The Role of Glutamine Synthetase (GS) and Glutamate Synthase (GOGAT) in the Improvement of Nitrogen Use Efficiency in Cereals

Cereals are the most broadly produced crops and represent the primary source of food worldwide. Nitrogen (N) is a critical mineral nutrient for plant growth and high yield, and the quality of cereal crops greatly depends on a suitable N supply. In the last decades, a massive use of N fertilizers has been achieved in the desire to have high yields of cereal crops, leading to damaging effects for the environment, ecosystems, and human health. To ensure agricultural sustainability and the required food source, many attempts have been made towards developing cereal crops with a more effective nitrogen use efficiency (NUE). NUE depends on N uptake, utilization, and lastly, combining the capability to assimilate N into carbon skeletons and remobilize the N assimilated. The glutamine synthetase (GS)/glutamate synthase (GOGAT) cycle represents a crucial metabolic step of N assimilation, regulating crop yield. In this review, the physiological and genetic studies on GS and GOGAT of the main cereal crops will be examined, giving emphasis on their implications in NUE.

Comparative Analysis of Bioactive Compounds in Two Globe Artichoke Ecotypes Sanitized and Non-Sanitized from Viral Infections

Globe artichoke ecotypes sanitized from plant pathogen infections are characterized by high vegetative vigor, productivity, and quality of capitula. The recent availability on the market of these plants has renewed the interest of farmers and pharmaceutical industries in the crop. Globe artichoke exhibits interesting nutraceutical properties due to the high content of health-promoting bioactive compounds (BACs), such as polyphenols, that could be extracted from waste biomass. The production of BACs depends on several factors including the plant portion considered, the globe artichoke variety/ecotype, and the physiological status of the plants, linked to biotic and abiotic stresses. We investigated the influence of viral infections on polyphenol accumulation in two Apulian late-flowering ecotypes "Locale di Mola tardivo" and "Troianella", comparing sanitized virus-free material (S) vs. naturally virus-infected (non-sanitized, NS) plants. Transcriptome analysis of the two ecotypes highlighted that differentially expressed genes (DEGs), in the two tested conditions, were mainly involved in primary metabolism and processing of genetic/environmental information. The up-regulation of the genes related to the biosynthesis of secondary metabolites and the analysis of peroxidase activity suggested that their modulation is influenced by the phytosanitary status of the plant and is ecotype-dependent. Conversely, the phytochemical analysis showed a remarkable decrease in polyphenols and lignin accumulation in S artichokes compared to NS plants. This unique study analyzes the potential of growing vigorous, sanitized plants, in order to have high amounts of 'soft and clean' biomass, finalized for BAC extraction for nutraceutical purposes. This, in turn, opens new perspectives for a circular economy of sanitized artichokes, in line with the current phytosanitary standards and sustainable development goals.

Chloroplast-localized GUN1 contributes to the acquisition of basal thermotolerance in Arabidopsis thaliana

Heat stress (HS) severely affects different cellular compartments operating in metabolic processes and represents a critical threat to plant growth and yield. Chloroplasts are crucial for heat stress response (HSR), signaling to the nucleus the environmental challenge and adjusting metabolic and biosynthetic functions accordingly. GENOMES UNCOUPLED 1 (GUN1), a chloroplast-localized protein, has been recognized as one of the main players of chloroplast retrograde signaling. Here, we investigate HSR in Arabidopsis wild-type and gun1 plantlets subjected to 2 hours of HS at 45°C. In wild-type plants, Reactive Oxygen Species (ROS) accumulate promptly after HS, contributing to transiently oxidize the cellular environment and acting as signaling molecules. After 3 hours of physiological recovery at growth temperature (22°C), the induction of enzymatic and non-enzymatic antioxidants prevents oxidative damage. On the other hand, gun1 mutants fail to induce the oxidative burst immediately after HS and accumulate ROS and oxidative damage after 3 hours of recovery at 22°C, thus resulting in enhanced sensitivity to HS. These data suggest that GUN1 is required to oxidize the cellular environment, participating in the acquisition of basal thermotolerance through the redox-dependent plastid-to-nucleus communication.

