INFLUENCE OF ULTRAVIOLET RADIATION ON PLANT SECONDARY METABOLITE PRODUCTION
Katerova Z. * , D. Todorova, K. Tasheva, I. Sergiev
Institute of Plant Physiology and Genetics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bldg. 21, 1113 Sofia, Bulgaria
Received: 22 January 2013 Accepted: 13 June 2013 Summary: Classification of major secondary metabolite groups is described. A short account is
also given to ultraviolet (UV) climatology and UV response in plants. Investigations regarding secondary metabolite production in plants, in vitro cultivated plant cell and tissue cultures under UV radiation, particularly UV-B (280-315 nm) and UV-C (200-280 nm) are reviewed. The capacity of plants, callus and tissue cultures to accumulate secondary metabolite compounds after exposure to UV is discussed. The cell and tissue cultures possess high potential for production of valuable secondary metabolites under controlled conditions, and it seems perspective to enlarge the investigations in this direction by using low doses of UV as elicitors of such compounds.
Citation: Katerova Z., D. Todorova, K. Tasheva, I. Sergiev. Influence of ultraviolet radiation on plant secondary metabolite production. Genetics and Plant Physiology, 2012, 2(3–4), 113–144.
Keywords: Calli; plant secondary metabolites; UV-B; UV-C. Abbreviations: PAR – photosyntheticaly active radiation; ROS – reactive oxygen species; UV
– ultraviolet radiation.
Contents
1. UV radiation – classification and impact on plants
2. Secondary metabolites – an introduction
3. UV radiation and secondary metabolites
3.1. Impact of UV-C on the synthesis of secondary metabolites
3.1.1. In vitro cultured plant cells, tissues and calli
3.1.2. Plant organs
3.2. Impact of UV-B on the synthesis of secondary metabolites
3.2.1. In vitro cultured plant cells, tissues and calli
3.2.2. Plant organs
4. Concluding remarks
114 Katerova et al.
1 . UV radiation – classification and impact on plants
Ultraviolet (UV) wavelength (400 – 200 nm) is a small part of the solar radiation reaching the Earth’s surface but with significant biological impact on the living organisms, including plants. According to the International Commission on Illumination this wavelength region is divided into UV-A (315 – 400nm), UV-B (280 – 315nm) and UV-C (200 – 280nm). The negative effect of UV radiation increases towards the shorter wavelengths. Therefore, due to its highest energy, UV-C quickly provokes high levels of injuries and it is most detrimental for the living organisms (Stapleton, 1992; Hollósy, 2002; Häder et al., 2007). Both UV-C and UV-B possess enough energy to damage different chemical bonds causing photochemical reactions, which is the main reason for the negative biological effects (Kovács and Keresztez, 2002). The composition of the UV radiation is modified due to its absorption by the atmosphere . Usually the short-wave UV-C radiation is fully absorbed with exception of the high mountain locations (Häder et al., 2007), while UV-B radiation is only absorbed by the stratospheric ozone and small part of it reaches the Earth’s surface. During the last decades the surface solar UV-B radiation was found to increase which corresponds with depletion of the stratospheric ozone caused by the increased release of anthropogenic pollutants such as chlorofluorocarbons and other gaseous emissions. Along with the atmospheric ozone amount, the spectral irradiance of the environmental UV depends also on the angle at which the solar radiation reaches the Earth’s atmosphere, i.e. the “solar zenith angle” (including time of day, season and
latitude), altitude, clouds, surface reflection, aerosols and even air pollution (Diffey, 1991; Paul, 2001; McKenzie et al., 2007). Since there is not selective absorber for the long-wave UV-A radiation, it is affected mainly by the light scattering, its intensity is much higher than UV-B but it is not so biologically relevant (Stapleton, 1992; Vass et al. 2005). Additionally, it was found that UV-A irradiation partially protected PSII reaction center from damages caused by UV-B by activating xanthophyll cycle by preserving the level of β-carotene in cluster bean chloroplasts during the steady phase of leaf development (Joshi et al., 2007). The report supported previous observations showing that environmentally relevant UV-A doses possess ameliorating effect on UV-B triggered damage (Newsham et al., 1998; Bischof et al., 2003). In fact, the presence of realistic doses of both UV-A and PAR are highly important in order to obtain environmentally adequate results since they both moderate the negative UV-B effects (Caldwell et al., 1994; Flint et al. 2003, Kakani et al., 2003; Dolzhenko et al., 2010).
High doses of UV-B and UV-C radiation affect negatively growth, development, photosynthesis, and other important processes in plants, leading to overproduction of reactive oxygen species (ROS) and development of oxidative stress, acting negatively on macromolecules, may decrease cell viability and cause cell death (Alexieva et al., 2001; Jansen, 2002; Frohnmeyer and Staiger, 2003; Zacchini and de Agazio, 2004; Procházková and Wilhelmová, 2007; Takeuchi et al., 2007; Danon and Gallois, 1998; Toncheva- Panova et al., 2010; Schreiner et al., 2012). However, low ROS concentrations were found to play a key role in the signaling
UV radiation and secondary metabolites
processes during plant acclimation (Dat et al., 2000). Along with the nucleic acids (Kucera et al., 2003; Takeuchi et al., 2007), proteins and lipids , the main target sites of UV radiation are known to be amino acids, membranes, quininones, pigments, photosynthetic machinery, mainly because of the UV absorbing aromatic chemical groups (Jansen et al., 1998; Hollósy, 2002; Jansen, 2002; Vass et al., 2005; Edreva, 2005). UV-induced effects depend also on the plant sensitivity (Lavola et al., 2003; Zu et al., 2011). Low UV-B or UV-C doses may trigger acclimation responses in plants, including activation of enzymatic and non- enzymatic defense systems (Loyall et al., 2000; Jansen, 2002; Lavola et al., 2003; Katerova and Todorova, 2009; Katerova et al., 2009; Katerova and Todorova, 2011; Rai et al., 2011), but high UV doses
could activate repair mechanisms in order to cope with the stress (Frohnmeyer and
Staiger, 2003). It was documented that the defense or tolerance to UV-B can
be related to the induction of different signal transduction pathways, secondary metabolite production, and DNA repair mechanisms (A-H Mackerness, 2000; Brown et al., 2005; Ishibashi et al., 2006). Application of low UV-C doses (0.5– 9.0kJm -2 ) has been considered to be of commercial prospect by causing hormetic (beneficial) effects to prevent pathogen diseases and delay senescence during fruit storage (Shama and Alderson, 2005).
