photoprotection revisited: genetic and molecular … · tection has long been a topic of...

P1: SKH/mbg P2: SKH/NBL/vks QC: NBL

March 10, 1999 15:26 Annual Reviews AR082-13

Annu. Rev. Plant Physiol. Plant Mol. Biol. 1999. 50:333–59Copyright c© 1999 by Annual Reviews. All rights reserved

PHOTOPROTECTION REVISITED:Genetic and Molecular Approaches

Krishna K. NiyogiDepartment of Plant and Microbial Biology, University of California, Berkeley,California 94720-3102; e-mail: [email protected]

KEY WORDS: antioxidant, oxidative stress, photosynthesis, photoinhibition, thermaldissipation

ABSTRACT

The involvement of excited and highly reactive intermediates in oxygenic photo-synthesis poses unique problems for algae and plants in terms of potential oxida-tive damage to the photosynthetic apparatus. Photoprotective processes prevent orminimize generation of oxidizing molecules, scavenge reactive oxygen speciesefficiently, and repair damage that inevitably occurs. This review summarizesseveral photoprotective mechanisms operating within chloroplasts of plants andgreen algae. The recent use of genetic and molecular biological approaches isproviding new insights into photoprotection, especially with respect to thermaldissipation of excess absorbed light energy, alternative electron transport path-ways, chloroplast antioxidant systems, and repair of photosystem II.

CONTENTS

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334

PHOTO-OXIDATIVE DAMAGE TO THE PHOTOSYNTHETIC APPARATUS. . . . . . . . . . 335Generation of Oxidizing Molecules in Photosynthesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . 335Targets of Photo-Oxidative Damage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340

ADJUSTMENT OF LIGHT-HARVESTING ANTENNA SIZE. . . . . . . . . . . . . . . . . . . . . . . . 340

THERMAL DISSIPATION OF EXCESS ABSORBED LIGHT ENERGY. . . . . . . . . . . . . . . 341

PHOTOPROTECTION THROUGH PHOTOCHEMISTRY. . . . . . . . . . . . . . . . . . . . . . . . . . . 344Assimilatory Linear Electron Transport. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344Oxygen-Dependent Electron Transport. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345Cyclic Electron Transport. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346

SCAVENGING OF REACTIVE OXYGEN SPECIES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347Antioxidant Molecules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347Scavenging Enzymes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349

3331040-2519/99/0601-0333$08.00

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334 NIYOGI

REPAIR OF PHOTODAMAGE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350

CONCLUSIONS AND PROSPECTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351

INTRODUCTION

Light is required for photosynthesis, yet plants need protection from light.Photosynthesis inevitably generates highly reactive intermediates and byprod-ucts that can cause oxidative damage to the photosynthetic apparatus (16, 58).This photo-oxidative damage, if not repaired, decreases the efficiency and/ormaximum rate of photosynthesis, termed “photoinhibition” (89; reviewed in13, 98, 118, 129).

Oxygenic photosynthetic organisms have evolved multiple photoprotectivemechanisms to cope with the potentially damaging effects of light, as dia-grammed in Figure 1. Some algae and plants avoid absorption of excessive lightby movement of leaves, cells (negative phototaxis), or chloroplasts. Within thechloroplast, regulation of photosynthetic light harvesting and electron transport

Figure 1 Schematic diagram of photoprotective processes occurring within chloroplasts.

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PHOTOPROTECTION REVISITED 335

balances the absorption and utilization of light energy. For example, adjust-ments in light-harvesting antenna size and photosynthetic capacity can decreaselight absorption and increase light utilization, respectively, during relativelylong-term acclimation to excessive light. Alternative electron transport path-ways and thermal dissipation can also help to remove excess absorbed lightenergy from the photosynthetic apparatus. Numerous antioxidant moleculesand scavenging enzymes are present to deal with the inevitable generation ofreactive molecules, especially reactive oxygen species. However, despite thesephotoprotective defenses, damage to the photosynthetic machinery still occurs,necessitating turnover and replacement of damaged proteins. The overall goalof photoprotection is, therefore, to prevent net damage from occurring.

Because of its importance for maintaining photosynthesis and ultimately forsurvival of photosynthetic organisms in many natural environments, photopro-tection has long been a topic of considerable interest in plant physiology andbiochemistry. The general subject of photoprotection, as well as several specificphotoprotective processes, have been extensively reviewed (9, 13, 18, 31, 37,47–49, 51, 57, 65, 76, 77, 98, 112, 118, 128), so one might ask why the topicneeds to be “revisited.” Although genetic methods have long been used inanalysis of photosynthesis (95, 142), only recently has there been widespreaduse of genetic and molecular techniques to dissect specific processes involvedin photoprotection. The roles of specific cloned genes are being tested throughreverse genetics, and classical (forward) genetics is uncovering new mutantsand providing insights into the complexity of photoprotection. This articlereviews several photoprotective processes that occur within chloroplasts of eu-karyotic photosynthetic organisms, with a particular emphasis on the use ofmolecular and genetic approaches in intact green algae and plants. Space con-straints direct my focus to results obtained with the most commonly used modelorganisms with good molecular genetics, the green algaChlamydomonas rein-hardtii (Table 1) and the C3 vascular plantsArabidopsis thalianaand tobacco(Table 2).

PHOTO-OXIDATIVE DAMAGETO THE PHOTOSYNTHETIC APPARATUS

Before discussing specific photoprotective mechanisms, I briefly summarize thetypes of oxidizing molecules that are involved in damaging the photosyntheticmachinery.

