Haemophilia A and B - evaluation of the Swedish prophylactic regimen by magnetic resonance imaging.

INTRODUCTION: Sweden has been a pioneer in the prophylactic treatment of haemophilia. Magnetic resonance imaging (MRI) can detect small changes in joints and can therefore give an indication of a risk of developing arthropathy. AIM: To use MRI to evaluate the outcome of the Swedish 'high-dose regimen' and correlate the findings to age, bleeds, joint score and physical activity. METHODS: The study group comprised 48 Swedish male patients, mean age 25 years (range 12-33 years), with severe or moderate haemophilia A or B. Data on the Haemophilia Joint Health Score (HJHS) were available and physical activity was evaluated by a self-reported questionnaire. RESULTS: MRI score was recorded in 188 joints. Twenty out of 48 patients had a score of ≥1 (range 1-13) in 31 joints of which 3/31 scores were in the knees and 28/31 in the ankles. No correlation was found between the number of recorded bleeds and the MRI score or between HJHS and MRI score. There was no correlation between the physical activity and the number of joint bleeds per se, but a trend (OR 3.0) that those most physically active (19/48; 39.6%), more frequently had an MRI score of ≥1 with an overweight for the right ankle. CONCLUSION: The Swedish prophylactic model offers protection against haemophilia joint arthropathy but will still not prevent osteochondral changes in some patients at young age. MRI of the ankles can signal risk of future arthropathy and indicate need to modify the prophylactic regimen.

Visualization of wrist ligaments with 3D and 2D magnetic resonance imaging at 3 Tesla.

BACKGROUND: Wrist ligaments are challenging to visualize using magnetic resonance imaging (MRI). Injuries involving the scapholunate ligament (SLL), the lunotriquetral ligament (LTL), and the triangular fibrocartilage complex (TFCC) are common and difficult to diagnose, often requiring diagnostic arthroscopy. PURPOSE: To compare the visualization of wrist ligaments on a three-dimensional (3D) sequence with two-dimensional (2D) sequences on 3-T MRI. MATERIAL AND METHODS: Eighteen healthy volunteers were examined with a 3D SPACE (sampling perfection with application optimized contrasts using different flip angle evolution) sequence and 2D coronal, axial, and sagittal proton density-weighted (PD) sequences. Four musculoskeletal radiologists graded the anatomical visibility of the SLL, LTL, TFCC, and the image quality, using five grades in a visual grading characteristics (VGC) evaluation. After Bonferroni correction, a P value ≤0.005 was considered statistically significant. RESULTS: The 3D images were graded significantly better than the 2D images in the visualization of the dorsal and palmar parts of the SLL and the LTL. Regarding the TFCC, the 3D images were graded significantly better for visualization of the foveal attachment. 2D imaging was not found significantly superior to 3D imaging in any aspect. CONCLUSION: The 3D SPACE sequence was scored as superior to the 2D sequences at 3 T in the assessment of the SLL, the LTL, and the foveal attachment of the TFCC. Thus, 3D SPACE can replace 2D PD sequences when these ligaments need to be assessed.

Critical appraisal of the International Prophylaxis Study Group magnetic resonance image scale for evaluating haemophilic arthropathy.

A goal of the International Prophylaxis Study Group (IPSG) is to provide an accurate instrument to measure MRI-based disease severity of haemophilic arthropathy at various time points, so that longitudinal changes in disease severity can be identified to support decisions on treatment management. We review and discuss in this paper the evaluative purpose of the IPSG MRI scale in relation to its development and validation processes so far. We also critically appraise the validity, reliability and responsiveness of using the IPSG MRI scale in different clinical and research settings, and whenever applicable, compare these clinimetric properties of the IPSG MRI scale with those of its precursors, the compatible additive and progressive MRI scales.

Poor outcome after a surgically treated chondral injury on the medial femoral condyle: early evaluation with dGEMRIC and 17-year radiographic and clinical follow-up in 16 knees.

