The roles of specific xanthophylls in photoprotection

Zeaxanthin accumulation in the absence of a functional xanthophyll cycle protects Chlamydomonas reinhardtii from photooxidative stress

Xanthophylls participate in light harvesting and are essential in protecting the chloroplast from photooxidative damage. To investigate the roles of xanthophylls in photoprotection, we isolated and characterized extragenic suppressors of the npq1 lor1 double mutant of Chlamydomonas reinhardtii, which lacks zeaxanthin and lutein and undergoes irreversible photooxidative bleaching and cell death at moderate to high light intensities. Here, we describe three suppressor strains that carry point mutations in the coding sequence of the zeaxanthin epoxidase gene, resulting in the constitutive accumulation of zeaxanthin in a range of concentrations. The presence of zeaxanthin in these strains was sufficient to prevent photooxidative damage in the npq1 lor1 background. The size of the light-harvesting antenna in the suppressors decreased in high light in a manner that was proportional to the relative content of zeaxanthin, with the strain having the most zeaxanthin showing a severe reduction in levels of the major light-harvesting complex II proteins in high light. We show that the effect of constitutive zeaxanthin on light harvesting is not the main cause of increased photoprotection, because in the absence of zeaxanthin, a strain with a smaller light-harvesting antenna showed only minor protection against photobleaching in high light. Furthermore, the zeaxanthin-accumulating suppressors were able to tolerate higher levels of exogenous reactive oxygen than their parental strain under conditions that did not affect light harvesting. Our results are consistent with an antioxidant role of zeaxanthin in the quenching of singlet oxygen and/or free radicals in the thylakoid membrane in vivo.

Wavelength-dependent induction of a mycosporine-like amino acid in a rice-field cyanobacterium, Nostoc commune: role of inhibitors and salt stress

Wavelength-dependent induction of a mycosporine-like amino acid (MAA) was studied in a nitrogen-fixing rice-field cyanobacterium, Nostoc commune. HPLC studies showed the presence of shinorine, a bisubstituted MAA containing both glycine and serine groups and having an absorption maximum at 334 nm. Exposure of cultures to simulated solar radiation in combination with various cut-off filters (WG 280, 295, 305, 320, 335, 345, GG 400, 420, 455, 475, OG 515, 530, 570, RG 645, 665; all Schott filter series) clearly indicated that MAAs were induced by UV-B radiation, while UV-A and PAR had very little effect on MAA induction in this organism. The ratio of the absorption at 334 nm (shinorine) to 665 nm (chlorophyll a) and the derived action spectrum also revealed the induction of MAAs to be UV-B dependent with a prominent peak at 290 nm and a second small peak at 310 nm. Various concentrations (50-300 mM) of NaCl were used to test whether another common stress factor, such as osmotic stress, also induces MAAs, as has been reported for other cyanobacterial species. The results indicate that cells grown at high concentration of NaCl but without UV-B did not show any MAA induction. In order to elucidate the possible photoreceptors, the effects of various inhibitors/quenchers on the induction of MAAs were studied. There was a marked reduction in the amount of MAA when the cells were irradiated with UV-B in the presence of inhibitors of the shikimate pathway (glyphosate, 1 mM), photosynthesis [3-(3,4-dichlorophenyl)-1,1-dimethylurea, 20 microM], protein synthesis (chloramphenicol, 25 microg ml(-1)), pterin synthesis (N-acetylserotonin, 5 mM, and 2,4-diamino-6-hydroxypyrimidine, 5 mM) and a quencher of the excited state of flavins and pterins (phenylacetic acid, 1 mM).

A mutant of the green alga Dunaliella salina constitutively accumulates zeaxanthin under all growth conditions

A novel mutant (zea1) of the halotolerant unicellular green alga Dunaliella salina is impaired in the zeaxanthin epoxidation reaction, thereby lacking a number of the beta-branch xanthophylls. HPLC analysis revealed that the zea1 mutant lacks neoxanthin (N), violaxanthin (V) and antheraxanthin (A) but constitutively accumulates zeaxanthin (Z). Under low-light physiological growth conditions, the zea1 (6 mg Z per g dry weight or 8 x 10(-16) mol Z/cell) had a substantially higher Z content than the wild type (0.2 mg Z per g dry weight or 0.5 x 10(-16) mol Z/cell). Lack of N, V, and A did not affect photosynthesis or growth of the zea1 strain. Biochemical analyses suggested that Z constitutively and quantitatively substitutes for N, V, and A in the zea1 strain. This mutant is discussed in terms of its commercial value and potential utilization by the algal biotechnology industry for the production of zeaxanthin, a high-value bioproduct.

PsbS-dependent enhancement of feedback de-excitation protects photosystem II from photoinhibition

Feedback de-excitation (qE) regulates light harvesting in plants to prevent inhibition of photosynthesis when light absorption exceeds photosynthetic capacity. Although the mechanism of qE is not completely understood, it is known to require a low thylakoid lumen pH, de-epoxidized xanthophylls, and the photosystem II protein PsbS. During a short-term 4-h exposure to excess light, three PsbS- and qE-deficient Arabidopsis thaliana mutants that differed in xanthophyll composition were more photoinhibited than the wild type. The extent of photoinhibition was the same in all of the mutants, suggesting that qE capacity rather than xanthophyll composition is critical for photoprotection in short-term high light, in contrast to longer-term high light conditions (days) when additional antioxidant roles of specific xanthophylls are evident. Plants with a 2-fold increase in qE capacity were generated by overexpression of PsbS, demonstrating that the level of PsbS limits the qE capacity in wild-type Arabidopsis. These results are consistent with the idea that variations in PsbS expression are responsible for species-specific and environmentally induced differences in qE capacity observed in nature. Furthermore, plants with higher qE capacity were more resistant to photoinhibition than the wild type. Increased qE was associated with decreased photosystem II excitation pressure and changes in the fractional areas of chlorophyll a fluorescence lifetime distributions, but not the lifetime centers, suggesting that qE protects from photoinhibition by preventing overreduction of photosystem II electron acceptors. Engineering of qE capacity by PsbS overexpression could potentially yield crop plants that are more resistant to environmental stress.

In vivo modulation of nonphotochemical exciton quenching (NPQ) by regulation of the chloroplast ATP synthase

Nonphotochemical quenching (NPQ) of excitation energy, which protects higher plant photosynthetic machinery from photodamage, is triggered by acidification of the thylakoid lumen as a result of light-induced proton pumping, which also drives the synthesis of ATP. It is clear that the sensitivity of NPQ is modulated in response to changing physiological conditions, but the mechanism for this modulation has remained unclear. Evidence is presented that, in intact tobacco or Arabidopsis leaves, NPQ modulation in response to changing CO(2) levels occurs predominantly by alterations in the conductivity of the CF(O)-CF(1) ATP synthase to protons (g(H)(+)). At a given proton flux, decreasing g(H)(+) will increase transthylakoid proton motive force (pmf), thus lowering lumen pH and contributing to the activation of NPQ. It was found that an approximately 5-fold decrease in g(H)(+) could account for the majority of NPQ modulation as atmospheric CO(2) was decreased from 2,000 ppm to 0 ppm. Data are presented that g(H)(+) is kinetically controlled, rather than imposed thermodynamically by buildup of DeltaG(ATP). Further results suggest that the redox state of the ATP synthase gamma-subunit thiols is not responsible for altering g(H)(+). A working model is proposed wherein g(H)(+) is modulated by stromal metabolite levels, possibly by inorganic phosphate.

