A knockdown mutation of YELLOW-GREEN LEAF2 blocks chlorophyll biosynthesis in rice

An insert mutation of YELLOW-GREEN LEAF2 , encoding Heme Oxygenase 1 , results in significant reduction of its transcript levels, and therefore impairs chlorophyll biosynthesis in rice. Heme oxygenase (HO) in higher plants catalyzes the degradation of heme to synthesize phytochrome precursor and its roles conferring the photoperiodic control of flowering in rice have been revealed. However, its involvement in regulating rice chlorophyll (Chl) synthesis is not fully explored. In this study, we isolated a rice mutant named yellow-green leaf 2 (ygl2) from a (60)Co-irradiated population. Normal grown ygl2 seedlings showed yellow-green leaves with reduced contents of Chl and tetrapyrrole intermediates whereas an increase of Chl a/b ratio. Ultrastructural analyses demonstrated grana were poorly stacked in ygl2 mutant, resulting in underdevelopment of chloroplasts. The ygl2 locus was mapped to chromosome 6 and isolated via map-based cloning. Sequence analysis indicated that it encodes the rice HO1 and its identity was verified by transgenic complementation test and RNA interference. A 7-Kb insertion was found in the first exon of YGL2/HO1, resulting in significant reduction of YGL2 expressions in the ygl2 mutant. YGL2 was constitutively expressed in a variety of rice tissues with the highest levels in leaves and regulated by temperature. In addition, we found expression levels of some genes associated with Chl biosynthesis and photosynthesis were concurrently altered in ygl2 mutant. These results provide direct evidence that YGL2 has a vital function in rice Chl biosynthesis.

Loss-of-function of OsSTN8 suppresses the photosystem II core protein phosphorylation and interferes with the photosystem II repair mechanism in rice (Oryza sativa)

STN8 kinase is involved in photosystem II (PSII) core protein phosphorylation (PCPP). To examine the role of PCPP in PSII repair during high light (HL) illumination, we characterized a T-DNA insertional knockout mutant of the rice (Oryza sativa) STN8 gene. In this osstn8 mutant, PCPP was significantly suppressed, and the grana were thin and elongated. Upon HL illumination, PSII was strongly inactivated in the mutants, but the D1 protein was degraded more slowly than in wild-type, and mobilization of the PSII supercomplexes from the grana to the stromal lamellae for repair was also suppressed. In addition, higher accumulation of reactive oxygen species and preferential oxidation of PSII reaction center core proteins in thylakoid membranes were observed in the mutants during HL illumination. Taken together, our current data show that the absence of STN8 is sufficient to abolish PCPP in osstn8 mutants and to produce all of the phenotypes observed in the double mutant of Arabidopsis, indicating the essential role of STN8-mediated PCPP in PSII repair.

Probing the consequences of antenna modification in cyanobacteria

Photosynthetic organisms rely on antenna systems to harvest and deliver energy from light to reaction centers. In fluctuating photic environments, regulation of light harvesting is critical for a photosynthetic organism's survival. Here, we describe the use of a suite of phycobilisome mutants to probe the consequences of antenna truncation in the cyanobacterium Synechocystis sp. PCC 6803. Studies using transmission electron microscopy (TEM), hyperspectral confocal fluorescence microscopy (HCFM), small-angle neutron scattering (SANS), and an optimized photobioreactor system have unraveled the adaptive strategies that cells employ to compensate for antenna reduction. As the phycobilisome antenna size decreased, changes in thylakoid morphology were more severe and physical segregation of the two photosystems increased. Repeating distances between thylakoid membranes measured by SANS were correlated with TEM data, and corresponded to the degree of phycobilisome truncation. Thylakoid membranes were found to have a high degree of structural flexibility, and changes in the membrane system upon illumination were rapid and reversible. Phycobilisome truncation in Synechocystis 6803 reduced the growth rate and lowered biomass accumulation. Together, these results lend a dynamic perspective to the intracellular membrane organization in cyanobacteria cells and suggest an adaptive mechanism that allows cells to adjust to altered light absorption capabilities, while highlighting the cell-wide implications of antenna truncation.