Generation of Oxidizing Molecules in PhotosynthesisBecause of the large differences in redox potential between reactants and prod-ucts and the involvement of excited intermediates, oxygenic photosynthesis

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336 NIYOGI

Table 1 Summary of Chlamydomonas mutants affecting photoprotection in the chloroplast

Photoprotective Photoprotectionsystem Mutant Description phenotype Reference

Thermal npq1 Lacks zeaxanthin in HLa Partly NPQ-deficient; 109dissipation and grows in HLxanthophylls npq2 Accumulates zeaxanthin; Faster induction of NPQ; 109

lacks antheraxanthin, grows in HLviolaxanthin, andneoxanthin

lor1 Lacks lutein and Partly NPQ-deficient; 38, 110loroxanthin grows in HL

npq1 lor1 Lacks zeaxanthin in HL; NPQ-deficient; 110lacks lutein and sensitive to HLloroxanthin

npq4 Normal xanthophylls NPQ-deficient; D Elrad, KK Niyogigrows in HL & AR Grossman,

unpublished results

npq2 lor1 Accumulates zeaxanthin; Grows in HL KK Niyogi,lacks antheraxanthin, unpublished resultsviolaxanthin,neoxanthin, lutein,and loroxanthin

Water-water PAR19 Overexpresses FeSOD n.d.b 85cycle andscavengingenzymes

PS II repair sr/spr Deficient in chloroplast Sensitive to HL 73protein synthesis

ag16.2 Accumulates D1 protein Sensitive to HL 158

psbT1 Deficient in PS II in HL Sensitive to HL 105

aHigh light.bNot determined.

poses unique problems for algae and plants with respect to the generation ofreactive oxygen species and other oxidizing molecules. Potentially damagingmolecules are generated at three major sites in the photosynthetic apparatus:the light-harvesting complex (LHC) associated with photosystem (PS) II, thePS II reaction center, and the PS I acceptor side.

Chlorophyll (Chl) molecules are critical participants in light-harvesting andelectron transfer reactions in photosynthesis, but Chl can act as a potentendogenous photosensitizer in algae and plants. Absorption of light causesChl to enter the singlet excited state (1Chl), and the excitation energy is rapidlytransferred (<ps time scale) between neighboring Chls in the LHC by res-onance transfer. Before excitation energy is trapped in the reaction center,triplet Chl (3Chl) can be formed from1Chl through intersystem crossing. Thisis an inherent physical property of1Chl (121), and the yield of3Chl formation

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depends on the average lifetime of1Chl (∼ns time scale) in the antenna (58). Incontrast to1Chl, 3Chl is relatively long-lived (∼ms time scale) and can interactwith O2 to produce singlet oxygen (1O2) (56). Because the average lifetime of1Chl in the PS II LHC is several times longer than in the PS I LHC, the potentialfor generation of1O2 is greater in the PS II LHC.

In the PS II reaction center, trapping of excitation energy involves a primarycharge separation between a Chl dimer (P680) and a pheophytin molecule(Pheo) that are bound to the D1 reaction center protein. The P680+/Pheo−

radical pair is reversible (44, 134), and the charge recombination (and otherbackreactions in PS II) can generate triplet P680 (3P680) (11, 13, 115, 148). Asin the LHC, energy exchange between3P680 and O2 results in formation of1O2.The P680+/Pheo− charge separation can be stabilized upon electron transferfrom Pheo− to the quinone acceptor QA, and P680+ is subsequently reducedby an electron derived from the oxidation of H2O via the secondary donor YZ(tyrosine 161 of the D1 protein). Although the very high oxidizing potential ofP680+ and Y+Z enables plants to use H2O as an electron donor, P680+ and Y+Zthemselves are also capable of oxidizing nearby pigments and proteins (11, 13).

In PS I, charge separation occurring between P700 and the primary acceptorChl A0 is stabilized by subsequent electron transfer to secondary acceptors, thephylloquinone A1 and three iron-sulfur clusters (FX, FA, and FB). In contrastto P680+, P700+ is less oxidizing and relatively stable; in fact, P700+ is a veryefficient quencher of excitation energy from the PS I LHC (44, 121). However,the acceptor side of PS I, which has a redox potential low enough to reduceNADP+ via ferredoxin, is also capable of reducing O2 to the superoxide anionradical (O−2 ) (102). O−2 can be metabolized to H2O2, and a metal-catalyzedHaber-Weiss or Fenton reaction can lead to production of the hydroxyl radical(OH·), an extremely toxic type of reactive oxygen species (16).

Generation of these oxidizing molecules occurs at all light intensities, butwhen absorbed light energy exceeds the capacity for light energy utilizationthrough photosynthesis, the potential for photo-oxidative damage is exacer-bated. In excessive light, accumulation of excitation energy in the PS II LHCwill increase the average lifetime of1Chl, thereby increasing the yield of3Chland1O2. Higher excitation pressure in PS II can increase the frequency of directdamage by P680+ and formation of1O2 as a result of P680+/Pheo− recombi-nation. Furthermore, the high1pH that builds up in excessive light can inhibitelectron donation to P680+ from the oxygen-evolving complex, resulting inlonger-lived P680+ and/or Y+Z . Overreduction of the PS I acceptor side in ex-cessive light favors direct reduction of O2 to form O−2 . Analysis of transgenictobacco plants with reduced levels of cytochromeb6/f complex has pointed tothe involvement of lumen acidification and/or overreduction of the PS I acceptorside in photoinhibition (80).

