Apart from Arabidopsis Atskp1, Phalaenopsis O108also shares significant homology with yeast SKP1
that encodes a novel kinetochore protein required for cell cycle progression [47,48]. Because ovule de-
velopment is initiated from a quiescent meristem in the orchid ovary that becomes activated by pollina-
tion, it is likely that some of the ovule-associated genes may be involved in cell cycle regulation, and
O108appears to be a potential candidate. Further experiments, such as complementation assays using
yeast skp1 mutants with O108, may establish the functional relationship between yeast skp1 and O108.
O126encodes an 18-kDa glycine-rich putative protein that contains a signal peptide sequence simi-
lar to those of other glycine-rich proteins that are thought to be structural components of the cell wall.
This suggests that the O126protein is a component of a specialized cell wall in the ovule. Wang et al. [42]
reported an O138gene regulated in a stage- and tissue-specific manner, but the sequence is not available
in the EMBL/GenBank database. O141encodes a putatative 40-kDa cysteine proteinase that is most sim-
ilar to endopeptidases of the papain family found in seed or fruit. Cysteine proteinases occur widely in
plants and are induced by various stress conditions, including cold, heat, salt, and drought, and by wound-
ing [49]. The expression of cysteine proteinase genes is also associated with ripening in a number of fruits
such as tomato and citrus, where they are predicted to play a role in fruit development. In situ hybridiza-
tion indicates that O141is specifically expressed in the outer integument of ovules during seed formation.
Because the formation of seed coat requires the degeneration of integument cells, it is conceivable that
O141may be involved in the developmentally regulated programmed cell death in a manner akin to the
mammalian cysteine proteases called caspases [50]. In agreement with this hypothesis, recent findings in
soybean cells show that cysteine proteases are involved in the regulation of programmed cell death in
plants [51].
B. Perianth Senescence
Pollination-induced senescence is a well-documented phenomenon in many flowers [52,53]. For Pha-
laenopsis, the longevity of intact unpollinated flowers can reach up to 3 months, but once pollinated, the
petals start to show signs of visible senescence sometimes within 1 day. Such an early display of senes-
cence symptoms has been taken as an indication that some pollination signals move through the flower,
eventually reaching the petals well before pollen germination and fertilization, both of which begin much
later after pollination. There is substantial evidence supporting a role for an increase in sensitivity to ethy-
lene following pollination in pollination-induced petal senescence [54–57]. This “sensitivity signal” is
thought to be the first signal moving into the petals, much earlier than the ethylene biosynthesis signal. In
Phalaenopsis, direct involvement of GTP-binding proteins, calcium, and protein phosphorylation has
been implicated in the regulation of ethylene sensitivity [58]. Studies suggest that short-chain saturated
fatty acids (SCSFAs), particularly octanoic acid, may be the ethylene “sensitivity factors” produced fol-
lowing pollination, and they are likely to act by altering the properties of the lipid bilayer membrane [59].
Other candidate molecules for ethylene sensitivity factors include auxin, but not ethylene, 1-aminocyclo-
propane-1-carboxylate (ACC), systemin, lipoxygenase, and jasmonates [41,60–62]. More recently, a
small molecular substance with molecular weight (MW) below 3000 that is distinct from auxin has been
extracted from the pollinia of Phalaenopsis[63]. Initial characterization of this substance indicates that it
is water soluble, unlikely to be proteinaceous in nature, and may be separated into at least five different
fractions by high-performance liquid chromatography (HPLC). The exact nature of the primary pollen
signals awaits further purification of these peaks and more detailed chromatographic analysis such as gas
chromatography coupled with mass spectrometry.
Clearly, the ethylene sensitivity pathway is closely associated with loss of membrane integrity, indi-
cating membrane changes. In connection with this, -oxidation of fatty acids plays an essential role in the
lipid metabolism of the cell membrane and is carried out exclusively by glyoxysomes in plants [64].
Catalysis of the dehydrogenation of fatty acyl-coenzyme (CoA) into hydrogen peroxide is achieved by
acyl-CoA oxidase, the rate-limiting enzyme of peroxisomal -oxidation [65]. Although acyl-CoA oxi-
dase has been identified in various plant species, little is understood about its expression and regulation
at the molecular level. A PhalaenopsiscDNApOACO31has been isolated by screening a library of
poly(A)RNA extracted from petals 1 day after pollination [8]. It encodes a 699-amino-acid putative per-
oxisomal acyl-CoA oxidase protein named PACO1 and is expressed specifically in petals after induction
by pollination. PACO1 shares significant sequence similarity with the peroxisomal acyl-CoA oxidase
from human, rat, and yeast acyl-CoA, particularly within 13 conserved regions and a putative flavin
GENES ASSOCIATED WITH ORCHID FLOWER 553