Singlet oxygen in plants: production, detoxification and signaling

High light acclimation in the secondary plastids containing diatom Phaeodactylum tricornutum is triggered by the redox state of the plastoquinone pool

In diatoms, the process of energy-dependent chlorophyll fluorescence quenching (qE) has an important role in photoprotection. Three components are essential for qE: (1) the light-dependent generation of a transthylakoidal proton gradient; (2) the deepoxidation of the xanthophyll diadinoxanthin (Dd) into diatoxanthin (Dt); and (3) specific nucleus-encoded antenna proteins, called Light Harvesting Complex Protein X (LHCX). We used the model diatom Phaeodactylum tricornutum to investigate the concerted light acclimation response of the qE key components LHCX, proton gradient, and xanthophyll cycle pigments (Dd+Dt) and to identify the intracellular light-responsive trigger. At high-light exposure, the up-regulation of three of the LHCX genes and the de novo synthesis of Dd+Dt led to a pronounced rise of qE. By inhibiting either the conversion of Dd to Dt or the translation of LHCX genes, qE amplification was abolished and the diatom cells suffered from stronger photoinhibition. Artificial modification of the redox state of the plastoquinone (PQ) pool via 3-(3,4-dichlorophenyl)-1,1-dimethylurea and 5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone resulted in a disturbance of Dd+Dt synthesis in an opposite way. Moreover, we could increase the transcription of two of the four LHCX genes under low-light conditions by reducing the PQ pool using 5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone. Altogether, our results underline the central role of the redox state of the PQ pool in the light acclimation of diatoms. Additionally, they emphasize strong evidence for the existence of a plastid-to-nucleus retrograde signaling mechanism in an organism with plastids that derived from secondary endosymbiosis.

He-Ne laser pretreatment protects wheat seedlings against cadmium-induced oxidative stress

The aim of the investigation is to determine the effect of He-Ne laser pretreatment of wheat seeds on the resistance of seedlings to cadmium stress. Changes in physiological, biochemical and molecular characters were measured. Our results showed that 150 μM Cd treatment significantly reduced plant height, root length, shoot fresh weight, shoot dry weight, root fresh weight, root dry weight, ascorbate acid (AsA) and glutathione (GSH) concentration, the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX) and gene expression levels of SOD, APX, enhanced the concentration of malondialdehyde (MDA), hydrogen peroxide (H(2)O(2)) and the rate of superoxide radical (O(2)(•-)) generation in the wheat seedlings when compared with the control. However, seeds with He-Ne laser pretreatment 5 min conferred tolerance to cadmium stress in wheat seedlings by decreasing the concentration of MDA and H(2)O(2), the rate of O(2)(•-) generation and increasing the gene expression levels of SOD, POD, APX, the activities of SOD, POD, CAT, APX and AsA and GSH concentration. These results suggest that those changes in MDA, O(2)(•-), H(2)O(2), anti-oxidative enzymes, gene expression level and anti-oxidative compounds are responsible for the increase in cadmium stress resistance observed in the experiments. The results also showed that the laser had a positive physiological effect on the growth of cadmium stressed seedlings. This is the first investigation reporting the use of He-Ne laser pretreatment to enhance of wheat seedlings tolerance to cadmium stress.

Proteins needed to activate a transcriptional response to the reactive oxygen species singlet oxygen

Singlet oxygen ((1)O(2)) is a reactive oxygen species generated by energy transfer from one or more excited donors to molecular oxygen. Many biomolecules are prone to oxidation by (1)O(2), and cells have evolved systems to protect themselves from damage caused by this compound. One way that the photosynthetic bacterium Rhodobacter sphaeroides protects itself from (1)O(2) is by inducing a transcriptional response controlled by ChrR, an anti-σ factor which releases an alternative sigma factor, σ(E), in the presence of (1)O(2). Here we report that induction of σ(E)-dependent gene transcription is decreased in the presence of (1)O(2) when two conserved genes in the σ(E) regulon are deleted, including one encoding a cyclopropane fatty acid synthase homologue (RSP2144) or one encoding a protein of unknown function (RSP1091). Thus, we conclude that RSP2144 and RSP1091 are each necessary to increase σ(E) activity in the presence of (1)O(2). In addition, we found that unlike in wild-type cells, where ChrR is rapidly degraded when (1)O(2) is generated, turnover of this anti-σ factor is slowed when cells lacking RSP2144, RSP1091, or both of these proteins are exposed to (1)O(2). Further, we demonstrate that the organic hydroperoxide tert-butyl hydroperoxide promotes ChrR turnover in both wild-type cells and mutants lacking RSP2144 or RSP1091, suggesting differences in the ways different types of oxidants increase σ(E) activity.

IMPORTANCE

Oxygen serves many crucial functions on Earth; it is produced during photosynthesis and needed for other pathways. While oxygen is relatively inert, it can be converted to reactive oxygen species (ROS) that destroy biomolecules, cause disease, or kill cells. When energy is transferred to oxygen, the ROS singlet oxygen is generated. To understand how singlet oxygen impacts cells, we study the stress response to this ROS in Rhodobacter sphaeroides, a bacterium that, like plants, generates this compound as a consequence of photosynthesis. This paper identifies proteins that activate a stress response to singlet oxygen and shows that they act in a specific response to this ROS. The identified proteins are found in many free-living, symbiotic, or pathogenic bacteria that can encounter singlet oxygen in nature. Thus, our findings provide new information about a stress response to a ROS of broad biological, agricultural, and biomedical importance.

Jasmonate: A decision maker between cell death and acclimation in the response of plants to singlet oxygen

Under stress conditions that bring about excessive absorption of light energy in the chloroplasts, the formation of singlet oxygen ( (1)O2) can be strongly enhanced, triggering programmed cell death. However, the (1)O2 signaling pathway can also lead to acclimation to photooxidative stress, when (1)O2 is produced in relatively low amounts. This acclimatory response is associated with a strong downregulation of the jasmonate biosynthesis pathway and the maintenance of low jasmonate levels, even under high light stress conditions that normally induce jasmonate synthesis. These findings suggest a central role for this phytohormone in the orientation of the (1)O2 signaling pathway toward cell death or acclimation. This conclusion is confirmed here in an Arabidopsis double mutant obtained by crossing the (1)O2-overproducing mutant ch1 and the jasmonate-deficient mutant dde2. This double mutant was found to be constitutively resistant to (1)O2 stress and to display a strongly stimulated growth rate compared with the single ch1 mutant. However, the involvement of other phytohormones, such as ethylene, cannot be excluded.

ROS-mediated lipid peroxidation and RES-activated signaling

Nonenzymatic lipid oxidation is usually viewed as deleterious. But if this is the case, then why does it occur so frequently in cells? Here we review the mechanisms of membrane peroxidation and examine the genesis of reactive electrophile species (RES). Recent evidence suggests that during stress, both lipid peroxidation and RES generation can benefit cells. New results from genetic approaches support a model in which entire membranes can act as supramolecular sinks for singlet oxygen, the predominant reactive oxygen species (ROS) in plastids. RES reprogram gene expression through a class II TGA transcription factor module as well as other, unknown signaling pathways. We propose a framework to explain how RES signaling promotes cell "REScue" by stimulating the expression of genes encoding detoxification functions, cell cycle regulators, and chaperones. The majority of the known biological activities of oxygenated lipids (oxylipins) in plants are mediated either by jasmonate perception or through RES signaling networks.

