75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

303 Regulation of AGAMOUS Expression by Histone Methylation
Joyce Cartagena 1 , Daisuke Kurihara 1 , Satoru Fujimoto 1 , Yoshitaka Azumi 2 , Susumu Uchiyama 1 , Sachihiro
Matsunaga 1 , Kiichi Fukui 1
1
Osaka University, 2 Kanagawa University
Plant SET domain proteins are known to be involved in the epigenetic control of gene expression during plant
development. In Arabidopsis, SET domain proteins are assigned to four classes, namely E(Z), SU(VAR)3-9, TRX and
ASH1, based on the homology of their SET domains. Here we report the potential role of an ASH1-related protein,
SML (stamen loss), in regulating the expression of the floral homeotic gene, AGAMOUS, and its function in stamen
development and differentiation. A loss-of-function phenotype of sml plants is characterized by defects in stamen
initiation and development. Furthermore, the level of AG expression was significantly reduced among sml flowers. Using
chromatin immunoprecipitation assays, it was revealed that SML does not directly bind to the regulatory regions of the
AG gene. Instead, it was shown that SML regulates AG expression by methylation of specific lysine residues. Based on
the functional and expression analyses performed on SML, we showed that SML is capable of regulating floral homeotic
genes in Arabidopsis, particularly those involved in specification of whorl 3 floral organs.
304 Two-step recruitment of RNA-directed DNA methylation to tandem repeat sequences
Simon Chan 1, 3 , Yana Bernatavichute 1 , Xiaoyu Zhang 1 , Carey Li 1 , Steven Jacobsen 1, 2
1
Dept of MCD Biology, UCLA, 2 Howard Hughes Medical Institute, 3 current address: Section of Plant Biology,
UC Davis
DNA methylation silences transposons, but also regulates expression of a select group of endogenous genes.
Arabidopsis FWA is silenced in adult tissues by DNA methylation on two tandem repeats, and a new copy of FWA
transformed into plants is an efficient target for de novo DNA methylation. We have used FWA transformation as a
model to study the initiation of DNA methylation, and have found that de novo DNA methylation is guided by siRNAs
produced by an RNA interference (RNAi) pathway. Little is known about why FWA is recognized by the cell as a target
for gene silencing. We show that tandem repeats are necessary and sufficient for de novo DNA methylation of FWA,
and that repeated character rather than intrinsic sequence is important. Endogenous FWA can adopt either of two stable
epigenetic states, methylated and silenced or unmethylated and expressed. Surprisingly, we found siRNAs associated
with FWA in both states. The unmethylated endogenous fwa gene was able to recruit siRNA producing machinery, but
could not easily regain DNA methylation. Preexisting CG DNA methylation was required to recruit further RNA directed
DNA methylation to endogenous FWA, or to cause efficient de novo methylation of transgenic FWA. This suggests
that tandem repeats are sufficient to recruit siRNA production, but a second event is required to recruit self-reinforcing
DNA methylation factors acting in cis or in trans. Tandem repeats throughout the Arabidopsis genome produce siRNAs,
suggesting that repeat acquisition may be a general mechanism for the evolution of gene silencing.