Mutation of the Arabidopsis NAC016 transcription factor delays leaf senescence

The highly ordered process of senescence forms the final stage of leaf development; a large set of senescence-associated genes (SAGs) execute this orderly dismantling of the photosynthetic apparatus and remobilization of cellular components. A number of transcription factors (TFs) modulate SAG expression to promote or delay senescence. Here we show that NAC016, the previously uncharacterized senescence-associated NAM/ATAF1/2/CUC2 (senNAC) TF in Arabidopsis thaliana, promotes senescence. Leaves of nac016 mutants remained green under senescence-inducing conditions, and leaves of NAC016-overexpressing (NAC016-OX) plants senesced early. Under dark-induced senescence (DIS) conditions, nac016 mutants had low ion leakage, and retained the proper balance of photosystem proteins and normal grana thylakoid shape much longer than wild-type plants, suggesting that nac016 acts as a functional stay-green type senescence mutant. Under DIS conditions, SAGs (NYC1, PPH, SGR1/NYE1 and WRKY22), including senNACs (JUB1, NAP, ORE1, ORS1 and VNI2), were down-regulated in nac016 mutants and up-regulated in NAC016-OX plants. In addition to its role in senescence, NAC016 also affects abiotic stress. Under salt and oxidative stress conditions, NAC016 expression rapidly increased in developing leaves, possibly to promote senescence. Indeed, under the stress conditions, nac016 mutants stayed green and NAC016-OX plants senesced rapidly. To identify direct targets of the NAC016 TF in the regulation of leaf senescence, we conducted yeast one-hybrid assays, which strongly suggested that NAC016 binds to the promoters of NAP and ORS1. Based on these results, we propose that NAC016 regulatory mechanisms promoting leaf senescence exhibit cross-talk with the salt and oxidative stress-responsive signaling pathways.

Architectural switches in plant thylakoid membranes

Recent progress in elucidating the structure of higher plants photosynthetic membranes provides a wealth of information. It allows generation of architectural models that reveal well-organized and complex arrangements not only on whole membrane level, but also on the supramolecular level. These arrangements are not static but highly responsive to the environment. Knowledge about the interdependency between dynamic structural features of the photosynthetic machinery and the functionality of energy conversion is central to understanding the plasticity of photosynthesis in an ever-changing environment. This review summarizes the architectural switches that are realized in thylakoid membranes of green plants.

Thylakoid membrane maturation and PSII activation are linked in greening Synechocystis sp. PCC 6803 cells

Thylakoid membranes are typical and essential features of both chloroplasts and cyanobacteria. While they are crucial for phototrophic growth of cyanobacterial cells, biogenesis of thylakoid membranes is not well understood yet. Dark-grown Synechocystis sp. PCC 6803 cells contain only rudimentary thylakoid membranes but still a relatively high amount of phycobilisomes, inactive photosystem II and active photosystem I centers. After shifting dark-grown Synechocystis sp. PCC 6803 cells into the light, "greening" of Synechocystis sp. PCC 6803 cells, i.e. thylakoid membrane formation and recovery of photosynthetic electron transport reactions, was monitored. Complete restoration of a typical thylakoid membrane system was observed within 24 hours after an initial lag phase of 6 to 8 hours. Furthermore, activation of photosystem II complexes and restoration of a functional photosynthetic electron transport chain appears to be linked to the biogenesis of organized thylakoid membrane pairs.

Arabidopsis thylakoid formation 1 is a critical regulator for dynamics of PSII-LHCII complexes in leaf senescence and excess light

In higher plants, photosystem II (PSII) is a large pigment-protein supramolecular complex composed of the PSII core complex and the plant-specific peripheral light-harvesting complexes (LHCII). PSII-LHCII complexes are highly dynamic in their quantity and macro-organization to various environmental conditions. In this study, we reported a critical factor, the Arabidopsis Thylakoid Formation 1 (THF1) protein, which controls PSII-LHCII dynamics during dark-induced senescence and light acclimation. Loss-of-function mutations in THF1 lead to a stay-green phenotype in pathogen-infected and senescent leaves. Both LHCII and PSII core subunits are retained in dark-induced senescent leaves of thf1, indicative of the presence of PSII-LHCII complexes. Blue native (BN)-polyacrylamide gel electrophoresis (PAGE) and immunoblot analysis showed that, in dark- and high-light-treated thf1 leaves, a type of PSII-LHCII megacomplex is selectively retained while the stability of PSII-LHCII supercomplexes significantly decreased, suggesting a dual role of THF1 in dynamics of PSII-LHCII complexes. We showed further that THF1 interacts with Lhcb proteins in a pH-dependent manner and that the stay-green phenotype of thf1 relies on the presence of LHCII complexes. Taken together, the data suggest that THF1 is required for dynamics of PSII-LHCII supramolecular organization in higher plants.