A major light-harvesting polypeptide of photosystem II functions in thermal dissipation

Under high-light conditions, photoprotective mechanisms minimize the damaging effects of excess light. A primary photoprotective mechanism is thermal dissipation of excess excitation energy within the light-harvesting complex of photosystem II (LHCII). Although roles for both carotenoids and specific polypeptides in thermal dissipation have been reported, neither the site nor the mechanism of this process has been defined precisely. Here, we describe the physiological and molecular characteristics of the Chlamydomonas reinhardtii npq5 mutant, a strain that exhibits little thermal dissipation. This strain is normal for state transition, high light-induced violaxanthin deepoxidation, and low light growth, but it is more sensitive to photoinhibition than the wild type. Furthermore, both pigment data and measurements of photosynthesis suggest that the photosystem II antenna in the npq5 mutant has one-third fewer light-harvesting trimers than do wild-type cells. The npq5 mutant is null for a gene designated Lhcbm1, which encodes a light-harvesting polypeptide present in the trimers of the photosystem II antennae. Based on sequence data, the Lhcbm1 gene is 1 of 10 genes that encode the major LHCII polypeptides in Chlamydomonas. Amino acid alignments demonstrate that these predicted polypeptides display a high degree of sequence identity but maintain specific differences in their N-terminal regions. Both physiological and molecular characterization of the npq5 mutant suggest that most thermal dissipation within LHCII of Chlamydomonas is dependent on the peripherally associated trimeric LHC polypeptides.

A major light-harvesting polypeptide of photosystem II functions in thermal dissipation

Under high-light conditions, photoprotective mechanisms minimize the damaging effects of excess light. A primary photoprotective mechanism is thermal dissipation of excess excitation energy within the light-harvesting complex of photosystem II (LHCII). Although roles for both carotenoids and specific polypeptides in thermal dissipation have been reported, neither the site nor the mechanism of this process has been defined precisely. Here, we describe the physiological and molecular characteristics of the Chlamydomonas reinhardtii npq5 mutant, a strain that exhibits little thermal dissipation. This strain is normal for state transition, high light-induced violaxanthin deepoxidation, and low light growth, but it is more sensitive to photoinhibition than the wild type. Furthermore, both pigment data and measurements of photosynthesis suggest that the photosystem II antenna in the npq5 mutant has one-third fewer light-harvesting trimers than do wild-type cells. The npq5 mutant is null for a gene designated Lhcbm1, which encodes a light-harvesting polypeptide present in the trimers of the photosystem II antennae. Based on sequence data, the Lhcbm1 gene is 1 of 10 genes that encode the major LHCII polypeptides in Chlamydomonas. Amino acid alignments demonstrate that these predicted polypeptides display a high degree of sequence identity but maintain specific differences in their N-terminal regions. Both physiological and molecular characterization of the npq5 mutant suggest that most thermal dissipation within LHCII of Chlamydomonas is dependent on the peripherally associated trimeric LHC polypeptides.

Identification and regulation of high light-induced genes in Chlamydomonas reinhardtii

We have used restriction fragment differential display for isolating genes of the unicellular green alga Chlamydomonas reinhardtii that exhibit elevated expression on exposure of cells to high light. Some of the high light-activated genes were also controlled by CO2 concentration. Genes requiring both elevated light and low CO2 levels for activation encoded both novel polypeptides and those that function in concentrating inorganic carbon (extracellular carbonic anhydrase, low CO2-induced protein, ABC transporter of the MRP subfamily). All the genes in this category were shown to be under the control of Cia5, a protein that regulates the responses of C. reinhardtii to low-CO2 conditions. Genes specifically activated by high light, even under high-CO2 conditions, encoded a 30 kDa chloroplast membrane protein, a serine hydroxymethyltransferase, a nuclease, and two proteins of unknown function. Experiments using DCMU, an inhibitor of photosynthetic electron transport, and mutants devoid of either photosystem I or photosystem II activity, showed aberrant expression of all the genes regulated by both CO2 and high light, suggesting that redox plays a role in controlling their expression. In contrast, there was little effect of DCMU or lesions that block photosynthetic electron transport on the activity of genes that were specifically controlled by high light.

Xanthophyll biosynthetic mutants of Arabidopsis thaliana: altered nonphotochemical quenching of chlorophyll fluorescence is due to changes in Photosystem II antenna size and stability

Xanthophylls (oxygen derivatives of carotenes) are essential components of the plant photosynthetic apparatus. Lutein, the most abundant xanthophyll, is attached primarily to the bulk antenna complex, light-harvesting complex (LHC) II. We have used mutations in Arabidopsis thaliana that selectively eliminate (and substitute) specific xanthophylls in order to study their function(s) in vivo. These include two lutein-deficient mutants, lut1 and lut2, the epoxy xanthophyll-deficient aba1 mutant and the lut2aba1 double mutant. Photosystem stoichiometry, antenna sizes and xanthophyll cycle activity have been related to alterations in nonphotochemical quenching of chlorophyll fluorescence (NPQ). Nondenaturing polyacrylamide gel electrophoresis indicates reduced stability of trimeric LHC II in the absence of lutein (and/or epoxy xanthophylls). Photosystem (antenna) size and stoichiometry is altered in all mutants relative to wild type (WT). Maximal DeltapH-dependent NPQ (qE) is reduced in the following order: WT>aba1>lut1 approximately lut2>lut2aba1, paralleling reduction in Photosystem (PS) II antenna size. Finally, light-activation of NPQ shows that zeaxanthin and antheraxanthin present constitutively in lut mutants are not qE active, and hence, the same can be inferred of the lutein they replace. Thus, a direct involvement of lutein in the mechanism of qE is unlikely. Rather, altered NPQ in xanthophyll biosynthetic mutants is explained by disturbed macro-organization of LHC II and reduced PS II-antenna size in the absence of the optimal, wild-type xanthophyll composition. These data suggest the evolutionary conservation of lutein content in plants was selected for due to its unique ability to optimize antenna structure, stability and macro-organization for efficient regulation of light-harvesting under natural environmental conditions.

Characterization of acclimation of Hordeum vulgare to high irradiation based on different responses of photosynthetic activity and pigment composition

The ability of spring barley (Hordeum vulgare cv. Akcent) to adjust the composition and function of the photosynthetic apparatus to growth irradiances of 25-1200 mumol m(-2) s(-1) was studied by gas exchange and chlorophyll a fluorescence measurements and high-performance liquid chromatography. The increased growth irradiance stimulated light- and CO(2)-saturated rates of CO(2) assimilation expressed on a leaf area basis up to 730 mumol m(-2) s(-1) (HL730), whereas at an irradiance of 1200 mumol m(-2) s(-1) (EHL1200) both rates decreased significantly. Further, the acclimation to EHL1200 was associated with an extremely high chlorophyll a/b ratio (3.97), a more than doubled xanthophyll cycle pool (VAZ) and a six-fold higher de-epoxidation state of the xanthophyll cycle pigments as compared to barley grown under 25 mumol m(-2) s(-1) (LL25). EHL1200 plants also exhibited a long-term inhibition of Photosystem II (PS II) photochemical efficiency (F (v)/F (m)). Photosynthetic capacity, chlorophyll a/b and VAZ revealed a linear trend of dependence on PS II excitation pressure in a certain range of growth irradiances (100-730 mumol m(-2) s(-1)). The deviation from linearity of these relationships for EHL1200 barley is discussed. In addition, the role of increased VAZ and/or accumulation of zeaxanthin and antheraxanthin in acclimation of barley to high irradiance is studied with respect to regulation of non-radiative dissipation and/or photochemical efficiency within PS II.

