Coordination of shoot apical meristem shape and identity by APETALA2 during floral transition in Arabidopsis

Abstract

Plants flower in response to environmental signals. These signals change the shape and developmental identity of the shoot apical meristem (SAM), causing it to form flowers and inflorescences. We show that the increases in SAM width and height during floral transition correlate with changes in size of the central zone (CZ), defined by CLAVATA3 expression, and involve a transient increase in the height of the organizing center (OC), defined by WUSCHEL expression. The APETALA2 (AP2) transcription factor is required for the rapid increases in SAM height and width, by maintaining the width of the OC and increasing the height and width of the CZ. AP2 expression is repressed in the SAM at the end of floral transition, and extending the duration of its expression increases SAM width. Transcriptional repression by SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) represents one of the mechanisms reducing AP2 expression during floral transition. Moreover, AP2 represses SOC1 transcription, and we find that reciprocal repression of SOC1 and AP2 contributes to synchronizing precise changes in meristem shape with floral transition.

Fig. 1. AP2 is a positive regulator of SAM size and morphology during floral transition.

a–b Measurement of (a) width and (b) height of the SAM in continuous LD-grown plants. c SAM morphology adjusted to parabolas. The parabolas are colored according to the identity of primordia that were formed at the SAM periphery. The number of meristems producing each kind of primordia are listed on the top-right corner in each genotype and time point. d–f Segmentation analysis of the SAM of Col-0, ap2-12 and rAP2-VENUS under continuous long days (LDs). n = 4 SAMs. d Top view of the heatmap quantification of cell area in the meristem region. White asterisks indicate the first time point at which floral primordia were detected in the analysis of the corresponding genotype. Scale bar = 50 μm. e–f Quantification of (e) meristem area and (f) cell number. a–b, e–f The horizontal bars represent the median value for each genotype. Significant differences between wild-type and mutants within each time point were determined via two-sided Mann-Whitney-Wilcoxon-test (p < 0.05). Significant differences among time points within each genotype were determined via one-way ANOVA (two-sided), followed by Tukey post-hoc comparisons (p < 0.05). Data sets that share a common letter do not differ significantly. The color of the dots and the letters correspond to the genotype. See Supplementary Data 7 for precise sample size and p-values of Mann-Whitney-Wilcoxon and ANOVA test. Source Data are provided as a Source Data file.

Fig. 2. AP2 is present in the SAM as it increases in height and width during floral transition.

a Pattern of protein accumulation of AP2::AP2:VENUS at the SAM in ap2-12 plants under continuous LDs. Each SAM is shown in longitudinal section. The outline of each acquired meristem and its peripheral organs is indicated with a dotted white line. Scale bar = 50 μm. b–c Quantification of AP2:VENUS concentration of fluorescence intensity (total fluorescence divided by volume) at the shoot apex from the tip (b) to 50 μm deep or (c) 25 μm deep in the basal direction in the ap2-12 mutant background during continuous LDs. The horizontal bars represent the median value for each time point. Significant differences among time points within each genotype were determined via one-way ANOVA (two-sided), followed by Tukey post-hoc comparisons (p < 0.05). Data sets that share a common letter do not differ significantly. d Normalized measurements of height and width of the SAMs from LD-grown plants in this study (data from 6 independent experiments were pooled). The point of maximum height of each genotype in each experiment was selected. Measurements were normalized by the median of that measurement in Col-0 in each experiment. The horizontal bars represent the median value for each genotype. The outlines of the dots are colored according to the measurement. The dots are colored according to the identity of primordia that were formed at the SAM periphery. Significant differences for each measurement among genotypes were determined via one-way ANOVA (two-sided), followed by Tukey post-hoc comparisons (p < 0.05). Data sets that share a common letter do not differ significantly. See Supplementary Data 7 for precise sample size and p-values of Mann-Whitney-Wilcoxon and ANOVA test. Source Data are provided as a Source Data file.

Fig. 3. AP2 affects central zone and peripheral zone during floral transition.

a Pattern expression of WUS::3xVENUS-NLS at Col-0, ap2-12 and rAP2 SAMs of plants grown under continuous longs days (LDs). Each SAM is shown from the side. The outline of each acquired meristem and its peripheral organs is indicated with a dotted white line. Scale bar = 50 μm. b–c Quantification of the size of CLV3::mCHERRY-NLS domain (b) height and (c) width in Col-0, ap2-12 and rAP2 backgrounds. d–e Quantification of the size of WUS::3xVENUS-NLS domain (d) height and (e) width in Col-0, ap2-12 and rAP2 backgrounds. f Mean meristem periphery calculated subtracting WUS domain width (Fig. 3e) to the meristem width (Supplementary Fig. 6c) and dividing by 2 that difference. g Ratio of meristem periphery and WUS domain width in Col-0, ap2-12 and rAP2 backgrounds. The horizontal bars represent the median value for each genotype. Significant differences among genotypes within each time point were determined via two-sided Mann-Whitney-Wilcoxon test with the wild type and across the same genotype at different time points via one-way ANOVA (two-sided), followed by Tukey post-hoc comparisons (p < 0.05). For the ANOVA test, data sets that share a common letter do not differ significantly. The color of the dots and the letters correspond to the genotype. See Supplementary Data 7 for precise sample size and p-values of Mann-Whitney-Wilcoxon and ANOVA test. Source Data are provided as a Source Data file.

