(A) SoPIN1, PIN1a, and PIN1b CRISPR-derived mutant alleles (see Materials and methods). Coding sequences are indicated by grey boxes. Arrowheads indicate CRISPR target sites and are labeled with the type of DNA lesion (C insertion, T insertion, or A deletion). All mutant alleles have frame shifts that result in premature stop codons at the positions indicated. (B–G) Inflorescence phenotypes of CRISPR-derived sopin1-1, pin1a-1, and pin1b-1 mutants. See Figure 2 for whole-plant phenotypes. (B–E) and (G) are scanning electron microscopy (SEM). (B) Immature wild-type (WT) (inbred line Bd21-3) Brachypodium inflorescence with several lateral spikelet meristems (lsm), and a terminal spikelet meristem (tsm). (C) sopin1-1 plants have severe organ initiation defects in the inflorescence. (D) Detail of a wild-type lateral spikelet meristem outlined by a box in (B) showing an immature lemma (l), which is the leaf-like organ that subtends the floral meristem (fm). (E) Detail of barren lateral spikelet meristem outlined by box in (C). (F) Mature inflorescence phenotypes of WT (Inbred Bd21-3), sopin1-1, pin1a-1, pin1b-1, and double pin1a-1/pin1b-1 mutants. The terminal spikelet (ts) of each inflorescence is indicated for comparison. Arrowhead indicates bent internode tissue in pin1b-1. Genotypes for (G–I) are indicated at the bottom of (I). (G) SEM details of representative spikelet meristems. (H) Box-plot of total whole-plant spikelet number at seed-set. (n = 22–53 plants each genotype). Samples with different letters are significantly different from each other (ANOVA, Tukey HSD, p<0.05). See ‘Figure 1H–I Source Data 1’ for source data. (I) Box-plot of the number of florets in each terminal spikelet of the central branch at seed set. (n = 22–53 plants each genotype). Samples with different letters are significantly different from each other (ANOVA, Tukey HSD, p<0.05). See ‘Figure 1H–I Source Data 1’ for source data. (J) Medial confocal Z-section of pZmUbi::DII-Venus (DII) expression in a WT spikelet co-expressing SoPIN1 tagged with Cerulean (a CFP variant) under the native SoPIN1 promoter. Organ primordia are numbered I2, I1, P1 from youngest to oldest. DII is normally degraded at SoPIN1 convergence points in I2 and I1 primordia (asterisks), and in response to auxin treatment (See Figure 1—figure supplement 2). Inset shows color look-up-table for all subsequent PIN images and color look-up-table for DII. (K) Medial confocal Z-section of pZmUbi::DII-Venus expression in a sopin1-1 spikelet meristem. DII degradation does not occur in the periphery of sopin1-1 meristems, and organs fail to initiate (arrow). Scale bars: 100 µm in (B) and (C), 20 µm in (D) and (E), 1 cm in (F), 50 µm each in (G), and 25 µm in (J) and (K).