Mechanisms of temperature acclimatisation in the psychrotolerant green alga Coccomyxa subellipsoidea C-169 (Trebouxiophyceae).

Despite the interest in different temperature acclimatisations of higher plants, few studies have considered the mechanisms that allow psychrotolerant microalgae to live in a cold environment. Although the analysis of the genomes of some algae revealed the presence of specific genes that encode enzymes that can be involved in the response to stress, this area has not been explored deeply. This work aims to clarify the acclimatisation mechanisms that enable the psychrotolerant green alga Coccomyxa subellipsoidea C-169 to grow in a broad temperature spectrum. The contents of various biochemical compounds in cells, the lipid composition of the biological membranes of entire cells, and the thylakoid fraction as well as the electron transport rate and PSII efficiency were investigated. The results demonstrate an acclimatisation mechanism that is specific for C. subellipsoidea and that allows the maintenance of appropriate membrane fluidity, for example, in thylakoid membranes. It is achieved almost exclusively by changes within the unsaturated fatty acid pool, like changes from C18:2 into C18:3 and C16:2 into C16:3 or vice versa. This ensures, for example, an effective transport rate through PSII and in consequence a maximum quantum yield of it in cells growing at different temperatures. Furthermore, reactions characteristic for both psychrotolerant and mesophilic microalgae, involving the accumulation of lipids and soluble sugars in cells at temperatures other than optimal, were observed. These findings add substantially to our understanding of the acclimatisation of psychrotolerant organisms to a wide range of temperatures and prove that this process could be accomplished in a species-specific manner.

How Light Modulates the Growth of Cyanidioschyzon merolae Cells by Changing the Function of Phycobilisomes.

The aim of this study was to examine how light intensity and quality affect the photosynthetic apparatus of Cyanidioschyzon merolae cells by modulating the structure and function of phycobilisomes. Cells were grown in equal amounts of white, blue, red, and yellow light of low (LL) and high (HL) intensity. Biochemical characterization, fluorescence emission, and oxygen exchange were used to investigate selected cellular physiological parameters. It was found that the allophycocyanin content was sensitive only to light intensity, whereas the phycocynin content was also sensitive to light quality. Furthermore, the concentration of the PSI core protein was not affected by the intensity or quality of the growth light, but the concentration of the PSII core D1 protein was. Finally, the amount of ATP and ADP was lower in HL than LL. In our opinion, both light intensity and quality are main factors that play an important regulatory role in acclimatization/adaptation of C. merolae to environmental changes, and this is achieved by balancing the amounts of thylakoid membrane and phycobilisome proteins, the energy level, and the photosynthetic and respiratory activity. This understanding contributes to the development of a mix of cultivation techniques and genetic changes for a future large-scale synthesis of desirable biomolecules.

How Light Reactions of Photosynthesis in C4 Plants Are Optimized and Protected under High Light Conditions.

Most C4 plants that naturally occur in tropical or subtropical climates, in high light environments, had to evolve a series of adaptations of photosynthesis that allowed them to grow under these conditions. In this review, we summarize mechanisms that ensure the balancing of energy distribution, counteract photoinhibition, and allow the dissipation of excess light energy. They secure effective electron transport in light reactions of photosynthesis, which will lead to the production of NADPH and ATP. Furthermore, a higher content of the cyclic electron transport components and an increase in ATP production are observed, which is necessary for the metabolism of C4 for effective assimilation of CO2. Most of the data are provided by studies of the genus Flaveria, where species belonging to different metabolic subtypes and intermediate forms between C3 and C4 are present. All described mechanisms that function in mesophyll and bundle sheath chloroplasts, into which photosynthetic reactions are divided, may differ in metabolic subtypes as a result of the different organization of thylakoid membranes, as well as the different demand for ATP and NADPH. This indicates that C4 plants have plasticity in the utilization of pathways in which efficient use and dissipation of excitation energy are realized.

The effect of temperature conditions during growth on the transformation frequency of Coccomyxa subellipsoidea C-169 obtained by electroporation.

In this study, we have shown that transformation efficiency of Coccomyxa subellipsoidea C-169 obtained by electroporation can be significantly increased by either supra- or sub-optimal growth temperatures.

Loss of function TRPV6 variants are associated with chronic pancreatitis in nonalcoholic early-onset Polish and German patients.