Enhanced Natural Attenuation of Groundwater Cr(VI) Pollution Using Electron Donors: Yeast Extract vs. Polyhydroxybutyrate

Remediation interventions based on the native bacteria’s capability to reduce Cr(VI) represent a valid strategy in terms of economic and environmental sustainability. In this study, a bioremediation test was carried out using viable microcosms set with groundwater and deep soil (4:1), collected from the saturated zone of an industrial site in Southern Italy that was polluted by ~130 µg L−1 of Cr(VI). Conditions simulating the potential natural attenuation were compared to the enhanced natural attenuation induced by supplying yeast extract or polyhydroxybutyrate. Sterile controls were set up to study the possible Cr(VI) abiotic reduction. No pollution attenuation was detected in the unamended viable reactors, whereas yeast extract provided the complete Cr(VI) removal in 7 days, and polyhydroxybutyrate allowed ~70% pollutant removal after 21 days. The incomplete abiotic removal of Cr(VI) was observed in sterile reactors amended with yeast extract, thus suggesting the essential role of native bacteria in Cr(VI) remediation. This was in accordance with the results of Pearson’s coefficient test, which revealed that Cr(VI) removal was positively correlated with microbial proliferation (n = 0.724), and also negatively correlated with pH (n = −0.646), dissolved oxygen (n = −0.828) and nitrate (n = −0.940). The relationships between the Cr(VI) removal and other monitored parameters were investigated by principal component analysis, which explained 76.71% of the total variance.

GUN1 involvement in the redox changes occurring during biogenic retrograde signaling

Chloroplast biogenesis requires a tight communication between nucleus and plastids. By retrograde signals, plastids transmit information about their functional and developmental state to adjust nuclear gene expression, accordingly. GENOMES UNCOUPLED 1 (GUN1), a chloroplast-localized protein integrating several developmental and stress-related signals, is one of the main players of retrograde signaling. Here, we focused on the interplay between GUN1 and redox regulation during biogenic retrograde signaling, by investigating redox parameters in Arabidopsis wild type and gun1 seedlings. Our data highlight that during biogenic retrograde signaling superoxide anion (O₂^-) and hydrogen peroxide (H₂O₂) play a different role in response to GUN1. Under physiological conditions, even in the absence of a visible phenotype, gun1 mutants show low activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX), with an increase in O₂^- accumulation and lipid peroxidation, suggesting that GUN1 indirectly protects chloroplasts from oxidative damage. In wild type seedlings, perturbation of chloroplast development with lincomycin causes H₂O₂ accumulation, in parallel with the decrease of ROS-removal metabolites and enzymes. These redox changes do not take place in gun1 mutants which, in contrast, enhance SOD, APX and catalase activities. Our results indicate that in response to lincomycin, GUN1 is necessary for the H₂O₂-dependent oxidation of cellular environment, which might contribute to the redox-dependent plastid-to nucleus communication.

Cyclic AMP mediates heat stress response by the control of redox homeostasis and ubiquitin-proteasome system

Heat stress (HS), causing impairment in several physiological processes, is one of the most damaging environmental cues for plants. To counteract the harmful effects of high temperatures, plants activate complex signalling networks, indicated as HS response (HSR). Expression of heat shock proteins (HSPs) and adjustment of redox homeostasis are crucial events of HSR, required for thermotolerance. By pharmacological approaches, the involvement of cAMP in triggering plant HSR has been recently proposed. In this study, to investigate the role of cAMP in HSR signalling, tobacco BY-2 cells overexpressing the 'cAMP-sponge', a genetic tool that reduces intracellular cAMP levels, have been used. in vivo cAMP dampening increased HS susceptibility in a HSPs-independent way. The failure in cAMP elevation during HS caused a high accumulation of reactive oxygen species, due to increased levels of respiratory burst oxidase homolog D, decreased activities of catalase and ascorbate peroxidase, as well as down-accumulation of proteins involved in the control of redox homeostasis. In addition, cAMP deficiency impaired proteasome activity and prevented the accumulation of many proteins of ubiquitin-proteasome system (UPS). By a large-scale proteomic approach together with in silico analyses, these UPS proteins were identified in a specific cAMP-dependent network of HSR.