Nonenzymic carotenoid oxidation and photooxidative stress signalling in plants

Carotenoids play a crucial protective role in photosynthetic organisms as quenchers of singlet oxygen ((1)O(2)). This function occurs either via a physical mechanism involving thermal energy dissipation or via a chemical mechanism involving direct oxidation of the carotenoid molecule. The latter mechanism can produce a variety of aldehydic or ketonic cleavage products containing a reactive carbonyl group. One such molecule, the volatile β-carotene derivative β-cyclocitral, triggers changes in the expression of (1)O(2)-responsive genes and leads to an enhancement of photooxidative stress tolerance. Thus, besides their well-known functions in light harvesting and photoprotection, carotenoids can also play a role through their nonenzymic oxidation in the sensing and signalling of reactive oxygen species and photooxidative stress in photosynthetic organisms. Enzymic carotenoid oxidation does not seem to play a significant role in this phenomenon. Elucidation of the carotenoid-mediated (1)O(2) signalling pathway could provide new targets for improving photooxidative stress tolerance of plants.

Getting a sense for signals: regulation of the plant iron deficiency response

Understanding the Fe deficiency response in plants is necessary for improving both plant health and the human diet, which relies on Fe from plant sources. In this review we focus on the regulation of the two major strategies for iron acquisition in plants, exemplified by the model plants Arabidopsis and rice. Critical to our knowledge of Fe homeostasis in plants is determining how Fe is sensed and how this signal is transmitted and integrated into a response. We will explore the evidence for an Fe sensor in plants and summarize the recent findings on hormones and signaling molecules which contribute to the Fe deficiency response. This article is part of a Special Issue entitled: Cell Biology of Metals.

Function of plastochromanol and other biological prenyllipids in the inhibition of lipid peroxidation-A comparative study in model systems

Lipid peroxidation is an oxidation reaction leading to the generation of lipid hydroperoxides. Here we present comparative data on the inhibition of lipid peroxidation by a variety of biological prenyllipids in liposomes prepared from natural lipid membranes. Lipid peroxidation was initiated by hydrophilic and hydrophobic azo initiators, as well as by singlet oxygen generated via photosensitized reaction of hydrophobic zinc tetraphenylporphine. When lipid peroxidation was initiated in the water phase, tocopherols and plastochromanol-8 were more effective than prenylquinols, such as plastoquinol-9, ubiquinol-10 or α-tocopherolquinol. However, if the peroxidation was initiated within the hydrophobic interior of liposome membranes, long-chain prenyllipids, such as plastoquinol-9 and plastochromanol-8, were considerably more active than tocopherols in the inhibition of the reaction. In the latter system, tocopherols showed even prooxidant activity. The prooxidant activity of α-tocopherol was prevented by plastoquinol, suggesting the reduction of α-tocopheroxyl radical by the quinol. All the investigated prenyllipids were able to inhibit singlet oxygen-mediated lipid peroxidation but the most active were prenylquinols in this respect. Among all the prenyllipids investigated, plastochromanol-8 was the most versatile antioxidant in the inhibition of lipid peroxidation initiated by the three different methods.

Reactive oxygen species-inducible ECF σ factors of Bradyrhizobium japonicum

Extracytoplasmic function (ECF) σ factors control the transcription of genes involved in different cellular functions, such as stress responses, metal homeostasis, virulence-related traits, and cell envelope structure. The genome of Bradyrhizobium japonicum, the nitrogen-fixing soybean endosymbiont, encodes 17 putative ECF σ factors belonging to nine different ECF σ factor families. The genes for two of them, ecfQ (bll1028) and ecfF (blr3038), are highly induced in response to the reactive oxygen species hydrogen peroxide (H₂O₂) and singlet oxygen (¹O₂). The ecfF gene is followed by the predicted anti-σ factor gene osrA (blr3039). Mutants lacking EcfQ, EcfF plus OsrA, OsrA alone, or both σ factors plus OsrA were phenotypically characterized. While the symbiotic properties of all mutants were indistinguishable from the wild type, they showed increased sensitivity to singlet oxygen under free-living conditions. Possible target genes of EcfQ and EcfF were determined by microarray analyses, and candidate genes were compared with the H₂O₂-responsive regulon. These experiments disclosed that the two σ factors control rather small and, for the most part, distinct sets of genes, with about half of the genes representing 13% of the members of H₂O₂-responsive regulon. To get more insight into transcriptional regulation of both σ factors, the 5′ ends of ecfQ and ecfF mRNA were determined. The presence of conserved sequence motifs in the promoter region of ecfQ and genes encoding EcfQ-like σ factors in related α-proteobacteria suggests regulation via a yet unknown transcription factor. By contrast, we have evidence that ecfF is autoregulated by transcription from an EcfF-dependent consensus promoter, and its product is negatively regulated via protein-protein interaction with OsrA. Conserved cysteine residues 129 and 179 of OsrA are required for normal function of OsrA. Cysteine 179 is essential for release of EcfF from an EcfF-OsrA complex upon H₂O₂ stress while cysteine 129 is possibly needed for EcfF-OsrA interaction.

Role of the G-box element in regulation of chlorophyll biosynthesis in Arabidopsis roots

Regulation of chlorophyll metabolism comprises strong transcriptional control together with a range of post-translational mechanisms during chloroplast biogenesis. Recently we reported that chlorophyll biosynthesis in Arabidopsis thaliana roots is regulated by auxin/cytokinin signaling via the combination of two transcription factors, LONG-HYPOCOTYL5 (HY5) and GOLDEN2-LIKE2 (GLK2). In this study, we examined the involvement of cis-elements in the expression of chlorophyll biosynthesis genes. Searches for predicted cis-elements in key chlorophyll biosynthesis genes and their co-expressed genes revealed coexistence of the G-box motif and the CCAATC motif, which may be targeted by HY5 and GLK factors, respectively, in their promoter regions. Deletion of the G-box from the promoter of the CHLH gene encoding the H subunit of Mg-chelatase resulted in the absence of its expression in roots but not in shoots, showing a differing involvement of the G-box in CHLH expression between shoots and roots. Our data suggest that transcription factors and cis-elements participating chlorophyll biosynthesis are substantially changed during organ differentiation, which may be linked to the differentiation of plastids.

Chloroplast-mediated activation of plant immune signalling in Arabidopsis

Chloroplasts have a critical role in plant immunity as a site for the production for salicylic acid and jasmonic acid, important mediators of plant immunity. However, the molecular link between chloroplasts and the cytoplasmic-nuclear immune system remains largely unknown. Here we show that pathogen-associated molecular pattern (PAMP) signals are quickly relayed to chloroplasts and evoke specific Ca(2+) signatures in the stroma. We further demonstrate that a chloroplast-localized protein, named calcium-sensing receptor (CAS), is involved in stromal Ca(2+) transients and responsible for both PAMP-induced basal resistance and R gene-mediated hypersensitive cell death. CAS acts upstream of salicylic acid accumulation. Transcriptome analysis demonstrates that CAS is involved in PAMP-induced expression of defence genes and suppression of chloroplast gene expression possibly through (1)O(2)-mediated retrograde signalling, allowing chloroplast-mediated transcriptional reprogramming during plant immune responses. The present study reveals a previously unknown chloroplast-mediated signalling pathway linking chloroplasts to cytoplasmic-nuclear immune responses.

