Molecular signatures of auxin response are detected prior to changes in morphology. (A) Normal progression of axillary meristem initiation in wild-type ears occurs in a developmental gradient from tip to base. SPMs are formed at 1 mm (B), and SMs are formed at 2 mm (C,E). (D) Expression of the DR5-ER∷RFP reporter is strongly polarized to either side of developing SPMs in wild-type ears and these maxima indicate where new SM primordia will form. There is no DR5 signal detected between maxima in wild-type ears (gray arrow). (F) SPMs in ra1 mutants take on a fate similar to indeterminate BMs, reiterating the SPM developmental program, and (G,H,J) do not produce SMs by 2 mm. (I) In ra1 mutants, a weak DR5 signal is observed spanning the central domain of indeterminate SPMs joining the two maxima (white arrow), similar to that observed in tassel BMs. (K–M) In tassels, basal BMs are initiated first before the IM switches to produce determinate SPMs. (N) DR5 expression is observed across the central domain of indeterminate BMs (white arrow), connecting the maxima formed on opposite flanks. (Red asterisks) Determinate spikelet pair meristem (SPM); (green asterisks) indeterminate branch meristem (BM); scale bars, 250 μm in all panels except D (right), I (right), and N, where scale bars = 100 μm; DR5 expression views in D and I are taken from the section in white boxes in C and H, respectively.