Although the role of some secondary metabolites such as anthocyanins is still under question (Sarma and Sharma, 1999; Hada et al., 2003), most authors claim that the production of these compounds (mainly flavonoids and UV-B absorbing metabolites) in plants subjected to low UV-B doses is a major part of the complex
plant defense system (Solovchenko and Schmitz-Eiberger, 2003; Kucera et al., 2003; Schmitz-Hoerner and Weissenböck, 2003; Frohnmeyer and Staiger, 2003; Jansen et al., 2008). Bashandy et al. (2009) also assume that the accumulation of non-pigmented flavonoids in leaves of the double ntra ntrb (lacking NADPH-
dependent thioredoxin reductases) Arabidopsis mutant might lead to the observed UV-C tolerance. The authors strongly support the proposed theory by the fact that UV-C tolerance was lost after crossing the double mutant with the tt4 (mutation in the gene encoding the first enzyme of the flavonoid biosynthesis) showing that production of flavonoids in
the ntra ntrb mutant could protect plants against UV-C. In addition, the mRNA
level of Chs gene (chalcone synthase gene, involved in flavonoid production) was also induced in UV-C treated plants. The authors suggest that NADPH- dependent thioredoxin reductases could
be a new negative regulator of flavonoid biosynthesis.
In opposite to the high UV-fluency rate, relatively low UV-B or UV-C doses led to an increased production of secondary metabolites (Kreft et al., 2002; Antognoni et al., 2007; Nadeau et al., 2012; Schreiner et al., 2012). As the pathways for secondary metabolite production are interrelated, the fact that some of these compounds increase and other decrease is not unexpected, and the biosynthesis prevails mostly to compounds possessing higher ROS scavenging activity or UV-shielding properties (Jansen et al., 2008). In the current review, we focus on secondary metabolites, which have been reported to alter predominantly after UV-B and/or UV-C treatment.
Katerova et al.
2. Secondary metabolites – an
reported (Bell, 1980; Gershenzon,
introduction
2002). They are comparable to common The primary metabolites are vital
amino acids and occur in a free form or for every living cell. On the other hand,
as ingredients of low molecular weight the secondary metabolites are present
compounds. The non-protein amino acids only incidentally and are not essential could be divided into several subgroups –
for plant life (Edreva et al., 2008). neutral aliphatic amino acids; acidic amino They are organic compounds derived
acids; basic amino acids; heterocyclic through methylation, hydroxylation or
amino acids; aromatic amino acids; imino glycosylation from primary metabolites
acids; sulphur-containing amino acids; (carbohydrates, proteins, amino acids,
and selenium-containing amino acids lipids) (Korkina, 2007). Secondary
(Bell, 1980).
metabolites could be classified into Plant amines often derive from amino several categories according to various acids by decarboxylation (Smith, 1980). features like their chemical structure,
Several subcategories of plant amines are solubility in different solvents, or the
identified in relation to number of amino pathway of their biosynthesis. Another
groups in their structure – simple aliphatic important classification is related to
monoamines; aliphatic diamines; aliphatic the presence or absence of nitrogen in
polyamines, amines containing various their chemical structure (Gershenzon
heterocyclic groups. Several plant amines 2002). Thus, secondary metabolites
serve as precursors in biosynthesis of form two major groups: 1) nitrogen
polyamine alkaloids.
containing – alkaloids, non-protein Cyanogenic glycosides are natural amino acids, amines, cyanogenic
compounds containing cyanide group in glycosides, and glucosinolates; and 2)
their structure and can release HCN by without nitrogen – terpenes (mono-,
hydrolysis (Conn, 1980; Gershenzon, sesqui-, di-, tri-, tetraterpenes, steroids,
2002). They also derive from common saponins), phenolics (phenolic acids
amino acids and could be classified on the and phenylpropanoids), polyketides and
basis of the glycosylated group. polyacetylenes.