Time-resolved fluorescence analysis of the photosystem II antenna proteins in detergent micelles and liposomes

We have studied the time-resolved fluorescence properties of the light-harvesting complexes (Lhc) of photosystem II (Lhcb) in order to obtain information on the mechanism of energy dissipation (non-photochemical quenching) which is correlated to the conversion of violaxanthin to zeaxanthin in excess light conditions. The chlorophyll fluorescence decay of Lhcb proteins LHCII, CP29, CP26, and CP24 in detergent solution is mostly determined by two lifetime components of 1.2-1.5 and 3.6-4 ns while the contribution of the faster component is higher in CP29, CP26, and CP24 with respect to LHCII. The xanthophyll composition of Lhc proteins affects the ratio of the lifetime components: when zeaxanthin is bound into the site L2 of LHCII, the relative amplitude of the faster component is increased and, consequently, the chlorophyll fluorescence quenching is enhanced. Analysis of quenching in mutants of Arabidopsis thaliana, which incorporate either violaxanthin or zeaxanthin in their Lhc proteins, shows that the extent of quenching is enhanced in the presence of zeaxanthin. The origin of the two fluorescence lifetimes was analyzed by their temperature dependence: since lifetime heterogeneity was not affected by cooling to 77 K, it is concluded that each lifetime component corresponds to a distinct conformation of the Lhc proteins. Upon incorporation of Lhc proteins into liposomes, a quenching of chlorophyll fluorescence was observed due to shortening of all their lifetime components: this indicates that the equilibrium between the two conformations of Lhcb proteins is displaced toward the quenched conformation in lipid membranes or thylakoids with respect to detergent solution. By increasing the protein density in the liposomes, and therefore the probability of protein-protein interactions, a further decrease of fluorescence lifetimes takes place down to values typical of quenched leaves. We conclude that at least two major factors determine the quenching of chlorophyll fluorescence in Lhcb proteins, i.e., intrasubunit conformational change and intersubunit interactions within the lipid membranes, and that these processes are both important in the photoprotection mechanism of nonphotochemical quenching in vivo.

Two distinct isopentenyl diphosphate isomerases in cytosol and plastid are differentially induced by environmental stresses in tobacco

Two distinct cDNA clones (IPI1 and IPI2) encoding IPI were isolated from Nicotiana tabacum. In situ expression of isopentenyl diphosphate isomerase-1 (IPI1)- and IPI2-green fluorescent protein fusion constructs revealed that IPI1 and IPI2 were localized in chloroplast and cytosol, respectively. The level of IPI1 mRNA was increased under high-salt and high-light stress conditions, while that of IPI2 mRNA was increased under high-salt and cold stress conditions. Both IPI transcripts were increased in an abscisic acid-independent manner. This is the first report of a cytosolic IPI. The results indicated that two distinct IPIs were differentially induced in response to stress.

Functional genomics of plant photosynthesis in the fast lane using Chlamydomonas reinhardtii

Oxygenic photosynthesis by algae and plants supports much of life on Earth. Several model organisms are used to study this vital process, but the unicellular green alga Chlamydomonas reinhardtii offers significant advantages for the genetic dissection of photosynthesis. Recent experiments with Chlamydomonas have substantially advanced our understanding of several aspects of photosynthesis, including chloroplast biogenesis, structure-function relationships in photosynthetic complexes, and environmental regulation. Chlamydomonas is therefore the organism of choice for elucidating detailed functions of the hundreds of genes involved in plant photosynthesis.

Time-resolution of the antheraxanthin- and delta pH-dependent chlorophyll a fluorescence components associated with photosystem II energy dissipation in Mantoniella squamata

The electronic excited-state behavior of photosystem II (PSII) in Mantoniella squamata, as influenced by the xanthophyll cycle and the transthylakoid pH gradient (delta pH), was examined in vivo. Mantoniella is distinguished from other photosynthetic organisms by two main features namely (1) a unique light-harvesting complex that serves both photosystems I (PSI) and II (PSII); and (2) a violaxanthin (V) cycle that undergoes only one de-epoxidation step in excess light to accumulate the monoepoxide antheraxanthin (A) as opposed to the epoxide-free zeaxanthin (Z). The cells were treated first with high light to induce the delta pH and A accumulation, followed by herbicide-induced closure of PSII traps and a chilling treatment, to sustain and stabilize the delta pH and nigericin-sensitive fluorescence level in the dark. De-epoxidation was controlled with subsaturating concentrations of dithiothreitol (DTT) and was 5-10 times more sensitive to DTT than higher plant thylakoids. The PSII energy dissipation involved two steps: (1) the pH activation of the xanthophyll binding site that was associated with a narrowing and slight attenuation of the main 2 ns (ns = 10(-9) s) fluorescence lifetime distribution; and (2) the concentration-dependent binding of A to the activated binding site yielding a second distribution centered around 0.9 ns. Consistent with the model of Gilmore et al. (1998) (Biochemistry 37, 13,582-13,593), the fractional intensity of the 0.9 ns component depended almost entirely on the A concentration and correlated linearly with the decrease of the steady-state chlorophyll alpha fluorescence intensity.

CHLAMYDOMONAS AS A MODEL ORGANISM

The unicellular green alga Chlamydomonas offers a simple life cycle, easy isolation of mutants, and a growing array of tools and techniques for molecular genetic studies. Among the principal areas of current investigation using this model system are flagellar structure and function, genetics of basal bodies (centrioles), chloroplast biogenesis, photosynthesis, light perception, cell-cell recognition, and cell cycle control. A genome project has begun with compilation of expressed sequence tag data and gene expression studies and will lead to a complete genome sequence. Resources available to the research community include wild-type and mutant strains, plasmid constructs for transformation studies, and a comprehensive on-line database.

