Abstract Gynoecium polarity establishment is regulated by auxin, a phytohormone whose signal is transduced through several pathways. The relationship between the ETT and canonical TIR1/AFB pathways, and their relevance for carpel development beyond Arabidopsis thaliana , have not been investigated. The data presented here show that the expression patterns of canonical and ETT-mediated signalling components, and phenotypes of higher order mutants are shared between Arabidopsis and Capsella rubella . tir1 afb2 ett mutants partially phenocopy ett arf4 double mutants, suggesting a role for AUXIN RESPONSE FACTOR 4 (ARF4) in the integration of canonical and ETT-mediated signalling. Comparative transcriptomics revealed that the auxin-independent mis-regulation of YABBY genes correlate with patterning defects observed in Arabidopsis ett arf4 mutants. Together, the data presented suggest conserved synergism between canonical and ETT-mediated pathways in gynoecium polarity establishment in the Brassicaceae . Finally, the data suggest that ETT/ARF4 function to prevent the auxin-induced expression of a range of targets in Arabidopsis , consistent with activator-repressor ARF antagonism, and implying that the maintenance of auxin insensitivity by repressive ARFs is important for a range of biological processes. Summary Statement Here the relationship between canonical and ETT-mediated auxin signalling machineries is investigated in Arabidopsis and Capsella revealing conserved synergism between these two pathways in Brassicaceae with distinct fruit shapes.

Canonical and ETT-mediated auxin signalling pathways synergistically promote gynoecium development inArabidopsis thaliana