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

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.