Characterization of photosystem I in rice (Oryza sativa L.) seedlings upon exposure to random positioning machine

To gain a better understanding of how photosynthesis is adapted under altered gravity forces, photosynthetic apparatus and its functioning were investigated in rice (Oryza sativa L.) seedlings grown in a random positioning machine (RPM). A decrease in fresh weight and dry weight was observed in rice seedlings grown under RPM condition. No significant changes were found in the chloroplast ultrastructure and total chlorophyll content between the RPM and control samples. Analyses of chlorophyll fluorescence and thermoluminescence demonstrate that PSII activity was unchanged under RPM condition. However, PSI activity decreased significantly under RPM condition. 77 K fluorescence emission spectra show a blue-shift and reduction of PSI fluorescence emission peak in the RPM seedlings. In addition, RPM caused a significant decrease in the amplitude of absorbance changes of P700 at 820 nm (A 820) induced by saturated far-red light. Moreover, the PSI efficiency (Φ I) decreased significantly under RPM condition. Immunoblot and blue native gel analyses further illustrate that accumulation of PSI proteins was greatly decreased in the RPM seedlings. Our results suggest that PSI, but not PSII, is down-regulated under RPM condition.

Promotion of cyclic electron transport around photosystem I during the evolution of NADP-malic enzyme-type C4 photosynthesis in the genus Flaveria

C4 plants display higher cyclic electron transport activity than C3 plants. This activity is suggested to be important for the production of ATPs required for C4 metabolism. To understand the process by which photosystem I (PSI) cyclic electron transport was promoted during C4 evolution, we conducted comparative analyses of the functionality of PSI cyclic electron transport among members of the genus Flaveria, which contains several C3, C3-C4 intermediate, C4-like and C4 species. The abundance of NDH-H, a subunit of NADH dehydrogenase-like complex, increased markedly in bundle sheath cells with the activity of the C4 cycle. By contrast, PROTON GRADIENT REGULATION5 (PGR5) and PGR5-LIKE1 increased in both mesophyll and bundle sheath cells in C4-like Flaveria palmeri and C4 species. Grana stacks were drastically reduced in bundle sheath chloroplasts of C4-like F. palmeri and C4 species; these species showed a marked increase in PSI cyclic electron transport activity. These results suggest that both the expression of proteins involved in PSI cyclic electron transport and changes in thylakoid structure contribute to the high activity of cyclic electron flow in NADP-malic enzyme-type C4 photosynthesis. We propose that these changes were important for the establishment of C4 photosynthesis from C3-C4 intermediate photosynthesis in Flaveria.

A TIR-NBS protein encoded by Arabidopsis Chilling Sensitive 1 (CHS1) limits chloroplast damage and cell death at low temperature

Survival of plants at low temperature depends on mechanisms for limiting physiological damage and maintaining growth. We mapped the chs1-1 (chilling sensitive1-1) mutation in Arabidopsis accession Columbia to the TIR-NBS gene At1g17610. In chs1-1, a single amino acid exchange at the CHS1 N-terminus close to the conserved TIR domain creates a stable mutant protein that fails to protect leaves against chilling stress. The sequence of another TIR-NBS gene (At5g40090) named CHL1 (CHS1-like 1) is related to that of CHS1. Over-expression of CHS1 or CHL1 alleviates chilling damage and enhances plant growth at moderate (24°C) and chilling (13°C) temperatures, suggesting a role for both proteins in growth homeostasis. chs1-1 mutants show induced salicylic acid production and defense gene expression at 13°C, indicative of autoimmunity. Genetic analysis of chs1-1 in combination with defense pathway mutants shows that chs1-1 chilling sensitivity requires the TIR-NBS-LRR and basal resistance regulators encoded by EDS1 and PAD4 but not salicylic acid. By following the timing of metabolic, physiological and chloroplast ultrastructural changes in chs1-1 leaves during chilling, we have established that alterations in photosynthetic complexes and thylakoid membrane integrity precede leaf cell death measured by ion leakage. At 24°C, the chs1-1 mutant appears normal but produces a massive necrotic response to virulent Pseudomonas syringae pv. tomato infection, although this does not affect bacterial proliferation. Our results suggest that CHS1 acts at an intersection between temperature sensing and biotic stress pathway activation to maintain plant performance over a range of conditions.