Glucosinolates are sulfur- and More than 12000 alkaloids are
nitrogen-containing compounds, dis- synthesized in plants (Gershenzon, 2002;
tributed predominantly in dicotyledonous Zhang and Bjorn, 2009), and derived from
plant families and like other nitrogen- amino acids such as ornithine, lysine,
secondary metabolites phenylalanine, tyrosine, tryptophan,
containing
are synthesized from common amino histidine, and aspartic acid. Five major
acids (Underhill, 1980; Gershenzon alkaloid subgroups are identified and
2002). Glucosinolates are precursors of representative alkaloids are shown in Fig.
mustard oils and similarly to cyanogenic
1. glycosides, they can also release toxic Beside the well known 20 essential
volatiles like isothiocyanate. amino acids involved into protein
Up to date, approximately 29000 structures more than 500 other uncommon
terpenoids/terpenes are discovered. non-proteinogenic amino acids are
Terpenes derive from their precursor
UV radiation and secondary metabolites
Figure 1. Major alkaloid subgroups and chemical structures of some representatives. isopentenyl diphosphate and are for generation of the ubiquinone prenyl
classified by the number of isoprene group in mitochondria (Croteau et al., units. There are two major pathways for
2000). The key biosynthetic enzymes biosynthesis of isopentenyl diphosphate:
are prenyltransferases and monoterpene
1) acetate/mevalonate pathway in cytosol synthase/cyclases. Some major repre- and endoplasmatic reticulum and 2) sentatives of the terpene group are
glyceraldehyde
presented in Fig. 2. Steroids and saponins pathway in plastids (Croteau et al., 2000;
phosphate/pyruvate
are secondary metabolites closely related Gershenzon, 2002). Additionaly, the
to terpenoids (Grunwald, 1980, Croteau, acetate/mevalonate pathway is implicated
et al. 2000).
Katerova et al.
Figure 2. Terpenes classification based on isoprene unit numbers. More than 11000 plant phenolics
derivatives (C 6 -C 1 skeleton), so called are synthesized mainly by two major
phenolic acids and phenylpropanoids (C 6 - pathways: shikimic acid pathway and
C 3 skeleton) (Fig. 3).
malonic acid pathway (Gershenzon, 2002). Phenolic acids are common substances The plant phenols could be classified
widespread in plant species. Several well- into two major categories – benzoic acid
known plant acids belong to this category:
UV radiation and secondary metabolites 119
Figure 3. Classification of plant phenolics and some important representatives.
Katerova et al.
salicylic acid, vanillic acid, gallic acid, 4-phenylcoumarins (neoflavanoids), and etc. (Harborne, 1980).
isocoumarins are recognized. Phenylpropanoids are classified into
Stilbenes (C 6 -C 2 -C 6 ) consist of a several subgroups: flavonoids, hydroxy-
trans (or cis ) ethene bond substituted cynnamic acids, cynnamic aldehydes,
with a phenyl group on both carbon coumarins, lignins, lignans, stilbenes and
atoms of the double bond. The most suberins.
abundant natural stilbenes are resveratrol
Hydroxycynnamic acids (C 6 -
and lunularic acid (Harborne, 1980).
C 3 ) are derivatives of cinnamic acid Suberins are lipid-derived bio- (Harborne, 1980). They are formed from
polymers consisting of long-chain (C 16 trans -cinnamic acid by a series of
to C 22 ) dicarboxylic acids, long-chain hydroxylations and O -methylations to
(C 20 to C 26 ) components like acids and yield compounds like p -coumaric, caffeic,
alcohols and substantial amount of ferulic and sinapic acid. Hydroxycynnamic
phenolic compounds (Thompson, 1980). acids rarely occur in free forms and
Among the phenylpropanoids in usually exist as conjugates, mostly as
plants flavonoids are the most plentiful esters of glucose or various organic acids
group including more than 9000 or amides, and less often as glycosides.
representatives. They are synthesized Lignins (C 6 -C 3 ) n are complex in plants via the flavonoid branch of the phenolic heteropolymers based on
phenylpropanoid and acetate-malonate phenylpropanoid units resulting pathway. Flavonoids comprise 15 carbon
from the oxidative polymerization of atoms - two aromatic rings (A and B) hydroxycinnamoyl alcohol derivatives connected with a 3-carbon bridge (C
(Ibrahim, 2001b; Gershenzon, 2002;Vogt, ring). The basic flavonoid skeleton can 2010).
tolerate a large number of substitutions,
for example hydroxyl groups, methyl group of phenylpropanoids which are
Lignans (C 6 -C 3 ) 2 constitute a different
groups, sugars (e.g. glucose, galactose, biochemically related to lignins. Lignans
rhamnose), etc. Introducing a second are synthesized from phenylalanine via
hydroxyl group at o -position in the B dimerization of substituted cinnamic
ring of flavonoids is responsible for alcohols to yield monolignol-derived
the enhancement of the antioxidant dimers and some oligomers (Harborne,
capacity of the resulting compounds 1980; Ibrahim, 2001b).
(Edreva et al., 2006, 2008). Sugars and Coumarins (C 6 -C 3 ) are structurally hydroxyl groups increase the water considered as the lactone derivatives of
solubility of flavonoids, while methyl 2-hydroxy-(cis)-cinnamic acids which
and isopentyl groups make flavonoids result in an apyrone nucleus. More than
lipophilc. Flavonoids are divided into 500 naturally occurring coumarins are
several subclasses: flavones, flavanones, known and they are mostly spread only
flavonols, flavanols, anthocyanidins, in few plant families like Umbelliferae
isoflavonoids (Ibrahim 2001a; Buer et and Rutaceae (Harborne, 1980, Ibrahim
al., 2010).
2001b). Beside the typical coumarins, Flavones are a class of flavonoids some other classes as furanocoumarins,
based on the basic flavone structure
UV radiation and secondary metabolites
with substituents mainly on 4’, 5, and pelargonidin, peonidin, and petunidin.
