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A Repressor Complex That Functions in Organogenesis

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KNOTTED1-like homeobox (KNOX) genes function in meristem maintenance in the plant shoot apical meristem (SAM), and organogenesis requires downregulation of KNOX genes in lateral organ primordia (reviewed in Byrne, 2005; Scofield and Murray, 2006). Principle features of organogenesis include a switch from indeterminate cell proliferation in the SAM to determinate organ growth (acquisition of cell fate) and establishment of organ polarity, and KNOX gene repression appears to play a key role in both of these processes. Continued silencing of KNOX genes is required in developing organs, and ectopic KNOX expression results in both patterning defects and overproliferation of cells (Byrne, 2005). A number of factors have been established as negative regulators of KNOX gene expression. In Arabidopsis, these include the MYB domain transcription factor ASYMMETRIC LEAVES1 (AS1), the LOB domain transcription factor AS2, and the chromatin-remodeling factor HIRA.

In this issue of The Plant Cell, Guo et al. (pages 48–58) investigate the molecular mechanism for the repression of KNOX expression by AS1 and AS2 and show that these proteins form a repressor complex that directly binds to specific regions of KNOX gene promoters. The authors show that the C-terminal non-MYB region of AS1 confers repressor function. Plants transformed with a chimeric construct (LFY_DB:AS1_CTD) consisting of the AS1 C-terminal domain fused to the DNA binding domain of LEAFY, which specifies floral meristem fate, exhibited floral defects similar to lfy mutants (see figure). They then used chromatin immunoprecipitation and an electrophoretic mobility shift assay to identify AS1 binding elements at the Arabidopsis KNOX target BREVIPEDICELLUS (BP). Results showed that the AS1 complex binds directly to two target sequences in the BP promoter and that similar AS1 complex binding sites are present in another KNOX target gene, KNAT2. In addition, an interaction between AS1 and AS2 facilitates binding to these sites. Because the two AS1-AS2 binding sites in the BP promoter were found to act nonredundantly, the authors propose that AS1-AS2 complexes interact to create a loop in the KNOX gene promoters and, together with other factors, form a repressive chromatin state that blocks enhancer activity during organogenesis.

View larger version (68K): [in this window] [in a new window] [as a PowerPoint slide]   Fusion to the C-terminal domain of AS1 converts the LFY DNA binding protein into a dominant repressor, and Arabidopsis transformed with this construct (right) shows lfy-like floral defects compared with the wild type (left).

www.plantcell.org/cgi/doi/10.1105/tpc.108.200110

REFERENCES

Byrne, M. (2005). Networks in leaf development. Curr. Opin. Plant Biol. 8: 59–66.[CrossRef][ISI][Medline]

Guo, M., Thomas, J., Collins, G., and Timmermans, M.C.P. (2008). Direct repression of KNOX loci by the ASYMMETRIC LEAVES1 complex of Arabidopsis. Plant Cell 20: 48–58.[Abstract/Free Full Text]

Scofield, S., and Murray, J.A.H. (2006). KNOX gene function in plant stem cell niches. Plant Mol. Biol. 60: 929–946.[CrossRef][ISI][Medline]

Related articles in Plant Cell:

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

Mengjuan Guo, Julie Thomas, Galen Collins, and Marja C.P. Timmermans
Plant Cell 2008 20: 48-58. [Abstract] [Full Text]  

This Article Full Text (PDF) All Versions of this Article: 20/1/5    most recent tpc.108.200110v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in Plant Cell Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Eckardt, N. A. Search for Related Content PubMed Articles by Eckardt, N. A. Agricola Articles by Eckardt, N. A.