PURPOSE: Loss of function variants of the transient receptor potential cation channel, subfamily V, member 6 (TRPV6) have been recently associated with chronic pancreatitis (CP) in Japanese, German and French patients. Here, we investigated the association of TRPV6 variants with CP in independent European cohorts of early-onset CP patients from Poland and Germany. PATIENTS AND METHODS: We enrolled 152 pediatric CP patients (median age 8.6 yrs) with no history of alcohol/smoking abuse and 472 controls from Poland as well as 157 nonalcoholic young CP patients (median age 20 yrs) and 750 controls from Germany. Coding regions of TRPV6 were screened by Sanger and next generation sequencing. Selected, potentially pathogenic TRPV6 variants were expressed in HEK293T cells and TRPV6 activity was analyzed using ratiometric Ca2+ measurements. RESULTS: Overall, we identified 10 novel (3 nonsense and 7 missenses) TRPV6 variants in CP patients. TRPV6 p.V239SfsX53 nonsense variant and the variants showing significant decrease in intracellular Ca2+ concentration in HEK293T cells (p.R174X, p.L576R, p.R342Q), were significantly overrepresented in Polish patients as compared to controls (6/152, 3.9% vs. 0/358, 0%; P = 0,0007). Nonsense TRPV6 variants predicted as loss of function (p.V239SfsX53 and p.R624X) were also significantly overrepresented in German patients (3/157; 2.0% vs 0/750; 0%, P = 0.005). CONCLUSIONS: We showed that TRPV6 loss of function variants are associated with elevated CP risk in early-onset Polish and German patients confirming that TRPV6 is a novel CP susceptibility gene.

Stable transformation of unicellular green alga Coccomyxa subellipsoidea C-169 via electroporation.

In this study, we have shown the applicability of electroporation and hygromycin B as a convenient selectable marker for stable nuclear transformation of Coccomyxa subellipsoidea C-169. Since it is the first sequenced eukaryotic microorganism from polar environment, this offers unique opportunities to study adaptation mechanisms to cold.

PEG-mediated, Stable, Nuclear and Chloroplast Transformation of Cyanidioschizon merolae.

The ability to achieve nuclear or chloroplast transformation in plants has been a long standing goal, especially in microalgae research. Over past years there has been only little success, but transient and stable nuclear transformation has been achieved in multiple species. Our newly developed method allows for relatively simple transformation of Cyanidioschizon merolae in both nuclear and chloroplast genome by means of homologous recombination between the genome and a transformation vector. The use of chloramphenicol resistance gene as the selectable marker allows for plate-based efficient selection of mutant colonies. Overall, the method allows the generation of mutant strains within 6 months.

Deletion of psbQ' gene in Cyanidioschyzon merolae reveals the function of extrinsic PsbQ' in PSII.

KEY MESSAGE: We have successfully produced single-cell colonies of C. merolae mutants, lacking the PsbQ' subunit in its PSII complex by application of DTA-aided mutant selection. We have investigated the physiological changes in PSII function and structure and proposed a tentative explanation of the function of PsbQ' subunit in the PSII complex. We have improved the selectivity of the Cyanidioschyzon merolae nuclear transformation method by the introduction of diphtheria toxin genes into the transformation vector as an auxiliary selectable marker. The revised method allowed us to obtained single-cell colonies of C. merolae, lacking the gene of the PsbQ' extrinsic protein. The efficiency of gene replacement was extraordinarily high, allowing for a complete deletion of the gene of interest, without undesirable illegitimate integration events. We have confirmed the absence of PsbQ' protein at genetic and protein level. We have characterized the physiology of mutant cells and isolated PSII protein complex and concluded that PsbQ' is involved in nuclear regulation of PSII activity, by influencing several parameters of PSII function. Among these: oxygen evolving activity, partial dissociation of PsbV, regulation of dimerization, downsizing of phycobilisomes rods and regulation of zeaxanthin abundance. The adaptation of cellular physiology appeared to favorite upregulation of PSII and concurrent downregulation of PSI, resulting in an imbalance of energy distribution, decrease of photosynthesis and inhibition of cell proliferation.

Chloramphenicol acetyltransferase-a new selectable marker in stable nuclear transformation of the red alga Cyanidioschyzon merolae.

