Brassinosteroids mediate proper coordination of sepal elongation [Spatial Transcriptomics]

To understand how this robust coordination occurs, we use the loss of robustness mutant development related myb-like1 (drmy1), which exhibits variable sepal initiation and growth, causing failure of the sepals to close the flower bud. Specifically, drmy1 has overgrown outer (abaxial) sepals and undergrown inner (adaxial) sepals, leading to a large discrepancy in the sizes of different sepals within individual flower buds.

Using single cell and spatial RNA-seq, we found changes in expression of key genes related to brassinosteroid (BR) signaling in drmy1, particularly in cell types important to young flower bud development such as epidermal cells, boundary cells, and meristematic cells. Confocal imaging of a BRI1-EMS-SUPPRESSOR1 (BES1) ratiometric reporter confirms that BR signaling is upregulated and more variable in young drmy1 sepals.