7 carbon atoms and lack of –OH group Isoflavonoids (isoflavones) are in position 3. Apigenin and luteolin and
similar to flavones, but the B ring is their respective glycosides are commonly
attached to C 3 of the C ring. The major found in many herbaceous plant species
representatives are the isoflavones (Harborne, 1980).
genistein and daidzein and their Flavanones are presented in high
respective glycosides daidzin and concentrations in citrus fruits. Flavanones
genistin. They are found almost
exclusively in leguminous plants with the flavone structure. The most common
have no double bond between C 2 and C 3 of
highest concentrations in soybean. flavonones are hesperetin, naringenin,
Tannins are secondary metabolites and their glycosides hesperidin and which can be divided in two categories naringin.
– hydrolyzable (gallotannins) and The molecule of flavonols has a
condensed (Harborne, 1980, Gershenzon double-bonded oxygen atom attached
2002). The hydrolyzable tannins usually to position 4 and double bond between
contain glucose and phenolic acids
2 C 3 and C of the flavone structure, (mainly gallic acid). Condensed tannins and –OH group at C 3 . Flavonols are (or polyflavonoid tannins, catechol-type
present in a wide variety of fruits and tannins, pyrocatecollic type tannins, vegetables mainly as O-glycosides. The
non-hydrolyzable tannins or flavolans) most abundant flavonols are quercetin
are polymers formed by the condensation (main glycosides – rutin and quercitrin),
of flavans and they do not contain myricetin (glycoside myricitrin), and
sugar residues. Tannins have molecular kaempferol. More than 200 different
weights ranging from 500 to over 3000 sugar conjugates of kaempferol are
(gallic acid esters) and up to 20000 discovered.
(proanthocyanidins).
Flavanols (catechins and epi- Quinones are aromatic dicarbonyl catechins) are flavonoids without double
compounds. The two carbonyl groups bonds in the C ring, and have –
usually are in p -position and form colored OH group at C 3 . They occur in a number
pigments. Most of the naturally occurring of plant species, but predominantly in
quinones contain a long isoprenoid side cocoa, green tea and some woody species
chain, and are divided in two major as birch, pine and apple (Harborne, 1980;
structural groups – naphtoquinones Kostina et al. 2001; Lavola et al. 2003;
and benzoquinones. Usually naturally Solovchenko and Schmitz-Eiberger,
occurring quinone pigments contain 2003). Another two classes of flavanols
phenolic or metoxyl constituents and are flavan-4-ol and flavan-3,4-diol.
have important role in vital physiological Anthocyanidins have a positive
processes in plants (Harborne, 1980). charge in the C ring and two double
Polyketides are secondary meta- bonds in the C ring. Anthocyanins are
bolites from plants that are usually anthocyanidin glycosides, and most
synthesized in a similar to fatty common in plants are the glycosides acids biosynthetic process through of cyanidin, delphinidin, malvidin,
decarboxylative
condensation of
122 Katerova et al.
malonyl-CoA. The polyketide chains produced by a polyketide synthase are often further modified into bioactive metabolites (Thompson, 1980). Poly-
acetylenes are natural products containing carbon-carbon triple bond functionality. There are discovered approximately 2000 polyacetylenes, and near 1200 of them are found in plants of Asteraceae (Compositae) family. Polyacetylenes derive from fatty acids and polyketide precursors (Minto and Blacklock, 2008).
Secondary metabolites play an important role in many plant physiological
and developmental processes such as root nodule formation; determination of pollen germination and pollen functionality; leaf and petal pigmentation; gravity responses; regulation of auxin binding and transport; inhibition of certain enzymatic activities; influence on cellular protein phosphorylation. They also contribute to stress responses as signaling molecules, potent scavengers of ROS, and to the
protection against pathogens and UV irradiation
(Winkel-Shirley,
Ibrahim, 2001a; Gershenzon, 2002; Velikova et al., 2004; Edreva, 2005; Velikova et al., 2007; Korkina, 2007, Edreva et al., 2007; Edreva et al., 2008; Buer et al., 2010; Samanta et al., 2011). Furthermore, plant secondary metabolites possess biological activities which are important for human life and health. A number of articles have documented the benefits of plant secondary metabolites and their use in traditional and modern medicine, food industry, perfumery and cosmetics (Havsteen, 2002; Korkina, 2007; Dinkova-Kostova, 2008; Jansen et al., 2008; Zhang and Bjorn, 2009; Caputi and Aprea, 2011; Wijesinghe and Jeon, 2011). Most of the known functions
of alkaloids are related to protection against insects, but they also contribute to animal metabolism as important neurotransmitters. Many alkaloids are used in medicine as antiarrhythmics, anticholinergics, antitumors, vasodilata- tors, antihypertensives, anesthetics, analgesics, as well as muscle relaxants, inhibitors
of acetylcholinesterase, antipyretics, and antiprotozoal agents. Terpenes and terpenoids are important components of plant essential oils that are also extensively used as
natural flavor additives in food, as fragrances in perfumery, and in traditional and alternative medicine
such as aromatherapy. Plant-derived phenylpropanoids (especially flavonoids) and their derivatives are among the most
common biologically active components in food, wines, beer, spices, aromas, fragrances, and essential oils. Taking in account their defensive roles, these compounds are of great medicinal interest, especially as free radical scavengers, antioxidants, UV screens, anticancer, antivirus, anti-inflammatory, wound healing, antibacterial, and metal chelating agents.
3. UV radiation and secondary metabolites
3.1. Impact of UV-C on the synthesis of secondary metabolites
3.1.1. In vitro cultured plant cells, tissues and calli
UV-C light exposure of sterile cultures of Scenedesmus quadricauda (Chlorophyceae) over 1h did not
influence total soluble phenols and flavonoids (Kováčik et al., 2010).