In this study, we have shown the applicability of chloramphenicol acetyltransferase as a new and convenient selectable marker for stable nuclear transformation as well as potential chloroplast transformation of Cyanidioschyzon merolae-a new model organism, which offers unique opportunities for studding the mitochondrial and plastid physiology as well as various evolutionary, structural, and functional features of the photosynthetic apparatus.

Differences in photosynthetic responses of NADP-ME type C4 species to high light.

MAIN CONCLUSION: Three species chosen as representatives of NADP-ME C4 subtype exhibit different sensitivity toward photoinhibition, and great photochemical differences were found to exist between the species. These characteristics might be due to the imbalance in the excitation energy between the photosystems present in M and BS cells, and also due to that between species caused by the penetration of light inside the leaves. Such regulation in the distribution of light intensity between M and BS cells shows that co-operation between both the metabolic systems determines effective photosynthesis and reduces the harmful effects of high light on the degradation of PSII through the production of reactive oxygen species (ROS). We have investigated several physiological parameters of NADP-ME-type C4 species (e.g., Zea mays, Echinochloa crus-galli, and Digitaria sanguinalis) grown under moderate light intensity (200 µmol photons m-2 s-1) and, subsequently, exposed to excess light intensity (HL, 1600 µmol photons m-2 s-1). Our main interest was to understand why these species, grown under identical conditions, differ in their responses toward high light, and what is the physiological significance of these differences. Among the investigated species, Echinochloa crus-galli is best adapted to HL treatment. High resistance of the photosynthetic apparatus of E. crus-galli to HL was accompanied by an elevated level of phosphorylation of PSII proteins, and higher values of photochemical quenching, ATP/ADP ratio, activity of PSI and PSII complexes, as well as integrity of the thylakoid membranes. It was also shown that the non-radiative dissipation of energy in the studied plants was not dependent on carotenoid contents and, thus, other photoprotective mechanisms might have been engaged under HL stress conditions. The activity of the enzymes superoxide dismutase and ascorbate peroxidase as well as the content of malondialdehyde and H2O2 suggests that antioxidant defense is not responsible for the differences observed in the tolerance of NADP-ME species toward HL stress. We concluded that the chloroplasts of the examined NADP-ME species showed different sensitivity to short-term high light irradiance, suggesting a role of other factors excluding light factors, thus influencing the response of thylakoid proteins. We also observed that HL affects the mesophyll chloroplasts first hand and, subsequently, the bundle sheath chloroplasts.

Transformation of the Cyanidioschyzon merolae chloroplast genome: prospects for understanding chloroplast function in extreme environments.

KEY MESSAGE: We have successfully transformed an exthemophilic red alga with the chloramphenicol acetyltransferase gene, rendering this organism insensitive to its toxicity. Our work paves the way to further work with this new modelorganism. Here we report the first successful attempt to achieve a stable, under selectable pressure, chloroplast transformation in Cyanidioschizon merolae-an extremophilic red alga of increasing importance as a new model organism. The following protocol takes advantage of a double homologous recombination phenomenon in the chloroplast, allowing to introduce an exogenous, selectable gene. For that purpose, we decided to use chloramphenicol acetyltransferase (CAT), as chloroplasts are particularly vulnerable to chloramphenicol lethal effects (Zienkiewicz et al. in Protoplasma, 2015, doi: 10.1007/s00709-015-0936-9 ). We adjusted two methods of DNA delivery: the PEG-mediated delivery and the biolistic bombardment based delivery, either of these methods work sufficiently with noticeable preference to the former. Application of a codon-optimized sequence of the cat gene and a single colony selection yielded C. merolae strains, capable of resisting up to 400 µg/mL of chloramphenicol. Our method opens new possibilities in production of site-directed mutants, recombinant proteins and exogenous protein overexpression in C. merolae-a new model organism.

The short-term response of Arabidopsis thaliana (C3) and Zea mays (C4) chloroplasts to red and far red light.