Comparison of different cells of Haematococcus pluvialis reveals an extensive acclimation mechanism during its aging process: from a perspective of photosynthesis

Both biomass dominated green vegetative cells (GV) and astaxanthin-dominated orange resting cells (OR) affect the final astaxanthin yield in industry. Examination of Haematococcus pluvialis revealed that the OR cells greatly varied from the GV cells at both cellular and subcellular levels. In particular, the thylakoid membranes in the OR were disassembled and fragmented. Furthermore, the OR conserved most of the photosynthetic pigments, with elevated concentrations of violaxanthin, antheraxanthin, and neoxanthin. Notably, moderate photosynthesis was detected in OR, even though most of the thylakoid membranes were disassembled, when compared with those in the GV. However, the energy distribution pattern between photosystem I and II (PSI and PSII) in the OR favored PSI, which was also confirmed by 77-K fluorescence. As zeaxanthin was not detected in the OR, we attribute the acclimation role to astaxanthin, instead of xanthophyll cycle. Additionally, proteomic-scale comparison analysis of thylakoids of the OR and GV indicated no photosynthetically remarkable variations. However, an extensive acclimation mechanism of H. pluvialis was proposed, in which proteins in thylakoid of GV were noted to be involved in biomass accumulation and those in OR were involved in stress response. Conclusions of the comparative analysis might provide some physiological background of OR for astaxanthin production by using H. pluvialis.

TAC7, an essential component of the plastid transcriptionally active chromosome complex, interacts with FLN1, TAC10, TAC12 and TAC14 to regulate chloroplast gene expression in Arabidopsis thaliana

Transcriptionally active chromosome (TAC) is a fraction of protein/DNA complexes with RNA polymerase activity in the plastid. However, the function of most TAC proteins remains unknown. Here, we isolated two allelic mutants of the gene for a TAC component, TAC7, and performed functional analysis in plastid gene expression and chloroplast development in Arabidopsis. tac7-1 is a mutant with a premature translation termination isolated from a population treated with ethyl methane sulfonate, and tac7-2 is a transfer-DNA tagging mutant. Both of them showed an albino phenotype when grown under normal light conditions, and a few appressed membranes were observed inside the defective chloroplasts. These data indicate that TAC7 is important for thylakoid biogenesis. The TAC7 gene encodes an uncharacterized 161 amino acids polypeptide localized in chloroplast. The transcriptional levels of plastid-encoded polymerase (PEP)-dependent genes were downregulated in tac7-2, suggesting that PEP activity was decreased in the mutant. Yeast two-hybrid assay shows that TAC7 can interact with the four TAC components including FLN1, TAC10, TAC12 and TAC14 which are involved in redox state changes, phosphorylation processes and phytochrome-dependent light signaling, respectively, These data indicate that TAC7 plays an important role for TAC to regulate PEP-dependent chloroplast gene expression and chloroplast development.

Kinetics of structural reorganizations in multilamellar photosynthetic membranes monitored by small-angle neutron scattering

We demonstrate the power of time-resolved small-angle neutron scattering experiments for the investigation of the structure and structural reorganizations of multilamellar photosynthetic membranes. In addition to briefly summarizing our results on thylakoid membranes isolated from higher plants and in unicellular organisms, we discuss the advantages and technical and methodological limitations of time-resolved SANS. We present a detailed and more systematical investigation of the kinetics of light-induced structural reorganizations in isolated spinach thylakoid membranes, which show how changes in the repeat distance and in the long-range order of the multilamellar membranes can be followed with a time resolution of seconds. We also present data from comparative measurements performed on thylakoid membranes isolated from tobacco.

Arabidopsis CURVATURE THYLAKOID1 proteins modify thylakoid architecture by inducing membrane curvature

Chloroplasts of land plants characteristically contain grana, cylindrical stacks of thylakoid membranes. A granum consists of a core of appressed membranes, two stroma-exposed end membranes, and margins, which connect pairs of grana membranes at their lumenal sides. Multiple forces contribute to grana stacking, but it is not known how the extreme curvature at margins is generated and maintained. We report the identification of the CURVATURE THYLAKOID1 (CURT1) protein family, conserved in plants and cyanobacteria. The four Arabidopsis thaliana CURT1 proteins (CURT1A, B, C, and D) oligomerize and are highly enriched at grana margins. Grana architecture is correlated with the CURT1 protein level, ranging from flat lobe-like thylakoids with considerably fewer grana margins in plants without CURT1 proteins to an increased number of membrane layers (and margins) in grana at the expense of grana diameter in overexpressors of CURT1A. The endogenous CURT1 protein in the cyanobacterium Synechocystis sp PCC6803 can be partially replaced by its Arabidopsis counterpart, indicating that the function of CURT1 proteins is evolutionary conserved. In vitro, Arabidopsis CURT1A proteins oligomerize and induce tubulation of liposomes, implying that CURT1 proteins suffice to induce membrane curvature. We therefore propose that CURT1 proteins modify thylakoid architecture by inducing membrane curvature at grana margins.