UV radiation and secondary metabolites
Phenolic acids were altered differently derivatives of boswellic acid - active by UV-C - vanillic acid increased;
metabolite produced in Boswellia serrata gallic,
Roxb. (endangered medicinal plant). p -coumaric acids were decreased,
The authors found that 5 min of UV-C while protocatechuic and salicylic acids
irradiation was effective for production did not change significantly. Selected
and accumulation of acetyl-11-keto-β- flavonols (quercetin and kaempferol)
boswellic acid (10-fold) and β-boswellic were not detected after UV-C treatment.
acid (7-fold) in the callus culture. The The authors concluded that the exposure
synthesis of stilbenes was extensively time to UV light was not sufficient
investigated in grape calli systems to
(Liu et al., 2010). The authors reported changes of the phenolic metabolites in
stimulate more
considerable
induced by UV-C in vitro production of Scenedesmus quadricauda. Exposure resveratrols and their glucoside (piceids)
of Chlamydomonas nivalis (so-called in four grape genotypes and three tissue snow alga) cells to UV-C light resulted
types of each genotype. UV-C irradiated in a three-fold increase in free proline
calli accumulated stilbenes which have occurred within two days after exposure
been already reported to have a number to UV-C, accompanied with a 12–24%
of health-beneficial properties, such as increase in phenolics after 7 days of
antioxidant capacity, cardioprotective exposure (Duval et al., 2000). The
effects, and anti-mutagenic, estrogenic authors report that UV-C light exposure
and anti-cancer activity (Hung et al., can stimulate phenolic-antioxidant
2000; Sgambato et al., 2001). Further by production in aplanospores of C. nivalis
methylation of resveratrol via O-methyl- which supports the idea that there is
transferases can be generated its methyl
a considerable biotechnological and ether pterostilbene (trans-3,5-dimethoxy- pharmaceutical potential incorporated 4′-hydroxystilbene) which is known to
within the genome of this UV-tolerant possess fungicidal, antioxidant, anti- snow alga. Moreover, UV-induced
cancer , and antiinfective properties. secondary
Xu et al. (2012) isolated VpROMT similar to that in C. nivalis , may provide
pseudoreticulata resveratrol
a valuable source of pharmacological O -methyltransferase) gene from the products targeted for anticancer,
Chinese wild plant V. pseudoreticulata, anticoagulant, antimicrobial, or anti-
which had 98.9% and 98.3% identity inflammatory treatments (Duval et
to the resveratrol O -methyltransferase al., 2000). Additionally, by using blue
gene of V. vinifera at the nucleotide and autofluorescence method Lesniewska
amino acid levels and found that gene et al. (2004) also found that UV-C light
expression level was rapidly induced by forced Vitis vinifera cells to produce UV-C irradiation in suspension culture phytoalexins - secondary metabolites
cells of Vitis romanetii. Ku et al. (2005) with antimicrobial properties.
also found that synthesis of resveratrol Ghorpade et al. (2011) used tissue
and piceatannol (stilbenoids) were culture techniques and examined the
promoted by UV-C radiation in callus effect of UV-C on the synthesis of four
cultures of peanut.
Katerova et al.
3.1.2. Plant organs Nadeau et al. (2012) studied the Total polyphenols and phenol-
effect of hormetic UV-C dose on carboxylic acids in potato and buckwheat
glucosinolates - secondary metabolites were examined by Orsák et al. (2001)
derived from amino acids which are who found that the content of secondary
the precursors of bioactive compounds metabolites studied was enhanced by
with anti-cancer properties such as UV-C irradiation. Schmidlin et al. (2008) sulforaphane and indole-3-carbinol.
reported that a whole range of stilbene The authors showed that UV-C tended derivatives (including trans -resveratrol,
to enhance 4-methoxyglucobrassicin, trans - and cis-piceid, trans-ε and trans-δ
4-hydroxyglucobrassicin and gluco- viniferins, and trans-pterostilbene),
raphanin in broccoli florets. So they are induced in leaves of grapevine
suggested that hormetic dose of UV-C (Vitis vinifera , Cabernet Sauvignon
had biochemical significance to enlarge variety) by UV-C (6 min, 90 mW cm -2 ).
potential health effect of broccoli in Balouchi et al. (2009) investigated the
cancer prevention by increasing bioactive changes in photosynthetic pigments and
compounds.
other physiological and biochemical traits of durum wheat leaves exposed
3.2. Impact of UV-B on the synthesis of
to UV-C radiation. Their results
secondary metabolites
showed that carotenoids, anthocyanins, flavonoids and proline content increased
3.2.1. In vitro cultured plant cells, tissues significantly by UV-C as compared
and calli
to the control. Other authors (Boveris Accumulation and tissue localization et al., 2001) found that the pigment
of phenolic compounds in response to apigeninidin -2 (3-deoxyanthocyanidin) UV-B radiation (up to 40 d, 0.74 W m )
accumulated in the epidermal areas of was studied in two strains of Camellia soybean cotyledons irradiated for 60
sinensis L. (tea plant) callus cultures, min with UV-C light. Interestingly, the
which varied in biosynthetic capacity authors stated that this pigment was not
(Zagoskina et al., 2003). UV-B treatment verified in soybean species principally
affected negatively culture growth and and only UV-C (not UV-B) led to its
size of the callus-forming cells. However, induced accumulation. The in vitro
UV-B radiation induced a considerable test showed that apigeninidin had the
increase in soluble phenolics and flavans ability to quench some semiquinone
but the rise in polymeric forms as lignin radicals as ascorbyl and lipid radicals in
was negligible. Phenolic deposition in
a dose-dependent manner and possessed cell walls and intercellular space as well antioxidant capacity. Twenty four hour-
as the deposition of lignin-resembling UV-C irradiation was also effective in the
substance on the callus cultures surface reddening of yellow saffron thistle florets
also rose. Further, the strain possessing to yield carthamin which is applied as a
a higher rate of phenolic compounds colour additive for processed foods, in
accumulation revealed greater tolerance cosmetic and medicinal industry (Saito,
to the UV-B radiation, demonstrating 2001).