Small molecule signaling agents: the integrated chemistry and biochemistry of nitrogen oxides, oxides of carbon, dioxygen, hydrogen sulfide, and their derived species

Several small molecule species formally known primarily as toxic gases have, over the past 20 years, been shown to be endogenously generated signaling molecules. The biological signaling associated with the small molecules NO, CO, H₂S (and the nonendogenously generated O₂), and their derived species have become a topic of extreme interest. It has become increasingly clear that these small molecule signaling agents form an integrated signaling web that affects/regulates numerous physiological processes. The chemical interactions between these species and each other or biological targets is an important factor in their roles as signaling agents. Thus, a fundamental understanding of the chemistry of these molecules is essential to understanding their biological/physiological utility. This review focuses on this chemistry and attempts to establish the chemical basis for their signaling functions.

Small CAB-like proteins prevent formation of singlet oxygen in the damaged photosystem II complex of the cyanobacterium Synechocystis sp. PCC 6803

The cyanobacterial small CAB-like proteins (SCPs) are single-helix membrane proteins mostly associated with the photosystem II (PSII) complex that accumulate under stress conditions. Their function is still ambiguous although they are assumed to regulate chlorophyll (Chl) biosynthesis and/or to protect PSII against oxidative damage. In this study, the effect of SCPs on the PSII-specific light-induced damage and generation of singlet oxygen ((1)O(2)) was assessed in the strains of the cyanobacterium Synechocystis sp. PCC 6803 lacking PSI (PSI-less strain) or lacking PSI together with all SCPs (PSI-less/scpABCDE(-) strain). The light-induced oxidative modifications of the PSII D1 protein reflected by a mobility shift of the D1 protein and by generation of a D1-cytochrome b-559 adduct were more pronounced in the PSI-less/scpABCDE(-) strain. This increased protein oxidation correlated with a faster formation of (1)O(2) as detected by the green fluorescence of Singlet Oxygen Sensor Green assessed by a laser confocal scanning microscopy and by electron paramagnetic resonance spin-trapping technique using 2, 2, 6, 6-tetramethyl-4-piperidone (TEMPD) as a spin trap. In contrast, the formation of hydroxyl radicals was similar in both strains. Our results show that SCPs prevent (1)O(2) formation during PSII damage, most probably by the binding of free Chl released from the damaged PSII complexes.

Carotenoid oxidation products are stress signals that mediate gene responses to singlet oxygen in plants

(1)O(2) (singlet oxygen) is a reactive O(2) species produced from triplet excited chlorophylls in the chloroplasts, especially when plants are exposed to excess light energy. Similarly to other active O(2) species, (1)O(2) has a dual effect: It is toxic, causing oxidation of biomolecules, and it can act as a signal molecule that leads to cell death or to acclimation. Carotenoids are considered to be the main (1)O(2) quenchers in chloroplasts, and we show here that light stress induces the oxidation of the carotenoid β-carotene in Arabidopsis plants, leading to the accumulation of different volatile derivatives. One such compound, β-cyclocitral, was found to induce changes in the expression of a large set of genes that have been identified as (1)O(2) responsive genes. In contrast, β-cyclocitral had little effect on the expression of H(2)O(2) gene markers. β-Cyclocitral-induced reprogramming of gene expression was associated with an increased tolerance to photooxidative stress. The results indicate that β-cyclocitral is a stress signal produced in high light that is able to induce defense mechanisms and represents a likely messenger involved in the (1)O(2) signaling pathway in plants.

Chloroplastic NADPH-dependent alkenal/one oxidoreductase contributes to the detoxification of reactive carbonyls produced under oxidative stress

Lipid peroxide-derived reactive carbonyls (RCs) can cause serious damage to plant functions. A chloroplastic NADPH-dependent alkenal/one oxidoreductase (AOR) detoxifies RCs, but its physiological significance remains unknown. In this study, we investigated the biological impacts of AOR using an AOR-knock out Arabidopsis line (aor). Methyl viologen treatment, mainly to enhance photosystem (PS) I-originated reactive oxygen species (ROS) production, caused more severe damage to aor than wild type (Col-0). In contrast, the high light treatment used to enhance PSII-originated ROS production resulted in no difference in PSII damage between Col-0 and aor. In conclusion, AOR can contribute to detoxify stromal RCs produced under oxidative stress.

Chemical quenching of singlet oxygen by carotenoids in plants

Carotenoids are considered to be the first line of defense of plants against singlet oxygen ((1)O(2)) toxicity because of their capacity to quench (1)O(2) as well as triplet chlorophylls through a physical mechanism involving transfer of excitation energy followed by thermal deactivation. Here, we show that leaf carotenoids are also able to quench (1)O(2) by a chemical mechanism involving their oxidation. In vitro oxidation of β-carotene, lutein, and zeaxanthin by (1)O(2) generated various aldehydes and endoperoxides. A search for those molecules in Arabidopsis (Arabidopsis thaliana) leaves revealed the presence of (1)O(2)-specific endoperoxides in low-light-grown plants, indicating chronic oxidation of carotenoids by (1)O(2). β-Carotene endoperoxide, but not xanthophyll endoperoxide, rapidly accumulated during high-light stress, and this accumulation was correlated with the extent of photosystem (PS) II photoinhibition and the expression of various (1)O(2) marker genes. The selective accumulation of β-carotene endoperoxide points at the PSII reaction centers, rather than the PSII chlorophyll antennae, as a major site of (1)O(2) accumulation in plants under high-light stress. β-Carotene endoperoxide was found to have a relatively fast turnover, decaying in the dark with a half time of about 6 h. This carotenoid metabolite provides an early index of (1)O(2) production in leaves, the occurrence of which precedes the accumulation of fatty acid oxidation products.

On the relationship between non-photochemical quenching and photoprotection of Photosystem II

Non-photochemical quenching (NPQ) of chlorophyll fluorescence is thought to be an indicator of an essential regulation and photoprotection mechanism against high-light stress in photosynthetic organisms. NPQ is typically characterized by modulated pulse fluorometry and it is often assumed implicitly to be a good proxy for the actual physiological photoprotection capacity of the organism. Using the results of previously published ultrafast fluorescence measurements on intact leaves of w.t. and mutants of Arabidopsis (Holzwarth et al. 2009) we have developed exact relationships for the fluorescence quenching and the corresponding Photosystem II acceptor side photoprotection effects under NPQ conditions. The approach based on the exciton-radical pair equilibrium model assumes that photodamage results from triplet states generated in the reaction center. The derived relationships allow one to distinguish and determine the individual and combined quenching as well as photoprotection contributions of each of the multiple NPQ mechanisms. Our analysis shows inter alia that quenching and photoprotection are not linearly related and that antenna detachment, which can be identified with the so-called qE mechanism, contributes largely to the measured fluorescence quenching but does not correspond to the most efficient photoprotective response. Conditions are formulated which allow simultaneously the maximal photosynthetic electron flow as well as maximal acceptor side photoprotection. It is shown that maximal photoprotection can be achieved if NPQ is regulated in such a way that PSII reaction centers are open under given light conditions. The results are of fundamental importance for a proper interpretation of the physiological relevance of fluorescence-based NPQ data.