Cyclic AMP: A Polyhedral Signalling Molecule in Plants

The cyclic nucleotide cAMP (3',5'-cyclic adenosine monophosphate) is nowadays recognised as an important signalling molecule in plants, involved in many molecular processes, including sensing and response to biotic and abiotic environmental stresses. The validation of a functional cAMP-dependent signalling system in higher plants has spurred a great scientific interest on the polyhedral role of cAMP, as it actively participates in plant adaptation to external stimuli, in addition to the regulation of physiological processes. The complex architecture of cAMP-dependent pathways is far from being fully understood, because the actors of these pathways and their downstream target proteins remain largely unidentified. Recently, a genetic strategy was effectively used to lower cAMP cytosolic levels and hence shed light on the consequences of cAMP deficiency in plant cells. This review aims to provide an integrated overview of the current state of knowledge on cAMP's role in plant growth and response to environmental stress. Current knowledge of the molecular components and the mechanisms of cAMP signalling events is summarised.

Plant Cell Cultures as Model Systems to Study Programmed Cell Death

The study of programmed cell death (PCD) activated in a certain group of cells is complex when analyzed in the whole plant. Plant cell suspension cultures are useful when investigating PCD triggered by environmental and developmental stimuli. Due to their homogeneity and the possibility to synchronize their responses induced by external stimuli, these cultures are used for studying the signaling pathways leading to PCD. The first problem in the analysis of PCD in cell cultures is the quantification of cell viability/death over time. Cultured cells from different plant species may have specific mitotic patterns leading to calli or cell chains mixed to single cell suspensions. For this reason, not all cell cultures allow morphological parameters to be investigated using microscopy analysis, and adapted or ad hoc methods are needed to test cell viability.Here we report on some accurate methods to establish and propagate cell cultures from different plant species, including crops, as well as to determine cell viability and PCD morphological and genetic markers. In particular, we describe a protocol for extracting nucleic acids required for real-time PCR analysis which has been optimized for those cell cultures that do not allow the use of commercial kits.

Gigaspora margarita with and without its endobacterium shows adaptive responses to oxidative stress

Arbuscular mycorrhizal (AM) fungi experience oxidative stress during the plant-fungal interaction, due to endogenous reactive oxygen species (ROS) produced by fungal metabolism and exogenous ROS produced by plant cells. Here, we examine the responses to H₂O₂ in Gigaspora margarita, an AM fungus containing the endobacterial symbiont Candidatus Glomeribacter gigasporarum (CaGg). Previous studies revealed that G. margarita with its endobacterium produces more ATP and has higher respiratory activity than a cured line that lacks the endobacterium. This higher bioenergetic potential leads to higher production of ROS and to a higher ROS-detoxifying capacity, suggesting a direct or indirect role of the endobacterium in modulating fungal antioxidant responses. To test the hypothesis that the fungal-endobacterial symbiosis may enhance the fitness of the AM fungus in the presence of oxidative stress, we treated the fungus with a sublethal concentration of H₂O₂ and performed RNA-seq analysis. Our results demonstrate that (i) irrespective of the endobacterium presence, G. margarita faces oxidative stress by activating multiple metabolic processes (methionine oxidation, sulfur uptake, the pentose phosphate pathway, activation of ROS-scavenger genes); (ii) in the presence of its endobacterium, G. margarita upregulates some metabolic pathways, like chromatin status modifications and iron metabolism; and (iii) contrary to our hypothesis, the cured line responds to H₂O₂ by activating the transcription of specific ROS scavengers. We confirmed the RNA-seq findings by measuring the glutathione and ascorbate concentration, which was the same in both lines after H₂O₂ treatment. We conclude that both fungal lines may face oxidative stress, but they activate alternative strategies.

