GENES ASSOCIATED WITH ORCHID FLOWER 555
Figure 2 Comparison of amino acid sequences among Ds-ACS1 (EMBL/GenBank Accession L07882), Ds-
ACS2 (EMBL/GenBank Accession L07883) from Doritaenopsissp. [3] DC-ACS (EMBL/GenBank Accession
U64031) from Dendrobium crumenatum[7] and Pt-ACS1 (EMBL/GenBank Accession Z77854) from Pha-
laenopsis[6]. 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 con-
servation of weak groups.
Despite the cloning of these ACC synthase and ACC oxidase genes in orchid, the nature of pollen-
pistil interactions leading to the expression of these genes in the floral organs is not well clarified. Based
on the current data, the latest model [3,75,77] for the interorgan regulation of ACC synthase and ACC ox-
idase gene expression in the pollinated orchid flower has been modified. This model, which is similar to
the one proposed in carnation [78] suggests that auxin and other pollen-borne factors are the primary in-
ducers of ACC synthase gene expression in the stigma and ovary, and the precursor ACC is the main sig-
nal translocated among the floral organs. However, the molecular events that follow ethylene perception
and gene expression are not clearly understood.
The important role of ethylene in agriculture has led to intensive investigation of signal transduc-
tion of this compound. The first putative ethylene receptor gene ETR1, thought to be the first compo-
nent of the ethylene signal transduction cascade, was isolated from Arabidopsis[79]. It encodes a pro-
tein resembling the two-component signal transducer in prokaryotes [80]. Subsequently, more than 30
putative ethylene receptors have been isolated from different plant species [69,81,82]. Sequence com-
parison of the Arabidopsisand tomato ethylene receptors reveals the existence of a second type of re-