Background and purpose - The optimal treatment for traumatic cartilage injuries remains unknown. Contrast-enhanced MRI of cartilage (dGEMRIC) evaluates cartilage quality and a low dGEMRIC index may predict radiographic osteoarthritis (OA). The purpose of this study was (a) to explore the results 17 years after surgical treatment of an isolated cartilage knee injury and (b) to evaluate the predictive value of dGEMRIC. Patients and methods - 16 knees with an isolated traumatic cartilage injury of the medial femoral condyle had cartilage repair surgery either by microfracture or autologous cartilage implantation. dGEMRIC of the injured knee was performed 2 years after surgery and radiographic examinations were performed 17 years after the operation. Results - Radiographic OA was present in 12 of 16 knees. Irrespective of surgical method, the dGEMRIC index was lower in repair tissue compared with adjacent cartilage in the medial compartment, 237 ms vs. 312 ms (p < 0.001), which in turn had lower value than in the non-injured lateral cartilage, 312 ms vs. 354 ms (p < 0.008). The dGEMRIC index in the cartilage adjacent to the repair tissue correlated negatively with radiographic osteophyte score, r = -0.75 (p = 0.03). Interpretation - A traumatic cartilage injury is associated with a high prevalence of OA after 17 years. The low dGEMRIC index in the repair tissue 2 years postoperatively indicates fibrocartilage of low quality. The negative correlation between the dGEMRIC index in the adjacent cartilage and future OA suggests that the quality of the surrounding cartilage influences outcome after cartilage repair surgery.

Knee dGEMRIC at 7 T: comparison against 1.5 T and evaluation of T1-mapping methods.

BACKGROUND: dGEMRIC (delayed Gadolinium Enhanced Magnetic Resonance Image of Cartilage) is a well-established technique for cartilage quality assessment in osteoarthritis at clinical field strengths. The method is robust, but requires injection of contrast agent and a cumbersome examination procedure. New non-contrast-agent-based techniques for cartilage quality assessment are currently being developed at 7 T. However, dGEMRIC remains an important reference technique during this development. The aim of this work was to compare T1 mapping for dGEMRIC at 7 T and 1.5 T, and to evaluate three T1-mapping methods at 7 T. METHODS: The knee of 10 healthy volunteers and 9 patients with early signs of cartilage degradation were examined at 1.5 T and 7 T after a single (one) contrast agent injection (Gd-(DTPA)2-). Inversion recovery (IR) sequences were acquired at both field strengths, and at 7 T variable flip angle (VFA) and Look-Locker (LL) sequences were additionally acquired. T1 maps were calculated and average T1 values were estimated within superficial and deep regions-of-interest (ROIs) in the lateral and medial condyles, respectively. RESULTS: T1 values were 1.8 (1.4-2.3) times longer at 7 T. A strong correlation was detected between 1.5 T and 7 T T1 values (r = 0.80). For IR, an additional inversion time was required to avoid underestimation (bias±limits of agreement - 127 ± 234 ms) due to the longer T1 values at 7 T. Out of the two 3D sequences tested, LL resulted in more accurate and precise T1 estimation compared to VFA (average bias±limits of agreement LL: 12 ± 202 ms compared to VFA: 25 ± 622 ms). For both, B1 correction improved agreement to IR. CONCLUSION: With an adapted sampling scheme, dGEMRIC T1 mapping is feasible at 7 T and correlates well to 1.5 T. If 3D is to be used for T1 mapping of the knee at 7 T, LL is preferred and VFA is not recommended. For VFA and LL, B1 correction is necessary for accurate T1 estimation.

PSB33 sustains photosystem II D1 protein under fluctuating light conditions.