the key role of these metabolites in cell
UV radiation and secondary metabolites
protection to UV-B light. -2 37.9 kJ m ) the optimal was found to be
UV-B irradiation induced a rise of -2 25.3 kJ m . It was reported that exposure nitric oxide (NO) production, activities of
of P. quadrangularis calli to the optimal nitric oxide synthase and phenylalanine dose increased the production of all ammonia lyase (leading to flavonoid
studied flavonoids, which was 6 to 40- synthesis), as well as flavonoid level in
fold higher than elicitation with methyl Ginkgo biloba callus (Hao et al., 2009).
jasmonate. In addition, UV-B treatment The authors reported that both inhibitors
led to a higher antioxidant activity of nitric oxide synthase and nitric oxide
compared to non-treated calli. Further, reduced phenylalanine ammonia lyase UV-B exposure of callus cultures for 7 activity and the production of flavonoids.
days caused production of isoorientin It was noted that both phenylalanine
similar to the quantities found in fresh ammonia lyase activation and flavonoids
leaves from glasshouse-grown plants. synthesis in UV-B treated G. biloba
The induction of monoterpenoid callus were induced mainly by NO
indole alkaloids camptothecin and signaling molecule. Further, Loyall et
10-hydroxycamptothecin by phyto- al. (2000) demonstrated a contribution
hormones, heavy metals, hydrogen of the non-enzymatic and enzymatic
peroxide and UV-B radiation were antioxidants glutathione and glutathione evaluated in the Chinese medicinal tree
S-transferase in the initial events of UV- Camptotheca acuminate cell culture (Pi dependent signaling to the gene encoding
et al., 2010). UV-B and salicylic acid the key enzyme chalcone synthase (Chs)
showed the most prominent effects as in parsley (Petroselinum crispum) cell elicitors of the studied alkaloids, which
cultures. Thus, using several short pulses are valuable due to their considerable of UV-B radiations, the authors proved
anti-tumor actions. UV-B irradiation that the oxidative cell status played a role
of Catharanthus roseus multiple shoot of a central regulating element.
cultures and cell suspension cultures was The effect of elicitation by methyl
shown to induce a considerable rise in the jasmonate and/or UV-B radiation on the
production of terpenoid indole alkaloids, production of four C-glycosyl flavonoids
along with precursors of the dimeric (isoorientin, orientin, isovitexin, vitexin)
alkaloids vinblastine and vincristine was examined in callus cultures from leaf
(composed of both vindoline and explants of Passiflora quadrangularis
catharanthine), known to be effective in (Antognoni et al., 2007). Flavonoids
the treatment of leukemia and lymphoma shield UV-B radiation, play a defense
(Binder et al., 2009). Other authors role against pathogen attacks, function
exposed stationary phase cell suspension as attractants to pollinators, and because
cultures of C. roseus to low UV-B dose of their high antioxidant activity, they are
and succeeded to enhance substantially considered to posses health-promoting
the amounts of catharanthine and assets for humans and to provide
vindoline without affecting cell growth protection against cardiovascular disease,
and viability (Ramani and Chelliah, cancer, and age-related disorders. Among
2008). The concentrations of these all UV-B doses tested (12.6, 25.3 and
secondary metabolites were found to be
Katerova et al.
highest 48-72h after UV-B treatment. authors assumed that the octadecanoid In general, cell cultures of C. roseus
pathway did not actively control the produce terpenoid indole alkaloids, but
generation of terpenoid indole alkaloids fail in vindoline production, noted to
under normal or UV-B stress conditions
be an important component of the anti- in C. roseus . Leaf concentration of tumor dimeric alkaloids. Further, Ramani
another monoterpene indole alkaloid and Chelliah (2007) showed that cell
brachycerine, possessing antioxidant and surface receptor(s), calcium, medium
antimutagenic activities was also noted alkalinization, ROS, Ca +2 -dependent to increase significantly in UV-B-treated
protein kinase and mitogen-activated cuttings of Psychotria brachyceras protein kinase have an important role Müll. Arg. (do Nascimento et al., 2013).
in UV-B signaling, in transcriptional The authors supposed that brachycerine activation of triptophan decarboxylase
probably participated in acute UV-B (Tdc) and strictosidine synthase responses and at least partially its
(Str ) genes, which encode enzymes accumulation might be regulated at participating in biosynthesis of terpenoid
transcriptional level. The expression of indole alkaloids, and subsequent
the majority genes involved in peppermint accumulation of catharanthine.