An interdomain network: the endobacterium of a mycorrhizal fungus promotes antioxidative responses in both fungal and plant hosts

Arbuscular mycorrhizal fungi (AMF) are obligate plant biotrophs that may contain endobacteria in their cytoplasm. Genome sequencing of Candidatus Glomeribacter gigasporarum revealed a reduced genome and dependence on the fungal host. RNA-seq analysis of the AMF Gigaspora margarita in the presence and absence of the endobacterium indicated that endobacteria have an important role in the fungal pre-symbiotic phase by enhancing fungal bioenergetic capacity. To improve the understanding of fungal-endobacterial interactions, iTRAQ (isobaric tags for relative and absolute quantification) quantitative proteomics was used to identify differentially expressed proteins in G. margarita germinating spores with endobacteria (B+), without endobacteria in the cured line (B-) and after application of the synthetic strigolactone GR24. Proteomic, transcriptomic and biochemical data identified several fungal and bacterial proteins involved in interspecies interactions. Endobacteria influenced fungal growth, calcium signalling and metabolism. The greatest effects were on fungal primary metabolism and respiration, which was 50% higher in B+ than in B-. A shift towards pentose phosphate metabolism was detected in B-. Quantification of carbonylated proteins indicated that the B- line had higher oxidative stress levels, which were also observed in two host plants. This study shows that endobacteria generate a complex interdomain network that affects AMF and fungal-plant interactions.

Cyclic AMP deficiency negatively affects cell growth and enhances stress-related responses in tobacco Bright Yellow-2 cells

Cyclic adenosine 3',5'-monophosphate (cAMP) is a recognized second messenger; however, knowledge of cAMP involvement in plant physiological processes originates primarily from pharmacological studies. To obtain direct evidence for cAMP function in plants, tobacco Bright Yellow-2 (BY-2) cells were transformed with the cAMP sponge, which is a genetically encoded tool that reduces cAMP availability. BY-2 cells expressing the cAMP sponge (cAS cells), showed low levels of free cAMP and exhibited growth inhibition that was not proportional to the cAMP sponge transcript level. Growth inhibition in cAS cells was closely related to the precocious inhibition of mitosis due to a delay in cell cycle progression. The cAMP deficiency also enhanced antioxidant systems. Remarkable changes occurred in the cAS proteomic profile compared with that of wild-type (WT) cells. Proteins involved in translation, cytoskeletal organization, and cell proliferation were down-regulated, whereas stress-related proteins were up-regulated in cAS cells. These results support the hypothesis that BY-2 cells sense cAMP deficiency as a stress condition. Finally, many proteasome subunits were differentially expressed in cAS cells compared with WT cells, indicating that cAMP signaling broadly affects protein degradation via the ubiquitin/proteasome pathway.

Involvement of DNA methylation in the control of cell growth during heat stress in tobacco BY-2 cells

The alteration of growth patterns, through the adjustment of cell division and expansion, is a characteristic response of plants to environmental stress. In order to study this response in more depth, the effect of heat stress on growth was investigated in tobacco BY-2 cells. The results indicate that heat stress inhibited cell division, by slowing cell cycle progression. Cells were stopped in the pre-mitotic phases, as shown by the increased expression of CycD3-1 and by the decrease in the NtCycA13, NtCyc29 and CDKB1-1 transcripts. The decrease in cell length and the reduced expression of Nt-EXPA5 indicated that cell expansion was also inhibited. Since DNA methylation plays a key role in controlling gene expression, the possibility that the altered expression of genes involved in the control of cell growth, observed during heat stress, could be due to changes in the methylation state of their promoters was investigated. The results show that the altered expression of CycD3-1 and Nt-EXPA5 was consistent with changes in the methylation state of the upstream region of these genes. These results suggest that DNA methylation, controlling the expression of genes involved in plant development, contributes to growth alteration occurring in response to environmental changes.

