Journal of Photochemistry and Photobiology B: Biology 122 (2013) 1–6

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Journal of Photochemistry and Photobiology B: Biology

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Preillumination of lettuce seedlings with red light enhances the resistanceof photosynthetic apparatus to UV-A

Vladimir D. Kreslavski a,b,⇑, Valery Yu. Lyubimov a, Galina N. Shirshikova a, Alexander N. Shmarev a,Anatoly A. Kosobryukhov a, Franz-Josef Schmitt c, Thomas Friedrich c, Suleyman I. Allakhverdiev a,b,⇑a Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Moscow Region 142290, Russiab Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya Street 35, Moscow 127276, Russiac Technical University of Berlin, Institute of Chemistry, Max-Volmer-Laboratory of Biophysical Chemistry, Straße des 17. Juni 135, D-10623 Berlin, Germany

a r t i c l e i n f o

Article history:Received 7 January 2013Received in revised form 22 February 2013Accepted 25 February 2013Available online 5 March 2013

Keywords:Lactuca sativa L.Peroxidase activityStress-resistancePhotosynthetic apparatusPhytochromeUV-A radiation

1011-1344/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jphotobiol.2013.02.016

⇑ Corresponding authors at: Institute of Basic BAcademy of Sciences, Pushchino, Moscow Region 142

E-mail addresses: vkreslav@rambler.ru (V.D. Kresla@google.com (S.I. Allakhverdiev).

a b s t r a c t

Seedlings of 10-day-old lettuce (Lactuca sativa L., cultivar Berlin) were preilluminated by low intensityred light (kmax = 660 nm, 10 min, 5 lmol quanta m�2 s�1) and far-red light (kmax = 730 nm, 10 min,5 lmol quanta m�2 s�1) to study the effect of pre-treatment on photosynthesis, photochemical activityof photosystem II (PSII), the contents of photosynthetic and UV-A-absorbing pigments (UAPs) andH2O2, as well as total and ascorbate peroxidase activities in cotyledonary leaves of seedlings exposedto UV-A. UV radiation reduced the photosynthetic rate (Pn), the activity of PSII, and the contents of Chla and b, carotenoids and UAPs in the leaves, but increased the content of H2O2 and the total peroxidaseactivity. Preillumination with red light removed these effects of UV. In turn, the illumination with redlight, then far-red light removed the effect of the red light. Illumination with red light alone increasedthe content of UAPs, as well as peroxidase activity. It is suggested that higher resistance of the lettucephotosynthetic apparatus to UV-A radiation is associated with involvement of the active form of phyto-chrome B, thereby increasing peroxidase activities as well as UAPs and saving preservation of photosyn-thetic pigment contents due to pre-illumination with red light.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

UV radiation damages the various target molecules and systemsof the photosynthetic apparatus (PA), especially PSII, first of allcomponents such as QA, QB, PQ, proteins D1 and D2 as well asthe manganese cluster [1–6]. Excitation of manganese, likelyhigh-valent manganese species, by UV induces an inactivation ofthe manganese-containing oxygen-evolving complex due to re-lease of Mn ion [4,5]. Here, normal reduction of Pþ680 is impaired[7] that leads to the generation of various reactive oxygen species(ROS). On the other hand, UV may induce a variety of protectionsystems, increasing the synthesis of sunscreen UV-absorbing com-pounds and enhancing the activity and/or synthesis of antioxidantenzymes and accumulation of low molecular weight antioxidants[8–10]. Photoreceptors, in particular phytochrome (Phy), can playan important role in the photoprotection of PA from UV radiationand photoreactivation of the PA [11,12]. In the so-called ‘‘low-energy’’ phytochrome-controlled reactions, short-time red lighttransforms the Phy into physiologically active form PFR [13,14].

ll rights reserved.

iological Problems, Russian290, Russia.vski), suleyman.allakhverdiev

Upon irradiation with red light (kmax = 660 nm, 5–10 min),and a subsequent irradiation with far-red light (kmax = 730 nm,5–10 min) effect of red light is eliminated.

It is believed that short-term exposure of the leaves to red light,inducing the formation of the active form of phytochrome – PFR,conserves the content of chlorophyll and other pigments, and alsohelps to maintain the structure and activity of chloroplasts duringaging of leaves [11,12,15]. Different types of phytochrome are in-volved in the formation of mechanisms of plant adaptation to envi-ronmental stress factors [16]. In particular, PhyB can participate incold and high temperature adaptation and is involved in the pro-tection of plants from high-intensity light and UV-B radiation[16–18].

