GENES ASSOCIATED WITH ORCHID FLOWER 551
Figure 1 Comparison of amino acid sequences among O-MADS (EMBL/GenBank Accession X69107)
fromAranda deborah[23] and DOMADS1 (EMBL/GenBank Accession AF198174), DOMADS2
(EMBL/GenBank Accession AF198175), and DOMADS3 (EMBL/GenBank Accession AF198176) from
Dendrobium grexMadame Thong-IN [22]. Alignment was performed by the CLUSTAL W program [36]. The
stars indicate perfectly conserved sequences among the genes, colons represent conservation of strong groups,
and dots represent conservation of weak groups.of tomato and Arabidopsis. Four other MADS box cDNAs have now been identified in Dendrobium grex
Madame Thong-IN during the floral transition [22]. Of these, two cDNAS, otg7andDOMADS1, both en-
code the same protein. They also possess very similar nucleotide sequences (97% similar) except that DO-
MADS1is a longer transcript with more 5 and 3 untranslated regions. DOMADS1, DOMADS2, and
DOMADS3 are of different sizes, comprising 174 amino acids, 247 amino acids, and 220 amino acids,
respectively, and are thought to belong to the APETALA1/AGL9 subfamily of the MADS box gene fam-
ily. Sequence comparison reveals that DOMADS1 shares 54% and 67% similarity with DOMADS2 and
DOMADS3, respectively, whereas DOMADS2 and DOMADS3 share only 49% similarity (Figure 1). In-
terestingly, O-MADS is more similar to DOMADS1 (78% similarity) and DOMADS3 (61% similarity)
than DOMADS2 (40% similarity), suggesting that O-MADS, DOMADS1, and DOMADS3 may share
some functional similarity and that DOMADS2 may play a different role than the others.
Substantial gaps remain in our knowledge of how these orchid floral homeotic genes are controlled.
Although meristem-identity genes such as FLOWERING LOCUS T(FT) and LEAFY(LFY) have been
identified in Arabidopsis, the orchid homologues have yet to be isolated. Furthermore, the link between
meristem-identity and homeotic genes is unclear. Recent studies in Arabidopsissuggest that FTandLFY
act in a parallel fashion to transduce floral initiating signals such as photoperiod [37,38]. The FT protein
is similar to the sequence of TERMINAL FLOWER 1 (TFL1), an inhibitor of flowering that also shares
sequence similarity with membrane-associated mammalian proteins including phosphatidyletha-
nolamine-binding protein (PEBP). As PEBP is a precursor of hippocampal cholinergic neurostimulating
peptide, this raises the intriguing possibility that peptide molecules may be generated as the transmissible
signals during flower development. Isolation of the orchid homologues of Arabidopsis FTandTFL1us-
ing heterologous probes may provide some insights.