Reduction-oxidation network for flexible adjustment of cellular metabolism in photoautotrophic cells

Photosynthesis generates the energy carriers NADPH and ATP to be consumed in assimilatory processes. Continuous energy conversion and optimal use of the available light energy are only guaranteed when all reduction-oxidation (redox) processes are tightly controlled. A robust network links metabolism with regulation and signalling. Information on the redox situation is generated and transferred by various redox components that are parts of this network. Any imbalance in the network is sensed, and the information is transmitted in order to elicit a response at the various levels of regulation and in the different cellular compartments. Redox information within the chloroplast is derived from intersystem electron transport, the ferredoxin-NADP oxidoreductase (FNR)/NADPH branch of the redox network, the thioredoxin branch and from reactive oxygen species (ROS), resulting in a high diversity of responses that are able to adjust photosynthesis, as well as poising and antioxidant systems accordingly in each specific situation. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) represents a central step in CO(2) reduction and in carbohydrate oxidation involving both forms of energy, namely NAD(P)H and ATP, with its various isoforms that are located in plastids, cytosol and nucleus. GAPDH is used as an example to demonstrate complexity, flexibility and robustness of the regulatory redox network in plants.

Accelerated cell death 2 suppresses mitochondrial oxidative bursts and modulates cell death in Arabidopsis

The Arabidopsis ACCELERATED CELL DEATH 2 (ACD2) protein protects cells from programmed cell death (PCD) caused by endogenous porphyrin-related molecules like red chlorophyll catabolite or exogenous protoporphyrin IX. We previously found that during bacterial infection, ACD2, a chlorophyll breakdown enzyme, localizes to both chloroplasts and mitochondria in leaves. Additionally, acd2 cells show mitochondrial dysfunction. In plants with acd2 and ACD2 (+) sectors, ACD2 functions cell autonomously, implicating a pro-death ACD2 substrate as being cell non-autonomous in promoting the spread of PCD. ACD2 targeted solely to mitochondria can reduce the accumulation of an ACD2 substrate that originates in chloroplasts, indicating that ACD2 substrate molecules are likely to be mobile within cells. Two different light-dependent reactive oxygen bursts in mitochondria play prominent and causal roles in the acd2 PCD phenotype. Finally, ACD2 can complement acd2 when targeted to mitochondria or chloroplasts, respectively, as long as it is catalytically active: the ability to bind substrate is not sufficient for ACD2 to function in vitro or in vivo. Together, the data suggest that ACD2 localizes dynamically during infection to protect cells from pro-death mobile substrate molecules, some of which may originate in chloroplasts, but have major effects on mitochondria.

The Mysterious Rescue of adg1-1/tpt-2 - an Arabidopsis thaliana Double Mutant Impaired in Acclimation to High Light - by Exogenously Supplied Sugars

An Arabidopsis thaliana double mutant (adg1-1/tpt-2) defective in the day- and night-path of photoassimilate export from the chloroplast due to a knockout in the triose phosphate/phosphate translocator (TPT; tpt-2) and a lack of starch [mutation in ADP glucose pyrophosphorylase (AGPase); adg1-1] exhibits severe growth retardation, a decrease in the photosynthetic capacity, and a high chlorophyll fluorescence (HCF) phenotype under high light conditions. These phenotypes could be rescued when the plants were grown on sucrose (Suc) or glucose (Glc). Here we address the question whether Glc-sensing hexokinase1 (HXK1) defective in the Glc insensitive 2 (gin2-1) mutant is involved in the sugar-dependent rescue of adg1-1/tpt-2. Triple mutants defective in the TPT, AGPase, and HXK1 (adg1-1/tpt-2/gin2-1) were established as homozygous lines and grown together with Col-0 and Landsberg erecta (Ler) wild-type plants, gin2-1, the adg1-1/tpt-2 double mutant, and the adg1-1/tpt-2/gpt2-1 triple mutant [additionally defective in the glucose 6-phosphate/phosphate translocator 2 (GPT2)] on agar in the presence or absence of 50 mM of each Glc, Suc, or fructose (Fru). The growth phenotype of the double mutant and both triple mutants could be rescued to a similar extent only by Glc and Suc, but not by Fru. All three sugars were capable of rescuing the HCF and photosynthesis phenotype, irrespectively of the presence or absence of HXK1. Quantitative RT-PCR analyses of sugar-responsive genes revealed that plastidial HXK (pHXK) was up-regulated in adg1-1/tpt-2 plants grown on sugars, but showed no response in adg1-1/tpt-2/gin2-1. It appears likely that soluble sugars are directly taken up by the chloroplasts and enter further metabolism, which consumes ATP and NADPH from the photosynthetic light reaction and thereby rescues the photosynthesis phenotype of the double mutant. The implication of sugar turnover and probably signaling inside the chloroplasts for the concept of retrograde signaling is discussed.

Fitting into the harsh reality: regulation of iron-deficiency responses in dicotyledonous plants

Iron is an essential element for life on Earth and its shortage, or excess, in the living organism may lead to severe health disorders. Plants serve as the primary source of dietary iron and improving crop iron content is an important step towards a better public health. Our review focuses on the control of iron acquisition in dicotyledonous plants and monocots that apply a reduction-based strategy in order to mobilize and import iron from the rhizosphere. Achieving a balance between shortage and excess of iron requires a tight regulation of the activity of the iron uptake system. A number of studies, ranging from single gene characterization to systems biology analyses, have led to the rapid expansion of our knowledge on iron uptake in recent years. Here, we summarize the novel insights into the regulation of iron acquisition and internal mobilization from intracellular stores. We present a detailed view of the main known regulatory networks defined by the Arabidopsis regulators FIT and POPEYE (PYE). Additionally, we analyze the root and leaf iron-responsive regulatory networks, revealing novel potential gene interactions and reliable iron-deficiency marker genes. We discuss perspectives and open questions with regard to iron sensing and post-translational regulation.

Functional roles of melatonin in plants, and perspectives in nutritional and agricultural science

The presence of melatonin in plants is universal. Evidence has confirmed that a major portion of the melatonin is synthesized by plants themselves even though a homologue of the classic arylalkylamine N-acetyltransferase (AANAT) has not been identified as yet in plants. Thus, the serotonin N-acetylating enzyme in plants may differ greatly from the animal AANAT with regard to sequence and structure. This would imply multiple evolutionary origins of enzymes with these catalytic properties. A primary function of melatonin in plants is to serve as the first line of defence against internal and environmental oxidative stressors. The much higher melatonin levels in plants compared with those found in animals are thought to be a compensatory response by plants which lack means of mobility, unlike animals, as a means of coping with harsh environments. Importantly, remarkably high melatonin concentrations have been measured in popular beverages (coffee, tea, wine, and beer) and crops (corn, rice, wheat, barley, and oats). Billions of people worldwide consume these products daily. The beneficial effects of melatonin on human health derived from the consumption of these products must be considered. Evidence also indicates that melatonin has an ability to increase the production of crops. The mechanisms may involve the roles of melatonin in preservation of chlorophyll, promotion of photosynthesis, and stimulation of root development. Transgenic plants with enhanced melatonin content could probably lead to breakthroughs to increase crop production in agriculture and to improve the general health of humans.