On Earth, solar irradiance varies as the sun rises and sets over the horizon, and sunlight is thus in constant fluctuation, following a slow dark-low-high-low-dark curve. Optimal plant growth and development are dependent on the capacity of plants to acclimate and regulate photosynthesis in response to these changes of light. Little is known of regulative processes for photosynthesis during nocturnal events. The nucleus-encoded plant lineage-specific protein PSB33 has been described as stabilizing the photosystem II complex, especially under light stress conditions, and plants lacking PSB33 have a dysfunctional state transition. To clarify the localization and function of this protein, we used phenomic, biochemical and proteomics approaches in the model plant Arabidopsis. We report that PSB33 is predominantly located in non-appressed thylakoid regions and dynamically associates with a thylakoid protein complex in a light-dependent manner. Moreover, plants lacking PSB33 show an accelerated D1 protein degradation in nocturnal periods, and show severely stunted growth when challenged with fluctuating light. We further show that the function of PSB33 precedes the STN7 kinase to regulate or balance the excitation energy of photosystems I and II in fluctuating light conditions.

Serine and threonine residues of plant STN7 kinase are differentially phosphorylated upon changing light conditions and specifically influence the activity and stability of the kinase.

STN7 kinase catalyzes the phosphorylation of the globally most common membrane proteins, the light-harvesting complex II (LHCII) in plant chloroplasts. STN7 itself possesses one serine (Ser) and two threonine (Thr) phosphosites. We show that phosphorylation of the Thr residues protects STN7 against degradation in darkness, low light and red light, whereas increasing light intensity and far red illumination decrease phosphorylation and induce STN7 degradation. Ser phosphorylation, in turn, occurs under red and low intensity white light, coinciding with the client protein (LHCII) phosphorylation. Through analysis of the counteracting LHCII phosphatase mutant tap38/pph1, we show that Ser phosphorylation and activation of the STN7 kinase for subsequent LHCII phosphorylation are heavily affected by pre-illumination conditions. Transitions between the three activity states of the STN7 kinase (deactivated in darkness and far red light, activated in low and red light, inhibited in high light) are shown to modulate the phosphorylation of the STN7 Ser and Thr residues independently of each other. Such dynamic regulation of STN7 kinase phosphorylation is crucial for plant growth and environmental acclimation.

A voltage-dependent chloride channel fine-tunes photosynthesis in plants.

In natural habitats, plants frequently experience rapid changes in the intensity of sunlight. To cope with these changes and maximize growth, plants adjust photosynthetic light utilization in electron transport and photoprotective mechanisms. This involves a proton motive force (PMF) across the thylakoid membrane, postulated to be affected by unknown anion (Cl(-)) channels. Here we report that a bestrophin-like protein from Arabidopsis thaliana functions as a voltage-dependent Cl(-) channel in electrophysiological experiments. AtVCCN1 localizes to the thylakoid membrane, and fine-tunes PMF by anion influx into the lumen during illumination, adjusting electron transport and the photoprotective mechanisms. The activity of AtVCCN1 accelerates the activation of photoprotective mechanisms on sudden shifts to high light. Our results reveal that AtVCCN1, a member of a conserved anion channel family, acts as an early component in the rapid adjustment of photosynthesis in variable light environments.

Each of the chloroplast potassium efflux antiporters affects photosynthesis and growth of fully developed Arabidopsis rosettes under short-day photoperiod.

In Arabidopsis thaliana, the chloroplast harbors three potassium efflux antiporters (KEAs), namely KEA1 and KEA2 in the inner envelope and KEA3 in the thylakoid membrane. They may play redundant physiological roles as in our previous analyses of young developing Arabidopsis rosettes under long-day photoperiod (16 h light per day), chloroplast kea single mutants resembled the wild-type plants, whereas kea1kea2 and kea1kea2kea3 mutants were impaired in chloroplast development and photosynthesis resulting in stunted growth. Here, we aimed to study whether chloroplast KEAs play redundant roles in chloroplast function of older Arabidopsis plants with fully developed rosettes grown under short-day photoperiod (8 h light per day). Under these conditions, we found defects in photosynthesis and growth in the chloroplast kea single mutants, and most dramatic defects in the kea1kea2 double mutant. The mechanism behind these defects in the single mutants involves reduction in the electron transport rate (kea1 and kea3), and stomata conductance (kea1, kea2 and kea3), which in turn affect CO2 fixation rates. The kea1kea2 mutant, in addition to these alterations, displayed reduced levels of photosynthetic machinery. Taken together, our data suggest that, in addition to the previously reported roles in chloroplast development in young rosettes, each chloroplast KEA affects photosynthesis and growth of Arabidopsis fully developed rosettes.

