ABCDEmodel elucidate the role of floral homeotic genes in sex determination andfloral development.
Expressionof the A-class genes specifies sepal formation in whorl 1. A- and B-class genescombinedly specifies the development of petals in whorl 2. In whorl 3, thecombination of B- and C-class genes form stamens. The expression of the C-classgenes in whorl 4 determines the development of carpels. D-type activity involved in the specificationof ovules and an E function necessary for the determination of the corolla,androecium and gynoecium (Angenentand Colombo, 1996; Pelaz et al., 2000).
B and C class genes belong to MADS-boxfamily which are highly important transcription factors with its role in floralorganogenesis. These transcriptionfactors have highly conserved DNA binding domain known as the MADS box and moderately conserved domain known as Kbox. MADS box genes have been isolated andcharacterized in both dioecious species (white campion and sorrel) and inmonoecious species maize and cucumber (Kater et al., 1998; Perl-Treves et al., 1998Hardenack et al.
, 1994) ,Schmidt et al., 1993; Mena et al., 1995, Ainsworth et al., 1995). Previous studies identified AGAMOUS,APETALA3 and PISTILLATA genes of B and C classes were differentially expressed in the male flowers in Thalictrumdioicum and Spinacia oleracea (Di Stilio, Pfent C, Sather DN).
In Elaeis guineensis mutationin AP3 and PI male tissues, allowing females to develop. In cucumber FLORAL BINDING PROTEIN 11 (FBP11), a D-class gene determines ovule.AG2 has mixed C/D function gene as it is expressed only in ovule primordia andcarpel in A. thaliana andElaeis guineensis (Favaro et al.
, 2003;Pinyopich et al., 2003). In monoeciousspecies Liquidambar styraciflua L and Rumex acetosa L, the expression of the C gene has been associatedwith the arresting of the sexual organs.
However, in Liquidambar styraciflua the arresting could be theresult of cell death. In Populustrichocarpa, a genetic switch at a sex locus controlsexpression of B and C-class genes, thereby controlling the development of maleor female flowers.