Plastoquinol is more active than α-tocopherol in singlet oxygen scavenging during high light stress of Chlamydomonas reinhardtii

In the present study, we have performed comparative analysis of different prenyllipids in Chlamydomonas reinhardtii cultures during high light stress under variety of conditions (presence of inhibitors, an uncoupler, heavy water). The obtained results indicate that plastoquinol is more active than α-tocopherol in scavenging of singlet oxygen generated in photosystem II. Besides plastoquinol, also its oxidized form, plastoquinone shows antioxidant action during the stress conditions, resulting in formation of plastoquinone-C, whose level can be regarded as an indicator of singlet oxygen oxidative stress in vivo. The pronounced stimulation of α-tocopherol consumption and α-tocopherolquinone formation by an uncoupler, FCCP, together with the results of additional model system studies, led to the suggestion that α-tocopherol can be recycled in thylakoid membranes under high light conditions from 8a-hydroperoxy-α-tocopherone, the primary oxidation product of α-tocopherol by singlet oxygen.

Does singlet oxygen activate cell death in Arabidopsis cell suspension cultures?: analysis of the early transcriptional defense responses to high light stress

Can Arabidopsis cell suspension cultures (ACSC) provide a useful working model to investigate genetically-controlled defense responses with signaling cascades starting in chloroplasts? In order to provide a convincing answer, we analyzed the early transcriptional profile of Arabidopsis cells at high light (HL). The results showed that ACSC respond to HL in a manner that resembles the singlet oxygen ((1)O(2))-mediated defense responses described for the conditional fluorescent (flu) mutant of Arabidopsis thaliana. The flu mutant is characterized by the accumulation of free protochlorophyllide (Pchlide) in plastids when put into darkness and the subsequent production of (1)O(2) when the light is on. In ACSC, (1)O(2) is produced in chloroplasts at HL when excess excitation energy flows into photosystem II (PSII). Other reactive oxygen species are also produced in ACSC at HL, but to a lesser extent. When the HL stress ceases, ACSC recovers the initial rate of oxygen evolution and cell growth continues. We can conclude that chloroplasts of ACSC are both photosynthetically active and capable of initiating (1)O(2)-mediated signaling cascades that activate a broad range of genetically-controlled defense responses. The upregulation of transcripts associated with the biosynthesis and signaling pathways of OPDA (12-oxophytodienoic acid) and ethylene (ET) suggests that the activated defense responses at HL are governed by these two hormones. In contrast to the flu mutant, the (1)O(2)-mediated defense responses were independent of the upregulation of EDS1 (enhanced disease susceptibility) required for the accumulation of salicylic acid (SA) and genetically-controlled cell death. Interestingly, a high correlation in transcriptional expression was also observed between ACSC at HL, and the aba1 and max4 mutants of Arabidopsis, characterized by defects in the biosynthesis pathways of abscisic acid (ABA) and strigolactones, respectively.

Photosystem II component lifetimes in the cyanobacterium Synechocystis sp. strain PCC 6803: small Cab-like proteins stabilize biosynthesis intermediates and affect early steps in chlorophyll synthesis

To gain insight in the lifetimes of photosystem II (PSII) chlorophyll and proteins, a combined stable isotope labeling (15N)/mass spectrometry method was used to follow both old and new pigments and proteins. Photosystem I-less Synechocystis cells were grown to exponential or post-exponential phase and then diluted in BG-11 medium with [15N]ammonium and [15N]nitrate. PSII was isolated, and the masses of PSII protein fragments and chlorophyll were determined. Lifetimes of PSII components ranged from 1.5 to 40 h, implying that at least some of the proteins and chlorophyll turned over independently from each other. Also, a significant amount of nascent PSII components accumulated in thylakoids when cells were in post-exponential growth phase. In a mutant lacking small Cab-like proteins (SCPs), most PSII protein lifetimes were unaffected, but the lifetime of chlorophyll and the amount of nascent PSII components that accumulated were decreased. In the absence of SCPs, one of the PSII biosynthesis intermediates, the monomeric PSII complex without CP43, was missing. Therefore, SCPs may stabilize nascent PSII protein complexes. Moreover, upon SCP deletion, the rate of chlorophyll synthesis and the accumulation of early tetrapyrrole precursors were drastically reduced. When [14N]aminolevulinic acid (ALA) was supplemented to 15N-BG-11 cultures, the mutant lacking SCPs incorporated much more exogenous ALA into chlorophyll than the control demonstrating that ALA biosynthesis was impaired in the absence of SCPs. This illustrates the major effects that nonstoichiometric PSII components such as SCPs have on intermediates and assembly but not on the lifetime of PSII proteins.

Carbon dynamics, development and stress responses in Arabidopsis: involvement of the APL4 subunit of ADP-glucose pyrophosphorylase (starch synthesis)

An Arabidopsis thaliana T-DNA insertional mutant was identified and characterized for enhanced tolerance to the singlet-oxygen-generating herbicide atrazine in comparison to wild-type. This enhanced atrazine tolerance mutant was shown to be affected in the promoter structure and in the regulation of expression of the APL4 isoform of ADP-glucose pyrophosphorylase, a key enzyme of the starch biosynthesis pathway, thus resulting in decrease of APL4 mRNA levels. The impact of this regulatory mutation was confirmed by the analysis of an independent T-DNA insertional mutant also affected in the promoter of the APL4 gene. The resulting tissue-specific modifications of carbon partitioning in plantlets and the effects on plantlet growth and stress tolerance point out to specific and non-redundant roles of APL4 in root carbon dynamics, shoot-root relationships and sink regulations of photosynthesis. Given the effects of exogenous sugar treatments and of endogenous sugar levels on atrazine tolerance in wild-type Arabidopsis plantlets, atrazine tolerance of this apl4 mutant is discussed in terms of perception of carbon status and of investment of sugar allocation in xenobiotic and oxidative stress responses.

Overexpression of protochlorophyllide oxidoreductase C regulates oxidative stress in Arabidopsis

Light absorbed by colored intermediates of chlorophyll biosynthesis is not utilized in photosynthesis; instead, it is transferred to molecular oxygen, generating singlet oxygen (¹O₂). As there is no enzymatic detoxification mechanism available in plants to destroy ¹O₂, its generation should be minimized. We manipulated the concentration of a major chlorophyll biosynthetic intermediate i.e., protochlorophyllide in Arabidopsis by overexpressing the light-inducible protochlorophyllide oxidoreductase C (PORC) that effectively phototransforms endogenous protochlorophyllide to chlorophyllide leading to minimal accumulation of the photosensitizer protochlorophyllide in light-grown plants. In PORC overexpressing (PORCx) plants exposed to high-light, the ¹O₂ generation and consequent malonedialdehyde production was minimal and the maximum quantum efficiency of photosystem II remained unaffected demonstrating that their photosynthetic apparatus and cellular organization were intact. Further, PORCx plants treated with 5-aminolevulinicacid when exposed to light, photo-converted over-accumulated protochlorophyllide to chlorophyllide, reduced the generation of ¹O₂ and malonedialdehyde production and reduced plasma membrane damage. So PORCx plants survived and bolted whereas, the 5-aminolevulinicacid-treated wild-type plants perished. Thus, overexpression of PORC could be biotechnologically exploited in crop plants for tolerance to ¹O₂-induced oxidative stress, paving the use of 5-aminolevulinicacid as a selective commercial light-activated biodegradable herbicide. Reduced protochlorophyllide content in PORCx plants released the protochlorophyllide-mediated feed-back inhibition of 5-aminolevulinicacid biosynthesis that resulted in higher 5-aminolevulinicacid production. Increase of 5-aminolevulinicacid synthesis upregulated the gene and protein expression of several downstream chlorophyll biosynthetic enzymes elucidating a regulatory net work of expression of genes involved in 5-aminolevulinicacid and tetrapyrrole biosynthesis.