It is assumed that the phytochrome is involved in the adaptiveresponses of PA to high intensity light [19]. It is also found that thelow intensity red light, acting presumably through the phyto-chrome system, modifies the effect of UV radiation on growth, pro-tects PSII against UV radiation and reduces the rate of degradationof Chl [12,15,20–22]. It has also been suggested that the protectiveeffect of red light is a consequence of the induction of weak oxida-tive stress at relatively high doses of red light (1 J cm�2) [22]. Be-sides that, studies available in the literature are mainly focusedon the protective effects of low intensity red light against UV-B[15,21,23,24], which usually demonstrated more significant

Fig. 1. Effect of preillumination of seedlings with red light and far-red light on UV-induced degradation of Chl a (A), Chl b (B) and carotenoids (C) in the dark. After thelast dark period, the seedlings were subjected to continuous white light (light) orkept under dark conditions (dark). A part of the seedlings was irradiated with UV-A(UV) or preilluminated with red light or red light ? far-red light, then irradiatedwith UV. Original contents of pigments were: 0.42 mg of Chl a, 0.144 mg of Chl band 0.131 mg of carotenoids per 1 g of fresh leaf mass. The values are the means ofthree biological replicates ±SE. Table 1

Effects of UV irradiation and preillumination with RL or RL ? FRL on the rate ofphotosynthesis (Pn) in lettuce seedlings. A part of the seedlings was exposed to UV orRL (RL ? FRL), followed by UV-A, and then kept in the dark 0.5 h (1) or 24 h (2).Another part of the seedlings was irradiated with RL, then kept in the dark for 2.5 h(1) or 26 h (2) or not irradiated (control). RL (10) – seedlings were exposed to RL, thenkept in the dark for 10 min. The values are the means of 3 biological replicates ±SE.

Variants Photosynthetic rate (lmol CO2 m�2 s�1)

1 2

Control 38.8 ± 1.8 44.3 ± 2.7RL (10) 33.1 ± 1.1 –RL 38.0 ± 2.0* 42.2 ± 2.3*

UV 21.4 ± 2.2 27.5 ± 1.9RL ? UV 31.2 ± 2.5* 35.8 ± 2.3*

RL ? FRL ? UV 26.9 ± 2.0 –

* Difference is insignificant (p > 0.05). Difference between UV and RL ? UV is sig-nificant (p < 0.05).

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damage to PA than UV-A [25]. However, UV-A also induced astrong decrease in the activity of PSII and radical formation, as ob-served in the unicellular green alga Dunaliella spp. [26]. Here, theoxygen-evolving complex, absorbing UV and blue light, can be apossible target of UV-A [27]. Thus, the protecting role of red lightagainst UV-A, especially at low intensities (60.05 J cm�2), has notbeen studied exhaustively. It is unclear, how and to which extentthe PFR, the antioxidant system, and the pool of UV-absorbing pig-ments contribute to the protective effects of red light.

To solve this problem, the effects of UV radiation, red light illu-mination and combined action of UV and red light on the photo-synthetic activity in cotyledonary lettuce leaves wereinvestigated. We hypothesized that the protective effect of red

light against UV radiation involves PFR, the antioxidant systemand UAPs.

2. Materials and methods

Lettuce seedlings (Lactuca sativa L., cultivar Berlin) were grownunder white fluorescent lamps (I = 60 lmol quanta m�2 s�1), witha photoperiod of 12 h and a temperature of 21 ± 1 �C during theday and 19 ± 1 �C at night for 10 days. Immediately after the lastnight period, the seedlings were exposed for 2 h to ultraviolet(UV)-A radiation (kmax = 365 nm, 10 W m�2). Before the exposureto UV-A, some of the seedlings were irradiated with red light(kmax = 660 nm, 10 min), or red light (kmax = 660 nm, 10 min) fol-lowed by far-red light (730 nm, 10 min). In some experiments, apart of the seedlings was exposed to green light (kmax = 550 nm,10 min) or far-red light (730 nm, 10 min), then subjected to UVirradiation. Another part of the plant was not subjected to the lighttreatments at all (control). After irradiation, all seedlings were keptin the dark for 0.5 h. In other part of experiments (Fig. 1), seedlingswere grown under white light of lower intensity (10 lmol quantam�2 s�1) and irradiated with either red light, or red light ? far-redlight, and then exposed to UV-A (1 h), or only with UV-A, and thenall seedlings were kept for 6 days in the dark.