(Mentha x piperita L.) terpenoid biosynthesis were also modulated by
3.2.2. Plant organs exposure to UV-B (7.1 kJm -2 day -1 UV BE ) Ouwerkerk et al. (1999) reported
radiation of plants grown in field and in a that UV-B specifically induced a Tdc-
growth chamber, but it did not correlate gusA construct in tobacco. In addition,
with the amount of most essential oil UV-B induced expression of Tdc gene compounds (Dolzhenko et al., 2010). and accumulation of terpenoid indole
The authors documented enhanced alkaloids, but the percentage induction
phenolic compounds like flavonoids of catharanthine and vindoline was
eriocitrin, hesperidin and kaempferol not markedly enhanced (14 and 11%,
7-O-rutinoside in UV-B treated plants. respectively) in C. roseus leaves. Binder
The interaction between terpenoid and et al. (2009) revealed that up to 168h
flavonoid production in response to after UV-B exposure an augmented
UV-B was proven by the higher essential lochnericine and reduced hörhammericine
oil amount in the growth chamber plants amounts were found in hairy roots of
associated with lower total phenolic
C. roseus . When UV-B exposure time contents; and the decreased terpenoid was increased up to 20 min a rise in
concentrations in field grown peppermint lochnericine, serpentine, and ajmalicine
related with increased content of phenolic and decline in hörhammericine was
compounds. Expectedly, it was concluded noted. Peebles et al. (2009) examined
that field grown plants were better the role of the endogenous production of
adapted to increasing UV-B irradiation jasmonic acid via octadecanoid pathway
than peppermints in the growth chamber in the production of terpenoid indole
due to enhanced flavonoid concentration alkaloids in C. roseus hairy roots using (Dolzhenko et al., 2010). Similarly, octadecanoid pathway inhibitors. The
Johnson et al., (1999) found that the
UV radiation and secondary metabolites
broad-leaf variety of sweet basilicum plants to different spectral quality of (Ocimum basilicum) containing light (red, blue, white) and UV-B showed phenylpropanoids in its essential oils,
the highest melatonin concentration after after UV-B exposure showed a strong
exposure to high intensity UV-B radiation increase of the phenylpropanoids
for 3 days, which decreased after longer (eugenol and methyl-eugenol) and the
exposure period. The authors assumed terpenoids (1,8-cineole, linalool, trans-
that melatonin protected G. uralensis β-ocimene, α- and β-pinene, sabinene,
plant against UV-B-triggered oxidative β-myrcene, limonene, α-terpinolene,
damage. In addition, Solhaug et al. borneol, α-terpineol, α-trans-bergamo-
(2003) showed that induction of melanin tene, γ-cadinene, germacrene D). UV-B
and parietin (anthraquinone, possessing induction of these secondary metabolites
antifungal activity) synthesis in Lobaria was found to be strongest in the five-
pulmonaria and Xanthoria parietina leaf than in two-leaf plants. Further,
lichens required presence of UV-B. UV-B-induced accumulation of the
Secondary metabolites, including monoterpene trans -ocimene was also
flavonoids and terpenoids, are important observed in leaves (predominantly in
for UV-B induced lessening of plant mature than in developing leaves) of
tissue quality required for microbial and linalool-rich commercial variety of sweet
herbivory pathogenes (Bassman, 2004; basilicum lacking phenylpropanoids in
Roberts and Paul, 2006; Izaguirre et al., its essential oil (Ioannidis et al., 2002). In
2007). Izaguirre et al. (2007) reported addition, it was reported that UV-B was
that both UV-B and simulated herbivory necessary for the normal development
induced the accumulation of several leaf of oil glands, in particular for the filling
phenolic compounds (chlorogenic acid of glandular trichomes of sweet basil.
and dicaffeoylspermidine isomers) in Other authors revealed that short term
Nicotiana attenuata and N. longiflora high intensity (3d, 1.13Wm -2 ) and long plants. The flavonoid rutin was specifically
term low intensity (15d, 0.43Wm −2 ) increased by UV-B irradiation. In another UV-B irradiation induced nearly 1.5-fold
study (Kreft et al., 2002), rutin and higher glycyrrhizin (an oleanane-type
tannin concentrations were reported to triterpenoid saponin, natural sweetener
be reduced in the following order: under possessing anti-tumor and anti-viral
ambient > UV-B enhanced (simulating activities) production in roots of only
17% O 3 depletion) > UV-B depleted
3 month-old Glycyrrhiza uralensis (using Mylar foil) conditions in field than in control plants (Afreen et al.,
grown Fagopyrum esculentum Moench 2005). Using a similar model system,
(buckwheat). The highest amounts of Afreen et al. (2006) compared the
these compounds were determined in concentration of melatonin (N-acetyl-5-
flowers, followed by leaves and stems. methoxytryptamine, an indole amine) in
Tattini et al. (2004) identified and different tissues (seed, root, leaf and stem)
quantified the polyphenol spectrum of G. uralensis and noted the highest of Ligustrum vulgare leaves grown amount in root tissues, which increased
outdoors and exposed to increasing with plant development. Exposure of
sunlight (receiving PAR, UV-A and UV-
Katerova et al.
B). A prominent rise in the accumulation availability in pine seedlings. A similar of singlet oxygen ( 1 O
investigation was done with silver birch flavonoids (quercetin 3-O-rutinoside and
2 )-scavengers as
(Betula pendula ) showing that UV-B luteolin 7-O-glucoside occurring in both
exposure did not increase phenolic acids adaxial epidermis and palisade tissue) or condensed tannins, but significantly and hydrohycinnamates (echinacoside,
enhanced flavonoids (quercitrin, hyperin, occurring mainly in abaxial tissues) was
kaempferol-3-rhamnoside and myricetin- reported in response to solar radiation. 3-galactoside), which are important
The authors assume that a coordinated UV-B shields (de la Rosa et al., 2001). control system exists between flavonoids
The concentration of some flavonoids and hydroxycinnamate pathways and
was also found to depend on UV-B dose. flavonoids may serve antioxidant
In another study with silver birch it was functions in L. vulgare exposed to excess
shown that the amount of condensed light. Different physiological parameters
tannins and anthocyanins in leaves was were measured in Scots pine (Pinus
not altered by UV-B light (Tegelberg et sylvestris ) subjected to different UV-B
al., 2004). The authors showed that UV-B levels for one growing season (Lavola
treatment increased the concentration of et al. 2003). The authors suggested that
quercetins, kaempferols, and chlorogenic pine plants was adequately protected
acids but along all measured plant growth against supplemental irradiation. It parameters only leaf area was negatively was noted that UV-B affected mainly
affected. In another study, Kostina et al. secondary metabolites. Under moderate
(2001) demonstrated a rise in (+)-catechin, nutrient availability, the accumulation
quercetin, cinnamic acid derivatives, of flavonols in P. sylvestris needles was
apigenin and pentagalloylglucose in highest at the ambient (4.3 kJ m -2 day -1 )
leaves of birch seedlings exposed or near to ambient UV-radiation doses.