Changes in the plant proteome resulting from salt stress: toward the creation of salt-tolerant crops?

Salinity in agricultural land is a major problem worldwide, placing a severe constraint on crop growth and productivity in many regions, and increased salinization of arable land is expected to have devastating global effects. Though plants vary in their sensitivity to salt stress, high salinity causes water deficit and ion toxicity in many plant species. Considerable efforts have therefore been made to investigate how genes respond to salt stress in various plants by using several approaches, including proteomics. Proteomic approaches for identifying proteins that are regulated in response to salt stress are becoming common in the post-genomics era of crop research. In this review, we describe the physiological and biological changes in the proteomes of several important food crops under salt stress. We also provide a viewpoint into how proteomics-based research is likely to develop in this field.

Antagonistic role of 9-lipoxygenase-derived oxylipins and ethylene in the control of oxidative stress, lipid peroxidation and plant defence

9-lipoxygenases (9-LOXs) initiate fatty acid oxygenation in plant tissues, with formation of 9-hydroxy-10,12,15-octadecatrienoic acid (9-HOT) from linolenic acid. A lox1 lox5 mutant, which is deficient in 9-LOX activity, and two mutants noxy6 and noxy22 (non-responding to oxylipins), which are insensitive to 9-HOT, have been used to investigate 9-HOT signalling. Map-based cloning indicated that the noxy6 and noxy22 mutations are located at the CTR1 (CONSTITUTIVE ETHYLENE RESPONSE1) and ETO1 (ETHYLENE-OVERPRODUCER1) loci, respectively. In agreement, the noxy6 and noxy22 mutants, renamed as ctr1-15 and eto1-14, respectively, showed enhanced ethylene (ET) production. The correlation between increased ET production and reduced 9-HOT sensitivity indicated by these results was supported by experiments in which exogenously added ethylene precursor ACC (1-aminocyclopropane-1-carboxylic acid) impaired the responses to 9-HOT. Moreover, a reciprocal interaction between ET and 9-HOT signalling was indicated by results showing that the effect of ACC was reduced in the presence of 9-HOT. We found that the 9-LOX and ET pathways regulate the response to the lipid peroxidation-inducer singlet oxygen. Thus, the massive transcriptional changes seen in wild-type plants in response to singlet oxygen were greatly affected in the lox1 lox5 and eto1-14 mutants. Furthermore, these mutants displayed enhanced susceptibility to both singlet oxygen and Pseudomonas syringae pv. tomato, in the latter case leading to increased accumulation of the lipid peroxidation product malondialdehyde. These findings demonstrate an antagonistic relationship between products of the 9-LOX and ET pathways, and suggest a role for the 9-LOX pathway in modulating oxidative stress, lipid peroxidation and plant defence.

Light as both an input and an output of wound-induced reactive oxygen formation in Arabidopsis leaves

The wound response of plants is characterised by rapid changes in gene expression, biochemistry and physiology, and is important both in its own right and as a model for studying events elicited by herbivory. We have recently identified links between light and the wound response in Arabidopsis leaves. This includes an influence of the external light environment on the molecular and biochemical response to wounding, and the observation that endogenous bioluminescence (light emission) is a consequence of tissue damage. Here, we show that this link extends to the production of reactive oxygen species. We show that wounding causes rapid, light-dependent production of superoxide and hydrogen peroxide in chloroplasts via disruption of photosynthesis, and that wound-induced bioluminescence is a consequence of the generation of singlet oxygen.

Tetrapyrrole Metabolism in Arabidopsis thaliana

Higher plants produce four classes of tetrapyrroles, namely, chlorophyll (Chl), heme, siroheme, and phytochromobilin. In plants, tetrapyrroles play essential roles in a wide range of biological activities including photosynthesis, respiration and the assimilation of nitrogen/sulfur. All four classes of tetrapyrroles are derived from a common biosynthetic pathway that resides in the plastid. In this article, we present an overview of tetrapyrrole metabolism in Arabidopsis and other higher plants, and we describe all identified enzymatic steps involved in this metabolism. We also summarize recent findings on Chl biosynthesis and Chl breakdown. Recent advances in this field, in particular those on the genetic and biochemical analyses of novel enzymes, prompted us to redraw the tetrapyrrole metabolic pathways. In addition, we also summarize our current understanding on the regulatory mechanisms governing tetrapyrrole metabolism. The interactions of tetrapyrrole biosynthesis and other cellular processes including the plastid-to-nucleus signal transduction are discussed.

Linoleic acid-induced ultra-weak photon emission from Chlamydomonas reinhardtii as a tool for monitoring of lipid peroxidation in the cell membranes

Reactive oxygen species formed as a response to various abiotic and biotic stresses cause an oxidative damage of cellular component such are lipids, proteins and nucleic acids. Lipid peroxidation is considered as one of the major processes responsible for the oxidative damage of the polyunsaturated fatty acid in the cell membranes. Various methods such as a loss of polyunsaturated fatty acids, amount of the primary and the secondary products are used to monitor the level of lipid peroxidation. To investigate the use of ultra-weak photon emission as a non-invasive tool for monitoring of lipid peroxidation, the involvement of lipid peroxidation in ultra-weak photon emission was studied in the unicellular green alga Chlamydomonas reinhardtii. Lipid peroxidation initiated by addition of exogenous linoleic acid to the cells was monitored by ultra-weak photon emission measured with the employment of highly sensitive charged couple device camera and photomultiplier tube. It was found that the addition of linoleic acid to the cells significantly increased the ultra-weak photon emission that correlates with the accumulation of lipid peroxidation product as measured using thiobarbituric acid assay. Scavenging of hydroxyl radical by mannitol, inhibition of intrinsic lipoxygenase by catechol and removal of molecular oxygen considerably suppressed ultra-weak photon emission measured after the addition of linoleic acid. The photon emission dominated at the red region of the spectrum with emission maximum at 680 nm. These observations reveal that the oxidation of linoleic acid by hydroxyl radical and intrinsic lipoxygenase results in the ultra-weak photon emission. Electronically excited species such as excited triplet carbonyls are the likely candidates for the primary excited species formed during the lipid peroxidation, whereas chlorophylls are the final emitters of photons. We propose here that the ultra-weak photon emission can be used as a non-invasive tool for the detection of lipid peroxidation in the cell membranes.

Early transcriptional defense responses in Arabidopsis cell suspension culture under high-light conditions

The early transcriptional defense responses and reactive oxygen species (ROS) production in Arabidopsis (Arabidopsis thaliana) cell suspension culture (ACSC), containing functional chloroplasts, were examined at high light (HL). The transcriptional analysis revealed that most of the ROS markers identified among the 449 transcripts with significant differential expression were transcripts specifically up-regulated by singlet oxygen ((1)O(2)). On the contrary, minimal correlation was established with transcripts specifically up-regulated by superoxide radical or hydrogen peroxide. The transcriptional analysis was supported by fluorescence microscopy experiments. The incubation of ACSC with the (1)O(2) sensor green reagent and 2',7'-dichlorofluorescein diacetate showed that the 30-min-HL-treated cultures emitted fluorescence that corresponded with the production of (1)O(2) but not of hydrogen peroxide. Furthermore, the in vivo photodamage of the D1 protein of photosystem II indicated that the photogeneration of (1)O(2) took place within the photosystem II reaction center. Functional enrichment analyses identified transcripts that are key components of the ROS signaling transduction pathway in plants as well as others encoding transcription factors that regulate both ROS scavenging and water deficit stress. A meta-analysis examining the transcriptional profiles of mutants and hormone treatments in Arabidopsis showed a high correlation between ACSC at HL and the fluorescent mutant family of Arabidopsis, a producer of (1)O(2) in plastids. Intriguingly, a high correlation was also observed with ABA deficient1 and more axillary growth4, two mutants with defects in the biosynthesis pathways of two key (apo)carotenoid-derived plant hormones (i.e. abscisic acid and strigolactones, respectively). ACSC has proven to be a valuable system for studying early transcriptional responses to HL stress.