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338 NIYOGI

Tabl

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mut

ants

and

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chlo

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eren

ce

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rmal

diss

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ion

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thin

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La

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-defi

cien

t;11

1an

dxa

ntho

phyl

lsse

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veto

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t-te

rmH

L

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(npq

2)A

ccum

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thin

;Fa

ster

indu

ctio

nof

81,1

11,1

44la

cks

anth

erax

anth

in,

viol

axan

thin

,and

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neox

anth

in

Toba

cco

aba2

Acc

umul

ates

zeax

anth

in;

n.d.

b10

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cks

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thin

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defic

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Ara

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Targets of Photo-Oxidative DamageAlthough the exact mechanism(s) of damage has not been determined, PSII is a major target of photo-oxidative damage (10, 11, 13, 115). Interestingly,photoinhibition of PS II, measured as a decrease in either the flash yield of O2evolution or the Chl fluorescence parameterFv/Fm, seems to depend on thenumber of absorbed photons rather than the rate of photon absorption. Thisimplies that there is a constant probability of photodamage for each absorbedphoton during conditions of steady-state photosynthesis (122, 123). However,the probability of photodamage can be modulated by changes in Chl antennasize and rate of electron transport (20, 124), which alter the excitation pressureon PS II (79) at a given light intensity.

Generation of1O2 within the LHCs can potentially lead to oxidation of lipids,proteins, and pigments in the immediate vicinity (87). Thylakoid membranelipids are especially susceptible to damage by1O2 because of the abundanceof unsaturated fatty acid side chains. Reaction between1O2 and these lipidsproduces hydroperoxides and initiates peroxyl radical chain reactions in thethylakoid membrane. Generation of1O2 and/or P680+ in the PS II reactioncenter can result in damage to lipids, critical pigment cofactors, and proteinsubunits associated with PS II, especially the D1 protein, resulting in photo-oxidative inactivation of entire reaction centers (13, 18).

On the acceptor (stromal) side of PS I, the targets of oxidative damage by O−2 ,

H2O2, and OH· include key enzymes of photosynthetic carbon metabolism suchas phosphoribulokinase, fructose-1,6-bisphosphatase, and NADP-glyceralde-hyde-3-phosphate dehydrogenase (16, 84). Photoinhibition due to damage tothe PS I reaction center itself can be observed under some circumstances, es-pecially during chilling stress (reviewed in 143).

ADJUSTMENT OF LIGHT-HARVESTINGANTENNA SIZE

Changes in the sizes of the Chl antennae associated with PS II and PS I areinvolved in balancing light absorption and utilization (reviewed in 7, 103). Dur-ing long-term acclimation to growth in different light intensities, changes inantenna size are due to changes in LHC gene expression (54, 101, 150) and/orLHC protein degradation (96). No mutants affecting acclimatory changes inChl antenna size have yet been reported.

Short-term alteration of the relative antenna sizes of PS II and PS I can occurbecause of state transitions. According to the state transition model (reviewedin 2), overexcitation of PS II relative to PS I reduces the plastoquinone pool andactivates a kinase that phosphorylates the peripheral LHC associated with PS II.Subsequent detachment of phospho-LHC from PS II decreases the effective size

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of the PS II antenna, and the phospho-LHC may then transfer excitation energyto PS I. Although the state transition is often suggested to be photoprotective(for example, see 8, 13), there is no convincing evidence for its role in photo-protection, at least in excessive light. In fact, the LHC kinase system seems tobe inactivated in high light (46, 130, 136). Mutants affecting the state transitionwould be useful to test the hypothesis.

THERMAL DISSIPATION OF EXCESSABSORBED LIGHT ENERGY

In excessive light, an increase in the thylakoid1pH regulates PS II lightharvesting by triggering the dissipation of excess absorbed light energy asheat (reviewed in 31, 47, 48, 49, 51, 65, 76, 77). Over 75% of absorbed pho-tons can be eliminated by this process of thermal dissipation (50), which in-volves de-excitation of1Chl and which is measured (and often referred to) asnonphotochemical quenching of Chl fluorescence (NPQ). Thermal dissipationis thought to protect photosynthesis by (a) decreasing the lifetime of1Chl tominimize generation of1O2 in the PS II LHC and reaction center, (b) preventingoveracidification of the lumen and generation of long-lived P680+, and (c) de-creasing the rate of O2reduction by PS I. Although thermal dissipation is usuallyreversible within seconds or minutes, sustained thermal dissipation (also calledqI) can be manifested as photoinhibition, which may actually be due to a pho-toprotective mechanism (52, 77, 82, 92, 116). I focus on the rapidly reversible(pH-dependent) type of thermal dissipation (also called qE) that predominatesunder most circumstances.

pH-dependent thermal dissipation occurs in the PS II antenna pigment bedand involves specific de-epoxidized xanthophyll pigments (31, 47–49, 51;65, 76, 77). The increase in the thylakoid1pH in excessive light is thoughtto result in protonation of specific LHC polypeptides associated with PS II,namely LHCB4 (CP29) and LHCB5 (CP26) (42, 75, 151, 152). The1pH alsoactivates the enzyme violaxanthin de-epoxidase, which converts violaxanthinassociated with the LHCs to zeaxanthin (and antheraxanthin) via the so-calledxanthophyll cycle (see Figure 2) (55, 125). Binding of zeaxanthin and protonsto the LHC may cause a conformational change, monitored by an absorbancechange at 535 nm (23–25, 132), that is necessary for thermal dissipation. Theactual mechanism of1Chl de-excitation may involve a direct transfer of energyfrom Chl to zeaxanthin (37, 62, 120). Alternatively, xanthophylls (and pro-tons) may act as allosteric effectors of LHC structure, leading to “concentrationquenching” by Chls (77) or quenching via Chl dimer formation (42).

Mutants ofChlamydomonasand Arabidopsis that affect thermal dissipa-tion have been isolated by video imaging of Chl fluorescence quenching (109,

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Figure 2 The pathway for carotenoid biosynthesis in green algae and plants. The steps blockedin mutants ofChlamydomonasand Arabidopsis are designated by the boxes, with the names of themutants shown in the left half of the box forChlamydomonasand the right half for Arabidopsis.Strains defective in some of these reactions have also been identified inScenedesmus obliquus(26, 28). The xanthophyll cycle that operates in theβ-carotene branch of the pathway involveszeaxanthin, antheraxanthin, and violaxanthin. The carotenoids that normally accumulate in thechloroplast are underlined; zeaxanthin accumulates in excessive light.