Absence of lutein, violaxanthin and neoxanthin affects the functional chlorophyll antenna size of photosystem-II but not that of photosystem-I in the green alga Chlamydomonas reinhardtii

Chlamydomonas reinhardtii double mutant npq2 lor1 lacks the beta, epsilon-carotenoids lutein and loroxanthin as well as all beta,beta-epoxycarotenoids derived from zeaxanthin (e.g. violaxanthin and neoxanthin). Thus, the only carotenoids present in the thylakoid membranes of the npq2 lor1 cells are beta-carotene and zeaxanthin. The effect of these mutations on the photochemical apparatus assembly and function was investigated. In cells of the mutant strain, the content of photosystem-II (PSII) and photosystem-I (PSI) was similar to that of the wild type, but npq2 lor1 had a significantly smaller PSII light-harvesting Chl antenna size. In contrast, the Chl antenna size of PSI was not truncated in the mutant. SDS-PAGE and Western blot analysis qualitatively revealed the presence of all LHCII and LHCI apoproteins in the thylakoid membrane of the mutant. The results showed that some of the LHCII and most of the LHCI were assembled and functionally connected with PSII and PSI, respectively. Photon conversion efficiency measurements, based on the initial slope of the light-saturation curve of photosynthesis and on the yield of Chl a fluorescence in vivo, showed similar efficiencies. However, a significantly greater light intensity was required for the saturation of photosynthesis in the mutant than in the wild type. It is concluded that zeaxanthin can successfully replace lutein and violaxanthin in most of the functional light-harvesting antenna of the npq2 lor1 mutant.

Lutein production by Muriellopsis sp. in an outdoor tubular photobioreactor

The effect of dilution rate, mixing and daily solar cycles on lutein and biomass productivity of the green unicellular alga Muriellopsis sp. has been studied, throughout the year, in an outdoor tubular photobioreactor. Highest productivity values, for both lutein (about 180 mg m(-2) per day) and biomass (about 40 g (dry weight) m(-2) per day) were achieved on May and July. Values for the optimal dilution rate varied, being lower in May (0.06 h(-1)) than in November (0.09 h(-1)). Similar values for photosynthetic efficiency (about 4%) were recorded throughout the year, indicating that optimization of culture conditions was achieved for each experimental period. Along the daily solar cycle, there was a fast increase of lutein content of Muriellopsis sp. in response to irradiance during the early hours of daytime, with maximal lutein content (about 6 mg (g dry weight)(-1)) being recorded at noon, and decreasing slowly, thereafter. An increase in cell growth was observed following the establishment of maximum lutein/chlorophyll ratio, which might indicate a role for lutein in protecting cells from photodamage.

The photosynthetic apparatus of Prochlorococcus: Insights through comparative genomics

Within the vast oceanic gyres, a significant fraction of the total chlorophyll belongs to the light-harvesting antenna systems of a single genus, Prochlorococcus. This organism, discovered only about 10 years ago, is an extremely small, Chl b-containing cyanobacterium that sometimes constitutes up to 50% of the photosynthetic biomass in the oceans. Various Prochlorococcus strains are known to have significantly different conditions for optimal growth and survival. Strains which dominate the surface waters, for example, have an irradiance optimum for photosynthesis of 200 mumol photons m(-2) s(-1), whereas those that dominate the deeper waters photosynthesize optimally at 30-50 mumol photons m(-2) s(-1). These high and low light adapted 'ecotypes' are very closely related - less than 3% divergent in their 16S rRNA sequences - inviting speculation as to what features of their photosynthetic mechanisms might account for the differences in photosynthetic performance. Here, we compare information obtained from the complete genome sequences of two Prochlorococcus strains, with special emphasis on genes for the photosynthetic apparatus. These two strains, Prochlorococcus MED4 and MIT 9313, are representatives of high- and low-light adapted ecotypes, characterized by their low or high Chl b/a ratio, respectively. Both genomes appear to be significantly smaller (1700 and 2400 kbp) than those of other cyanobacteria, and the low-light-adapted strain has significantly more genes than its high light counterpart. In keeping with their comparative light-dependent physiologies, MED4 has many more genes encoding putative high-light-inducible proteins (HLIP) and photolyases to repair UV-induced DNA damage, whereas MIT 9313 possesses more genes associated with the photosynthetic apparatus. These include two pcb genes encoding Chl-binding proteins and a second copy of the gene psbA, encoding the Photosystem II reaction center protein D1. In addition, MIT 9313 contains a gene cluster to produce chromophorylated phycoerythrin. The latter represents an intermediate form between the phycobiliproteins of non-Chl b containing cyanobacteria and an extremely modified beta phycoerythrin as the sole derivative of phycobiliproteins still present in MED4. Intriguing features found in both Prochlorococcus strains include a gene cluster for Rubisco and carboxysomal proteins that is likely of non-cyanobacterial origin and two genes for a putative varepsilon and beta lycopene cyclase, respectively, explaining how Prochlorococcus may synthesize the alpha branch of carotenoids that are common in green organisms but not in other cyanobacteria.

Very high light resistant mutants of Chlamydomonas reinhardtii: Responses of Photosystem II, nonphotochemical quenching and xanthophyll pigments to light and CO(2)

We have isolated very high light resistant nuclear mutants (VHL (R)) in Chlamydomonas reinhardtii, that grow in 1500-2000 mumol photons m(-2) s(-1) (VHL) lethal to wildtype. Four nonallelic mutants have been characterized in terms of Photosystem II (PS II) function, nonphotochemical quenching (NPQ) and xanthophyll pigments in relation to acclimation and survival under light stress. In one class of VHL (R) mutants isolated from wild type (S4 and S9), VHL resistance was accompanied by slower PS II electron transfer, reduced connectivity between PS II centers and decreased PS II efficiency. These lesions in PS II function were already present in the herbicide resistant D1 mutant A251L (L (*)) from which another class of VHL (R) mutants (L4 and L30) were isolated, confirming that optimal PS II function was not critical for survival in very high light. Survival of all four VHL (R) mutants was independent of CO(2) availability, whereas photoprotective processes were not. The de-epoxidation state (DPS) of the xanthophyll cycle pigments in high light (HL, 600 mumol photons m(-2) s(-1)) was strongly depressed when all genotypes were grown in 5% CO(2). In S4 and S9 grown in air under HL and VHL, high DPS was well correlated with high NPQ. However when the same genotypes were grown in 5% CO(2), high DPS did not result in high NPQ, probably because high photosynthetic rates decreased thylakoid DeltapH. Although high NPQ lowered the reduction state of PS II in air compared to 5% CO(2) at HL in wildtype, S4 and S9, this did not occur during growth of S4 and S9 in VHL. L (*) and VHL (R) mutants L4 and L30, also showed high DPS with low NPQ when grown air or 5% CO(2), possibly because they were unable to maintain sufficiently high DeltapH due to constitutively impaired PS II electron transport. Although dissipation of excess photon energy through NPQ may contribute to VHL resistance, there is little evidence that the different genes conferring the VHL (R) phenotype affect this form of photoprotection. Rather, the decline of chlorophyll per biomass in all VHL (R) mutants grown under VHL suggests these genes may be involved in regulating antenna components and photosystem stoichiometries.