The Arabidopsis Thylakoid Chloride Channel AtCLCe Functions in Chloride Homeostasis and Regulation of Photosynthetic Electron Transport.

Chloride ions can be translocated across cell membranes through Cl(-) channels or Cl(-)/H(+) exchangers. The thylakoid-located member of the Cl(-) channel CLC family in Arabidopsis thaliana (AtCLCe) was hypothesized to play a role in photosynthetic regulation based on the initial photosynthetic characterization of clce mutant lines. The reduced nitrate content of Arabidopsis clce mutants suggested a role in regulation of plant nitrate homeostasis. In this study, we aimed to further investigate the role of AtCLCe in the regulation of ion homeostasis and photosynthetic processes in the thylakoid membrane. We report that the size and composition of proton motive force were mildly altered in two independent Arabidopsis clce mutant lines. Most pronounced effects in the clce mutants were observed on the photosynthetic electron transport of dark-adapted plants, based on the altered shape and associated parameters of the polyphasic OJIP kinetics of chlorophyll a fluorescence induction. Other alterations were found in the kinetics of state transition and in the macro-organization of photosystem II supercomplexes, as indicated by circular dichroism measurements. Pre-treatment with KCl but not with KNO3 restored the wild-type photosynthetic phenotype. Analyses by transmission electron microscopy revealed a bow-like arrangement of the thylakoid network and a large thylakoid-free stromal region in chloroplast sections from the dark-adapted clce plants. Based on these data, we propose that AtCLCe functions in Cl(-) homeostasis after transition from light to dark, which affects chloroplast ultrastructure and regulation of photosynthetic electron transport.

Phantom based qualitative and quantitative evaluation of artifacts in MR images of metallic hip prostheses.

PURPOSE: To develop methods for qualitative and quantitative evaluation of MRI artifacts near metallic prostheses, and to compare the efficiency of different artifact suppression techniques with different types of hip prostheses. METHODS: Three hip prostheses of cobalt-chromium, stainless steel, and titanium were embedded in agarose gel together with a rectilinear grid. Coronal MR images of the prostheses were acquired on a 1.5T scanner. Three pulse sequences were evaluated; TSE: a high-bandwidth turbo spin echo; VAT: TSE with view angle tilting, SEMAC: TSE with both VAT and slice distortion correction (6, 10 or 16 z-phase-encoding steps). Through-plane distortions were assessed as the length of visible gridlines, in-plane artifacts as the artifact area, and total artifacts by subtraction of an ideal, undistorted image from the actual image. RESULTS: VAT reduced in-plane artifacts by up to 50% compared to TSE, but did not reduce through-plane artifacts. SEMAC reduced through-plane artifacts by 60-80% compared to TSE and VAT. SEMAC in-plane artifacts were from 20% higher (6 encoding steps) to 50% lower (16 steps) than VAT. Total artifacts were reduced by 60-80% in the best sequence (SEMAC, 16 steps) compared to the worst (TSE). The titanium prosthesis produced 3-4 times lower artifact scores than the other prostheses. CONCLUSIONS: A rectilinear grid phantom is useful for qualitative and quantitative evaluation of artifacts provoked by different MRI protocols and prosthesis models. VAT and SEMAC were superior to TSE with high bandwidth. A proper number of z-encoding steps in SEMAC was critical. The titanium prosthesis caused least artifacts.