111, 140). Characterization of mutants affected in xanthophyll metabolism(Figure 2) has confirmed a role for zeaxanthin in thermal dissipation (81,109, 111, 144). Thenpq2 mutants ofChlamydomonasand Arabidopsis, to-gether with the existingaba1mutant of Arabidopsis (90) andaba2mutant oftobacco (100), are defective in zeaxanthin epoxidase activity; they accumulatezeaxanthin and contain only trace amounts of antheraxanthin, violaxanthin,and neoxanthin (53, 100, 109, 111, 131). The constitutive presence of zeaxan-thin in the PS II LHCs ofnpq2andaba1mutants is not sufficient for ther-mal dissipation, which also requires the1pH. However, induction of NPQby illumination with high light is more rapid in the mutants compared towild type (81, 109, 111, 144), presumably because it is driven solely by thebuild-up of the1pH. During short-term illumination with high light, someaba1mutants exhibited the same sensitivity to photoinhibition as the wild type

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(81), whereas anaba1mutant with a different allele appeared more sensitive(144).

TheChlamydomonas npq1mutant, which is unable to convert violaxanthin tozeaxanthin, is partially defective in NPQ but retains substantial, pH-dependentNPQ, which suggests that some but not all thermal dissipation depends onoperation of the xanthophyll cycle (109). Characterization of thelor1 mu-tant, which lacks xanthophylls derived fromα-carotene (38, 110), indicates apossible role for lutein in thermal dissipation (110). Neithernpq1 nor lor1is particularly sensitive to photoinhibition during growth in high light. How-ever, annpq1 lor1double mutant lacks almost all pH-dependent NPQ and isvery susceptible to photo-oxidative bleaching in high light. Although this re-sult seems to provide evidence for the importance of thermal dissipation forphotoprotection in vivo, the phenotype is complicated by the fact that xantho-phylls are also involved in quenching of3Chl and1O2 and inhibition of lipidperoxidation, as described below. Another mutant,npq4, lacks NPQ but hasnormal xanthophyll composition. It is able to survive in high light, suggest-ing that thermal dissipation itself is not required for photoprotection (D Elrad,KK Niyogi & AR Grossman, unpublished results).

Results with Arabidopsis xanthophyll mutants, although generally similarto results withChlamydomonas, revealed that the relative contributions of dif-ferent xanthophylls to thermal dissipation vary in different organisms. Likenpq1of Chlamydomonas, Arabidopsisnpq1mutants are unable to convert vi-olaxanthin to zeaxanthin in high light. Genetic and molecular analyses (111)demonstrated that the phenotype ofnpq1is due to a recessive mutation in theArabidopsis violaxanthin de-epoxidase gene (35). In contrast to the resultswith Chlamydomonas, induction of pH-dependent thermal dissipation in theArabidopsisnpq1mutant is almost completely inhibited, suggesting that mostof the thermal dissipation in Arabidopsis depends on de-epoxidation of vio-laxanthin (111). Leaves ofnpq1plants sustain more photoinhibition than wildtype following short-term illumination with high light, in agreement with ex-periments that used dithiothreitol as an inhibitor of violaxanthin de-epoxidasein detached leaves (23, 156). However, growth ofnpq1plants that are accli-mated to high light is not noticeably different from that of the wild type, whichsuggests that, in the long term, other photoprotective processes can compensatefor the defect innpq1(111).

As in Chlamydomonas, analysis of Arabidopsis mutants lacking lutein sug-gests that the residual pH-dependent NPQ innpq1may be attributable to a con-tribution of lutein-dependent NPQ (127). Thelut2 mutant, which lacks luteinowing to a mutation in the lycopeneε-cyclase gene (126), exhibits slower in-duction and a lower maximum extent of NPQ (127), and annpq1 lut2doublemutant lacks almost all pH-dependent NPQ (O Bj¨orkman, C Shih, B Pogson,

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344 NIYOGI

D DellaPenna & KK Niyogi, unpublished results). Lutein may have a direct rolein thermal dissipation, or the lack of lutein inlor1 andlut2 mutants may havean indirect effect. In mutants lacking lutein, zeaxanthin may be sequestered inbinding sites that are normally occupied by lutein but inactive in thermal dissi-pation, thereby lowering the relative effectiveness of zeaxanthin (AM Gilmore,KK Niyogi & O Bj orkman, unpublished results).

The requirement for different proteins in the PS II LHC for thermal dissipa-tion has been addressed using mutants lacking Chlb. These mutants, such asbarleychlorina f2and Arabidopsisch1, are impaired in the synthesis and/or as-sembly of LHC proteins, especially the peripheral LHC of PS II, due to the lackof Chl b (32, 93, 108). Measurements of NPQ and Chl fluorescence lifetimecomponents in these mutants suggest that only the minor LHC proteins, such asLHCB4 and LHCB5, are necessary for thermal dissipation, although maximalNPQ requires the presence of the entire LHC (12, 34, 66, 68, 69, 97; KK Niyogi,unpublished results). To test the role of specific LHC proteins in thermal dissipa-tion, mutants or antisense plants affecting individual genes will be very useful.

Several othernpqmutants exhibit normal pigment composition and xantho-phyll interconversions. These mutants presumably identify factors besides the1pH and xanthophylls (perhaps LHC components) that are required for ther-mal dissipation inChlamydomonas(109; D Elrad, KK Niyogi & AR Grossman,unpublished results) and Arabidopsis (KK Niyogi, C Shih & O Bjorkman, un-published results). For example, the Arabidopsisnpq4 mutant exhibits thesame lack of NPQ as annpq1 lut2double mutant, which suggests that it isdefective in all pH- and xanthophyll-dependent thermal dissipation, yet it isindistinguishable from wild type in terms of xanthophyll interconversion andgrowth in low light. In addition,npq4lacks the absorbance change that presum-ably reflects the conformational change that is necessary for thermal dissipation(KK Niyogi, C Shih & O Bjorkman, unpublished results). Determination ofthe molecular basis for mutations such asnpq4will likely provide new insightsinto the molecular mechanism of thermal dissipation.