Antisense inhibition of the beta-carotene hydroxylase enzyme in Arabidopsis and the implications for carotenoid accumulation, photoprotection and antenna assembly

The xanthophylls are oxygenated carotenoids and are important structural components of the photosynthetic apparatus. Xanthophylls contribute to the assembly and stability of light-harvesting complex apoproteins (LHC) and contribute to photoprotection via non-photochemical quenching of chlorophyll fluorescence (NPQ) in oxygenic photosynthetic organisms. Previously, mutations have been described that disrupt many steps in the xanthophyll biosynthetic pathway. However, there are no definitive reports of a lesion that effects the beta-hydroxylase enzyme, which catalyzes hydroxylation of the beta-rings of beta-carotene and alpha-carotene, and is thus necessary for synthesis of essentially all xanthophylls of higher plant chloroplasts. We have utilized an antisense approach to effectively reduce levels of beta-hydroxylase in Arabidopsis thaliana in order to examine how a reduction in this enzyme impacts carotenoid biosynthesis and plant viability. Expression of the antisense beta-hydroxylase transgene resulted in a maximal reduction in violaxanthin of 64% and a maximal reduction in neoxanthin of 41%. This reduction was reflected in a 22% increase in beta-carotene and a reduction in the total carotenoid pool, whereas lutein levels were relatively unaltered. Despite the reduction in violaxanthin and neoxanthin, the antisense beta-hydroxylase plants had a wild-type complement of chlorophylls and LHCs on a fresh weight basis. Under high light stress, the unconverted pool of violaxanthin was the same size as in wild type and thus there was an even greater proportional reduction in zeaxanthin of 75%. Despite this marked decrease in zeaxanthin, NPQ only declined by 16%.

Photoprotection in a zeaxanthin- and lutein-deficient double mutant of Arabidopsis

When light absorption by a plant exceeds its capacity for light utilization, photosynthetic light harvesting is rapidly downregulated by photoprotective thermal dissipation, which is measured as nonphotochemical quenching of chlorophyll fluorescence (NPQ). To address the involvement of specific xanthophyll pigments in NPQ, we have analyzed mutants affecting xanthophyll metabolism in Arabidopsis thaliana. An npq1 lut2 double mutant was constructed, which lacks both zeaxanthin and lutein due to defects in the violaxanthin de-epoxidase and lycopene in-cyclase genes. The npq1 lut2 strain had normal Photosystem II efficiency and nearly wild-type concentrations of functional Photosystem II reaction centers, but the rapidly reversible component of NPQ was completely inhibited. Despite the defects in xanthophyll composition and NPQ, the npq1 lut2 mutant exhibited a remarkable ability to tolerate high light.

Genetic manipulation of carotenoid biosynthesis and photoprotection

There are multiple complementary and redundant mechanisms to provide protection against photo-oxidative damage, including non-photochemical quenching (NPQ). NPQ dissipates excess excitation energy as heat by using xanthophylls in combination with changes to the light-harvesting complex (LHC) antenna. The xanthophylls are oxygenated carotenoids that in addition to contributing to NPQ can quench singlet or triplet chlorophyll and are necessary for the assembly and stability of the antenna. We have genetically manipulated the expression of the epsilon-cyclase and beta-carotene hydroxylase carotenoid biosynthetic enzymes in Arabidopsis thaliana. The epsilon-cyclase overexpression confirmed that lut2 (lutein deficient) is a mutation in the epsilon-cyclase gene and demonstrated that lutein content can be altered at the level of mRNA abundance with levels ranging from 0 to 180% of wild-type. Also, it is clear that lutein affects the induction and extent of NPQ. The deleterious effects of lutein deficiency on NPQ in Arabidopsis and Chlamydomonas are additive, no matter what the genetic background, whether npq1 (zeaxanthin deficient), aba1 or antisense beta-hydroxylase (xanthophyll cycle pool decreased). Additionally, increasing lutein content causes a marginal, but significant, increase in the rate of induction of NPQ despite a reduction in the xanthophyll cycle pool size.

Molecular genetics of xanthophyll-dependent photoprotection in green algae and plants

The involvement of excited and highly reactive intermediates in oxygenic photosynthesis inevitably results in the generation of reactive oxygen species. To protect the photosynthetic apparatus from oxidative damage, xanthophyll pigments are involved in the quenching of excited chlorophyll and reactive oxygen species, namely 1Chl*, 3Chl*, and 1O2*. Quenching of 1Chl* results in harmless dissipation of excitation energy as heat and is measured as non-photochemical quenching (NPQ) of chlorophyll fluorescence. The multiple roles of xanthophylls in photoprotection are being addressed by characterizing mutants of Chlarnydomonas reinhardtii and Arabidopsis thaliana. Analysis of Arabidopsis mutants that are defective in 1Chl* quenching has shown that, in addition to specific xanthophylls, the psbS gene is necessary for NPQ. Double mutants of Chlamydomonas and Arabidopsis that are deficient in zeaxanthin, lutein and NPQ undergo photo-oxidative bleaching in high light. Extragenic suppressors of the Chlamydomonas npq1 lor1 double mutant identify new mutations that restore varying levels of zeaxanthin accumulation and allow survival in high light.

Allosteric regulation of the light-harvesting system of photosystem II

Non-photochemical quenching of chlorophyll fluorescence (NPQ) is symptomatic of the regulation of energy dissipation by the light-harvesting antenna of photosystem II (PS II). The kinetics of NPQ in both leaves and isolated chloroplasts are determined by the transthylakoid delta pH and the de-epoxidation state of the xanthophyll cycle. In order to understand the mechanism and regulation of NPQ we have adopted the approaches commonly used in the study of enzyme-catalysed reactions. Steady-state measurements suggest allosteric regulation of NPQ, involving control by the xanthophyll cycle carotenoids of a protonation-dependent conformational change that transforms the PS II antenna from an unquenched to a quenched state. The features of this model were confirmed using isolated light-harvesting proteins. Analysis of the rate of induction of quenching both in vitro and in vivo indicated a bimolecular second-order reaction; it is suggested that quenching arises from the reaction between two fluorescent domains, possibly within a single protein subunit. A universal model for this transition is presented based on simple thermodynamic principles governing reaction kinetics.

Xanthophylls of the major photosynthetic light-harvesting complex of plants: identification, conformation and dynamics

The electronic transitions of lutein and neoxanthin in the major light-harvesting complex, LHCIIb, have been identified for the first time. It was found that 0-0, 0-1 and 0-2 transitions of neoxanthin were located around 486, 457 and 430 nm, whilst those for lutein were dependent on the oligomerisation state. For the monomer, the absorption bands of lutein were found at 495, 466 and 437 nm. Trimerisation caused a decrease in lutein absorption and the parallel appearance of an additional absorption band around 510 nm, which was identified by resonance Raman excitation spectra to originate from lutein. Circular dichroism measurements together with analysis of the nu(4) resonance Raman region of xanthophylls suggested that this lutein molecule is distorted in the trimer. This feature is not predicted by the LHCIIb atomic model of Kühlbrandt and co-workers [Kühlbrandt, W., Wang, D.N. and Fugiyoshi, Y. (1994) Nature 367, 614-621] and is an important step in understanding pigment dynamics of the complex. Oligomerisation of trimers led to a specific distortion of the neoxanthin molecule. These observations suggest that the xanthophylls of LHCIIb sense the protein conformation and which may reflect their special role in the assembly and function of the light-harvesting antenna of higher plants.