MAIN CONCLUSION: Light quality has various effects on photochemistry and protein phosphorylation in Zea mays and Arabidopsis thaliana thylakoids due to different degrees of light penetration across leaves and redox status in chloroplasts. The effect of the spectral quality of light (red, R and far red, FR) on the function of thylakoid proteins in Zea mays and Arabidopsis thaliana was investigated. It was concluded that red light stimulates PSII activity in A. thaliana thylakoids and in maize bundle sheath (BS) thylakoids, but not in mesophyll (M) thylakoids. The light quality did not change PSI activity in M thylakoids of maize. FR used after a white light period increased PSI activity significantly in maize BS and only slightly in A. thaliana thylakoids. As shown by blue native (BN)-PAGE followed by SDS-PAGE, proteins were differently phosphorylated in the thylakoids, indicating their different functions. FR light increased dephosphorylation of LHCII proteins in A. thaliana thylakoids, whereas in maize, dephosphorylation did not occur at all. The rate of phosphorylation was higher in maize BS than in M thylakoids. D1 protein phosphorylation increased in maize and decreased in A. thaliana upon irradiation with both R and growth light (white light, W). Light variations did not change the level of proteins in thylakoids. Our data strongly suggest that response to light quality is a species-dependent phenomenon. We concluded that the maize chloroplasts were differently stimulated, probably due to different degrees of light penetration across the leaf and thereby the redox status in the chloroplasts. These acclimation changes induced by light quality are important in the regulation of chloroplast membrane flexibility and thus its function.

Metabolic responses to lead of metallicolous and nonmetallicolous populations of Armeria maritima.

Metabolic responses to Pb(NO3)2 (Pb) ions of excised leaves of metallicolous (MPs) and nonmetallicolous populations (NMPs) of Armeria maritima, cultivated on normal soil, were examined. Detached leaves were exposure to Pb for 24 h, and metabolic parameters were investigated. Pb decreased the photosynthesis (Pn) rate and photosystem II (PSII) activity, whereas the photochemical efficiency of PSII remained unchanged. In both populations, Pb ions caused increase in O2 uptake of dark-treated leaves; however, respiration after Pn was not affected. Pb increased superoxide dismutase activity in MP leaves and malondialdehyde content in NMP leaves. Other metabolites after Pb treatment were increased (proline or H2O2) or decreased (malate). Ascorbate peroxidase activity and adenosine triphosphate content decreased more in MP than in NMP leaves. Our results indicate that A. maritima is well adapted to heavy metal-contaminated soils, and we discuss potential causes of the stimulation of respiration by Pb ions and possible reasons for the tolerance to oxidative stress of plants growing in a metal-rich habitat.

Light intensity and quality stimulated Deg1-dependent cleavage of PSII components in the chloroplasts of maize.

Recent studies have revealed that photo damages inducing high white light illumination of C3-type plant Arabidopsis thaliana promotes Deg1-mediated degradation of not only photosystem II core proteins D1/D2 but also minor LHCII proteins CP26, CP29 and PSII-associated PsbS protein. Using biochemical and immunological approaches we show that that the substrate pool of the heterologously expressed Deg1 ortholog protease from C4-type plant Zea mays is very similar to that of the A. thaliana in both mesophyll and bundle sheath chloroplasts. The Deg1-mediated degradation of photosystem II components has been observed after high light and red light treatment of maize leaves, while far red light did not induce Deg1-mediated degradation. Moreover, two isoforms of the Deg1 protease have been identified. Their genes are localized in chromosomes 6 and 8. The Pull-Down assay indicated that both proteins were able to bind the same set of chloroplast proteins, nevertheless in vitro digestion of Z. mays thylakoids in the form of inside-out vesicles has raveled that only Deg1 found in chromosome 8 exhibited proteolytic activity. Interestingly, the relative amount of Deg1 proteases in Z. mays bundle sheath chloroplasts (BS) is significantly higher than in mesophyll chloroplasts (M) in spite of lower content of PSII (∼20%) in BS.

[C4 type photosynthesis]. / Fotosynteza typu C4.

C4 photosynthesis includes several anatomical and biochemical modifications that allow plants to concentrate CO2 at the site of Rubisco. The photorespiratory pathway is repressed in C4 plants, since the rates of photosynthesis and biomass production are increased. This is an adaptation to high light intensities, high temperatures and dryness. C4 plants contain two distinct types of photosynthetic cells, mesophyll and bundle sheath. The processes of assimilation and reduction of CO2 are separated spatiality and catayzed by two different enzymes. Only the bundle sheath chloroplasts perform the reactions of the Calvin-Benson cycle with the help of the Rubisco enzyme present exclusively in this cell type. The primary CO2 fixation occurs in mesophyll cells through the action of the phosphoenolpyruvate carboxylase. The light-dependent reactions of the photosynthesis occur exclusively in the latter cell type. These differences in photochemistry lead to distinct redox profiles in both types of cells. C4 plants are divided into three biochemical subtypes on the basis of differences in the mechanisms of decarboxylation of the C4 acids. C4 plants will provide the main source of food for humans and animals in the nearest decade.