PHOTOPROTECTION THROUGH PHOTOCHEMISTRY

Assimilatory Linear Electron TransportMuch of the light energy absorbed by the LHCs is utilized through photochem-istry that drives linear electron transport from H2O to NADPH, resulting inO2 evolution and reduction of CO2, NO−3 , and SO2−

4 . The maximum rate ofphotosynthesis is a dynamic parameter that can be altered during acclimationto growth in different light environments (reviewed in 30, 37) through changesin enzyme activities and gene expression. These acclimation responses gener-ally occur during a period of several days, and there are few or no molecular

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genetic data that address the importance of long-term acclimation responses forphotoprotection.

Oxygen-Dependent Electron TransportThere is abundant evidence that nonassimilatory electron transport to oxygenplays an important role in consuming excess excitation energy. Oxygen canfunction as an electron acceptor either through the oxygenase reaction catalyzedby Rubisco (photorespiration) or by direct reduction of oxygen by electrons onthe acceptor side of PS I (102), and there is much debate about which processis more important for photoprotection (for example, see 22, 71, 119, 124, 155).

PHOTORESPIRATION In C3 plants, especially under conditions of CO2 limita-tion, photorespiratory oxygen metabolism is capable of maintaining consider-able linear electron transport and utilization of light energy (reviewed in 117).The role of photorespiration in photoprotection can be conveniently assessedby varying the gas composition to inhibit the oxygenation reaction of Rubisco.Blocking photorespiration with mutations or inhibitors leads to inhibition ofphotosynthesis and photo-oxidative damage, and the cause of inhibition varies(14). Accumulation of photorespiratory metabolites or depletion of carbon in-termediates can inhibit the Calvin cycle and shut down the photochemical sinkfor excitation energy. To assess the importance of photorespiration without thecomplications of accumulating toxic intermediates, mutants of Rubisco thatlack oxygenase activity would be required.

Manipulation of glutamine synthetase activity, a rate-limiting step in pho-torespiratory metabolism, in transgenic tobacco plants has provided additionalevidence for the role of photorespiration in photoprotection (91). Antisenseplants with less glutamine synthetase are more sensitive to photo-oxidationunder conditions of CO2 limitation because of accumulation of photorespira-tory NH3, in agreement with previous results with mutant barley plants lackingglutamine synthetase (149). In contrast, plants that overexpress glutamine syn-thetase have a higher capacity for photorespiration and are more resistant tophotoinhibition and photo-oxidative damage (91).

PHOTOREDUCTION OF OXYGEN BY PHOTOSYSTEM I Direct reduction of O2 byPS I is the first step in an alternative electron transport pathway that has beenvariously termed pseudocyclic electron transport, the Mehler-ascorbate perox-idase reaction, and the water-water cycle. Because a comprehensive review ofthis pathway appears in this volume (17), it is only briefly outlined here. The O−

2produced on the acceptor side of PS I by reduction of O2 is efficiently metabo-lized by thylakoid-bound isozymes of superoxide dismutase (SOD) and ascor-bate peroxidase (APX) to generate H2O and monodehydroascorbate, which canitself be reduced directly by PS I to regenerate ascorbate (15–17). Thus the four

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346 NIYOGI

electrons generated by oxidation of H2O by PS II are consumed by reductionof O2 to H2O by PS I. This pseudocyclic pathway generates a1pH for ATPsynthesis, but neither NADPH nor net O2 is produced.

Like photorespiration, the water-water cycle may help to dissipate excitationenergy through electron transport. However, the capacity of this pathway tosupport electron transport is unclear; estimates range between 10% and 30%of normal linear electron transport in algae and C3 plants (22, 99, 119). Thepathway may also be involved in maintaining a1pH necessary for thermaldissipation of excess absorbed light energy (135).

Genes encoding SOD and APX have been identified from several plants,including Arabidopsis (83, 86), and this should enable dissection of the water-water cycle by analysis of antisense plants or mutants. However, the mutantanalysis will be complicated by the dual roles of these enzymes in scavengingreactive oxygen intermediates and in electron transport.

Cyclic Electron TransportWithin PS II, cyclic electron transport pathways, possibly involving cytochromeb559, have been suggested to dissipate excitation energy (reviewed in 37, 154),but convincing evidence for their occurrence in vivo is lacking. Cytochromeb559may function to oxidize Pheo− or reduce P680+ to protect against photodamageto PS II (154). Site-directed mutagenesis of the chloroplast genes encoding theα andβ polypeptides of cytochromeb559 in Chlamydomonas(1, 107) will beuseful to dissect the possible role of this cytochrome in cyclic electron transportand photoprotection.

Cyclic electron transport around PS I is also suggested to have an importantrole in photoprotection. In addition to dissipating energy absorbed by PS I,cyclic electron transport may be involved in generating or maintaining the1pH that is necessary for downregulation of PS II by thermal dissipation ofexcess absorbed light energy (72). Biochemical approaches have led to theconclusion that there are at least two pathways of PS I cyclic electron transport,one involving a ferredoxin-plastoquinone oxidoreductase (FQR) and the otherinvolving an NADPH/NADH dehydrogenase (NDH) complex (reviewed in 21).The FQR has not yet been identified, although the PsaE subunit of PS I ispossibly involved. The NDH pathway involves a protein complex bound to thethylakoid membrane that is homologous to the NADH dehydrogenase complex Iof mitochondria. Several subunits of this complex are encoded by genes on thechloroplast genome of many plants (reviewed in 63).

