Vernalization

Arabidopsis accessions have adopted different reproductive strategies:

Winter annuals require vernalization to flower while rapid-cyclers do not require vernalization.

In Arabidopsis, a prolonged period of cold reduces mRNA levels of the floral repressor FLOWERING LOCUS C (FLC; Michaels and Amasino, 1999; Sheldon et al. 1999).

The Dean lab initiated the investigation of the molecular mechanism of vernalization through the identification of novel Arabidopsis vrn mutants defective in the cold-induced repression of FLC (Chandler et al., 1996). We now use an FLC-luciferase fusion whose expression mimics the endogenous FLC gene to identify and analyse genes that regulate FLC.

	Examples of FLC misregulation after vernalization of different vrn mutants.

We have cloned the corresponding VERNALIZATION (VRN) genes using map-based techniques. VRN2 is a protein with homology to the Polycomb protein Su(z)12 (Gendall et al., 2001).

In contrast, VRN1 is a plant-specific DNA binding protein (Levy et al., 2002).

We also showed that the stable repression of FLC is maintained by LHP1 (LIKE HETEROCHROMATIN PROTEIN 1) a homologue of the human heterochromatin protein HP1 (Mylne et al., 2006).

Recently, we characterized VRN7 which is a homologue of VERNALIZATION INDEPENDENT 3 (VIN3; Sung and Amasino, 2004), and VRN5 both of which are a PHD finger proteins involved in FLC repression (Greb et al., 2007). These are two members of the five member Arabidopsis VEL gene family.

 

We have been using chromatin immunoprecipitation to explore the changes in the histone code at FLC after vernalization (Bastow et al. 2004). Our results indicate that the VRN proteins mediate changes in histone modifications that specify an evolutionarily conserved silent chromatin state.

Current areas of activity:

1. Biochemical characterization of the chromatin complexes associated with FLC.
2. Targeting mutations disrupting the inital steps of vernalization using the FLC-luciferase transgene.
3. Investigating the role of small RNAs and non-coding RNAs in vernalization.
4. Characterization of the epigenetic reprogramming of FLC.