Fig. 4. AP2 is a negative regulator of SOC1 expression at the SAM before and during floral transition.

a–b Global transcriptome profiling via RNA-Seq of Col-0 and ap2-12 dissected meristems. a Venn diagram showing the overlap between the list of differentially expressed genes (DEGs) at 14 long days (LDs) between Col-0 and ap2-12, the list of DEGs in Col-0 between 10 LD and 14 LD, and the AP2-bound loci. b Expression profiles under LDs for dissected plant apices of the genes that are present in the three lists compared in a. Error bars represent the range between the maximum and minimum values among the three replicates. Significant differences between genotypes at the same time point were determined via two-sided likelihood ratio test (adjusted p-value < 0.05). c Pattern of protein accumulation of SOC1::SOC1:GFP at the SAM in soc1-2 (AP2/AP2) and soc1-2 ap2-12 (ap2-12/ap2-12) under LDs. In the side views, the shape of the acquired meristem and its peripheral organs is indicated with a dotted white line. In the top views, the orthogonal projection on xz plane of the same meristem is shown (projection of 50 μm from the top) and the meristematic region was highlighted using a dotted line. Scale bar = 50 μm. d Quantification of SOC1:GFP concentration of fluorescence intensity (total fluorescence divided by volume) at the shoot apex (from the tip to 50 μm deep in the basal direction) in soc1-2 and soc1-2 ap2-12 mutants during LDs. The horizontal bars represent the median value. Comparisons within each time point between genotypes were performed via two-sided Mann-Whitney-Wilcoxon-test (p < 0.05). Significant differences among time points within each genotype were determined via one-way ANOVA (two-sided), followed by Tukey post-hoc comparisons (p < 0.05). Data sets that share a common letter do not differ significantly. The color of the dots and the letters correspond to the genotype. The quantification of concentration of fluorescence intensity is consistent when performing the analysis with a depth of 20 μm, as shown in Supplementary Fig. 7b. See Supplementary Data 7 for precise sample size and p-values of Mann-Whitney-Wilcoxon and ANOVA test. Source Data are provided as a Source Data file.

Fig. 5. Mutual repression of SOC1 and AP2 affects flowering time and floral primordium identity.

a Top view of the heatmap quantification of cell area in the meristem region via segmentation in SAMs of Col-0, ap2-12, soc1-2 and soc1-2 ap2-12. White asterisks indicate the first time point at which floral primordia were detected in the analysis of the corresponding genotype. Scale bar = 50 μm. n = 4 SAMs. b–c Measurement of (b) width and (c) height of the SAM in continuous long day (LD)-grown plants. Significant differences between wild-type and mutants within each time point were determined via two-sided Mann-Whitney-Wilcoxon-test (p < 0.05). Significant differences among time points within each genotype were determined via one-way ANOVA (two-sided), followed by Tukey post-hoc comparisons (p < 0.05). Data sets that share a common letter do not differ significantly. The color of the dots and the letters correspond to the genotype. d–e SAM morphology of (d) Col-0, rAP2-V, soc1-2, rAP2-V / soc1-2 and (e) 35::SOC1:9xMyc under continuous LDs. The parabolas are colored according to the identity of primordia that were formed at the SAM periphery. The number of meristems producing each kind of primordia are listed on the top-right corner in each genotype and time point. See Supplementary Data 7 for precise sample size and p-values of Mann-Whitney-Wilcoxon and ANOVA test. Source Data are provided as a Source Data file.

Fig. 6. SOC1 is a negative regulator of AP2 expression at the SAM during floral transition.

a Pattern of protein accumulation of AP2::AP2:VENUS at the SAM in ap2-12 (SOC1/SOC1) and soc1-2 ap2-12 (soc1-2/soc1-2) plants under continuous longs days (LDs). In the side views, the outline of each acquired meristem and its peripheral organs is indicated with a dotted white line. In the top views, the orthogonal projection on xz plane of the same meristem is shown (projection of 50 μm from the top) and the meristematic region was highlighted using a dotted line. Scale bar = 50 μm. b Quantification of AP2:VENUS concentration of fluorescence intensity (total fluorescence divided by volume) at the shoot apex (from the tip to 50 μm deep in the basal direction) in ap2-12 and soc1-2 ap2-12 mutant backgrounds during continuous LDs. The dots are colored according to the mutant background of the analyzed plant. The horizontal bars represent the median value for each genotype. Comparisons within each time point between genotypes were performed via a two-sided Mann-Whitney-Wilcoxon-test (p < 0.05). Significant differences among time points within each genotype were determined via one-way ANOVA (two-sided), followed by Tukey post-hoc comparisons (p < 0.05). Data sets that share a common letter do not differ significantly. The color of the dots and the letters correspond to the genotype. The quantification of the concentration of fluorescence intensity is consistent when performing the analysis with a depth of 20 μm, as shown in Supplementary Fig. 10c. See Supplementary Data 7 for precise sample size and p-values of Mann-Whitney-Wilcoxon and ANOVA test. Source Data are provided as a Source Data file. c–e Schematic representation of AP2 and SOC1 regulation of SAM morphology and flowering at the (c) vegetative, (d) floral transition, and (e) inflorescence stages.