Dynamic control of auxin distribution imposes a bilateral-to-radial symmetry switch during gynoecium development

Abstract

Symmetry formation is a remarkable feature of biological life forms associated with evolutionary advantages and often with great beauty. Several examples exist in which organisms undergo a transition in symmetry during development. Such transitions are almost exclusively in the direction from radial to bilateral symmetry. Here, we describe the dynamics of symmetry establishment during development of the Arabidopsis gynoecium. We show that the apical style region undergoes an unusual transition from a bilaterally symmetric stage ingrained in the gynoecium due to its evolutionary origin to a radially symmetric structure. We also identify two transcription factors, INDEHISCENT and SPATULA, that are both necessary and sufficient for the radialization process. Our work furthermore shows that these two transcription factors control style symmetry by directly regulating auxin distribution. Establishment of specific auxin-signaling foci and the subsequent development of a radially symmetric auxin ring at the style are required for the transition to radial symmetry, because genetic manipulations of auxin transport can either cause loss of radialization in a wild-type background or rescue mutants with radialization defects. Whereas many examples have described how auxin provides polarity and specific identity to cells in a range of developmental contexts, our data presented here demonstrate that auxin can also be recruited to impose uniform identity to a group of cells that are otherwise differentially programmed.

Graphical abstract

Figure 1

Radial Symmetry in the Arabidopsis Gynoecium Is Imposed by the Activities of IND and SPT (A) SEM image of the apical region of wild-type Col-0 gynoecia at stage 13. (B and C) KLU::GUS in Col-0 at stage 9 (B) and stage 12 (C). (D) SEM image of the apical region of spt-12 gynoecium at stage 13. (E) KLU::GUS in spt-12 at stage 12. The scale bars in (A)–(E) represent 100 μm. (F) KLU quantitative RT-PCR in Col-0, spt-12, and ind-2 spt-12. Error bars show SDs. Student’s t test; ^∗p < 0.05; ^∗∗p < 0.01. WT, wild-type. (G–I) Col-0 gynoecia at stage 13. SEM (G) and Toluidine blue-stained cross-sections of the style (H) and ovary (I). (J–L) ind-2 spt-12 double-mutant gynoecium at stage 13. SEM (J) and Toluidine blue-stained cross-sections of the style (K) and ovary (L). In (G) and (J), white arrow indicates the style region and red arrow indicates the ovary. The scale bars in (G)–(L) represent 100 μm. (M and N) SEM images of rosette leaf from 35S::IND:GR in Col-0 (M) and spt-12 (N) without DEX. (O and P) 35S::IND:GR in Col-0 (O) and spt-12 (P) with 10 μM DEX. The scale bars in (M)–(P) represent 200 μm. (Q–T) SEM of rosette leaf epidermal cells from genotypes and treatments depicted in (M)–(P). Note that induction of IND imposes a change from jigsaw-shaped leaf epidermal cells to cylindrical-shaped cells resembling wild-type style cells in the inset (S’). The scale bars in (Q)–(T) represent 20 μm. See also Figure S1.

Figure 2

Auxin Is Dynamically Distributed at the Apex of the Developing Gynoecium and Functions in Sustaining Apical-Basal Growth and Establishing Radial Symmetry (A–F) Confocal images of DR5::GFP in Col-0 at stage 5 (A and B), stage 8 (C and D), and stage 10 (E and F). Upper images are longitudinal views (A, C, and E), and lower images are top views (B, D, and F). l indicates the position of the lateral auxin foci, and m indicates the position of the medial auxin foci. Insets in (B), (D), and (F) indicate the position of the GFP signal in the outline of the gynoecium viewed from the top. The scale bars in (A), (C), and (E) represent 50 μm and in (B), (D), and (F) represent 25 μm. (G) PIN1::PIN1:GFP stage 9 showing ovary expression in medial region and apical localization presumably transporting auxin toward the top. The scale bar represents 10 μm. (H) PIN1::PIN1:GFP stage 8 showing strongest expression in medial style region and apolar localization of the PIN1:GFP protein (top view). The scale bar represents 25 μm. (I) PIN1 apolar localization in ovary of PIN1::PIN1:GFP S1,3A pin1-5 at stage 9. The scale bar represents 10 μm. (J) Lateral view of PIN3::PIN3:GFP stage 5 with expression in lateral foci (arrows). (K) Top view of PIN3::PIN3:GFP stage 9 showing expansion of expression in a ring at the position of the presumptive style and apolar localization. (L) Top view of PIN7::PIN7:GFP stage 7 showing expression in the medial foci (arrows). The scale bars in (J)–(L) represent 25 μm. (M–R) Confocal images of DR5::RFP at stage 8 (M–O) and stage 10 (P–R) in Col-0 (M and P), pid-8 (N and Q), and ind-2 spt-12 (O and R). The scale bars in (M)–(R) represent 50 μm. (S–V) SEM images of stage 11 gynoecia from Col-0 (S), PIN1::PIN1:GFP S1,3A pin1-5 (T), pid-8 (U), and ind-2 spt-12 (V). White arrows indicate the base of the ovary. The scale bars in (S)–(V) represent 100 μm. See also Figure S2.

Figure 3

Control of PID-Directed PIN Phosphorylation Is Required for Radial Symmetry, and Induced Apolar Transport at the Style Is Sufficient to Rescue Mutants with Radial Defect (A and B) PID::GUS expression in Col-0 (A) and spt-12 (B) at stage 9. The scale bars represent 25 μm. (C) SEM image of PIN1::PIN1:GFP S1,2,3E pin1 at stage 13. (D) Confocal top-view image of PIN1::PIN1:GFP S1,2,3E pin1 at stage 8. (E and F) PIN1::PIN1:GFP pin1 with confocal top view at stage 8 (E) and SEM at stage 13 (F). (G and H) PIN1::PIN1:GFP S1,3A pin1-5 with confocal top view at stage 8 (G) and SEM at stage 13 (H). The scale bars in (C), (F), and (H) represent 100 μm and in (D), (E), and (G) represent 25 μm. (I) Confocal images of DR5::RFP at stage 10 from PIN1::PIN1:GFP S1,2,3E pin1. The scale bar represents 50 μm. (J–L) SEM of stage 10 gynoecia from pid-8 spt-12 (J), PIN1::PIN1:GFP S1,3A spt-12 (K), and PIN1::PIN1:GFP S1,3A spt-12 pin1 (L). The scale bars in (J)–(L) represent 100 μm. See also Figure S3.

Figure 4

Model for Radiality Establishment at the Top End of a Growing Organ Model showing how auxin-signaling accumulation (green) through polar auxin transport (PAT, arrows) presides over the bilateral-to-radial symmetry switch during gynoecium development. At stage 5, auxin signaling peaks at the lateral top part of the gynoecium, sustaining the apical-basal growth. At stages 8 and 9, SPT and IND repress PID expression, thus promoting apolar PIN localization leading to accumulation of auxin signaling at the medial top and subsequently formation of the radial auxin ring at stages 10 and 11.