are thought to be involved in flower initiation. Second, homeotic genes, expressed later, are responsible
for the identity of individual floral organs. On the basis of studies of homeotic mutants that develop flo-
ral organs at positions normally occupied by other organs, floral homeotic genes are divided into three
different groups that cooperatively determine floral organ formation: the AGAMOUS group, the
APETALA3/PISTILLATA group, and the APETALA1/AGL1 group [25]. The majority of the homeotic
genes have a highly conserved DNA binding domain known as the MADS box motif (MCM1, AGA-
MOUS, DEFICIENS, SRF) within the N-terminus region [26–31]. In vitro binding studies have shown
that the MADS box proteins bind to a consensus sequence of CC(A/T) 6 GG, although each protein pos-
sesses a distinct binding specificity. The DNA target sequence of the MADS box proteins is located in the
promoters of numerous genes, including MADS box genes themselves and the genes controlled by them
[32]. Another conserved region within plant MADS box proteins is the K box domain, an approximately
70-amino-acid region that has the potential to form an amphipathic -helical structure that is thought to
facilitate the formation of a functional MADS transcription factor dimer [33,34].
Studies of MADS box proteins have principally been performed in dicotyledonous species such as
Arabidopsis, Antirrhinum, and Petunia[35]. It is now clear that flower development in monocotyledons
such as orchids and gymnosperms is also based on a conserved organization of developmental factors in
which MADS box proteins play a crucial role. The first orchid MADS box gene was isolated from Aranda
deborahby screening the mature flower complementary DNA (cDNA) library with an agamouscDNA
probe from Arabidopsis[23]. The gene, designated om1, is expressed in the petals and sepals of mature
flowers but not in young developing inflorescences or young floral buds. The 250-amino-acid O-MADS
protein encoded by the om1gene possesses a MADS box domain that is highly similar (95%) to those
550 NEO AND HO
TABLE 1 Genomic and cDNA Clones from Orchids
Identity Accession number Reference
ACC oxidase Doritaenopsissp. L07912 3
Doritaenopsissp. L37103 4
Dendrobium crumenatumAF038840 5
ACC synthase PhalaenopsisZ77854 6
Doritaenopsissp. L07882; L07883 3
Dendrobium crumenatumU64031 7
Acyl-CoA oxidase PhalaenopsisU66299 8
Bibenzyl synthase Bromheadia finlaysonianaAJ131830 9
Phalaenopsissp. X79903; X79904 10
Chalcone synthase Phalaenopsissp. True Lady’ U88077 11 Bromheadia finlaysoniana 12 AF007097; AF007098, AF007099 4-Coumarate:CoA ligase Vanilla plamifoliaX75542 13 Cymbidium hybridaAF017451 14 Cysteine proteinase Phalaenopsissp.hybrid SM 9108’ U34747 15
Cytochrome P-450 monooxygenase Phalaenopsissp. hybrid SM 9108’ U34744 15 Ethylene receptor Phalaenopsissp.KCbutterfly’ AF113541 16
Phalaenopsissp. True Lady’ AF055894 17 Flavanone 3-hydroxylase Bromheadia finlaysonianaX89199 18 Methyltranferase Vanilla planifoliaX78703; X73587; 19 X87099; X69192 20 Phenylalanine ammonia-lyase Bromheadia finlaysonianaX99997 21 Putative transcription factor Dendrobium grexMadame Phong-In 22 AF198174; AF198175; AF198176; AF107588 ArandaDeborah X69107 23 Phalaenopsissp.hybrid SM 9108’ U34743 15
S-Adenosyl homocysteine hydrolase Phalaenopsissp. X79905 10
0108; 0126 (function unknown) Phalaenopsissp. `hybrid SM 9108’ 15
U34745; U34746