Molecular architecture of plant thylakoids under physiological and light stress conditions: a study of lipid-light-harvesting complex II model membranes

In this study, we analyzed multibilayer lipid-protein membranes composed of the photosynthetic light-harvesting complex II (LHCII; isolated from spinach [Spinacia oleracea]) and the plant lipids monogalcatosyldiacylglycerol and digalactosyldiacylglycerol. Two types of pigment-protein complexes were analyzed: those isolated from dark-adapted leaves (LHCII) and those from leaves preilluminated with high-intensity light (LHCII-HL). The LHCII-HL complexes were found to be partially phosphorylated and contained zeaxanthin. The results of the x-ray diffraction, infrared imaging microscopy, confocal laser scanning microscopy, and transmission electron microscopy revealed that lipid-LHCII membranes assemble into planar multibilayers, in contrast with the lipid-LHCII-HL membranes, which form less ordered structures. In both systems, the protein formed supramolecular structures. In the case of LHCII-HL, these structures spanned the multibilayer membranes and were perpendicular to the membrane plane, whereas in LHCII, the structures were lamellar and within the plane of the membranes. Lamellar aggregates of LHCII-HL have been shown, by fluorescence lifetime imaging microscopy, to be particularly active in excitation energy quenching. Both types of structures were stabilized by intermolecular hydrogen bonds. We conclude that the formation of trans-layer, rivet-like structures of LHCII is an important determinant underlying the spontaneous formation and stabilization of the thylakoid grana structures, since the lamellar aggregates are well suited to dissipate excess energy upon overexcitation.

Molecular architecture of plant thylakoids under physiological and light stress conditions: a study of lipid-light-harvesting complex II model membranes

In this study, we analyzed multibilayer lipid-protein membranes composed of the photosynthetic light-harvesting complex II (LHCII; isolated from spinach [Spinacia oleracea]) and the plant lipids monogalcatosyldiacylglycerol and digalactosyldiacylglycerol. Two types of pigment-protein complexes were analyzed: those isolated from dark-adapted leaves (LHCII) and those from leaves preilluminated with high-intensity light (LHCII-HL). The LHCII-HL complexes were found to be partially phosphorylated and contained zeaxanthin. The results of the x-ray diffraction, infrared imaging microscopy, confocal laser scanning microscopy, and transmission electron microscopy revealed that lipid-LHCII membranes assemble into planar multibilayers, in contrast with the lipid-LHCII-HL membranes, which form less ordered structures. In both systems, the protein formed supramolecular structures. In the case of LHCII-HL, these structures spanned the multibilayer membranes and were perpendicular to the membrane plane, whereas in LHCII, the structures were lamellar and within the plane of the membranes. Lamellar aggregates of LHCII-HL have been shown, by fluorescence lifetime imaging microscopy, to be particularly active in excitation energy quenching. Both types of structures were stabilized by intermolecular hydrogen bonds. We conclude that the formation of trans-layer, rivet-like structures of LHCII is an important determinant underlying the spontaneous formation and stabilization of the thylakoid grana structures, since the lamellar aggregates are well suited to dissipate excess energy upon overexcitation.

Loss of plastoglobule kinases ABC1K1 and ABC1K3 causes conditional degreening, modified prenyl-lipids, and recruitment of the jasmonic acid pathway

Plastoglobules (PGs) are plastid lipid-protein particles. This study examines the function of PG-localized kinases ABC1K1 and ABC1K3 in Arabidopsis thaliana. Several lines of evidence suggested that ABC1K1 and ABC1K3 form a protein complex. Null mutants for both genes (abc1k1 and abc1k3) and the double mutant (k1 k3) displayed rapid chlorosis upon high light stress. Also, k1 k3 showed a slower, but irreversible, senescence-like phenotype during moderate light stress that was phenocopied by drought and nitrogen limitation, but not cold stress. This senescence-like phenotype involved degradation of the photosystem II core and upregulation of chlorophyll degradation. The senescence-like phenotype was independent of the EXECUTER pathway that mediates genetically controlled cell death from the chloroplast and correlated with increased levels of the singlet oxygen-derived carotenoid β-cyclocitral, a retrograde plastid signal. Total PG volume increased during light stress in wild type and k1 k3 plants, but with different size distributions. Isolated PGs from k1 k3 showed a modified prenyl-lipid composition, suggesting reduced activity of PG-localized tocopherol cyclase (VTE1), and was consistent with loss of carotenoid cleavage dioxygenase 4. Plastid jasmonate biosynthesis enzymes were recruited to the k1 k3 PGs but not wild-type PGs, while pheophytinase, which is involved in chlorophyll degradation, was induced in k1 k3 and not wild-type plants and was localized to PGs. Thus, the ABC1K1/3 complex contributes to PG function in prenyl-lipid metabolism, stress response, and thylakoid remodeling.

