ceptor ERS, which shares a high degree of identity with the N-terminal domain and putative histidine
protein kinase domain of ETR1 but lacks the C-terminal response regulator domain present in ETR1.
Using the N-terminal domain of the ETR1gene as a probe, an orchid ethylene receptor homologue
PER1 [16] and another distinct putative orchid ethylene receptor [17] have been isolated in Pha-
laenopsissp. ‘True Lady’ and Phalaenopsissp. ‘KCbutterfly’, respectively. Sequence comparison in-
dicates that PER1 represents a novel ERS type of ethylene receptor in that it lacks the C-terminal do-
main, but its N-terminus and histidine kinase domain are more closely related to ArabidopsisETR1
than to ERS. These putative orchid ethylene receptors are also highly similar to ETR1 from bacteria
such as E. coliandPseudomonasthat is responsible for environmental sensing for the presence of nu-
trients or changes in osmolarity [83]. Whether these orchid ethylene receptors are responsible for the
perception of ethylene signal remains to be clarified.
IV. FLORAL COLORATION
Flavonoids are secondary metabolites, and the importance of some of these in plant pigmentation has been
well documented [84]. There are several types of flavonoid molecules, the chalcones, flavanones,
flavones, flavonols, and anthocyanins. Among the different flavonoids, anthocyanins are the most com-
mon and important. Some of these are highly colored and tend to have marked effects in colors or pat-
terns. The biosynthesis of flavonoids has been reviewed several times [85–87] and will not be described
in detail here. Briefly, it starts with the condensation of three malonyl-CoA molecules with one molecule
of hydroxycinnamic acid. The resulting products, chalcones, are converted to flavanones, which give rise
to flavones, flavonols, and anthocyanins.
Our understanding of the molecular and biochemical determinants affecting floral coloration of or-
chid is not as advanced as that of some other ornamental plants. Nevertheless, studies are beginning to
shed light on some of the genes that encode enzymes involved in the flavonoid pathway of orchid petal
tissue. To date, the enzymes for which cDNA clones have been isolated are phenylalanine ammonia-lyase
556 NEO AND HO
Figure 3 Comparison of amino acid sequences among OAO1 (EMBL/GenBank Accession L07912) from
Doritaenopsissp. [3], D-ACO2 (EMBL/GenBank Accession L37103) from Doritaenopsissp. [4] and DC-
ACO (EMBL/GenBank Accession AF038840) from Dendrobium crumenatum[5]. 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.