Summary

Plant growth is directed by the activity of stem cells within meristems. The first meristems are established during early embryogenesis, and this process involves the specification of both stem cells and their organizer cells. One of the earliest events in root meristem initiation is marked by re-specification of the uppermost suspensor cell as hypophysis, the precursor of the organizer. The transcription factor MONOPTEROS (MP) is a key regulator of hypophysis specification, and does so in part by promoting the transport of the plant hormone auxin and by activating the expression of TARGET OF MP (TMO) transcription factors, both of which are required for hypophysis specification. The mechanisms leading to the activation of these genes by MP in a chromatin context are not understood. Here, we show that the PHD-finger proteins OBERON (OBE) and TITANIA (TTA) are essential for MP-dependent embryonic root meristem initiation. TTA1 and TTA2 are functionally redundant and function in the same pathway as OBE1 and OBE2. These PHD-finger proteins interact with each other, and genetic analysis shows that OBE-TTA heterotypic protein complexes promote embryonic root meristem initiation. Furthermore, while MP expression is unaffected by mutations in OBE/TTA genes, expression of MP targets TMO5 and TMO7 is locally lost in obe1 obe2 embryos. PHD-finger proteins have been shown to act in initiation of transcription by interacting with nucleosomes. Indeed, we found that OBE1 binds to chromatin at the TMO7 locus, suggesting a role in its MP-dependent activation. Our data indicate that PHD-finger protein complexes are crucial for the activation of MP-dependent gene expression during embryonic root meristem initiation, and provide a starting point for studying the mechanisms of developmental gene activation within a chromatin context in plants.