Evaluation of metal artifacts in clinical MR images of patients with total hip arthroplasty using different metal artifact-reducing sequences.

PURPOSE: To evaluate the distortion and artifact area of metal in MR images and to compare artifact reduction using different metal artifact-reducing sequences in patients with metal-on-metal (MoM) and non-MoM total hip prostheses. MATERIALS AND METHODS: Thirty-six MoM and 15 non-MoM prostheses were examined in a 1.5-T MR scanner using T1-weighted (T1-w) sequences: turbo spin echo (TSE) high-readout bandwidth (hiBW), T1-w; TSE view angle tilting (VAT), T1-w; TSE VAT + slice encoding for metal artifact correction (SEMAC); short tau inversion recovery (STIR) hiBW or matched RF pulses (mRFp). Distortion was quantified using a new method measuring the acetabular roof angle (ARA). The artifact area was defined in the mid-coronal plane of the artifact. RESULTS: The T1 VAT + SEMAC sequence showed the least distortion compared to T1 VAT and T1-hiBW (150°, 127° and 102°, p < 0.001, in MoM; 152°, 143° and 128°, p ≤ 0.014, in non-MoM). The artifact area was smaller in MoM prostheses using the T1 VAT sequence compared to T1 hiBW and T1 VAT + SEMAC (2506 mm(2), 3160 mm(2) and 3214 mm(2), p < 0.001) and smaller in non-MoM prostheses using T1 VAT compared to T1-hiBW (4296 mm(2) and 4831 mm(2), p = 0.041). STIR-mRFp substantially reduced the artifact size compared with STIR-hiBW (MoM 4559 mm(2) and 6323 mm(2); non-MoM 5625 mm(2) and 8764 mm(2), p < 0.001). CONCLUSION: Metal artifacts in MR imaging examinations of hip prostheses can be evaluated for distortion using a distortion angle (ARA) and the degree of signal artifact as determined by measuring the largest cross-sectional artifact area. T1 VAT + SEMAC showed the least distortion; T1 VAT and STIR-mRFp were most efficient for reduction of the artifact area.

PHOTOSYSTEM II PROTEIN33, a protein conserved in the plastid lineage, is associated with the chloroplast thylakoid membrane and provides stability to photosystem II supercomplexes in Arabidopsis.

Photosystem II (PSII) is a multiprotein complex that catalyzes the light-driven water-splitting reactions of oxygenic photosynthesis. Light absorption by PSII leads to the production of excited states and reactive oxygen species that can cause damage to this complex. Here, we describe Arabidopsis (Arabidopsis thaliana) At1g71500, which encodes a previously uncharacterized protein that is a PSII auxiliary core protein and hence is named PHOTOSYSTEM II PROTEIN33 (PSB33). We present evidence that PSB33 functions in the maintenance of PSII-light-harvesting complex II (LHCII) supercomplex organization. PSB33 encodes a protein with a chloroplast transit peptide and one transmembrane segment. In silico analysis of PSB33 revealed a light-harvesting complex-binding motif within the transmembrane segment and a large surface-exposed head domain. Biochemical analysis of PSII complexes further indicates that PSB33 is an integral membrane protein located in the vicinity of LHCII and the PSII CP43 reaction center protein. Phenotypic characterization of mutants lacking PSB33 revealed reduced amounts of PSII-LHCII supercomplexes, very low state transition, and a lower capacity for nonphotochemical quenching, leading to increased photosensitivity in the mutant plants under light stress. Taken together, these results suggest a role for PSB33 in regulating and optimizing photosynthesis in response to changing light levels.

MR imaging with metal artifact-reducing sequences and gadolinium contrast agent in a case-control study of periprosthetic abnormalities in patients with metal-on-metal hip prostheses.