Mutants affecting the NDH complex have been generated by disruptingndhgenes in the chloroplast genome of tobacco by homologous recombination(36, 88, 139). These mutants have no obvious phenotype under normal growthconditions. However, measurements of Chl fluorescence and PS I reduction

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kinetics revealed that cyclic electron transport is partially impaired (36, 139).Induction of thermal dissipation upon sudden illumination was slightly delayedin mutants subjected to water stress, consistent with the idea that PS I cyclicelectron transport is involved in maintaining a1pH for thermal dissipation (36).Thorough examination of photoprotection in these mutants under less favorablegrowth conditions should be very informative, although the existence of othercyclic pathways may complicate interpretations.

SCAVENGING OF REACTIVE OXYGEN SPECIES

Several antioxidant systems in the chloroplast can scavenge reactive oxygenspecies that are inevitably generated by photosynthesis (15, 57, 128). In manycases, increases in antioxidant molecules and enzymes have been observedduring acclimation to excessive light (for example, see 67), and the specificroles of these molecules are starting to be tested using mutant and transgenicorganisms (3–5, 57).

Antioxidant MoleculesCAROTENOIDS Carotenoids, including the xanthophylls, are membrane-boundantioxidants that can quench3Chl and1O2, inhibit lipid peroxidation, and stabi-lize membranes (reviewed in 51, 60, 61, 70). The genes and enzymes involvedin their biosynthesis have been reviewed recently in this series (43). Xantho-phylls bound to the LHC proteins are located in close proximity to Chl forefficient quenching of3Chl and1O2 (94). β-carotene in the PS II reaction cen-ter quenches1O2 produced from interaction of3P680 and O2 but is not thoughtto quench3P680 itself (145). As discussed above, specific xanthophylls arealso involved in quenching of1Chl during thermal dissipation.

Carotenoids in general have essential functions in photosynthesis and pho-toprotection, as demonstrated by the bleached phenotypes of algae and plantsthat are unable to synthesize any carotenoids owing to mutations affecting earlysteps of carotenoid biosynthesis (6, 133). However, characterization of xantho-phyll mutants blocked in later steps in the carotenoid pathway (Figure 2) hasdemonstrated that no single xanthophyll is absolutely required for photopro-tection (109–111, 126). For example,Chlamydomonasmutants lacking luteinand loroxanthin (lor1), antheraxanthin, violaxanthin, and neoxanthin (npq2),or zeaxanthin (npq1) are all able to grow as well as wild type in high light(109, 110).

Eliminating combinations of xanthophylls in double mutants has revealeda redundancy among xanthophylls in terms of photoprotection. Constructionof an npq1 lor1 double mutant has demonstrated that accumulation ofeither zeaxanthin or lutein is necessary for photoprotection and survival of

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348 NIYOGI

Chlamydomonasin high light (110). The light sensitivity of thenpq1 lor1strainprobably reflects the multiple roles of specific xanthophylls in photoprotection;quenching of1Chl, 1O2, and possibly also inhibition of lipid peroxidation areimpaired in the absence of both zeaxanthin and lutein (110). A similar sensi-tivity to very high light is observed in older leaves of the Arabidopsisnpq1 lut2double mutant, but the photo-oxidative damage is not lethal to the entire plant(O Bjorkman, C Shih, B Pogson, D DellaPenna & KK Niyogi, unpublishedresults).

Zeaxanthin is sufficient for photoprotection. Double mutants ofChlamy-domonas(npq2 lor1) and Arabidopsis (aba1 lut2), as well as the C-2A′-67,3bstrain ofScenedesmus obliquus(26), that contain zeaxanthin as the only chloro-plast xanthophyll are viable, although the photosynthetic efficiency of theScenedesmusstrain is decreased (27), and growth of the Arabidopsisaba1 lut2is impaired (127). In the case ofChlamydomonas npq2 lor1, the mutant growsas well as wild type in high light (KK Niyogi, unpublished results). Selectiveelimination of all the xanthophylls derived fromβ-carotene to test whetherlutein is sufficient for photoprotection may be difficult using mutants, becausethe sameβ-hydroxylase seems to be involved in synthesis of both zeaxanthinand lutein (43).

TOCOPHEROLS Another important thylakoid membrane antioxidant isα-toco-pherol (vitamin E), which can physically quench or chemically scavenge1O2,O−2 and OH· in the membrane to prevent lipid peroxidation (57). Whereas thexanthophylls are largely bound to proteins,α-tocopherol is free in the lipidmatrix of the membrane and appears to have a role in controlling membranefluidity and stability (64). In addition,α-tocopherol participates in the efficienttermination of lipid peroxidation chain reactions with concomitant formationof its α-chromanoxyl radical (64).

Althoughα-tocopherol is the most abundant tocopherol in the chloroplast,other tocopherols such asβ- andγ -tocopherols are also present at low lev-els. The minor tocopherols are intermediates in the synthesis ofα-tocopherolthat differ in the number of methyl groups on the chromanol head group. Therelative abundance of the tocopherols (α > β > γ ) parallels their effective-ness as chemical scavengers of reactive oxygen species and as chain reactionterminators (64).

Unfortunately, there are few genetic data that address the importance of spe-cific tocopherols, or tocopherols in general, in photoprotection. AScenedesmusmutant lacking all tocopherols has been reported (29), but the lesion in themutant appeared to affect the synthesis of phylloquinones and phytylbenzo-quinones in addition to tocopherols (74). The Arabidopsispds1andpds2mu-tants lack both tocopherols and quinones due to blocks in the biosynthetic

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pathway common to both types of molecules (113). Interestingly, the mutantsalso lacked carotenoids, uncovering a role for plastoquinone in the desaturationof phytoene (113). A gene encoding one of the later steps of tocopherol biosyn-thesis has recently been identified in cyanobacteria and Arabidopsis (140a), andapplication of reverse genetics promises insights into the photoprotective roleof specific tocopherols in the near future.

