ORIGINAL ARTICLE
Laura Jaakola Æ Kaisu Ma
¨
a
¨
tta
¨
-Riihinen
Sirpa Ka
¨
renlampi Æ Anja Hohtola
Activation of flavonoid biosynthesis by solar radiation in bilberry
(
Vaccinium myrtillus
L.) leaves
Received: 15 July 2003 / Accepted: 20 October 2003 / Published online: 10 December 2003
Ó Springer-Verlag 2003
Abstract The effect of solar radiation on flavonoid bio-
synthesis was studied in bilberry (Vaccinium myrtillus L.)
leaves. Expression of flavonoid pathway genes of bilberry
was studied in the upper leaves of bilberry, exposed to
direct sunlight, in the shaded leaves growing lower in the
same plants and in fruits. Bilberry-specific digoxigenin–
dUTP-labeled cDNA fragments of five genes from the
general phenylpropanoid pathway coding phenylalanine
ammonia-lyase and from the flavonoid pathway coding
chalcone synthase, flavanone 3-hydroxylase, dihydrofl-
avonol 4-reductase, and anthocyanidin synthase were
used as probes in gene expression analysis. Anthocyanins,
catechins, proanthocyanidins, flavonols and hydroxy-
cinnamic acids from the leaves and fruits were identified
and quantified using high-performance liquid chroma-
tography combined with a diode array detector. An in-
crease in the expression of the studied flavonoid pathway
genes was observed in leaves growing under direct sun
exposure. Also, the concentrations of anthocyanins,
catechins, flavonols and hydroxycinnamic acids were
higher in the leaves exposed to direct sunlight. However,
the concentration of polymeric procyanidins was lower in
sun-exposed leaves, whereas that of prodelphinidins was
slightly increased. The results give further support for the
protective role of flavonoids and hydroxy cinnamic acids
against high solar radiation in plants. Also, the roles of
different flavonoid compounds as a defense against stress
caused by sun exposure is discussed.
Keywords Anthocyanin Æ Flavonol Æ Gene expression Æ
Hydroxycinnamic acid Æ Proanthocyanidin Æ Vaccinium
Abbreviations ANS Anthocyanidin synthase Æ CHS
Chalcone synthase Æ DFR Dihydroflavonol 4-reductase
F3H Flavanone 3-hydroxylase Æ GPD Glyceraldehyde-
3-phosphate dehydrogenase Æ PAL Phenylalanine
ammonia-lyase
Introduction
Flavonoids are a large group of phenolic secondary
metabolites that are widespread among plants and in-
volved in many plant functions. Anthocyanins, a flavo-
noid subclass, are the main pigments in flowers and
fruits, where they act as insect and animal attractants.
Red colors in the leaves are often also due to anthocy-
anins. Different flavonoid subclasses have been found to
possess protective roles in plant tissues and the biosyn-
thesis of flavonoids often increases in response to
external stress factors such as drought, cold tempera-
tures, wounding or excess UV light (Bohm 1988; Chal-
ker-Scott 1999; Winkel-Shirley 2002). Flavonoids are
synthesized via the phenylpropanoid pathway (Fig. 1).
The main steps of the pathway are well known (for re-
view, see Holton and Cornish 1995; Weisshaar and
Jenkins 1998; Winkel-Shirley 2001).
Bilberry, or European blueberry (Vaccinium myrt-
illus L.) is a deciduous dwarf shrub growing in Europe
and Northern Asia. In Northern Europe, it is among
the most important of the wild berries. Anthocyanin
pigments, which bilberry contains, have been investi-
gated for their beneficial effect on human health,
especially their antioxidant properties, and vascular
protective activity. Vaccinium flavonoids are also
recognized for their anti-carcinogenic properties (Kalt
and Dufour 1997; Katsube et al. 2003). Bilberry is the
characteristic field-layer species in boreal forests, and
is preferably a shade plant. When growing in the
open, the upper leaves of the plant turn red (see
Fig. 2). This phenomenon is understood to provide
protection against UV-B irradiation (Chalker-Scott
1999; Winkel-Shirley 2002). In addition to colored
Planta (2004) 218: 721–728
DOI 10.1007/s00425-003-1161-x
L. Jaakola (&) Æ A. Hohtola
Department of Biology/Botany,
University of Oulu, P.O.B. 3000, 90014 Oulu, Finland
E-mail: laura.jaakola@oulu.fi
Fax: +358-8-5531061
K. Ma
¨
a
¨
tta
¨
-Riihinen Æ S. Ka
¨
renlampi
Institute of Applied Biotechnology,
University of Kuopio, P.O.B. 1627, 70211 Kuopio, Finland