Björn Lundin

Arabidopsis ANTR1 Is a Thylakoid Na+-dependent Phosphate Transporter FUNCTIONAL CHARACTERIZATION IN ESCHERICHIA COLI

In this study, the putative anion transporter 1 (ANTR1) from Arabidopsis thaliana was shown to be localized to the chloroplast thylakoid membrane by Western blotting with two different peptide-specific antibodies. ANTR1 is homologous to the type I of mammalian Na+-dependent inorganic phosphate (Pi) transporters. The function of ANTR1 as a Na+-dependent Pi transporter was demonstrated by heterologous expression and uptake of radioactive Pi into Escherichia coli cells. The expression of ANTR1 conferred increased growth rates to the transformed cells and stimulated Pi uptake in a pH- and Na+-dependent manner as compared with the control cells. Among various tested effectors, Pi was the preferred substrate. Although it competed with the uptake of Pi, glutamate was not transported by ANTR1 into E. coli. In relation to its function as a Pi transporter, several physiological roles for ANTR1 in the thylakoid membrane are proposed, such as export of Pi produced during nucleotide metabolism in the thylakoid lumen back to the chloroplast stroma and balance of the trans-thylakoid H+ electrochemical 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.

Towards understanding the functional difference between the two PsbO isoforms in Arabidopsis thaliana—insights from phenotypic analyses of psbo knockout mutants

The extrinsic PsbO subunit of the water-oxidizing photosystem II (PSII) complex is represented by two isoforms in Arabidopsis thaliana, namely PsbO1 and PsbO2. Recent analyses of psbo1 and psbo2 knockout mutants have brought insights into their roles in photosynthesis and light stress. Here we analyzed the two psbo mutants in terms of PsbOs expression pattern, organization of PSII complexes and GTPase activity. Both PsbOs are present in wild-type plants, and their expression is mutually controlled in the mutants. Almost all PSII complexes are in the monomeric form not only in the psbo1 but also in the psbo2 mutant grown under high-light conditions. This results either from an enhanced susceptibility of PSII to photoinactivation or from malfunction of the repair cycle. Notably, the psbo1 mutant displays such problems even under growth-light conditions. These results together with the finding that PsbO2 has a threefold higher GTPase activity than PsbO1 have significance for the turnover of the PSII D1 subunit in Arabidopsis.

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.

Comparison of the electron transport properties of the psbo1 and psbo2 mutants of Arabidopsis thaliana.

Genome sequence of Arabidopsis thaliana (Arabidopsis) revealed two psbO genes (At5g66570 and At3g50820) which encode two distinct PsbO isoforms: PsbO1 and PsbO2, respectively. To get insights into the function of the PsbO1 and PsbO2 isoforms in Arabidopsis we have performed systematic and comprehensive investigations of the whole photosynthetic electron transfer chain in the T-DNA insertion mutant lines, psbo1 and psbo2. The absence of the PsbO1 isoform and presence of only the PsbO2 isoform in the psbo1 mutant results in (i) malfunction of both the donor and acceptor sides of Photosystem (PS) II and (ii) high sensitivity of PSII centers to photodamage, thus implying the importance of the PsbO1 isoform for proper structure and function of PSII. The presence of only the PsbO2 isoform in the PSII centers has consequences not only to the function of PSII but also to the PSI/PSII ratio in thylakoids. These results in modification of the whole electron transfer chain with higher rate of cyclic electron transfer around PSI, faster induction of NPQ and a larger size of the PQ-pool compared to WT, being in line with apparently increased chlororespiration in the psbo1 mutant plants. The presence of only the PsbO1 isoform in the psbo2 mutant did not induce any significant differences in the performance of PSII under standard growth conditions as compared to WT. Nevertheless, under high light illumination, it seems that the presence of also the PsbO2 isoform becomes favourable for efficient repair of the PSII complex.

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.

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

The study of a previously undescribed protein shows its involvement in the maintenance of the photosystem II light-harvesting supercomplex. 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.

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.

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.

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.