Plants under continuous light

Continuous light is an essential tool for understanding the plant circadian clock. Additionally, continuous light might increase greenhouse food production. However, using continuous light in research and practice has its challenges. For instance, most of the circadian clock-oriented experiments were performed under continuous light; consequently, interactions between the circadian clock and the light signaling pathway were overlooked. Furthermore, in some plant species continuous light induces severe injury, which is only poorly understood so far. In this review paper, we aim to combine the current knowledge with a modern conceptual framework. Modern genomic tools and rediscovered continuous light-tolerant tomato species (Solanum spp.) could boost the understanding of the physiology of plants under continuous light.

ROS signaling: the new wave?

Reactive oxygen species (ROS) play a multitude of signaling roles in different organisms from bacteria to mammalian cells. They were initially thought to be toxic byproducts of aerobic metabolism, but have now been acknowledged as central players in the complex signaling network of cells. In this review, we will attempt to address several key questions related to the use of ROS as signaling molecules in cells, including the dynamics and specificity of ROS signaling, networking of ROS with other signaling pathways, ROS signaling within and across different cells, ROS waves and the evolution of the ROS gene network.

Molecular mechanisms of production and scavenging of reactive oxygen species by photosystem II

Photosystem II (PSII) is a multisubunit protein complex in cyanobacteria, algae and plants that use light energy for oxidation of water and reduction of plastoquinone. The conversion of excitation energy absorbed by chlorophylls into the energy of separated charges and subsequent water-plastoquinone oxidoreductase activity are inadvertently coupled with the formation of reactive oxygen species (ROS). Singlet oxygen is generated by the excitation energy transfer from triplet chlorophyll formed by the intersystem crossing from singlet chlorophyll and the charge recombination of separated charges in the PSII antenna complex and reaction center of PSII, respectively. Apart to the energy transfer, the electron transport associated with the reduction of plastoquinone and the oxidation of water is linked to the formation of superoxide anion radical, hydrogen peroxide and hydroxyl radical. To protect PSII pigments, proteins and lipids against the oxidative damage, PSII evolved a highly efficient antioxidant defense system comprising either a non-enzymatic (prenyllipids such as carotenoids and prenylquinols) or an enzymatic (superoxide dismutase and catalase) scavengers. It is pointed out here that both the formation and the scavenging of ROS are controlled by the energy level and the redox potential of the excitation energy transfer and the electron transport carries, respectively. The review is focused on the mechanistic aspects of ROS production and scavenging by PSII. This article is part of a Special Issue entitled: Photosystem II.

Molecular mechanisms of photodamage in the Photosystem II complex

Light induced damage of the photosynthetic apparatus is an important and highly complex phenomenon, which affects primarily the Photosystem II complex. Here the author summarizes the current state of understanding of the molecular mechanisms, which are involved in the light induced inactivation of Photosystem II electron transport together with the relevant mechanisms of photoprotection. Short wavelength ultraviolet radiation impairs primarily the Mn₄Ca catalytic site of the water oxidizing complex with additional effects on the quinone electron acceptors and tyrosine donors of PSII. The main mechanism of photodamage by visible light appears to be mediated by acceptor side modifications, which develop under conditions of excess excitation in which the capacity of light-independent photosynthetic processes limits the utilization of electrons produced in the initial photoreactions. This situation of excess excitation facilitates the reduction of intersystem electron carriers and Photosystem II acceptors, and thereby induces the formation of reactive oxygen species, especially singlet oxygen whose production is sensitized by triplet chlorophyll formation in the reaction center of Photosystem II. The highly reactive singlet oxygen and other reactive oxygen species, such as H₂O₂ and O₂⁻, which can also be formed in Photosystem II initiate damage of electron transport components and protein structure. In parallel with the excess excitation dependent mechanism of photodamage inactivation of the Mn₄Ca cluster by visible light may also occur, which impairs electron transfer through the Photosystem II complex and initiates further functional and structural damage of the reaction center via formation of highly oxidizing radicals, such as P 680(+) and Tyr-Z(+). However, the available data do not support the hypothesis that the Mn-dependent mechanism would be the exclusive or dominating pathway of photodamage in the visible spectral range. This article is part of a Special Issue entitled: Photosystem II.

The role of the xanthophyll cycle and of lutein in photoprotection of photosystem II

Photoprotection of photosystem II (PSII) is essential to avoid the light-induced damage of the photosynthetic apparatus due to the formation of reactive oxygen species (=photo-oxidative stress) under excess light. Carotenoids are known to play a crucial role in these processes based on their property to deactivate triplet chlorophyll (³Chl*) and singlet oxygen (¹O₂*). Xanthophylls are further assumed to be involved either directly or indirectly in the non-photochemical quenching (NPQ) of excess light energy in the antenna of PSII. This review gives an overview on recent progress in the understanding of the photoprotective role of the xanthophylls zeaxanthin (which is formed in the light in the so-called xanthophyll cycle) and lutein with emphasis on the NPQ processes associated with PSII of higher plants. The current knowledge supports the view that the photoprotective role of Lut is predominantly restricted to its function in the deactivation of ³Chl*, while zeaxanthin is the major player in the deactivation of excited singlet Chl (¹Chl*) and thus in NPQ (non-photochemical quenching). Additionally, zeaxanthin serves important functions as an antioxidant in the lipid phase of the membrane and is likely to act as a key component in the memory of the chloroplast with respect to preceding photo-oxidative stress. This article is part of a Special Issue entitled: Photosystem II.

Enhanced levels of vitamin B(6) increase aerial organ size and positively affect stress tolerance in Arabidopsis

Vitamin B₆ is an essential nutrient in the human diet derived primarily from plant sources. While it is well established as a cofactor for numerous metabolic enzymes, more recently, vitamin B₆ has been implicated as a potent antioxidant. The de novo vitamin B₆ biosynthesis pathway in plants has recently been unraveled and involves only two proteins, PDX1 and PDX2. To provide more insight into the effect of the compound on plant development and its role as an antioxidant, we have overexpressed the PDX proteins in Arabidopsis, generating lines with considerably higher levels of the vitamin in comparison with other recent attempts to achieve this goal. Interestingly, it was possible to increase the level of only one of the two catalytically active PDX1 proteins at the protein level, providing insight into the mechanism of vitamin B₆ homeostasis in planta. Vitamin B₆ enhanced lines have considerably larger vegetative and floral organs and although delayed in pre-reproductive development, do not have an altered overall morphology. The vitamin was observed to accumulate in seeds and the enhancement of its levels was correlated with an increase in their size and weight. This phenotype is predominantly a consequence of embryo enlargement as reflected by larger cells. Furthermore, plants that overaccumulate the vitamin have an increased tolerance to oxidative stress providing in vivo evidence for the antioxidant functionality of vitamin B₆. In particular, the plants show an increased resistance to paraquat and photoinhibition, and they attenuate the cell death response observed in the conditional flu mutant.

