Diversification of functional requirements for proteolysis of auxin response factors

Auxin signaling through the Nuclear Auxin Pathway is essential for plant development and is mediated by competing A-class and B-class Auxin Response Factor (ARF) transcription factors. Recently, proteasomal ARF degradation through a degradation signal embedded within the DNA-Binding Domain, was identified as a key component of auxin response. Here, we investigate the structural requirements and biological relevance of ARF degradation in the bryophyte Marchantia polymorpha. We identify a critical residue for proteolysis of the repressive, B-class MpARF2, and find it to be functionally conserved in degradation of the activating, A-class MpARF1. Unlike MpARF2, however, impaired MpARF1 degradation had little effect on auxin response and development, suggesting differential integration in biological function. We find that MpARF2 degradation occurs across all developmental stages of the life cycle and is required for MpARF2 function during development. Our findings reveal a degradation mechanism targeting A- and B-class ARFs that shares the same origin but has evolved and diversified along unique trajectories. Auxin-dependent ARF transcription factors are subject to proteolytic degradation. Using the liverwort Marchantia polymorpha, de Roij et al. show that this degradation evolved early in the ARF family, but its relevance for ARF function diverged.