Significant involvement of PEP-CK in carbon assimilation of C4 eudicots

BACKGROUND AND AIMS

C4 eudicot species are classified into biochemical sub-types of C4 photosynthesis based on the principal decarboxylating enzyme. Two sub-types are known, NADP-malic enzyme (ME) and NAD-ME; however, evidence for the occurrence or involvement of the third sub-type (phosphoenolpyruvate carboxykinase; PEP-CK) is emerging. In this study, the presence and activity of PEP-CK in C4 eudicot species of Trianthema and Zaleya (Sesuvioideae, Aizoaceae) is clarified through analysis of key anatomical features and C4 photosynthetic enzymes.

METHODS

Three C4 species (T. portulacastrum, T. sheilae and Z. pentandra) were examined with light and transmission electron microscopy for leaf structural properties. Activities and immunolocalizations of C4 enzymes were measured for biochemical characteristics.

KEY RESULTS

Leaves of each species possess atriplicoid-type Kranz anatomy, but differ in ultrastructural features. Bundle sheath organelles are centripetal in T. portulacastrum and Z. pentandra, and centrifugal in T. sheilae. Bundle sheath chloroplasts in T. portulacastrum are almost agranal, whereas mesophyll counterparts have grana. Both T. sheilae and Z. pentandra are similar, where bundle sheath chloroplasts contain well-developed grana while mesophyll chloroplasts are grana deficient. Cell wall thickness is significantly greater in T. sheilae than in the other species. Biochemically, T. portulacastrum is NADP-ME, while T. sheilae and Z. pentandra are NAD-ME. Both T. portulacastrum and Z. pentandra exhibit considerable PEP-CK activity, and immunolocalization studies show dense and specific compartmentation of PEP-CK in these species, consistent with high PEP-CK enzyme activity.

CONCLUSIONS

Involvement of PEP-CK in C4 NADP-ME T. portulacastrum and NAD-ME Z. petandra occurs irrespective of biochemical sub-type, or the position of bundle sheath chloroplasts. Ultrastructural traits, including numbers of bundle sheath peroxisomes and mesophyll chloroplasts, and degree of grana development in bundle sheath chloroplasts, coincide more directly with PEP-CK recruitment. Discovery of high PEP-CK activity in C4 Sesuvioideae species offers a unique opportunity for evaluating PEP-CK expression and suggests the possibility that PEP-CK recruitment may exist elsewhere in C4 eudicots.

Structural constraints for protein repair in plant photosynthetic membranes

The thylakoid membrane system inside plants chloroplasts defines the structural framework for photosynthetic conversion of sunlight into metabolic energy forms (ATP, NADPH + H(+)). An architectural hallmark of these thylakoid membranes is the tight stacking of part of the membrane into cylindrical flat grana thylakoids, with a diameter of about 500 nm, that are interconnected by unstacked stroma lamellae forming a complex 3D network of alternating grana piles and stroma lamellae. The structural differentiation in the stacked and unstacked thylakoid regions is the basis for a pronounced spatial separation of multisubunit pigment-protein complexes that catalyze energy transformation. The main part of photosystem II (PSII) associated with light-harvesting complex II (LHCII) is concentrated in the grana thylakoids whereas PSI-LHCI and the ATPase complex are excluded from the stacked grana and accumulate in the unstacked thylakoid regions. The fifth protein complex, the cytochrome b 6f complex, is assumed to be homogenously distributed. It is important to recognize that this structural arrangement is not static but highly dynamic and responsive to environmental factors like light intensity and quality or temperature. Knowledge about the interplay between dynamic structural features of the intricate thylakoid architecture, and the functionality, regulation, repair and biogenesis of the photosynthetic machinery is essential for understanding the plasticity of energy conversion in plants living in a fluctuating multi-factorial environment.

