The correct establishment of adaxial-abaxial patterning is crucial for leaf expansion and growth. The AUXIN RESPONSE FACTOR (ARF) family of proteins are key determinants of organ symmetry and abaxial patterning in Arabidopsis thaliana and are subject to complex regulatory control at both the transcriptional and translational level. Here (), Chiyoko Machida and colleagues uncover an additional, dual mechanism for the regulation of ETT (also known as AFR3) by an ASYMMETRIC LEAVES1 (AS1)-AS2 complex. First, the authors show that the AS1-AS2 complex directly represses ETT expression via binding of AS1 to the promoter region. Second, they provide evidence for an indirect mechanism of regulation via miR390- and RDR6-dependent post-transcriptional gene silencing of both ETT and ARF4. The authors also suggest a possible epigenetic mechanism, as AS1-AS2 maintains the methylation status of ETT within the coding region. These discoveries shed light on the molecular framework of early leaf patterning events and help to uncover the events that lead to the specification of distinct adaxial and abaxial fates during leaf development.
