A dominant mutation reveals asymmetry in MP/ARF5 function along the adaxial-abaxial axis of shoot lateral organs

Abstract

The establishment of adaxial-abaxial polarity in plant lateral organs involves elaborate interactions between members of several transcription factor families, including the Auxin Response Factors (ARFs). We previously described a dominant allele of ARF5/MONOPTEROS (MP), termed MPΔ, which causes severe vascular hypertrophy in shoot lateral organs. Here we report that these organs are also disrupted in adaxial-abaxial polarity. Other MPΔ lateral organs with decreased vasculature show similar disruptions, suggesting that MP impinges on organ polarity through pathways separate from its role in promoting vascularization. Furthermore, we demonstrate that MPΔ exhibits an adaxial-abaxial asymmetry in its ability to influence organ development. Since ARFs previously implicated in polarity establishment function as transcriptional repressors, the transcriptional activator MP represents a novel link between auxin signal transduction and adaxial-abaxial polarity.

Figure 1.MPΔ disrupts leaf adaxial-abaxial polarity. (A) Wild-type first leaf. (B) MPΔ first leaf lacking an expanded lamina. (C,D) MPΔ-3 rosette leaves displaying cup/trumpet-shape (arrowheads). Bars: (A,B) 2 mm; (C,D) 1 mm.

Figure 2. Radialized MPΔ floral organs are defective in vascular production. (A) Wild-type petal. (B) MPΔ petal with laminar expansion at its distal end (arrowhead). (C,D) Cleared petal shown in (B) with vascular production (xylem) restricted to laterally expanded region (arrowhead). (E) Cleared wild-type petal with looping vasculature. (F) Radialized MPΔ floral organ. (G) Cleared radialized MPΔ floral organ devoid of vasculature. (H) Close view of organ depicted in (G). (C,D,E,G,H) Dark field images. Bars: (A-C,E,F) 0.5 mm; (G) 0.2 mm; (D,H) 0.05 mm.

Figure 3.MP expression and MPΔ function show an adaxial bias in leaf development. (A) MP::MP:GUS reporter expression in leaf primordia, with stronger signal in the adaxial/proximal region (arrowhead). (B) Cleared UAS::MPΔ leaf with a normal reticulate venation pattern. (C) Bright field and (D) GFP fluorescence images of leaf primordia of GAL4-GFP enhancer trap line CS70055. Arrowhead in (D) indicates GFP expression in adaxial/proximal region. (E) UAS::MPΔ; CS70055 cleared leaf exhibiting vascular hypertrophy (arrowheads) and ectopic lobing of the lamina (arrows). (F) Bright field and (G) GFP fluorescence images of leaf primordia of GAL4-GFP enhancer trap line CS70128. Arrowhead in (G) shows GFP expression in the abaxial leaf domain. (H) UAS::MPΔ; CS70128 cleared leaf showing a lack of vascular enhancement. (B,E,H) Dark field images. Bars: (A) 0.05 mm; (B,E,H) 2 mm; (C,F) 0.1 mm.