ASCORBATE The soluble antioxidant ascorbate (vitamin C) has a centralrole in preventing oxidative damage through direct quenching of1O2, O−2and OH·, in regeneration ofα-tocopherol from theα-chromanoxyl radical,and as a substrate in both the violaxanthin de-epoxidase and APX reactions(112, 141). Although ascorbate is very abundant in chloroplasts (∼25 mM),the biosynthetic pathway for ascorbate in plants was elucidated only very re-cently (153), and the importance of ascorbate in photoprotection has not beendetermined. Ascorbate-deficient mutants affecting the biosynthetic pathwayhave been identified recently in a screen for ozone-sensitive mutants in Ara-bidopsis (40, 41). One mutant, originally calledsoz1but renamedvtc1, accu-mulates approximately 30% as much ascorbate as wild type. In addition to itsozone sensitivity, thevtc1mutant is more sensitive to exogenous H2O2, SO2,and illumination with UV-B, but sensitivity to high light has not yet beenexamined.

GLUTATHIONE Another important soluble antioxidant in the chloroplast is glu-tathione, which is capable of detoxifying1O2 and OH·. Glutathione protectsthiol groups in stromal enzymes, and it is also involved inα-tocopherol re-generation and ascorbate regeneration through the glutathione-ascorbate cycle(16, 59). Biosynthesis of glutathione proceeds by the reaction of glutamineand cysteine to formγ -glutamylcysteine, followed by the addition of glycinecatalyzed by glutathione synthetase. The Arabidopsiscad2mutant is deficientin glutathione (78) owing to a defect in the gene encodingγ -glutamylcysteinesynthetase (39). The level of glutathione incad2leaves is approximately 30%of that in the wild type. As in the case of the ascorbate-deficient mutant, thesensitivity ofcad2to photoinhibition remains to be determined.

Scavenging EnzymesThe enzymes SOD and APX are involved in scavenging reactive oxygen speciesin the chloroplast. As discussed above, O−2 generated by reduction of O2 byPS I is metabolized enzymatically by SOD to produce H2O2. The subsequentreduction of H2O2 by APX produces the monodehydroascorbate radical, whichcan be directly reduced by PS I (via ferredoxin) in the water-water cycle (17).Ascorbate can also be regenerated in the stroma by the set of enzymes com-prising the glutathione-ascorbate cycle (16, 59).

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350 NIYOGI

Both SOD and APX exist in multiple isoforms within the chloroplast. TheSOD isozymes are generally classified according to their active site metal (33):copper/zinc (Cu/ZnSOD), iron (FeSOD), or manganese (MnSOD). Most plantshave FeSOD and Cu/ZnSOD in their chloroplasts, whereas most algae appearto lack the Cu/ZnSOD completely (45). Thylakoid-bound forms of SOD (114)and APX (104) may efficiently detoxify O−2 and H2O2 at their site of production(16, 58) and prevent inactivation of Calvin cycle enzymes (84). Soluble formsof SOD and APX react with O−2 and H2O2 that diffuse into the stroma from thethylakoid membrane.

Mutants or antisense plants have the potential to reveal roles of differentSODs and APXs in photoprotection. Arabidopsis has at least seven SOD genes:three encoding Cu/ZnSOD, three encoding FeSOD, and one encoding MnSOD(86). One of the Cu/ZnSOD and all three of FeSOD gene products are locatedin the chloroplast. Mutants that are deficient in expression of plastidic andcytosolic Cu/ZnSODs or plastidic FeSODs have been isolated (DJ Klieben-stein & RL Last, unpublished results), but the phenotypes of these mutants inhigh light have not been tested. Genes encoding the stromal and thylakoid-bound forms of APX have been cloned from Arabidopsis (83), but mutants ortransgenic plants with altered expression of chloroplast APX have not yet beenreported.

Transgenic plants overexpressing SOD have been generated by severalgroups, and the results of these experiments were recently reviewed (3–5, 57).Photoprotection has been examined specifically in only a few cases. An en-hancement of tolerance to a combination of high light and chilling stresshas been observed in tobacco plants overexpressing chloroplast Cu/ZnSOD(137, 138) but not in plants overexpressing chloroplast FeSOD (147). This isconsistent with the localization of Cu/ZnSOD at the thylakoid membrane whereO−2 is reduced by PS I (114). However, chloroplast APX activity is induced inthe Cu/ZnSOD transgenic plants; the combination of increased Cu/ZnSOD andAPX activities may result in enhanced photoprotection (138). The photopro-tection phenotype of aChlamydomonasmutant that overexpresses chloroplast-localized FeSOD has not been reported (85).

REPAIR OF PHOTODAMAGE

Despite multiple lines of defense, damage to the photosynthetic apparatus is aninevitable consequence of oxygenic photosynthesis, and the PS II reaction cen-ter is especially susceptible to photo-oxidative damage. Therefore, oxygenicphotosynthetic organisms have evolved an elaborate but efficient system forrepairing PS II that involves selective degradation of damaged proteins (pri-marily D1) and incorporation of newly synthesized proteins to reconstitute

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functional PS II (13). Under some circumstances, damaged PS II reactioncenters may also be sites of thermal dissipation (qI) (92, 116).