The Crd1 gene encodes a putative di-iron enzyme required for photosystem I accumulation in copper deficiency and hypoxia in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii adapts to copper deficiency by degrading apoplastocyanin and inducing Cyc6 and Cpx1 encoding cytochrome c(6) and coproporphyrinogen oxidase, respectively. To identify other components in this pathway, colonies resulting from insertional mutagenesis were screened for copper- conditional phenotypes. Twelve crd (copper response defect) strains were identified. In copper-deficient conditions, the crd strains fail to accumulate photosystem I and light-harvesting complex I, and they contain reduced amounts of light-harvesting complex II. Cyc6, Cpx1 expression and plastocyanin accumulation remain copper responsive. The crd phenotype is rescued by a similar amount of copper as is required for repression of Cyc6 and Cpx1 and for maintenance of plastocyanin at its usual stoichiometry, suggesting that the affected gene is a target of the same signal transduction pathway. The crd strains represent alleles at a single locus, CRD1, which encodes a 47 kDa, hydrophilic protein with a consensus carboxylate-bridged di-iron binding site. Crd1 homologs are present in the genomes of photosynthetic organisms. In Chlamydomonas, Crd1 expression is activated in copper- or oxygen-deficient cells, and Crd1 function is required for adaptation to these conditions.

Chlorophyll fluorescence quenching in isolated light harvesting complexes induced by zeaxanthin

Non-photochemical quenching of chlorophyll fluorescence in plants occurs in the light harvesting antenna of photosystem II and is regulated by the xanthophyll cycle. A new in vitro model for this process has been developed. Purified light harvesting complexes above the detergent critical micelle concentration have a stable high fluorescence yield but a rapidly inducible fluorescence quenching occurs upon addition of zeaxanthin. Violaxanthin was without effect, lutein and antheraxanthin induced a marginal response, whereas the violaxanthin analogue, auroxanthin, induced strong quenching. Quenching was not caused by aggregation of the complexes but was accompanied by a spectral broadening and red shift, indicating a zeaxanthin-dependent alteration in the chlorophyll environment.

Efficiency of light utilization of Chlamydomonas reinhardtii under medium-duration light/dark cycles

The light regime inside a photobioreactor is characterized by a light gradient with full (sun)light at the light-exposed surface and darkness in the interior of the bioreactor. Consequently, depending on the mixing characteristics, algae will be exposed to certain light/dark cycles. In this study the green alga Chlamydomonas reinhardtii was cultivated under five different light regimes: (1) continuous illumination; (2) a square-wave light/dark cycle with a light fraction (epsilon) of 0.5 and a duration (t(c)) of 6.1 s; (3) epsilon=0.5, t(c)=14.5 s; (4) epsilon=0.5, t(c)=24.3 s and (5) epsilon=0.8, t(c)=15.2 s. The biomass yield on light energy, protein per photons, decreased under light/dark cycles (epsilon=0. 5) in comparison to continuous light (CL), from 0.207 (CL) to 0.117-0.153 g mol(-1) (epsilon=0.5). Concomitantly, the maximal specific photosynthetic activity, oxygen production per protein, decreased from 0.94 (CL) to 0.64-0.66 g g(-1) h(-1) (epsilon=0.5). Also the quantum yield of photochemistry, yield of the conversion of light energy into chemical energy, decreased from 0.47 (CL) to 0. 23 (epsilon=0.5, t(c)=24.3 s). Apparently, C. reinhardtii is not able to maintain a high photosynthetic capacity under medium-duration light/dark cycles and since specific light absorption did not change, light utilization efficiency decreased in comparison to continuous illumination.

Prospects for crop improvement through the genetic manipulation of photosynthesis: morphological and biochemical aspects of light capture

The prospects for genetic manipulation of photosynthesis are assessed with an emphasis on the biochemical and morphological aspects of light capture. The connection between different parts of the photosynthetic process is considered together with the influence of environmental factors, development and acclimation, and metabolic regulation. The sites of real and potential photosynthetic losses are identified, using tropical rice as a case study. The important interaction between photosynthetic capacity, acclimation to the light environment, nitrogen accumulation and canopy architecture are discussed. The possibility of genetic intervention to increase both biomass accumulation and improve nitrogen economy simultaneously are considered. Finally, the numerous procedures for genetic manipulation of light harvesting are also discussed, with a view of improving radiation-use efficiency in crops.

A pigment-binding protein essential for regulation of photosynthetic light harvesting

Photosynthetic light harvesting in plants is regulated in response to changes in incident light intensity. Absorption of light that exceeds a plant's capacity for fixation of CO2 results in thermal dissipation of excitation energy in the pigment antenna of photosystem II by a poorly understood mechanism. This regulatory process, termed nonphotochemical quenching, maintains the balance between dissipation and utilization of light energy to minimize generation of oxidizing molecules, thereby protecting the plant against photo-oxidative damage. To identify specific proteins that are involved in nonphotochemical quenching, we have isolated mutants of Arabidopsis thaliana that cannot dissipate excess absorbed light energy. Here we show that the gene encoding PsbS, an intrinsic chlorophyll-binding protein of photosystem II, is necessary for nonphotochemical quenching but not for efficient light harvesting and photosynthesis. These results indicate that PsbS may be the site for nonphotochemical quenching, a finding that has implications for the functional evolution of pigment-binding proteins.

Carotenoid binding sites in LHCIIb. Relative affinities towards major xanthophylls of higher plants

The major light-harvesting complex of photosystem II can be reconstituted in vitro from its bacterially expressed apoprotein with chlorophylls a and b and neoxanthin, violaxanthin, lutein, or zeaxanthin as the only xanthophyll. Reconstitution of these one-carotenoid complexes requires low-stringency conditions during complex formation and isolation. Neoxanthin complexes (containing 30-50% of the all-trans isomer) disintegrate during electrophoresis, exhibit a largely reduced resistance against proteolytic attack; in addition, energy transfer from Chl b to Chl a is easily disrupted at elevated temperature. Complexes reconstituted in the presence of either zeaxanthin or lutein contain nearly two xanthophylls per 12 chlorophylls and are more resistant against trypsin. Lutein-LHCIIb also exhibits an intermediate maintenance of energy transfer at higher temperature. Violaxanthin complexes approach a xanthophyll/12 chlorophyll ratio of 3, similar to the ratio in recombinant LHCIIb containing all xanthophylls. On the other hand, violaxanthin-LHCIIb exhibits a low thermal stability like neoxanthin complexes, but an intermediate accessibility towards trypsin, similar to lutein-LHCIIb and zeaxanthin-LHCIIb. Binary competition experiments were performed with two xanthophylls at varying ratios in the reconstitution. Analysis of the xanthophyll contents in the reconstitution products yielded information about relative carotenoid affinities of three assumed binding sites. In lutein/neoxanthin competition experiments, two binding sites showed a strong preference (> 200-fold) for lutein, whereas the third binding site had a higher affinity (25-fold) to neoxanthin. Competition between lutein and violaxanthin gave a similar result, although the specificities were lower: two binding sites have a 36-fold preference for lutein and one has a fivefold preference for violaxanthin. The lowest selectivity was between lutein and zeaxanthin: two binding sites had a fivefold higher affinity for lutein and one has a threefold higher affinity to zeaxanthin.