A pleiotropic drug resistance transporter in Nicotiana tabacum is involved in defense against the herbivore Manduca sexta.

Pleiotropic drug resistance (PDR) transporters are a group of membrane proteins belonging to the ABCG sub-family of ATP binding cassette (ABC) transporters. There is clear evidence for the involvement of plant ABC transporters in resistance to fungal and bacterial pathogens, but not in the biotic stress response to insect or herbivore attack. Here, we describe a PDR transporter, ABCG5/PDR5, from Nicotiana tabacum. GFP fusion and subcellular fractionation studies revealed that ABCG5/PDR5 is localized to the plasma membrane. Staining of transgenic plants expressing the GUS reporter gene under the control of the ABCG5/PDR5 transcription promoter and immunoblotting of wild-type plants showed that, under standard growth conditions, ABCG5/PDR5 is highly expressed in roots, stems and flowers, but is only expressed at marginal levels in leaves. Interestingly, ABCG5/PDR5 expression is induced in leaves by methyl jasmonate, wounding, pathogen infiltration, or herbivory by Manduca sexta. To address the physiological role of ABCG5/PDR5, N. tabacum plants silenced for the expression of ABCG5/PDR5 were obtained. No phenotypic modification was observed under standard conditions. However, a small increase in susceptibility to the fungus Fusarium oxysporum was observed. A stronger effect was observed in relation to herbivory: silenced plants allowed better growth and faster development of M. sexta larvae than wild-type plants, indicating an involvement of this PDR transporter in resistance to M. sexta herbivory.

Nicotiana plumbaginifolia plants silenced for the ATP-binding cassette transporter gene NpPDR1 show increased susceptibility to a group of fungal and oomycete pathogens.

SUMMARY The behaviour of Nicotiana plumbaginifolia plants silenced for the ATP-binding cassette transporter gene NpPDR1 was investigated in response to fungal and oomycete infections. The importance of NpPDR1 in plant defence was demonstrated for two organs in which NpPDR1 is constitutively expressed: the roots and the petal epidermis. The roots of the plantlets of two lines silenced for NpPDR1 expression were clearly more sensitive than those of controls to the fungal pathogens Botrytis cinerea, Fusarium oxysporum sp., F. oxysporum f. sp. nicotianae, F. oxysporum f. sp. melonis and Rhizoctonia solani, as well as to the oomycete pathogen Phytophthora nicotianae race 0. The Ph gene-linked resistance of N. plumbaginifolia to P. nicotianae race 0 was totally ineffective in NpPDR1-silenced lines. In addition, the petals of the NpPDR1-silenced lines were spotted 15%-20% more rapidly by B. cinerea than were the controls. The rapid induction (after 2-4 days) of NpPDR1 expression in N. plumbaginifolia and N. tabacum mature leaves in response to pathogen presence was demonstrated for the first time with fungi and one oomycete: R. solani, F. oxysporum and P. nicotianae. With B. cinerea, such rapid expression was not observed in healthy mature leaves. NpPDR1 expression was not observed during latent infections of B. cinerea in N. plumbaginifolia and N. tabacum, but was induced when conditions facilitated B. cinerea development in leaves, such as leaf ageing or an initial root infection. This work demonstrates the increased sensitivity of NpPDR1-silenced N. plumbaginifolia plants to all of the fungal and oomycete pathogens investigated.

Differential turnover of the photosystem II reaction centre D1 protein in mesophyll and bundle sheath chloroplasts of maize.

