Coregulation of light-harvesting complex II phosphorylation and lhcb mRNA accumulation in winter rye

Winter rye plants grown under contrasting environmental conditions or just transiently shifted to varying light and temperature conditions, were studied to elucidate the chloroplast signal involved in regulation of photosynthesis genes in the nucleus. Photosystem II excitation pressure, reflecting the plastoquinone redox state, and the phosphorylation level of thylakoid light-harvesting proteins (LHCII and CP29) were monitored together with changes occurring in the accumulation of lhcb, rbcS and psbA mRNAs. Short-term shifts of plants to changed conditions, from 1 h to 2 d, were postulated to reveal signals crucial for the initiation of the acclimation process. Comparison of these results with those obtained from plants acclimated during several weeks' growth at contrasting temperature and in different light regimes, allow us to make the following conclusions: (1) LHCII protein phosphoylation is a sensitive tool to monitor redox changes in chloroplasts; (2) LHCII protein phosphorylation and lhcb mRNA accumulation occur under similar conditions and are possibly coregulated via an activation state of the same kinase (the LHCII kinase); (3) Maximal accumulation of lhcb mRNA during the diurnal light phase seems to require an active LHCII kinase whereas inactivation of the kinase is accompanied by dampening of the circadian oscillation in the amount of lhcb mRNA; (4) Excitation pressure of photosystem II (reduction state of the plastoquinone pool) is not directly involved in the regulation of lhcb mRNA accumulation. Instead (5) the redox status of the electron acceptors of photosystem I in the stromal compartment seems to be highly regulated and crucial for the regulation of lhcb gene expression in the nucleus.     

Electron transport pathways in isolated chromoplasts from Narcissus pseudonarcissus L.

During daffodil flower development, chloroplasts differentiate into photosynthetically inactive chromoplasts having lost functional photosynthetic reaction centers. Chromoplasts exhibit a respiratory activity reducing oxygen to water and generating ATP. Immunoblots revealed the presence of the plastid terminal oxidase (PTOX), the NAD(P)H dehydrogenase (NDH) complex, the cytochrome b₆f complex, ATP synthase and several isoforms of ferredoxin‐NADP⁺ oxidoreductase (FNR), and ferredoxin (Fd). Fluorescence spectroscopy allowed the detection of chlorophyll a in the cytochrome b₆f complex. Here we characterize the electron transport pathway of chromorespiration by using specific inhibitors for the NDH complex, the cytochrome b₆f complex, FNR and redox‐inactive Fd in which the iron was replaced by gallium. Our data suggest an electron flow via two separate pathways, both reducing plastoquinone (PQ) and using PTOX as oxidase. The first oxidizes NADPH via FNR, Fd and cytochrome b_h of the cytochrome b₆f complex, and does not result in the pumping of protons across the membrane. In the second, electron transport takes place via the NDH complex using both NADH and NADPH as electron donor. FNR and Fd are not involved in this pathway. The NDH complex is responsible for the generation of the proton gradient. We propose a model for chromorespiration that may also be relevant for the understanding of chlororespiration and for the characterization of the electron input from Fd to the cytochrome b₆f complex during cyclic electron transport in chloroplasts.     

Posttranslational Modifications of Chloroplast Proteins: An Emerging Field

Posttranslational modifications of proteins are key effectors of enzyme activity, protein interactions, targeting, and turnover rate, but despite their importance, they are still poorly understood in plants. Although numerous reports have revealed the regulatory role of protein phosphorylation in photosynthesis, various other protein modifications have been identified in chloroplasts only recently. It is known that posttranslational N^α-acetylation occurs in both nuclear- and plastid-encoded chloroplast proteins, but the physiological significance of this acetylation is not yet understood. Lysine acetylation affects the localization and activity of key metabolic enzymes, and it may work antagonistically or cooperatively with lysine methylation, which also occurs in chloroplasts. In addition, tyrosine nitration may help regulate the repair cycle of photosystem II, while N-glycosylation determines enzyme activity of chloroplastic carbonic anhydrase. This review summarizes the progress in the research field of posttranslational modifications of chloroplast proteins and points out the importance of these modifications in the regulation of chloroplast metabolism.The constantly changing environment challenges plants both in the short and long term. Environmental fluctuations affect gene expression by relaying signals that adjust the accumulation of proteins in various cell compartments according to ambient requirements. In addition to these relatively slow changes, rapid shifts in enzyme activities are sometimes required to meet the needs of plant metabolism at a given moment. Enzyme activity, interactions with other molecules, targeting, and turnover rate of a protein may be determined by a number of different posttranslational modifications (PTMs), which refer to the covalent processing of a protein after it has been fully translated. Although PTMs have long been known to provide enormous functional diversity and regulatory complexity in mammalian systems, an emerging interest in plant PTMs has developed only recently. Studies focusing on PTMs in plants are required not only because of fundamental interest in the regulation of physiological responses, but also because of the possibility of producing correctly modified biological compounds in plants for medical purposes. Despite the importance of chloroplast metabolism in the growth and development of the plant, PTMs of chloroplast proteins, apart from phosphorylation, have been poorly studied. It appears from recent progress that the numerous recently identified PTMs of chloroplast proteins provide additional regulation and signaling in chloroplasts. This review will introduce and summarize current developments in this emerging research field and will emphasize the importance of PTMs in chloroplast metabolism (Fig. 1).Open in a separate windowFigure 1.Overview of the possible PTMs of chloroplast proteins. Changes in internal and external conditions (yellow stars) require rapid regulation of many processes inside the chloroplast. The main processes regulated via PTMs are shown in gray boxes. Different types of modifications affecting these processes are marked with colored pins, and the enzymes/molecules executing the modifications are marked with dark-gray ellipses. Enzymes as yet uncharacterized are indicated with question marks. Black arrows mark known interactions, and hypothetical relations are marked with dashed gray arrows. KAT, Lys-acetyltransferase; KDAC, Lys-deacetylase; NAT, N^α-acetyltransferase; MT, methyltransferase; DM, demethylase; SUMO, sumoylation machinery.     

