as brassinosteroids, which play roles in cell and organ growth in plants. Interestingly, changes were particularly notable in cell types important to young flower bud development such as epidermal cells, boundary cells, and meristematic cells. Confocal imaging of a BES1 ratiometric reporter confirms that brassinosteroid signaling is upregulated and more variable in young drmy1 sepals. Additionally, increasing or decreasing brassinosteroid signaling with chemical treatments can partially rescue the outgrowth defect in drmy1 by differentially altering the growth rates of the inner and outer sepals. These results suggest that brassinosteroids mediate the coordination of the growth rates between inner and outer sepals necessary to properly enclose the flower bud during early development.