Lhc proteins and the regulation of photosynthetic light harvesting function by xanthophylls

Photoprotection of the chloroplast is an important component of abiotic stress resistance in plants. Carotenoids have a central role in photoprotection. We review here the recent evidence, derived mainly from in vitro reconstitution of recombinant Lhc proteins with different carotenoids and from carotenoid biosynthesis mutants, for the existence of different mechanisms of photoprotection and regulation based on xanthophyll binding to Lhc proteins into multiple sites and the exchange of chromophores between different Lhc proteins during exposure of plants to high light stress and the operation of the xanthophyll cycle. The use of recombinant Lhc proteins has revealed up to four binding sites in members of Lhc families with distinct selectivity for xanthophyll species which are here hypothesised to have different functions. Site L1 is selective for lutein and is here proposed to be essential for catalysing the protection from singlet oxygen by quenching chlorophyll triplets. Site L2 and N1 are here proposed to act as allosteric sites involved in the regulation of chlorophyll singlet excited states by exchanging ligand during the operation of the xanthophyll cycle. Site V1 of the major antenna complex LHC II is here hypothesised to be a deposit for readily available substrate for violaxanthin de-epoxidase rather than a light harvesting pigment. Moreover, xanthophylls bound to Lhc proteins can be released into the lipid bilayer where they contribute to the scavenging of reactive oxygen species produced in excess light.

Increased production of zeaxanthin and other pigments by application of genetic engineering techniques to Synechocystis sp. strain PCC 6803

The psbAII locus was used as an integration platform to overexpress genes involved in carotenoid biosynthesis in Synechocystis sp. strain PCC 6803 under the control of the strong psbAII promoter. The sequences of the genes encoding the yeast isopentenyl diphosphate isomerase (ipi) and the Synechocystis beta-carotene hydroxylase (crtR) and the linked Synechocystis genes coding for phytoene desaturase and phytoene synthase (crtP and crtB, respectively) were introduced into Synechocystis, replacing the psbAII coding sequence. Expression of ipi, crtR, and crtP and crtB led to a large increase in the corresponding transcript levels in the mutant strains, showing that the psbAII promoter can be used to drive transcription and to overexpress various genes in Synechocystis. Overexpression of crtP and crtB led to a 50% increase in the myxoxanthophyll and zeaxanthin contents in the mutant strain, whereas the beta-carotene and echinenone contents remained unchanged. Overexpression of crtR induced a 2.5-fold increase in zeaxanthin accumulation in the corresponding overexpressing mutant compared to that in the wild-type strain. In this mutant strain, zeaxanthin becomes the major pigment (more than half the total amount of carotenoid) and the beta-carotene and echinenone amounts are reduced by a factor of 2. However, overexpression of ipi did not result in a change in the carotenoid content of the mutant. To further alter the carotenoid content of Synechocystis, the crtO gene, encoding beta-carotene ketolase, which converts beta-carotene to echinenone, was disrupted in the wild type and in the overexpressing strains so that they no longer produced echinenone. In this way, by a combination of overexpression and deletion of particular genes, the carotenoid content of cyanobacteria can be altered significantly.

Antisense suppression of violaxanthin de-epoxidase in tobacco does not affect plant performance in controlled growth conditions

Violaxanthin de-epoxidase (VDE) catalyzes the de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin in the xanthophyll cycle. Tobacco was transformed with an antisense VDE construct under control of the cauliflower mosaic virus 35S promoter to determine the effect of reduced levels of VDE on plant growth. Screening of 40 independent transformants revealed 18 antisense lines with reduced levels of VDE activity with two in particular (TAS32 and TAS39) having greater than 95% reduction in VDE activity. Northern analysis demonstrated that these transformants had greatly suppressed levels of VDE mRNA. De-epoxidation of violaxanthin was inhibited to such an extent that no zeaxanthin and only very low levels of antheraxanthin could be detected after exposure of leaves to high light (2000 mumol m(-2) s(-1) for 20 min) with no observable effect on levels of other carotenoids and chlorophyll. Non-photochemical quenching was greatly reduced in the antisense VDE tobacco, demonstrating that a significant level of the non-photochemical quenching in tobacco requires de-epoxidation of violaxanthin. Although the antisense plants demonstrated a greatly impaired de-epoxidation of violaxanthin, no effect on plant growth or photosynthetic rate was found when plants were grown at a photon flux density of 500 or 1000 mumol m(-2) s(-1) under controlled growth conditions as compared to wild-type tobacco.

Algae displaying the diadinoxanthin cycle also possess the violaxanthin cycle

According to general agreement, all photosynthetic organisms using xanthophyll cycling for photoprotection contain either the violaxanthin (Vx) cycle or the diadinoxanthin (Ddx) cycle instead. Here, we report the temporal accumulation of substantial amounts of pigments of the Vx cycle under prolonged high-light stress in several microalgae thought to possess only the Ddx cycle. In the diatom Phaeodactylum tricornutum, used as a model organism, these pigments also participate in xanthophyll cycling, and their accumulation depends on de novo synthesis of carotenoids and on deepoxidase activity. Furthermore, our data strongly suggest a biosynthetic sequence from Vx via Ddx to fucoxanthin in P. tricornutum. This gives experimental support to the long-stated hypothesis that Vx is a common precursor of all carotenoids with an allenic or acetylenic group, including the main light-harvesting carotenoids in most chlorophyll a/c-containing algae. Thus, another important function for xanthophyll cycling may be to optimize the biosynthesis of light-harvesting xanthophylls under fluctuating light conditions.

PHOTOPROTECTION REVISITED:Genetic and Molecular Approaches

The involvement of excited and highly reactive intermediates in oxygenic photosynthesis poses unique problems for algae and plants in terms of potential oxidative damage to the photosynthetic apparatus. Photoprotective processes prevent or minimize generation of oxidizing molecules, scavenge reactive oxygen species efficiently, and repair damage that inevitably occurs. This review summarizes several photoprotective mechanisms operating within chloroplasts of plants and green algae. The recent use of genetic and molecular biological approaches is providing new insights into photoprotection, especially with respect to thermal dissipation of excess absorbed light energy, alternative electron transport pathways, chloroplast antioxidant systems, and repair of photosystem II.

Isolation of pigment-binding early light-inducible proteins from pea

The early light-inducible proteins (ELIPs) in chloroplasts possess a high sequence homology with the chlorophyll a/b-binding proteins but differ from those proteins by their substoichiometric and transient appearance. In the present study ELIPs of pea were isolated by a two-step purification strategy: perfusion chromatography in combination with preparative isoelectric focussing. Two heterogeneous populations of ELIPs were obtained after chromatographic separation of solubilized thylakoid membranes using a weak anion exchange column. One of these populations contained ELIPs in a free form providing the first isolation of these proteins. To prove whether the isolated and pure forms of ELIP bind pigments, spectroscopic and chromatographic analysis were performed. Absorption spectra and TLC revealed the presence of chlorophyll a and lutein. Measurements of steady-state fluorescence emission spectra at 77 K exhibited a major peak at 674 nm typical for chlorophyll a bound to the protein matrix. The action spectrum of the fluorescence emission measured at 674 nm showed several peaks originating mainly from chlorophyll a. It is proposed that ELIPs are transient chlorophyll-binding proteins not involved in light-harvesting but functioning as scavengers for chlorophyll molecules during turnover of pigment-binding proteins.

