AINTEGUMENTA and AINTEGUMENTA-LIKE6/PLETHORA3 Induce LEAFY Expression in Response to Auxin to Promote the Onset of Flower Formation in Arabidopsis.

Proper timing of the onset to flower formation is critical for reproductive success. Monocarpic plants like Arabidopsis (Arabidopsis thaliana) switch from production of branches in the axils of leaves to that of flowers once in their lifecycle, during the meristem identity transition. The plant-specific transcription factor LEAFY (LFY) is necessary and sufficient for this transition. Previously, we reported that the plant hormone auxin induces LFY expression through AUXIN RESPONSE FACTOR5/MONOPTEROS (ARF5/MP). It is not known whether MP is solely responsible for auxin-directed transcriptional activation of LFY. Here, we show that two transcription factors belonging to the AINTEGUMENTA-LIKE/PLETHORA family, AINTEGUMENTA (ANT) and AINTEGUMENTA-LIKE6/PLETHORA3 (AIL6/PLT3), act in parallel with MP to upregulate LFY in response to auxin. ant ail6 mutants display a delay in the meristem identity transition and in LFY induction. ANT and AIL6/PLT3 are expressed prior to LFY and bind to the LFY promoter to control LFY mRNA accumulation. Genetic and promoter/reporter studies suggest that ANT/AIL6 act in parallel with MP to promote LFY induction in response to auxin sensing. Our study highlights the importance of two separate auxin-controlled pathways in the meristem identity transition.