OBJECTIVE: To apply and compare magnetic resonance imaging (MRI) metal artifact reducing sequences (MARS) including subtraction imaging after contrast application in patients with metal-on-metal (MoM) hip prostheses, investigate the prevalence and characteristics of periprosthetic abnormalities, as well as their relation with pain and risk factors. MATERIALS AND METHODS: Fifty-two MoM prostheses (35 cases with pain and or risk factors, and 17 controls) in 47 patients were examined in a 1.5-T MR scanner using MARS: turbo spin echo (TSE) with high readout bandwidth with and without view angle tilting (VAT), TSE with VAT and slice encoding for metal artifact correction (SEMAC), short tau inversion recovery (STIR) with matched RF pulses, and post-contrast imaging. The relations of MRI findings to pain and risk factors were analyzed and in five revised hips findings from operation, histology, and MRI were compared. RESULTS: TSE VAT detected the highest number of osteolyses. Soft tissue mass, effusion, and capsular thickening were common, whereas osteolysis in acetabulum and femur were less frequent. Contrast enhancement occurred in bone, synovia, joint capsule, and the periphery of soft tissue mass. There was no significant relation between MRI findings and pain or risk factors. CONCLUSIONS: MARS and gadolinium subtraction imaging are useful for evaluation of complications to MoM prosthesis. TSE VAT had the highest sensitivity for osteolysis. Contrast enhancement might indicate activation of aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL). Pain, small head, or steep prosthesis inclination angle are not useful predictors of periprosthetic abnormalities, and wide indications for MR follow-up are warranted.

Quantitative versus semiquantitative MR imaging of cartilage in blood-induced arthritic ankles: preliminary findings.

BACKGROUND: Recent advances in hemophilia prophylaxis have raised the need for accurate noninvasive methods for assessment of early cartilage damage in maturing joints to guide initiation of prophylaxis. Such methods can either be semiquantitative or quantitative. Whereas semiquantitative scores are less time-consuming to be performed than quantitative methods, they are prone to subjective interpretation. OBJECTIVE: To test the feasibility of a manual segmentation and a quantitative methodology for cross-sectional evaluation of articular cartilage status in growing ankles of children with blood-induced arthritis, as compared with a semiquantitative scoring system and clinical-radiographic constructs. MATERIALS AND METHODS: Twelve boys, 11 with hemophilia (A, n = 9; B, n = 2) and 1 with von Willebrand disease (median age: 13; range: 6-17), underwent physical examination and MRI at 1.5 T. Two radiologists semiquantitatively scored the MRIs for cartilage pathology (surface erosions, cartilage loss) with blinding to clinical information. An experienced operator applied a validated quantitative 3-D MRI method to determine the percentage area of denuded bone (dAB) and the cartilage thickness (ThCtAB) in the joints' MRIs. Quantitative and semiquantitative MRI methods and clinical-radiographic constructs (Hemophilia Joint Health Score [HJHS], Pettersson radiograph scores) were compared. RESULTS: Moderate correlations were noted between erosions and dAB (r = 0.62, P = 0.03) in the talus but not in the distal tibia (P > 0.05). Whereas substantial to high correlations (r range: 0.70-0.94, P < 0.05) were observed between erosions, cartilage loss, HJHS and Pettersson scores both at the distal tibia and talus levels, moderate/borderline substantial (r range: 0.55-0.61, P < 0.05) correlations were noted between dAB/ThCtAB and clinical-radiographic constructs. CONCLUSION: Whereas the semiquantitative method of assessing cartilage status is closely associated with clinical-radiographic scores in cross-sectional studies of blood-induced arthropathy, quantitative measures provide independent information and are therefore less applicable for that research design.

Evidence for nucleotide-dependent processes in the thylakoid lumen of plant chloroplasts--an update.

