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

Chapter 2
Introduction
Mechanisms of flower development are under control of a complex genetic
system. The formation of organs in the four whorls of a typical eudicotyledonous
flower, consisting of sepals, petals, stamens and carpels, requires many genes for
proper organ and tissue development (Blázquez, 2000).
Transcription factors play major roles in the genetic regulation, being
responsible for orchestrating the cascade of processes for cellular development,
differentiation and maintenance. Homeotic genes encode transcription factors involved
in the specification of organ identity; their loss of function results in the replacement of
one type of organ by another.
Several studies with Arabidopsis thaliana and Antirrhinum majus led to the so-
called ABC model of flower development (Coen and Meyerowitz, 1991). This model
postulates that three classes of homeotic genes control floral organ formation in a
combinatorial fashion. Expression of type A genes alone, A plus B, B plus C and C
alone triggers the formation of sepal, petal, stamen and carpel, respectively. In
addition, A- and C-functions are considered antagonists since activity of one type
represses the activity of the other (Bowman et al., 1991).
The ABC model of flower development has been fine-tuned in time. Important
extensions in the model were the addition of a D function for ovule development
(Angenent et al., 1995) and, more recently, the inclusion of an E function of which
transcription is active in the inner three whorls of the flower. Together with other
functional gene classes, the E function is indispensable for the development of petals,
stamens and carpels and contributes to protein complex formation in order to trigger
proper differentiation of those organs (Pelaz et al., 2000, 2001; Honma and Goto,
2001). New proposals for transcriptional protein interaction, among others the “quartet
model” which postulates the multimeric complex formation of MADS box proteins
and DNA regions for different organ development in the floral whorls also arose
recently due to the forementioned discoveries on the ABC model, hence called
ABCDE model (Theissen and Saedler, 2001; Honma and Goto, 2001).
All the genes involved in the ABCDE model are MADS-box genes, with the
exception of the type A AP2 gene. The MADS-box genes constitute a superfamily of
transcription factors found in very simple organisms e.g. yeast as well as in complex
species, such as plants and animals (Schwarz-Sommer et al., 1990; Shore and
Sharrocks, 1995; Ng and Yanofsky, 2001). In plants, they are involved in many
developmental processes, especially in the reproductive organs (Ng and Yanofsky,
2001), but also in roots, leaves and other organs (Alvarez-Buylla et al., 2000; Causier
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