to enhanced UV-B radiation. The At elevated nutrient level the UV-B
significant negative correlations between doses higher than ambient (up to 13.1
apigenin, and mainly quercetin amounts kJ m -2 day -1 ) specifically enhanced the
and levels of lipid peroxidation revealed accumulation of diacylated flavonols
the antioxidant role of these secondary (dicoumaroyl-trifolin,
dicoumaroyl- metabolites (Kostina et al. 2001). Wulff isorhamnetin,
dicoumaroyl-astragalin et al. (1999) also documented UV-B- and dicoumaroyl-isoquercitin) in a
dependent accumulation of quercetin dose-dependant manner. Non-acylated
3-glycoside in European silver birch flavonols were increased to a lesser extent
(Betula pendula Roth.) seedlings by UV-B treatment but condensed tannins
exposed to high UV-B radiation
were not enhanced. The major effects -1 levels (14.4 or 22.5 kJ m d UV-B BE ) of UV-B radiation were on the pathway
accompanied with a transient increase division converting dihydroflavonols to
of Chs mRNA assuming induction of flavonols. The authors assume that the
flavonoid biosynthesis. The reported production of secondary metabolites
weaker induction of Chs mRNA levels through flavonoid pathway is multi-step
in the higher UV-B dose exposure led regulated by UV-B exposure and nutrient
to the suggestion that a substantial DNA
UV radiation and secondary metabolites
damage took place, which could partially deciduous as V. myrtillus , but possessed inhibit the transcription of Chs as well.
leaves with structural similarity to V. The concentrations of flavonol conjugates
vitis-idaea . Kumari and Agrawal (2010) and betacyanins in the halophyte
also applied supplemental to the ambient Mesembryanthemum crystallinum
was -2 d UV-B radiation (1.8 and 3.6 kJ m -1 ) on also reported to increase after exposure
the aromatic perennial herb Cymbopogon to very low wavelength UV-B (like
citratus (D.C.) Staph in field conditions 280 or 295 nm) and the amount of
and reported that only the high dose feruloylglucose (the precursor of flavonol
inhibited biomass production. A reduction conjugates and acylated betacyanins)
of chlorophyll content without significant was much higher in leaves than in leaf
alteration in photosynthesis, and increase tips (Ibdah et al., 2002). The authors
of carotenoids and phenolic compounds reported that accumulation of flavonols
was noted in UV-B treated plants. and betacyanins could be illustrated by a
The authors demonstrated the positive weakly sigmoid dose function along with
outcome of the supplemental low dose of an exponential reduction of the response
UV-B radiation on volatile oil production function of the plant with increasing
acompanied by dense waxy deposition wavelength.
on the adaxial surface of the leaves. Other authors observed that three
Further, using 45 species from different closely related species of the sub-Arctic
genera, Holmes and Keiller (2002) dwarf shrubs (Vaccinium myrtillus L.,
demonstrated that one of the functions Vaccinium vitis-idaea L., and Vaccinium
of leaf waxes is shielding of UV-B. In uliginosum L.) grown outdoors showed
another study, a reduction of epicuticular different strategies in UV-B response
wax, photosynthetic pigments and concerning the content and distribution
flavonoid content in needles of Korean of UV-absorbing phenolic compounds
pine (Pinus koraiensis Sieb. et Zucc) was in leaves (Semerdjieva et al., 2003). • associated with a rise in ROS ( OH, H
2 O 2 ) Methanol-extractable UV-B absorbing
and malondialdehyde amounts as well as compounds were highest in V. myrtillus.
catalase activity after UV-B exposure (Zu They increased with UV-B irradiation,
et al., 2011). The authors reported that and were distributed all over the leaf
Korean pine had considerable sensitivity but concentrated in cells containing
to supplementary UV-B exposure and chlorophyll. The majority of phenolic
concluded that the induced antioxidant compounds in V. vitis-idaea were cell-
defense system was not efficient against wall bound, concentrated in the walls of
UV-B triggered injuries. Supplemental epidermis and their pool was enhanced
UV-B radiation applied for 3 months with UV-B dose. The authors assumed
to three-year-old Taxus chinensis var. that the difference in strategies for UV
mairei led to a significant augmentation screening found in those two plants could
of taxol (a diterpenoid) and flavonoid
be connected with leaf longevity. The content in fully expanded leaves, which response of V. uliginosum to UV-B was
play an important role in the observed found to be flexible. This plasticity was
plant tolerance (Zu et al., 2010). The taxol explained with the fact that the plant is
production of this shrub is well known
130 Katerova et al.
and it is used as an anti-tumor agent for treating breast and ovarian cancer, however up to now the major source of this molecule is obtained via extraction from T. chinensis var. mairei.