The thylakoid lumen is an aqueous chloroplast compartment enclosed by the thylakoid membrane network. Bioinformatic and proteomic studies indicated the existence of 80-90 thylakoid lumenal proteins in Arabidopsis thaliana, having photosynthetic, non-photosynthetic or unclassified functions. None of the identified lumenal proteins had canonical nucleotide-binding motifs. It was therefore suggested that, in contrast to the chloroplast stroma harboring nucleotide-dependent enzymes and other proteins, the thylakoid lumen is a nucleotide-free compartment. Based on recent findings, we provide here an updated view about the presence of nucleotides in the thylakoid lumen of plant chloroplasts, and their role in function and dynamics of photosynthetic complexes.

Assay of photoinhibition and heat inhibition of photosystem II in higher plants.

When thylakoids of higher plant chloroplasts are exposed to excessive light or moderate heat stress, photosystem II reaction center-binding protein D1 is damaged. The photodamage of the D1 protein is caused by reactive oxygen species, mostly singlet oxygen, and also by endogenous cationic radicals generated by the photochemical reactions of photosystem II. Moreover, it was shown recently that the damage to the D1 protein by moderate heat stress is due to reactive oxygen species produced by lipid peroxidation near photosystem II. To maintain photosystem II activity, the oxidatively damaged D1 protein must be replaced by a newly synthesized copy, and thus degradation and removal of the photo- or heat-damaged D1 protein are essential for maintaining the viability of photosystem II. In this chapter, we describe the methods for assaying photoinhibition and heat inhibition of photosystem II in higher plant materials.

Role of thylakoid ATP/ADP carrier in photoinhibition and photoprotection of photosystem II in Arabidopsis.

The chloroplast thylakoid ATP/ADP carrier (TAAC) belongs to the mitochondrial carrier superfamily and supplies the thylakoid lumen with stromal ATP in exchange for ADP. Here, we investigate the physiological consequences of TAAC depletion in Arabidopsis (Arabidopsis thaliana). We show that the deficiency of TAAC in two T-DNA insertion lines does not modify the chloroplast ultrastructure, the relative amounts of photosynthetic proteins, the pigment composition, and the photosynthetic activity. Under growth light conditions, the mutants initially displayed similar shoot weight, but lower when reaching full development, and were less tolerant to high light conditions in comparison with the wild type. These observations prompted us to study in more detail the effects of TAAC depletion on photoinhibition and photoprotection of the photosystem II (PSII) complex. The steady-state phosphorylation levels of PSII proteins were not affected, but the degradation of the reaction center II D1 protein was blocked, and decreased amounts of CP43-less PSII monomers were detected in the mutants. Besides this, the mutant leaves displayed a transiently higher nonphotochemical quenching of chlorophyll fluorescence than the wild-type leaves, especially at low light. This may be attributed to the accumulation in the absence of TAAC of a higher electrochemical H(+) gradient in the first minutes of illumination, which more efficiently activates photoprotective xanthophyll cycle-dependent and independent mechanisms. Based on these results, we propose that TAAC plays a critical role in the disassembly steps during PSII repair and in addition may balance the trans-thylakoid electrochemical H(+) gradient storage.

Quality control of photosystem II: Thylakoid unstacking is necessary to avoid further damage to the D1 protein and to facilitate D1 degradation under light stress in spinach thylakoids.

Photosystem II is vulnerable to light damage. The reaction center-binding D1 protein is impaired during excessive illumination and is degraded and removed from photosystem II. Using isolated spinach thylakoids, we investigated the relationship between light-induced unstacking of thylakoids and damage to the D1 protein. Under light stress, thylakoids were expected to become unstacked so that the photodamaged photosystem II complexes in the grana and the proteases could move on the thylakoids for repair. Excessive light induced irreversible unstacking of thylakoids. By comparing the effects of light stress on stacked and unstacked thylakoids, photoinhibition of photosystem II was found to be more prominent in stacked thylakoids than in unstacked thylakoids. In accordance with this finding, EPR spin trapping measurements demonstrated higher production of hydroxyl radicals in stacked thylakoids than in unstacked thylakoids. We propose that unstacking of thylakoids has a crucial role in avoiding further damage to the D1 protein and facilitating degradation of the photodamaged D1 protein under light stress.