Interaction between the bHLH transcription factor FIT and ETHYLENE INSENSITIVE3/ETHYLENE INSENSITIVE3-LIKE1 reveals molecular linkage between the regulation of iron acquisition and ethylene signaling in Arabidopsis

Understanding the regulation of key genes involved in plant iron acquisition is of crucial importance for breeding of micronutrient-enriched crops. The basic helix-loop-helix protein FER-LIKE FE DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT), a central regulator of Fe acquisition in roots, is regulated by environmental cues and internal requirements for iron at the transcriptional and posttranscriptional levels. The plant stress hormone ethylene promotes iron acquisition, but the molecular basis for this remained unknown. Here, we demonstrate a direct molecular link between ethylene signaling and FIT. We identified ETHYLENE INSENSITIVE3 (EIN3) and ETHYLENE INSENSITIVE3-LIKE1 (EIL1) in a screen for direct FIT interaction partners and validated their physical interaction in planta. We demonstrate that the ein3 eil1 transcriptome was affected to a greater extent upon iron deficiency than normal iron compared with the wild type. Ethylene signaling by way of EIN3/EIL1 was required for full-level FIT accumulation. FIT levels were reduced upon application of aminoethoxyvinylglycine and in the ein3 eil1 background. MG132 could restore FIT levels. We propose that upon ethylene signaling, FIT is less susceptible to proteasomal degradation, presumably due to a physical interaction between FIT and EIN3/EIL1. Increased FIT abundance then leads to the high level of expression of genes required for Fe acquisition. This way, ethylene is one of the signals that triggers Fe deficiency responses at the transcriptional and posttranscriptional levels.

Role of charge recombination processes in photodamage and photoprotection of the photosystem II complex

Light-induced damage of the photosynthetic apparatus is an important and complex phenomenon, which affects primarily the photosystem II (PSII) complex. Here, the author summarizes the current state of understanding, which concerns the role of charge recombination reactions in photodamage and photoprotection. The main mechanism of photodamage induced by visible light appears to be mediated by acceptor side modifications, which develop under light intensity conditions when the capacity of light-independent photosynthetic processes limits the utilization of electrons produced in the initial photoreactions. This situation facilitates triplet chlorophyll formation and singlet oxygen production in the reaction center of PSII, which initiates the damage of electron transport components and protein structure. This mechanism is an important, but not exclusive, pathway of photodamage, and light-induced inactivation of the Mn cluster of water oxidation may occur in parallel with the singlet oxygen-dependent pathway.

A small zinc finger thylakoid protein plays a role in maintenance of photosystem II in Arabidopsis thaliana

This work identifies LOW QUANTUM YIELD OF PHOTOSYSTEM II1 (LQY1), a Zn finger protein that shows disulfide isomerase activity, interacts with the photosystem II (PSII) core complex, and may act in repair of photodamaged PSII complexes. Two mutants of an unannotated small Zn finger containing a thylakoid membrane protein of Arabidopsis thaliana (At1g75690; LQY1) were found to have a lower quantum yield of PSII photochemistry and reduced PSII electron transport rate following high-light treatment. The mutants dissipate more excess excitation energy via nonphotochemical pathways than wild type, and they also display elevated accumulation of reactive oxygen species under high light. After high-light treatment, the mutants have less PSII-light-harvesting complex II supercomplex than wild-type plants. Analysis of thylakoid membrane protein complexes showed that wild-type LQY1 protein comigrates with the PSII core monomer and the CP43-less PSII monomer (a marker for ongoing PSII repair and reassembly). PSII repair and reassembly involve the breakage and formation of disulfide bonds among PSII proteins. Interestingly, the recombinant LQY1 protein demonstrates a protein disulfide isomerase activity. LQY1 is more abundant in stroma-exposed thylakoids, where key steps of PSII repair and reassembly take place. The absence of the LQY1 protein accelerates turnover and synthesis of PSII reaction center protein D1. These results suggest that the LQY1 protein may be involved in maintaining PSII activity under high light by regulating repair and reassembly of PSII complexes.

Reactive oxygen species and transcript analysis upon excess light treatment in wild-type Arabidopsis thaliana vs a photosensitive mutant lacking zeaxanthin and lutein

BACKGROUND

Reactive oxygen species (ROS) are unavoidable by-products of oxygenic photosynthesis, causing progressive oxidative damage and ultimately cell death. Despite their destructive activity they are also signalling molecules, priming the acclimatory response to stress stimuli.

RESULTS

To investigate this role further, we exposed wild type Arabidopsis thaliana plants and the double mutant npq1lut2 to excess light. The mutant does not produce the xanthophylls lutein and zeaxanthin, whose key roles include ROS scavenging and prevention of ROS synthesis. Biochemical analysis revealed that singlet oxygen (1O2) accumulated to higher levels in the mutant while other ROS were unaffected, allowing to define the transcriptomic signature of the acclimatory response mediated by 1O2 which is enhanced by the lack of these xanthophylls species. The group of genes differentially regulated in npq1lut2 is enriched in sequences encoding chloroplast proteins involved in cell protection against the damaging effect of ROS. Among the early fine-tuned components, are proteins involved in tetrapyrrole biosynthesis, chlorophyll catabolism, protein import, folding and turnover, synthesis and membrane insertion of photosynthetic subunits. Up to now, the flu mutant was the only biological system adopted to define the regulation of gene expression by 1O2. In this work, we propose the use of mutants accumulating 1O2 by mechanisms different from those activated in flu to better identify ROS signalling.

CONCLUSIONS

We propose that the lack of zeaxanthin and lutein leads to 1O2 accumulation and this represents a signalling pathway in the early stages of stress acclimation, beside the response to ADP/ATP ratio and to the redox state of both plastoquinone pool. Chloroplasts respond to 1O2 accumulation by undergoing a significant change in composition and function towards a fast acclimatory response. The physiological implications of this signalling specificity are discussed.

Comparative proteomic analysis of canola leaves under salinity stress

Although canola is a moderately salt-tolerant species, its growth, seed yield, and oil production are markedly reduced under salt stress, particularly during the early vegetative growth stage. To identify the mechanisms of salt responsiveness in canola, the proteins expressed in the second and third newly developed leaves of salt-tolerant, Hyola 308, and salt-sensitive, Sarigol, cultivars were analyzed. Plants were exposed to 0, 175, and 350 mM NaCl during the vegetative stage. An increase in the Na content and a reduction in growth were observed in the third leaves compared to the second leaves. The accumulation of Na was more pronounced in the salt-sensitive compared with the salt-tolerant genotype. Out of 900 protein spots detected on 2-DE gels, 44 and 31 proteins were differentially expressed in the tolerant and susceptible genotypes, respectively. Cluster analysis based on the expression level of total and responsive proteins indicated that the second leaves had a discriminator role between the two genotypes at both salinity levels. Using MS analysis, 46 proteins could be identified including proteins involved in responses to oxidative stress, energy production, electron transport, translation, and photosynthesis. Our results suggest that these proteins might play roles in canola adaptation to salt stress.