The repair of PS II is an important photoprotective mechanism, because therate of repair must match the rate of damage to avoid photoinhibition resultingfrom net loss of functional PS II centers. Therefore, continual new synthesis ofchloroplast-encoded proteins, especially D1, is critical for photoprotection at alllight intensities. Blocking chloroplast protein synthesis with inhibitors duringexposure of algae or plants to light results in photoinhibition and net loss of D1protein (for example, see 19, 146). Similarly, attenuation of chloroplast proteinsynthesis in rRNA mutants ofChlamydomonasleads to chronic photoinhibitionand lower levels of D1 during growth in high light (73). To analyze the effectof a specific limitation of D1 synthesis, rather than a general inhibition ofall chloroplast protein synthesis, it may be possible to isolate partial loss-of-function alleles of mutations such asChlamydomonasF35, which specificallyaffects expression of D1 (157).

Other factors involved in repair of PS II are being identified by various geneticapproaches. A screen forChlamydomonasmutants that are more sensitive toPS II photodamage has uncovered mutants that may be defective in PS II repair,including a mutant that accumulates more D1 protein than wild type (158). Apossible role for thepsbTgene (originally known asycf8) in the chloroplastgenome ofChlamydomonasin protection of PS II in high light has been sug-gested by characterization of apsbTdisruption mutant (105). In transgenictobacco plants, decreasing the unsaturation of chloroplast glycerolipids resultsin greater sensitivity to low-temperature photoinhibition, apparently because ofinhibition of PS II repair (106).

CONCLUSIONS AND PROSPECTS

Photoprotection of photosynthesis is a balancing act. Thermal dissipation andalternative electron transport pathways, together with changes in antenna sizeand overall photosynthetic capacity, help to balance light absorption and utiliza-tion in the constantly changing natural environment. The generation of reactiveoxygen species is balanced by the capacity of antioxidant systems. The capacityfor repair must match the damage that is not prevented by other photoprotectiveprocesses.

Multiple, partially redundant mechanisms acting in concert help to preventnet photo-oxidative damage that results from generation of reactive moleculesthrough photosynthesis. The redundancy is not surprising given how crit-ical photoprotection is for fitness and survival of algae and plants in mostenvironments. Indeed, many of the molecules and enzymes involved in photo-protection have roles in more than one photoprotective mechanism. For

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example, xanthophylls are involved in both thermal dissipation and quench-ing of 1O2, whereas SOD and APX enzymes scavenge reactive oxygen specieswhile maintaining electron transport via the water-water cycle.

The use of molecular genetic approaches to study photoprotection is justbeginning, but the potential for rapid progress is obvious. Mutants affectingmany chloroplast processes are now available, and assessment of their relativeimportance for photoprotection is under way in several laboratories. Becausephotoprotective processes comprise several lines of defense against the damag-ing effects of light, construction of double and perhaps even triple mutants insome cases may be necessary in order to obtain clear phenotypes. In the future,other genetic approaches, such as screening for suppressors or enhancers ofthese phenotypes, may uncover previously unrecognized mechanisms of pho-toprotection.

ACKNOWLEDGMENTS

I thank Anastasios Melis and Catharina Casper-Lindley for comments on themanuscript and Dan Kliebenstein and Dean DellaPenna for communicatingunpublished results. Unpublished work in the author’s laboratory is supportedby grants from the Searle Scholars Program/The Chicago Community Trustand the USDA NRICGP (#9801685).

Visit the Annual Reviews home pageathttp://www.AnnualReviews.org

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Annual Review of Plant Physiology and Plant Molecular Biology Volume 50, 1999

CONTENTSEducator and Editor, Martin Gibbs 1Phosphate Translocators in Plastids, Ulf-Ingo Flügge 27The 1-Deoxy-D-xylulose-5-phosphate Pathway of Isoprenoid Biosynthesisin Plants, Hartmut K. Lichtenthaler 47

Chlorophyll Degradation, Philippe Matile, Stefan Hörtensteiner, Howard Thomas 67

Plant Protein Serine/Threonine Kinases: Classification and Functions, D. G. Hardie 97

Improving the Nutrient Composition of Plants to Enhance Human Nutrition and Health, Michael A. Grusak, Dean DellaPenna 133

Gametophyte Development in Ferns, Jo Ann Banks 163C4 Gene Expression, Jen Sheen 187Genetic Analysis of Hormone Signaling, Peter McCourt 219Cellulose Biosynthesis: Exciting Times for a Difficult Field of Study, Deborah P. Delmer 245

Nitrate Reductase Structure, Function, and Regulation: Bridging the Gap Between Biochemistry and Physiology, Wilbur H. Campbell 277

Crassulacean Acid Metabolism: Molecular Genetics, John C. Cushman, Hans J. Bohnert 305

Photoprotection Revisited: Genetic and Molecular Approaches, Krishna K. Niyogi 333

Molecular and Cellular Aspects of the Arbuscular Mycorrhizal Symbiosis, Maria J. Harrison 361

Enzymes and Other Agents that Enhance Cell Wall Extensibility, Daniel J. Cosgrove 391

Leaf Development in Angiosperms, Neelima Sinha 419The Pressure Probe: A Versatile Tool in Plant Cell Physiology, A. Deri Tomos, Roger A. Leigh 447

The Shikimate Pathway, Klaus M. Herrmann, Lisa M. Weaver 473Asymmetric Cell Division in Plants, Ben Scheres, Philip N. Benfey\ 505CO2-Concentrating Mechanisms in Photosynthetic Microorganisms, Aaron Kaplan, Leonora Reinhold 539

Plant Cold Acclimation: Freezing Tolerance Genes and Regulatory Mechanisms, Michael F. Thomashow 571

The Water-Water Cycle in Chloroplasts: Scavenging of Active Oxygens and Dissipation of Excess Photons, Kozi Asada 601

Silicon, Emanuel Epstein 641Phosphate Acquisition, K. G. Raghothama 665Roots in Soil: Unearthing the Complexities of Roots and Their Rhizospheres, Margaret E. McCully 695

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photoprotection revisited: genetic and molecular … · tection has long been a topic of...

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