Immediately before and after exposure to UV light, as well asduring the subsequent dark incubation of the seedlings, the photo-synthetic activity of PSII and the contents of photosynthetic pig-ments, UAPs and H2O2, as well as total peroxidase and ascorbateperoxidase activities, and the fresh weight of the leaves weremeasured.

To assess the state of the PA under stress and normal physiolog-ical conditions, methods of variable fluorescence of Chl a and themeasurement of CO2 gas exchange were used. Determination ofthe rate of photosynthesis was conducted on seedlings with aninfrared gas analyzer Infralyt 4 (Infralytic GmbH, Marburg,Germany) in a closed system under conditions of a saturating lightintensity. The photochemical activity of PSII was evaluated withthe use of a PAM fluorimeter (XE-PAM, Heinz Walz, Germany).The signal was recorded by means of a computer. The measuredparameters of variable florescence were Fo and Fv, respectively,background and variable fluorescence, and the ratio Fv/Fo charac-terizing PSII quantum efficiency.

UV-A irradiation used an ultraviolet lamp T8 18 W BLB (Selecta)with kmax = 365 nm (I = 12 W m�2). Red light (kmax = 660 nm),green light (kmax = 570 nm) and far-red light (kmax = 730 nm) wereobtained by the use of LED light sources (Dk = 10–20 nm). The lightintensity at the level of leaf surface was 5 lmol quanta m�2 s�1 forred light and green light, and 7.5 lmol quanta m�2 s�1 for far-redlight.

Table 2Effect of UV irradiation and preillumination with RL or RL ? FRL on the contents of photosynthetic pigments and UAPs in cotyledonary leaves and theirfresh mass. The values are the means of four biological replicates ±SE.

No. Parameters Control UV RL ? UV RL ? FRL ? UV

1 Chl a (mg) 0.91 0.81 0.93 0.88Chl a (%) 100 ± 1.5 89.5 ± 1.2 102 ± 1.2 96.5 ± 1.1

2 Chl b (mg) 0.405 0.36 0.41 0.39Chl b (%) 100 ± 1.9 88.4 ± 1.5 102 ± 1.4 96.2 ± 1.6

3 Carotenoids (mg) 0.350 0.317 0.35 0.336Carotenoids (%) 100 ± 2.7 90.7 ± 1.6 100 ± 2.0 96.0 ± 1.5

4 UAPs (%) 100 82.2 ± 2 92.5 ± 1.8 –5 Mass (mg) 2.00 ± 0.022 1.91 ± 0.015 2.0 ± 0.01 1.96 ± 0.016

Mass (%) 100 ± 1.1 95.5 ± 0.75 100 ± 0.6 98 ± 0.08

The differences between control and UV as well as UV and RL ? UV for pigment content and fresh mass are significant (p < 0.05).

Fig. 2. Effect on UV-absorbing pigment contents in lettuce leaves by 10 minillumination of seedlings with red light (1) and red light ? far-red light (2),followed by incubation in the dark. The content of the pigments in the non-irradiated leaves was set to 100%. The values are the means of three biologicalreplicates ±SE.

Table 3Effect of UV irradiation and preillumination with RL or RL ? FRL on the content ofH2O2 in lettuce leaves. A part of seedlings was exposed to RL (10 min), then kept inthe dark for 110 min. Another part was irradiated with UV-A or RL (RL ? FRL), thenUV-A. Control – no irradiations. The values are the means of six replicates ±SE.

Variant H2O2 (lmol g�1) H2O2 (% to control)

Control 0.520 ± 0.020 100 ± 4RL 0.562 ± 0.018* 108 ± 3*

UV 1.207 ± 0.036 232 ± 7RL ? UV 1.070 ± 0.027 206 ± 5RL ? FRL ? UV 1.180 ± 0.050 227 ± 10

* Insignificant difference (p > 0.05). The difference between the UV and RL ? UV issignificant (p < 0.05).

V.D. Kreslavski et al. / Journal of Photochemistry and Photobiology B: Biology 122 (2013) 1–6 3

Total activities of peroxidase and ascorbate peroxidase (AP)were measured in total protein extracts (TPEs). To obtain TPE,leaves (100 mg) were homogenized in 3 ml of medium containing50 mM K-phosphate buffer (pH 7.0), 2 mM Na4 EDTA, 0.5% PVP,5 mM Na ascorbate. After centrifugation for 10 min at 10,000g,the supernatant was used to measure the activity of enzymes.3,30-Diaminobenzidine�4HCl as a substrate (1 mM) and hydrogenperoxide (10 mM) were used to determine the total peroxidaseactivity, which was calculated from the changes in optical densityat km = 450 nm for 1 min, using E = 4.4 mM�1 cm�1, and then ex-pressed as a percentage relative to the control. AP activity wasdetermined by using 0.5 mM sodium ascorbate and 0.1 mM hydro-gen peroxide. The enzyme activity was calculated from the changesin optical density at k = 290 nm for 1 min, using E = 2.8 mM�1 -cm�1, and then expressed as a percentage relative to the control.All measurements were performed using a spectrophotometerGenesis 10UV (Termo Spectronic, USA).