Unusual carotenoid composition and a new type of xanthophyll cycle in plants

The capture of photons by the photosynthetic apparatus is the first step in photosynthesis in all autotrophic higher plants. This light capture is dominated by pigment-containing proteins known as light-harvesting complexes (LHCs). The xanthophyll-carotenoid complement of these LHCs (neoxanthin, violaxanthin, and lutein) is highly conserved, with no deletions and few, uncommon additions. We report that neoxanthin, considered an integral component of LHCs, is stoichiometrically replaced by lutein-5,6-epoxide in the parasitic angiosperm Cuscuta reflexa, without compromising the structural integrity of the LHCs. Lutein-5,6-epoxide differs from neoxanthin in that it is involved in a light-driven deepoxidation cycle similar to the deepoxidation of violaxanthin in the xanthophyll cycle, which is implicated in protection against photodamage. The absence of neoxanthin and its replacement by lutein-5,6-epoxide changes our understanding of the structure-function relationship in LHCs, has implications for biosynthetic pathways involving neoxanthin (such as the plant hormone abscisic acid), and identifies one of the early steps associated with the evolution of heterotrophy from autotrophy in plants.

The xanthophyll cycle modulates the kinetics of nonphotochemical energy dissipation in isolated light-harvesting complexes, intact chloroplasts, and leaves of spinach

We analyzed the kinetics of nonphotochemical quenching of chlorophyll fluorescence (qN) in spinach (Spinacia oleracea) leaves, chloroplasts, and purified light-harvesting complexes. The characteristic biphasic pattern of fluorescence quenching in dark-adapted leaves, which was removed by preillumination, was evidence of light activation of qN, a process correlated with the de-epoxidation state of the xanthophyll cycle carotenoids. Chloroplasts isolated from dark-adapted and light-activated leaves confirmed the nature of light activation: faster and greater quenching at a subsaturating transthylakoid pH gradient. The light-harvesting chlorophyll a/b-binding complexes of photosystem II were isolated from dark-adapted and light-activated leaves. When isolated from light-activated leaves, these complexes showed an increase in the rate of quenching in vitro compared with samples prepared from dark-adapted leaves. In all cases, the quenching kinetics were fitted to a single component hyperbolic function. For leaves, chloroplasts, and light-harvesting complexes, the presence of zeaxanthin was associated with an increased rate constant for the induction of quenching. We discuss the significance of these observations in terms of the mechanism and control of qN.

Altered xanthophyll compositions adversely affect chlorophyll accumulation and nonphotochemical quenching in Arabidopsis mutants

Collectively, the xanthophyll class of carotenoids perform a variety of critical roles in light harvesting antenna assembly and function. The xanthophyll composition of higher plant photosystems (lutein, violaxanthin, and neoxanthin) is remarkably conserved, suggesting important functional roles for each. We have taken a molecular genetic approach in Arabidopsis toward defining the respective roles of individual xanthophylls in vivo by using a series of mutant lines that selectively eliminate and substitute a range of xanthophylls. The mutations, lut1 and lut2 (lut = lutein deficient), disrupt lutein biosynthesis. In lut2, lutein is replaced mainly by a stoichiometric increase in violaxanthin and antheraxanthin. A third mutant, aba1, accumulates normal levels of lutein and substitutes zeaxanthin for violaxanthin and neoxanthin. The lut2aba1 double mutant completely lacks lutein, violaxanthin, and neoxanthin and instead accumulates zeaxanthin. All mutants were viable in soil and had chlorophyll a/b ratios ranging from 2.9 to 3.5 and near wild-type rates of photosynthesis. However, mutants accumulating zeaxanthin exhibited a delayed greening virescent phenotype, which was most severe and often lethal when zeaxanthin was the only xanthophyll present. Chlorophyll fluorescence quenching kinetics indicated that both zeaxanthin and lutein contribute to nonphotochemical quenching; specifically, lutein contributes, directly or indirectly, to the rapid rise of nonphotochemical quenching. The results suggest that the normal complement of xanthophylls, while not essential, is required for optimal assembly and function of the light harvesting antenna in higher plants.

Arabidopsis mutants define a central role for the xanthophyll cycle in the regulation of photosynthetic energy conversion

A conserved regulatory mechanism protects plants against the potentially damaging effects of excessive light. Nearly all photosynthetic eukaryotes are able to dissipate excess absorbed light energy in a process that involves xanthophyll pigments. To dissect the role of xanthophylls in photoprotective energy dissipation in vivo, we isolated Arabidopsis xanthophyll cycle mutants by screening for altered nonphotochemical quenching of chlorophyll fluorescence. The npq1 mutants are unable to convert violaxanthin to zeaxanthin in excessive light, whereas the npq2 mutants accumulate zeaxanthin constitutively. The npq2 mutants are new alleles of aba1, the zeaxanthin epoxidase gene. The high levels of zeaxanthin in npq2 affected the kinetics of induction and relaxation but not the extent of nonphotochemical quenching. Genetic mapping, DNA sequencing, and complementation of npq1 demonstrated that this mutation affects the structural gene encoding violaxanthin deepoxidase. The npq1 mutant exhibited greatly reduced nonphotochemical quenching, demonstrating that violaxanthin deepoxidation is required for the bulk of rapidly reversible nonphotochemical quenching in Arabidopsis. Altered regulation of photosynthetic energy conversion in npq1 was associated with increased sensitivity to photoinhibition. These results, in conjunction with the analysis of npq mutants of Chlamydomonas, suggest that the role of the xanthophyll cycle in nonphotochemical quenching has been conserved, although different photosynthetic eukaryotes rely on the xanthophyll cycle to different extents for the dissipation of excess absorbed light energy.

Arabidopsis mutants define a central role for the xanthophyll cycle in the regulation of photosynthetic energy conversion

A conserved regulatory mechanism protects plants against the potentially damaging effects of excessive light. Nearly all photosynthetic eukaryotes are able to dissipate excess absorbed light energy in a process that involves xanthophyll pigments. To dissect the role of xanthophylls in photoprotective energy dissipation in vivo, we isolated Arabidopsis xanthophyll cycle mutants by screening for altered nonphotochemical quenching of chlorophyll fluorescence. The npq1 mutants are unable to convert violaxanthin to zeaxanthin in excessive light, whereas the npq2 mutants accumulate zeaxanthin constitutively. The npq2 mutants are new alleles of aba1, the zeaxanthin epoxidase gene. The high levels of zeaxanthin in npq2 affected the kinetics of induction and relaxation but not the extent of nonphotochemical quenching. Genetic mapping, DNA sequencing, and complementation of npq1 demonstrated that this mutation affects the structural gene encoding violaxanthin deepoxidase. The npq1 mutant exhibited greatly reduced nonphotochemical quenching, demonstrating that violaxanthin deepoxidation is required for the bulk of rapidly reversible nonphotochemical quenching in Arabidopsis. Altered regulation of photosynthetic energy conversion in npq1 was associated with increased sensitivity to photoinhibition. These results, in conjunction with the analysis of npq mutants of Chlamydomonas, suggest that the role of the xanthophyll cycle in nonphotochemical quenching has been conserved, although different photosynthetic eukaryotes rely on the xanthophyll cycle to different extents for the dissipation of excess absorbed light energy.