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Plant Cell Advance Online Publication
Published on January 18, 2008; 10.1105/tpc.107.056127

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Received October 6, 2007
Returned for revision December 5, 2007
Accepted December 30, 2007

Direct Repression of KNOX Loci by the ASYMMETRIC LEAVES1 Complex of Arabidopsis

Mengjuan Guo 1, Julie Thomas 1, Galen Collins 2, and Marja C.P. Timmermans 2*

1 Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 11724
2 Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 11724; Watson School of Biological Sciences, Cold Spring Harbor, New York 11724

* To whom correspondence should be addressed. E-mail: timmerma{at}cshl.edu.

KNOTTED1-like homeobox (KNOX) genes promote stem cell activity and must be repressed to form determinate lateral organs. Stable KNOX gene silencing during organogenesis is known to involve the predicted DNA binding proteins ASYMMETRIC LEAVES1 (AS1) and AS2 as well as the chromatin-remodeling factor HIRA. However, the mechanism of silencing is unknown. Here, we show that AS1 and AS2 form a repressor complex that binds directly to the regulatory motifs CWGTTD and KMKTTGAHW present at two sites in the promoters of the KNOX genes BREVIPEDICELLUS (BP) and KNAT2. The two binding sites act nonredundantly, and interaction between AS1-AS2 complexes at these sites is required to repress BP. Promoter deletion analysis further indicates that enhancer elements required for BP expression in the leaf are located between the AS1-AS2 complex binding sites. We propose that AS1-AS2 complexes interact to create a loop in the KNOX promoter and, likely through recruitment of HIRA, form a repressive chromatin state that blocks enhancer activity during organogenesis. Our model for AS1-AS2–mediated KNOX gene silencing is conceptually similar to the action of an insulator. This regulatory mechanism may be conserved in simple leafed species of monocot and dicot lineages and constitutes a potential key determinant in the evolution of compound leaves.




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A Repressor Complex That Functions in Organogenesis
PLANT CELL, January 1, 2008; 20(1): 5 - 5.
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