Control of embryonic meristem initiation in Arabidopsis by PHD-finger protein complexes

Abstract

SUMMARYPlant 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.

ARF-Aux/IAA interactions through domain III/IV are not strictly required for auxin-responsive gene expression

“…Other results suggest that the reversal of this process (i.e., induction of an active chromatin state through histone acetylation and methylation/demethylation) plays an important role in the activation of auxin response genes 21 - 23 . It seems likely that chromatin modifications could impact on the binding of ARFs to their AuxRE target sites.…”

Targets of the StBEL5 Transcription Factor Include the FT Ortholog StSP6A

“…NAC2 and NAM are members of a large family of plantspecific TFs that function in both development and stress responses (Puranik et al, 2012). OBERON3 is a PHD-finger protein that functions as a transcriptional coregulator and is crucial for the activation of MONOPTEROS-dependent gene expression during embryonic root meristem initiation (Saiga et al, 2012). CLAVATA1 is a receptor protein kinase that maintains the population of stem cells in shoot meristems (Nimchuk et al, 2015), and BAK1 is a BRASSINOSTEROID INSENSITIVE1-associated receptor kinase (Wang et al, 2008).…”

PHDs govern plant development

“…31 In fact, OBE1 has been shown to bind TMO7 chromatin, suggesting a role for OBE proteins in its MP-dependent activation. 32 The PICKLE (PKL) CHD3 chromatin-remodeling factor also contains a PHD domain and is required to prevent the expression of embryonic traits in germinated seedlings. 33 Although PKL promotes the accumulation of the H3K27me3 mark, 34 it has been recently proposed that PKL and Polycomb Group (PcG) proteins antagonistically regulate root meristem activity in Arabidopsis.…”