In order to determine the content of H2O2, the leaf disks (50–100 mg) were cooled in liquid nitrogen for 2–3 min. The frozendisks were transferred to 0.4 mL of 2 M trichloroacetic acid (TCA)and homogenized. The homogenate was triply flushed into a testtube using 1 mL of 0.05 M potassium–phosphate buffer (pH 7.0).Next, 3.5 mL of the homogenate was mixed with 100 mg of acti-vated coal for the sorption of pheophytin and carotenoids, and cen-trifuged for 20 min at 10,000g. The supernatant was decanted andtitrated with 2 M KOH to pH 7.0. The content of H2O2 in 100 lL ofthe extract was determined by measuring bioluminescence in the

mixture of 10�6 M horseradish peroxidase and 10�4 M luminol(1:1, total volume 1 mL) [28].

The contents of Chl a and b and carotenoids were measured inethanol extracts, using the known absorption coefficients [29].The content of UAPs was determined by the method described in[30]. 15–20 leaves were kept 24 h in acidic methanol (metha-nol:water:HCl, 78:20:2) at 4 �C. Then, the optical density at327 nm was determined by a spectrophotometer Genesis 10UV(Termo Spectronic, USA) and expressed as number of UAPs.

The tables and graphs show the arithmetic means of the ob-tained values and their standard errors (±SE). At least three biolog-ical and 4–6 analytical replicates were used. To determine thebiochemical parameters and photosynthesis rate, at least 5–10seedlings were used for each condition. 50 Seedlings for each op-tion were used for determination of fresh biomass. The significanceof differences between the variants was described by t-test at the5% significance level.

3. Results

3.1. Photosynthetic activity

In our experiments, we discovered a UV-induced decrease in therate of photosynthesis (Pn) (Table 1). Preillumination with red lightreduced the inhibitory effect of UV. This effect was observed imme-diately after exposure and lasted for at least 24 h after exposure ofseedlings to the dark. Photosynthesis decreased in leaves of seed-lings exposed to illumination with red light, then 10 min darkexposure decreased Pn, but after 2 h dark exposure there was nodifference between red light-illuminated and control samples.The protective effect of red light was eliminated, if preirradiationwith red light was followed by subsequent irradiation with far-red light. Preillumination of seedlings with far-red light and greenlight did not reduce the degree of inhibition of photosynthesis withUV irradiation (data not shown).

Fig. 3. Effect of preillumination with red light on peroxidase activity in leaves ofseedlings irradiated with UV-A (UV) and non-irradiated with light (control(K) = 100%). A part of seedlings was preilluminated with red light, then UV (redlight ? UV) or red light only. After exposure of seedlings to UV or red light ? UV(red light ? far-red light ? UV) they were exposed to the dark for 0.5 h. Afterexposure to red light, only the time of dark exposure was 2.5 h up to determinationof the activity, which was calculated in relative units at equal protein content in thecontrol and experiment. The values are the means of four biological replicates ±SE.

Fig. 4. Dependence of the activity of leaf ascorbate peroxidase (AP) on the time ofdark exposure of seedlings after 10 min illumination with red light. The values arethe means of three biological replicates ±SE.

4 V.D. Kreslavski et al. / Journal of Photochemistry and Photobiology B: Biology 122 (2013) 1–6

As a result of UV irradiation, the contents of chlorophylls a andb, and carotenoids decreased as compared to the dark control with-out irradiation (Table 2). Preillumination with red light resulted ina reduced loss of Chl and carotenoids in leaves of seedlings exposedto UV-A. By itself, red light (without UV) had no appreciable effect.Illumination of seedlings with red light, then far-red light partlyeliminated the effect of red light on UV-induced inhibition, in ef-fect reducing the protective effect of red light. Initially, before irra-diations, the ratio Fv/Fo, estimated on the basis of the fluorescenceinduction curves was equal to 3.4 ± 0.1. After 5 day dark exposure,the ratio Fv/Fo decreased in control and red light preilluminatedleaves to 3.0 ± 0.2. In UV-irradiated leaves without red light pre-treatment the ratio was 1.5 ± 0.2.

