Differential expression of MYB gene (OgMYB1) determines color
patterning in floral tissue of Oncidium Gower Ramsey
Chung-Yi Chiou Æ Kai-Wun Yeh
Received: 13 June 2007 / Accepted: 14 December 2007 / Published online: 27 December 2007
Ó Springer Science+Business Media B.V. 2007
Abstract The yellow coloration pattern in Oncidium
floral lip associated with red sepal and petal tissues is an
ideal model to study coordinate regulation of anthocyanin
synthesis. In this study, chromatography analysis revealed
that the red coloration in floral tissues was composed
of malvidin-3-O-galactoside, peonidin-3-O-glucoside,
delphinidin-3-O-glucoside and cyanidin-3-O-glucoside
compounds. By contrary, these pigments were not detected
in yellow lip tissue. Four key genes involved in anthocy-
anin biosynthetic pathway, i.e. chalcone synthase
(OgCHS), chalcone isomerase (OgCHI), dihydroflavonol
4-reductase (OgDFR) and anthocyanidin synthase
(OgANS) were isolated and their expression patterns were
characterized. Northern blot analysis confirmed that
although they are active during floral development, OgCHI
and OgDFR genes are specifically down-regulated in yel-
low lip tissue. Bombardment with OgCHI and OgDFR
genes into lip tissue driven by a flower-specific promoter,
Pchrc (chromoplast-specific carotenoid-associated gene),
demonstrated that transient expression of these two genes
resulted in anthocyanin production in yellow lip. Further
analysis of a R2R3 MYB transcription factor, OgMYB1,
revealed that although it is actively expressed during floral
development, it is not expressed in yellow lip tissue.
Transient expression of OgMYB1 in lip tissues by bom-
bardment can also induce formation of red pigments
through the activation of OgCHI and OgDFR transcription.
These results demonstrate that differential expression of
OgMYB1 is critical to determine the color pattern of floral
organ in Oncidium Gower Ramsey.
Keywords Oncidium Gower Ramsey Á Anthocyanin Á
OgMYB1 Á Transcription factor
Introduction
Flower pigments, composed of carotenoids, anthocyanins
and betalains, are responsible for the natural attractive
display of plant colors. These three groups of pigments
play important ecological function, such as to attract
animal pollinators (Schaefer et al. 2004). Of the three
pigments, anthocyanins have the broadest distribution in
the flowering plants and their biosynthetic pathway has
been well characterized in several plant species (Grote-
wold 2006). Anthocyanins are derived from the
phenylpropanoid pathway. Chalcone synthase (CHS) is
the first key enzyme to produce a tetrahydroxychalcone,
which acts as the precursor for all classes of flavonoids.
A subsequent enzymatic reaction from chalcone to
naringenin is catalyzed by chalcone isomerase (CHI),
which is further converted to dihydrokaempferol by fla-
vanone 3-hydroxylase (F3H). Finally, three classes of
anthocyanidin end products are completed by consecutive
enzymatic activities, including flavonoid 3
0
-hydroxylase
(F3
0
H), flavonoid 3
0
,5
0
-hydroxylase (F3
0
5
0
H), and di-
hydroflavonol 4-reductase (DFR). Anthocyanidin
synthase (ANS) catalyzes the reaction from the colorless
leucoanthocyanidin to the colored anthocyanidin. Almost
all anthocyanidins undergo several modifications, such as
glycosylation or methylation, by UDP-glucoside:flavo-
noid 3-O-glucosyltransferase (3GT) and anthocyanin
methyltransferase (AMT) (Fig. 1a). These water-soluble
pigments are eventually accumulated in the vacuoles of
epidermal cell and are responsible for color appearance.
Delphinidin derivatives usually make the flower color
C.-Y. Chiou Á K.-W. Yeh (&)
Institute of Plant Biology, College of Life Science, National
Taiwan University, Taipei 106, Taiwan
e-mail: ykwbppp@ntu.edu.tw
123
Plant Mol Biol (2008) 66:379–388
DOI 10.1007/s11103-007-9275-3