Dual targeting of a mature plastoglobulin/fibrillin fusion protein to chloroplast plastoglobules and thylakoids in transplastomic tobacco plants

Plastoglobules (PG) are lipid droplets in chloroplasts and other plastid types having important functions in lipid metabolism. Plastoglobulins (PGL) also known as fibrillins (FBN) are evolutionary conserved proteins present at the PG surface but also to various extents at the thylakoid membrane. PGLs are thought to have structural functions in PG formation and maintenance. The targeting of an Arabidopsis PGL (PGL34) to PG required the full protein sequence with the exception of a short C-terminal stretch. This indicated that PGL targeting relies on correct folding rather than a discrete sequence. PGLs lack strongly hydrophic regions and may therefore extrinsically associate with PG and thylakoid membranes via interaction with hydrophilic headgroups of surface lipids. Here, we report on the expression of the Arabidopsis plastoglobulin of 35kD (PGL35 or FBN1a) expressed as a mature protein fused to HIVp24 (human immunodeficiency virus capsid particle p24) or HCV (hepatitis C virus core protein) in transplastomic tobacco. A PGL35-HIVp24 fusion targeted in part to plastoglobules but a larger proportion was recovered in the thylakoid fraction. The findings indicate that transplastomic PGL35-HIVp24 folded correctly after its synthesis inside the chloroplast and then dually targeted to plastoglobules as well as thylakoid membranes.

Young Leaf Chlorosis 1, a chloroplast-localized gene required for chlorophyll and lutein accumulation during early leaf development in rice

Chlorophyll (Chl) and lutein are the two most abundant and essential components in photosynthetic apparatus, and play critical roles in plant development. In this study, we characterized a rice mutant named young leaf chlorosis 1 (ylc1) from a ⁶⁰Co-irradiated population. Young leaves of the ylc1 mutant showed decreased levels of Chl and lutein compared to those of wild type, and transmission electron microscopy analysis revealed that the thylakoid lamellar structures were obviously loosely arranged. Whereas, the mutant turns green gradually and approaches normal green at the maximum tillering stage. The Young Leaf Chlorosis 1 (YLC1) gene was isolated via map-based cloning and identified to encode a protein of unknown function belonging to the DUF3353 superfamily. Complementation and RNA-interference tests confirmed the role of the YLC1 gene, which expressed in all tested rice tissues, especially in the leaves. Real-time PCR analyses showed that the expression levels of the genes associated with Chl biosynthesis and photosynthesis were affected in ylc1 mutant at different temperatures. In rice protoplasts, the YLC1 protein displayed a typical chloroplast location pattern. The N-terminal 50 amino acid residues were confirmed to be necessary and sufficient for chloroplast targeting. These data suggested that the YLC1 protein may be involved in Chl and lutein accumulation and chloroplast development at early leaf development in rice.

Phosphorylation controls the localization and activation of the lumenal carbonic anhydrase in Chlamydomonas reinhardtii

Background

Cah3 is the only carbonic anhydrase (CA) isoform located in the thylakoid lumen of Chlamydomonas reinhardtii. Previous studies demonstrated its association with the donor side of the photosystem II (PSII) where it is required for the optimal function of the water oxidizing complex. However this enzyme has also been frequently proposed to perform a critical function in inorganic carbon acquisition and CO₂ fixation and all mutants lacking Cah3 exhibit very poor growth after transfer to low CO₂ conditions.

Results/Conclusions

In the present work we demonstrate that after transfer to low CO₂, Cah3 is phosphorylated and that phosphorylation is correlated to changes in its localization and its increase in activity. When C. reinhardtii wild-type cells were acclimated to limiting CO₂ conditions, the Cah3 activity increased about 5–6 fold. Under these conditions, there were no detectable changes in the level of the Cah3 polypeptide. The increase in activity was specifically inhibited in the presence of Staurosporine, a protein kinase inhibitor, suggesting that the Cah3 protein was post-translationally regulated via phosphorylation. Immunoprecipitation and in vitro dephosphorylation experiments confirm this hypothesis. In vivo phosphorylation analysis of thylakoid polypeptides indicates that there was a 3-fold increase in the phosphorylation signal of the Cah3 polypeptide within the first two hours after transfer to low CO₂ conditions. The increase in the phosphorylation signal was correlated with changes in the intracellular localization of the Cah3 protein. Under high CO₂ conditions, the Cah3 protein was only associated with the donor side of PSII in the stroma thylakoids. In contrast, in cells grown at limiting CO₂ the protein was partly concentrated in the thylakoids crossing the pyrenoid, which did not contain PSII and were surrounded by Rubisco molecules.

Significance

This is the first report of a CA being post-translationally regulated and describing phosphorylation events in the thylakoid lumen.