3.2. UV-absorbing pigments

UV irradiation resulted in a decrease in the content of UAPs(mainly flavonoids [21]) in the leaves. Pretreatment with red lightpartially removed this effect (Table 2), probably due to the light-in-duced synthesis of these pigments. This is consistent with the factthat upon irradiation with only red light, the level of UAPs in-creased only after a dark lag phase of at least 1 h duration

(Fig. 2). Upon illumination with red light then far-red light, no sig-nificant changes in UAPs were observed.

3.3. The contents of H2O2 and peroxidase activity

Irradiation of seedlings with UV-A was accompanied by in-creased H2O2 content (Table 3). Preillumination with red light ledto the partial reduction of the increase in H2O2, whereas the se-quence of red light, then far-red light illumination did not elimi-nate the elevation of the H2O2 level caused by UV treatment ofseedlings. Reduced activity of PSII in the leaves irradiated withUV was accompanied by an increase in peroxidase activity(Fig. 3). Irradiation with red light only also increased the total per-oxidase activity. Upon measuring the specific ascorbate peroxidaseactivity, the activating effect of red light was also observed with amaximum after 1–2 h dark exposure (Fig. 4). After combined irra-diation (red light ? UV), the enhancement of activity was largercompared to the increase in case of exposure to the two light qual-ities individually. Far-red light partly eliminated the effect of redlight on peroxidase activity.

4. Discussion

Investigation of effects of increased solar ultraviolet radiationon terrestrial plants is one of the focuses of modern plant-stressphysiology [31]. Photosynthesis, first of all PSII and the system ofCO2 fixation, is a global sensor of the majority of environmentallystressful factors including UV-radiation, as well as it is importantfor development of plant adaptive responses [32]. Phytochromesmay be involved in the protection of PA to UV-radiation. Both,the high content of PhyB [17] and the fractional increase of the ac-tive form PFR may be important to initiate protective mechanismsagainst UV radiation [21,22].

Our data on the effects of UV and red light (far-red light)-preillumination on photosynthesis of native leaves strongly sug-gest that an increased PFR level due to pre-illumination with redlight leads to elevated resistance of PA to UV-A radiation, both atthe level of the primary processes of photosynthesis and the CO2

fixation pathway (Table 1). The higher resistance of PA to UV-Awas eliminated, if seedlings were pre-illuminated with red light,then far-red light. This, together with the absence of a protectingeffect of pre-illumination with green light strongly hints at theinvolvement of PhyB in the mechanism of resistance of PA to UVradiation [33]. Short-term illumination of Vigna sinensis L. withred light (kmax = 612 nm) during photoperiod protected PSII of cot-yledonary leaves of V. sinensis L. from UV-B radiation [15]. Theauthors suggested that PhyB is involved in the protection effectof red light irradiation from UV-B-induced damage. Experimentaldata obtained on detached leaves of spinach also suggest theinvolvement of PhyB in the protective response of PSII to UV-B[21] and UV-A [22]. Transformation of PhyB into its active form(PFR) serves as a signal to develop protective responses of PA tostrong sunlight, which is usually accompanied with a high redlight/far-red light ratio and, hence, high content of PFR. This situa-tion can occur at noon, on a sunny day, when the sunlight has ahigh intensity and significant percentage of UV. Under these condi-tions, the ratio of red light/far-red light is also high, which shouldalso lead to increased abundance of the active form of Phy [34,35].

It is known that the leaves of plants exposed to dark for a longtime show a significant loss of Chl and proteins [12]. Both UV-Band UV-A can accelerate the loss, and short-term exposure to redlight slows down this process. It is assumed that synthesis of pro-teolytic enzymes plays an important role in the degradation of Chlin the leaves [36]. The synthesis can be induced by the formation of

V.D. Kreslavski et al. / Journal of Photochemistry and Photobiology B: Biology 122 (2013) 1–6 5

ROS. Moreover, UV irradiation which induces ROS (Table 3) mayenhance the degradation of Chl in the dark (Fig. 1, Table 2).

