Flower development of Lilium longiflorum - The Lilium information ...

Chapter 3
Introduction
Transcription factors play important roles in many biological processes,
binding to regulatory regions of the DNA and to other protein factors in order to
inhibit or assist RNA polymerase in initiation or maintenance of transcription. The
MADS-box genes represent a large family of highly conserved transcription factors
found in plants, animals and yeast involved in a range of developmental processes
(Riechmann and Meyerowitz, 1997; Schwarz-Sommer et al., 1990; Theissen et al.,
2000). In plants, they play pivotal roles in the regulation of flowering time, meristem
identity, floral organ and fruit development (Causier et al., 2002) and, next to these
reproductive functions, they are also active in root architecture (Zhang and Forde,
1998) and may be necessary for guard cell and trichome development, as deduced by
expression analysis (Alvarez-Buylla et al., 2000).
Flower development has fascinated biologists for a long time but only in the
last decade a neat, elegant and satisfactory model for it was conceived. The identified
genes from the ABC model work in a combinatorial way for proper floral organ
development in which A type genes lead to sepal formation, A and B type genes
together trigger petal development, B and C type form stamens and the C type gene
expression constitutes carpels (Coen and Meyerowitz, 1991). The studies were first
performed in Arabidopsis thaliana and Antirrhinum majus, but soon they were
extrapolated to many other plants, confirming that the ABC model was a general
model for flower development throughout angiosperm species. However, the
simplicity of the original model demanded further elaboration to cope with the
complexity involved in flower development. This led to the addition of new functions
to the model, like the D function, which is involved in ovule development (Angenent
et al., 1995), and the E function, which was shown to be essential for petal, stamen and
carpel formation (Jack, 2001; Pelaz et al., 2000). The ABCDE model is of great value
for developmental studies and many MADS-box orthologues have been studied in
other species, including monocots such as sugarcane, sorghum, maize and rice
(Dornelas and Rodriguez, 2001; Greco et al., 1997; Mena et al., 1995; Pelucchi et al.,
2002).
One hundred and seven MADS-box genes have been identified in the
Arabidopsis genome (Parenicová et al., 2003). The MADS domain comprehends 56
amino acids and is the most conserved portion of the protein, which is involved in
DNA binding at the cis-elements motifs known as CArG boxes (Schwarz-Sommer et
al., 1990; Treisman, 1990). Many MADS-box genes contain a so-called MIKC
structure, presenting additionally to the MADS-box, a K-box that is a conserved
domain capable of mediating protein-protein interactions. The carboxy-terminal
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