We are interested in how cells in a multicellular organism assume their   developmental fates and form distinct patterns.  We have chosen to address  this fundamental question in the context of Arabidopsis flower development   by focusing on the regulatory mechanism of the floral homeotic gene AGAMOUS   (AG) .  AG is a key regulatory gene for flower development   with at least three functions: the specification of carpel and stamen identity,   the repression of sepal and petal identity, and the control of floral meristem   determinacy.  Previous studies established that the activity of   AG  is mainly regulated at the transcription level.  Negative regulation   of AG transcription is crucial in ensuring stage- and tissue-specific   AG mRNA expression and hence proper floral organ identity and floral   organ pattern.  However, little is known about the mechanism underlying   AG transcriptional repression.  

    We have isolated and characterized three key negative regulators  of   AG.  They are LEUNIG (LUG) , SEUSS      (SEU)  and  LARSON (LSN).  We showed that   the  homeotic transformation  in the floral organ identity  of    lug     and seu mutants was  mediated  by precocious and ectopic   AG  expression (Liu and Meyerowitz, 1995;   Franks    et al.,  2002).  Further, using a map-based approach, we have  isolated    LUG  and SEU.      LUG encodes a putative  transcriptional      co-repressor with    sequence similarity to a class of transcription co-repressors     including  Tup1  from yeast, Groucho from  Drosophila, and TLE from mammals     (Conner and  Liu,  2000).  SEU  encodes a novel protein with   at  least two glutamine-rich    domains  and a conserved domain that shares   sequence  identity with the dimerization     domain of the LIM-domain-binding   transcription  co-regulators in animals   (Franks  et al., 2002).  We   are actively testing  the hypothesis that    LUG   and SEU may   form a co-repressor  complex, that is recruited  to the  AG cis-regulatory   element by other DNA-binding  transcription    factors that  bind to AG   cis-regulatory  elements.

   LSN appears to encode an excellent candidate DNA-binding partner  of the LUG/SEU co-repressor.  lsn mutations enhance the  defects of lug , and LSN encodes a putative transcription factor with a DNA-binding motif (Xiaozhong Bao and Zhongchi Liu, unpublished result).  Further molecular analyses of how LSN, LUG and SEU interact will provide important insights into the repression mechanism of floral homeotic  geneexpression.

  in situ hybridization showing AG expression in a wild-type inflorescence.

AG expression in seu-1 lug-1 double mutant inflorescence.

      A second emphasis in the lab was the genetic and molecular     characterization  of tso1 and tso2 mutants.  We  are  interested in understanding  how inflorescence and  floral  meristems  are organized and what genes  regulate the cell division activity  and cell division orientation in  inflorescence  and floral meristems.  Mutations  in TSO1  cause defects  in cell division in Arabidopsis   floral  meristems and  cause inflorescence   meristem fasciation. We have isolated the TSO1   gene using the map-based   approach and showed that TSO1   encodes two putative    DNA-binding cysteine-rich repeats and is  localized to the nucleus (Song et   al.,    2000).  Thus,  TSO1   likely encodes   a transcriptional factor that regulates  the expression of genes  involved in cell division and meristem organization.  Our lab is further   characterizing the TSO1 protein, its localization,  and the regulation  of its expression.  We have a postdoctoral position  open for this  project.  If interested, please contact Dr. Zhongchi Liu  via email  ( ZL17@umail.umd.edu ).

How are the genes named?

   LEUNIG was named after the famous Australian catoonist Michael Leunig because of the resemblance of the horn-like protrutions   at the tip of leunig mutant gynoecium to the horn-like protruions  in Michael Leunig's cartoon characters.  Dr. David Smyth was responsible for naming the gene.

   SEUSS and LARSON were both initially identified by mutations   that enhance leunig, we decided to name them following the tradition   of naming after cartoonist.  Bob Franks (a former postdoctoral fellow   in the lab) decided to name the two genes after the American cartoonists  Dr. SEUSS and Gary Larson.

    TSO1 and TSO2 genes are named so because of the ugly floral morphology in  tso1 and tso2 mutants.  TSO means ugly in Chinese. 

Current Lab members

Some Former Lab members

         Bob Franks, postdoctoral fellow,  rgfranks@uclink.berkeley.edu


Joann  Conner, postdoctoral fellow
                                      
Jaiyoung Song, postdocteral fellow

Terri Leung,   technician, tsleung00@yahoo.com

   Some lab pictures




Lab picture, 1999 fall, in front of the H.J. Patterson  Hall.   From left: Terri Leung, Sarah Brooks, Bob Franks, Seth Glatstein,  Jai-Young   Song, Black  Wang, and Zhongchi Liu






        



Lab picture, 2000 spring, at Dumbartom   Oaks , Washington D.C.  From  left,   Anandkumar Surendrarao, Zhongchi Liu, Black Wang, Alex Bao, and Bob Franks.
   
  
   
 

Lab picture, 2000 summer, at the Hirshhorn Sculpture Garden, Smithsonian, Washington D.C.  From front to back, Anandkumar Surendrarao, Black Wang, and Bob Franks.

Recent Publications

Franks, R.G. and Liu, Z. (2001) Floral homeotic gene regulation. Horticultural Reviews vol. 27, 41-77.

Song J, Leung T, Ehler LK, Wang C, Liu Z. (2000) Regulation of meristem        organization    and cell division by TSO1 , an Arabidopsis gene  with cysteine-rich    repeats. DEVELOPMENT 127:2207-2217. ( pdf/fulltext )

Liu, Z., Franks, R.G. and Klink, V. P. (2000) Regulation of marginal  tissue formation by LEUNIG and AINTEGUMENTA . PLANT CELL   12, 1893-1902. (with Cover ).

Conner, J. and Liu Z. (2000) LEUNIG , a putative transcriptional      co-repressor    that regulates AGAMOUS expression during flower   development.   PNAS    97, 12902-12907. (click here to download the pdf file) .

Scovel, G., Altshuler, T., Liu, Z., and Vainstein, A. (2000) The EVERGREEN gene is essential for flower initiation in carnation. J. of Heredity 91, 487-491.

Liu, Z., Running, M. R., and Meyerowitz, E. M. (1997) TSO1 function in cell division during Arabidopsis flower development, DEVELOPMENT 124:    665-672. ( pdf/fulltext )

Liu, Z., and Meyerowitz E. M. (1995). LEUNIG regulates AGAMOUS expression in Arabidopsis flowers. DEVELOPMENT 121:   975-991.