Photoinhibition is caused by an imbalance between the rates of the damage and repair cycle of photosystem II D1 protein in thylakoid membranes. The PSII repair processes include (i) disassembly of damaged PSII-LHCII supercomplexes and PSII core dimers into monomers, (ii) migration of the PSII monomers to the stroma regions of thylakoid membranes, (iii) dephosphorylation of the CP43, D1 and D2 subunits, (iv) degradation of damaged D1 protein, and (v) co-translational insertion of the newly synthesized D1 polypeptide and reassembly of functional PSII complex. Here, we studied the D1 turnover cycle in maize mesophyll and bundle sheath chloroplasts using a protein synthesis inhibitor, lincomycin. In both types of maize chloroplasts, PSII was found as the PSII-LHCII supercomplex, dimer and monomer. The PSII core and the LHCII proteins were phosphorylated in both types of chloroplasts in a light-dependent manner. The rate constants for photoinhibition measured for lincomycin-treated leaves were comparable to those reported for C3 plants, suggesting that the kinetics of the PSII photodamage is similar in C3 and C4 species. During the photoinhibitory treatment the D1 protein was dephosphorylated in both types of chloroplasts but it was rapidly degraded only in the bundle sheath chloroplasts. In mesophyll chloroplasts, PSII monomers accumulated and little degradation of D1 protein was observed. We postulate that the low content of the Deg1 enzyme observed in mesophyll chloroplasts isolated from moderate light grown maize may retard the D1 repair processes in this type of plastids.

Structural organization of photosynthetic apparatus in agranal chloroplasts of maize.

We investigated the organization of photosystem II (PSII) in agranal bundle sheath thylakoids from a C(4) plant maize. Using blue native/SDS-PAGE and single particle analysis, we show for the first time that PSII in the bundle sheath (BS) chloroplasts exists in a dimeric form and forms light-harvesting complex II (LHCII).PSII supercomplexes. We also demonstrate that a similar set of photosynthetic membrane complexes exists in mesophyll and agranal BS chloroplasts, including intact LHCI.PSI supercomplexes, PSI monomers, PSII core dimers, PSII monomers devoid of CP43, LHCII trimers, LHCII monomers, ATP synthase, and cytochrome b(6)f complex. Fluorescence functional measurements clearly indicate that BS chloroplasts contain PSII complexes that are capable of performing charge separation and are efficiently sensitized by the associated LHCII. We identified a fraction of LHCII present within BS thylakoids that is weakly energetically coupled to the PSII reaction center; however, the majority of BS LHCII is shown to be tightly connected to PSII. Overall, we demonstrate that organization of the photosynthetic apparatus in BS agranal chloroplasts of a model C(4) plant is clearly distinct from that of the stroma lamellae of the C(3) plants. In particular, supramolecular organization of the dimeric LHCII.PSII in the BS thylakoids strongly suggests that PSII in the BS agranal membranes may donate electrons to PSI. We propose that the residual PSII activity may supply electrons to poise cyclic electron flow around PSI and prevent PSI overoxidation, which is essential for the CO(2) fixation in BS cells, and hence, may optimize ATP production within this compartment.

High light induced accumulation of two isoforms of the CF1 alpha-subunit in mesophyll and bundle sheath chloroplasts of C4 plants.

The effect of light irradiance on the amount of ATP synthase alpha-subunit in mesophyll (M) and bundle sheath (BS) chloroplasts of C(4) species such as maize (Zea mays L., type NADP-ME), millet (Panicum miliaceum, type NAD-ME) and guinea grass (Panicum maximum, type PEP-CK) was investigated in plants grown under high, moderate and low light intensities equal to 800, 350 and 50 micromol photons m(-2) s(-1), respectively. The results demonstrate that alpha-subunit of ATP synthase in both M and BS chloroplasts is altered by light intensity, but differently in the investigated species. Moreover, we identified two isoforms of the CF(1) alpha-subunit, called alpha and alpha. The CF(1) alpha-subunit was the major isoform and was present in all light conditions, whereas alpha was the minor isoform in low light. A strong increase in the level of the alpha-subunit in maize mesophyll and bundle sheath thylakoids was observed after 50 h of high light treatment. The alpha and alpha-subunits from investigated C(4) species displayed apparent molecular masses of 64 and 67 kDa, respectively, on SDS/PAGE. The presence of the alpha-subunit of ATPase was confirmed in isolated CF(1) complex, where it was recognized by antisera to the alpha-subunit. The N-terminal sequence of alpha-subunit is nearly identical to that of alpha. Our results indicate that both isoforms coexist in M and BS chloroplasts during plant growth at all irradiances. We suggest the existence in M and BS chloroplasts of C(4) plants of a mechanism(s) regulating the ATPase composition in response to light irradiance. Accumulation of the alpha isoform may have a protective role under high light stress against over protonation of the thylakoid lumen and photooxidative damage of PSII.