Preillumination with red light reduced the degree of degrada-tion of photosynthetic pigments induced by UV radiation, probablyby reducing the rate of synthesis of proteolytic enzymes. Thishypothesis is consistent with the fact that red light preilluminationreduced H2O2 levels (Table 3), The reduction is likely a result of anincrease of UAPs (Fig. 2) and peroxidase activities in leaves exposedto red light (Figs. 3 and 4). According to the hypothesis suggestedin the study of [37] it is conceivable that UAPs and peroxidasesrather than protecting PSII from photodamage, stimulate proteinsynthesis, with resultant repair of PSII and mitigation ofphotoinhibition.

The ratio of pro- and antioxidants is the means by which theplant can regulate many processes of growth and adaptation ofPA to stress [38]. Changing this ratio is also one of the nonspecificsigns of development of stress response, in particular, upon UVirradiation [39]. We tried to link the observed protective effectsof red light to the induction of the synthesis of antioxidant en-zymes. Activation of antioxidant enzymes by irradiation of plantwith red light is known, but the mechanism is largely unclear[23,24,40]. One of the key antioxidant enzymes during UV-inducedoxidative stress is a peroxidase, and, above all, ascorbate peroxi-dase [41]. Peroxidase activity may be regulated by the phyto-chrome system [33,40,42], which is consistent with our dataabout the effect of red light preillumination on the total peroxidaseand ascorbate peroxidase activities (Figs. 3 and 4). We also ob-served a reversion of the effect of red light by subsequent far-redlight (data not shown) that suggests the participation of PhyB ininduction of total peroxidase activity.

One can assume that there is a partial similarity between theaction of different effectors such as a stressor (UV radiation) anda protective factor (red light-irradiation) on the antioxidant activ-ity. Red light triggers a chain of phytochrome signaling, whichleads to an increase in peroxidase activity, possibly as a result ofthe preliminary short-time increase in the content of H2O2, whichis consistent with short-term reduction of photosynthesis after10 min exposure to red light (Table 1). Such a mechanism was dis-cussed in our previous paper [22]. Similarly, an irradiation of plantswith UV is accompanied with an increased amount of H2O2 thatserves as the primary trigger signal to induce a cascade of eventsleading to elevated peroxidase activity.

Regarding the mechanism of phytochrome effects, there is astudy available, which demonstrated that the phytochrome-medi-ated enhancement of peroxidase activity in maize leaves was re-pressed by inhibitors of cytoplasmic protein synthesis, whereasinhibitors of RNA and organelle protein synthesis were ineffective[42]. These results indicate that the phytochrome regulation ofperoxidase activity occurs at the translational level. However, itis possible that the short-term exposure to red light leads to theactivation of transcription factors (TFs) such as PIF3 or PIF4, pro-viding direct photoregulation of the expression of genes responsi-ble for stress-protection [33,43,44]. Both, UV irradiation and redlight can enhance peroxidase activity by increasing the synthesisof peroxidase substrates, such as phenolic compounds, especiallyflavonoids, possibly by light-induced activation of the enzymesphenylalaninammonium-lyase and chalcone synthase [45].

5. Conclusion

Preirradiation of lettuce seedlings with red light increases resis-tance of PA to UV-A radiation. The induction of stress resistance ofPA to UV-A is likely due to the activation of PhyB by preillumina-tion of seedlings with red light, which leads to an increase of theactivity of antioxidant enzymes and of the content of UAPs, as well

as a decrease of the ROS pool and the loss of leaf mass in responseto irradiation with UV. Moreover, based on a new scheme devel-oped during the last decade that environmental stress involvingROS production inhibits the de novo synthesis of proteins involvedin the repair of photodamaged photosystem II [46,47], one can sug-gest that preillumination with red light before UV exposure com-pared to only UV illumination leads to reduced formation of ROSand, hence, smaller suppression of the synthesis of photosyntheticand antioxidant proteins.

Red light stimulates the PR ? PFR conversion, which inducestranscription factors (and Ca2+ ions), in effect increasing the activ-ity of antioxidant enzymes. This process increases the UAPs con-tent and preserves the carotenoid and Chl content, eventuallyleading to an increased resistance of PA against UV-A radiation.

6. Abbreviations

AP

ascorbate peroxidase Chl chlorophyll TFs transcription factors PA photosynthetic apparatus PSII photosystem II Phy (P) phytochrome PFR the active form of phytochrome ROS reactive oxygen species UAPs ultraviolet-absorbing pigments

Acknowledgements

This work was supported by Grants from the Russian Founda-tion for Basic Research, Molecular and Cell Biology Programs ofthe Russian Academy of Sciences, and by the Bilateral CooperationProgram between Germany and Russia of the German Bundesmin-isterium für Bildung und Forschung (BMBF Project RUS 11/014) .

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