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\section*{Experimental observations}
Figure S1 A-D shows four transverse sections of the flower buds shown in Figure 1E in the main text. In early stages (Figure S1 A,B), the signal is mostly confined to the L1 layer. At later stages of sepal development, DR5 expression becomes mostly confined to L1 cells in tips of the growing sepal organs (Figure S1 D), and to internal cells likely giving rise to pro-vascular strands (Figure S1 C, y-z section).



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