Chloroplast-targeted ferredoxin-NADP(+) oxidoreductase (FNR): structure, function and location

Ferredoxin-NADP(+) oxidoreductase (FNR) is a ubiquitous flavin adenine dinucleotide (FAD)-binding enzyme encoded by a small nuclear gene family in higher plants. The chloroplast targeted FNR isoforms are known to be responsible for the final step of linear electron flow transferring electrons from ferredoxin to NADP(+), while the putative role of FNR in cyclic electron transfer has been under discussion for decades. FNR has been found from three distinct chloroplast compartments (i) at the thylakoid membrane, (ii) in the soluble stroma, and (iii) at chloroplast inner envelope. Recent in vivo studies have indicated that besides the membrane-bound FNR, also the soluble FNR is photosynthetically active. Two chloroplast proteins, Tic62 and TROL, were recently identified and shown to form high molecular weight protein complexes with FNR at the thylakoid membrane, and thus seem to act as the long-sought molecular anchors of FNR to the thylakoid membrane. Tic62-FNR complexes are not directly involved in photosynthetic reactions, but Tic62 protects FNR from inactivation during the dark periods. TROL-FNR complexes, however, have an impact on the photosynthetic performance of the plants. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.     

Regulatory subunit B'gamma of protein phosphatase 2A prevents unnecessary defense reactions under low light in Arabidopsis

Light is an important environmental factor that modulates acclimation strategies and defense responses in plants. We explored the functional role of the regulatory subunit B'γ (B'γ) of protein phosphatase 2A (PP2A) in light-dependent stress responses of Arabidopsis (Arabidopsis thaliana). The predominant form of PP2A consists of catalytic subunit C, scaffold subunit A, and highly variable regulatory subunit B, which determines the substrate specificity of PP2A holoenzymes. Mutant leaves of knockdown pp2a-b'γ plants show disintegration of chloroplasts and premature yellowing conditionally under moderate light intensity. The cell-death phenotype is accompanied by the accumulation of hydrogen peroxide through a pathway that requires CONSTITUTIVE EXPRESSION OF PR GENES5 (CPR5). Moreover, the pp2a-b'γ cpr5 double mutant additionally displays growth suppression and malformed trichomes. Similar to cpr5, the pp2a-b'γ mutant shows constitutive activation of both salicylic acid- and jasmonic acid-dependent defense pathways. In contrast to cpr5, however, pp2a-b'γ leaves do not contain increased levels of salicylic acid or jasmonic acid. Rather, the constitutive defense response associates with hypomethylation of DNA and increased levels of methionine-salvage pathway components in pp2a-b'γ leaves. We suggest that the specific B'γ subunit of PP2A is functionally connected to CPR5 and operates in the basal repression of defense responses under low irradiance.     

Arabidopsis tic62 trol Mutant Lacking Thylakoid- Bound Ferredoxin-NADP＋ Oxidoreductase Shows Distinct Metabolic Phenotype

Ferredoxin-NADP＋ oxidoreductase （FNR）, functioning in the last step of the photosynthetic electron transfer chain, exists both as a soluble protein in the chloroplast stroma and tightly attached to chloroplast membranes. Surface plasmon resonance assays showed that the two FNR isoforms, LFNR1 and LFNR2, are bound to the thylakoid membrane via the C-terminal domains of Tic62 and TROL proteins in a pH-dependent manner. The tic62 trol double mutants contained a reduced level of FNR, exclusively found in the soluble stroma. Although the mutant plants showed no visual phenotype or defects in the function of photosystems under any conditions studied, a low ratio of NADPH/NADP~ was detected. Since the CO2 fixation capacity did not differ between the tic62 trol plants and wild-type, it seems that the plants are able to funnel reducing power to most crucial reactions to ensure survival and fitness of the plants. However, the activity of malate dehydrogenase was down-regulated in the mutant plants. Apparently, the plastid metabolism is able to cope with substantial changes in directing the electrons from the light reactions to stromal metabolism and thus only few differences are visible in steady-state metabolite pool sizes of the tic62 trol plants.