Effects of salt stress on the structure and function of the photosynthetic apparatus in Cucumis sativus and its protection by exogenous putrescine

With the objective to clarify the physiological significance of polyamines (PAs) in the photosynthetic apparatus, the present study investigated the effects of salt stress with and without foliar application of putrescine (Put) on the structure and function of the photosynthetic apparatus in cucumber. Salt stress at 75 mM NaCl for 7 days resulted in a severe reduction of photosynthesis. The fast chlorophyll afluorescence transient analysis showed that salt stress inhibited the maximum quantum yield of PSII photochemistry (F(v)/F(m)), mainly due to damage at the receptor side of PSII. In addition, salt stress decreased the density of active reaction centers and the structure performance. The microscopic analysis revealed that salt stress-induced destruction of the chloroplast envelope and increased the number of plastoglobuli along with aberrations in thylakoid membranes. Besides, salt stress caused a decrease in the content of endogenous PAs, conjugated and bound forms of spermidine and spermine in particular, in thylakoid membranes. However, applications of 8 mM Put alleviated the salt stress-mediated decrease in net photosynthetic rates (Pn) and actual efficiency of PSII(Φ(PSII)). Put increased PAs in thylakoid membranes and overcame the damaging effects of salt stress on the structure and function of the photosynthetic apparatus in salt-stressed plant leaves. Put application to control plants neither increased PAs in thylakoid membranes nor affected photosynthesis. These results indicate that PAs in chloroplasts play crucial roles in protecting the thylakoid membranes against the deleterious influences of salt stress. In addition, the present results point to the probability that the salt-induced dysfunction of photosynthesis is largely attributable to the loss of PAs in the photosynthetic apparatus.

Chilling-induced ultrastructural changes to mesophyll cells of Arabidopsis grown under short days are almost completely reversible by plant re-warming

Exposure of plants to chilling (low temperatures above freezing) limits growth and development in all environments outside the lowest latitudes. Cell ultrastructure and morphometric studies may allow associations to be made between chilling-induced changes at the ultrastructural level, molecular events and their physiological consequences. We examined changes in the shape, size and membrane organization of the organelles of mesophyll cells in Arabidopsis thaliana (Col 0), a cold-resistant species, after subjecting 6-week-old plants grown at normal growth temperatures to chilling (2.5-4°C; 14-h dark/10-h light cycle) for 6, 24 and 72 h and after a re-warming period of 50 h. No ultrastructural differences were seen in the first 6 h of chilling but after 24 h we observed swollen and rounded chloroplasts with larger starch grains and dilated thylakoids compared to control plants. By 72 h, chilling had resulted in a large accumulation of starch in chloroplasts, an apparent crowding of the cytosol and a lower abundance of peripheral reticulum than in the controls. The average area per chloroplast in cell sections increased after 72-h chilling while the number of chloroplasts remained the same. Ring-shaped and other morphologically aberrant mitochondria were present in significantly higher abundance in plants given 72 h chilling than in the controls. Plant re-warming for 50 h reduced chloroplast size to those of the controls and returned mitochondria to standard morphology, but peripheral reticulum remained less abundant than in plants never given a cold treatment. The near full return to normal ultrastructure upon plant re-warming indicates that the morphological changes may be part of acclimation to cold.

Molecular cloning and characterization of OsCHR4, a rice chromatin-remodeling factor required for early chloroplast development in adaxial mesophyll

Mi-2 protein, the central component of the NuRD nucleosome remodeling and histone deacetylase complex, plays a role in transcriptional repression in animals. Mi-2-like genes have been reported in Arabidopsis, though their function in monocots remains largely unknown. In the present study, a rice Mi-2-like gene, OsCHR4 (Oryza sativa Chromatin Remodeling 4, LOC_Os07g03450), was cloned from a rice mutant with adaxial albino leaves. The Oschr4 mutant exhibited defective chloroplasts in adaxial mesophyll, but not in abaxial mesophyll. Ultrastructural observations indicated that proplastid growth and/or thylakoid membrane formation in adaxial mesophyll cells was blocked in the Oschr4 mutant. Subcellular localization revealed that OsCHR4::GFP fusion protein was targeted to the nuclei. OsCHR4 was mainly expressed in the root meristem, flower, vascular bundle, and mesophyll cells by promoter::GUS analysis in transgenic rice. The transcripts of some nuclear- and plastid-encoded genes required for early chloroplast development and photosynthesis were decreased in the adaxial albino mesophyll of the Oschr4 mutant. These observations provide evidence that OsCHR4, the rice Mi-2-like protein, plays an important role in early chloroplast development in adaxial mesophyll cells. The results increase our understanding of the molecular mechanism underlying tissue-specific chloroplast development in plants.