Role of redox homeostasis in thermo-tolerance under a climate change scenario

BACKGROUND

Climate change predictions indicate a progressive increase in average temperatures and an increase in the frequency of heatwaves, which will have a negative impact on crop productivity. Over the last decade, a number of studies have addressed the question of how model plants or specific crops modify their metabolism when exposed to heat stress.

SCOPE

This review provides an overview of the redox pathways that contribute to how plants cope with heat stress. The focus is on the role of reactive oxygen species (ROS), redox metabolites and enzymes in the signalling pathways leading to the activation of defence responses. Additional attention is paid to the regulating mechanisms that lead to an increase in specific ROS-scavenging systems during heat stress, which have been studied in different model systems. Finally, increasing thermo-tolerance in model and crop plants by exposing them to heat acclimation or to exogenous treatments is discussed.

CONCLUSIONS

Although there is clear evidence that several strategies are specifically activated according to the intensity and the duration of heat stress, as well as the capacity of the different species or genotypes to overcome stress, an alteration in redox homeostasis seems to be a common event. Different mechanisms that act to enhance redox systems enable crops to overcome heat stress more effectively. Knowledge of thermo-tolerance within agronomic biodiversity is thus of key importance to enable researchers to identify new strategies for overcoming the impacts of climate change, and for decision-makers in planning for an uncertain future with new choices and options open to them.

Changes in antioxidants are critical in determining cell responses to short- and long-term heat stress

Heat stress can have deleterious effects on plant growth by impairing several physiological processes. Plants have several defense mechanisms that enable them to cope with high temperatures. The synthesis and accumulation of heat shock proteins (HSPs), as well as the maintenance of an opportune redox balance play key roles in conferring thermotolerance to plants. In this study changes in redox parameters, the activity and/or expression of reactive oxygen species (ROS) scavenging enzymes and the expression of two HSPs were studied in tobacco Bright Yellow-2 (TBY-2) cells subjected to moderate short-term heat stress (SHS) and long-term heat stress (LHS). The results indicate that TBY-2 cells subjected to SHS suddenly and transiently enhance antioxidant systems, thus maintaining redox homeostasis and avoiding oxidative damage. The simultaneous increase in HSPs overcomes the SHS and maintains the metabolic functionality of cells. In contrast the exposure of cells to LHS significantly reduces cell growth and increases cell death. In the first phase of LHS, cells enhance antioxidant systems to prevent the formation of an oxidizing environment. Under prolonged heat stress, the antioxidant systems, and particularly the enzymatic ones, are inactivated. As a consequence, an increase in H2 O2 , lipid peroxidation and protein oxidation occurs. This establishment of oxidative stress could be responsible for the increased cell death. The rescue of cell growth and cell viability, observed when TBY-2 cells were pretreated with galactone-γ-lactone, the last precursor of ascorbate, and glutathione before exposure to LHS, highlights the crucial role of antioxidants in the acquisition of basal thermotolerance.

Phenols and Antioxidant Activity in Vitro and in Vivo of Aqueous Extracts Obtained by Ultrasound-Assisted Extraction from Artichoke By-Products

Artichoke by-products are rich in phenolic compounds although they represent a waste for the food industry. This paper examines the application of ultrasound-assisted extraction (UAE) for obtaining organic solvent-free extracts rich in nutraceuticals from artichoke scraps. Application of ultrasounds for 60 minutes on test samples, using water as a solvent, improved recovery of phenolic substances compared with untreated samples. Among the phenols detected by high performance liquid chromatography, 5- O-caffeoylquinic and 1,5-di- O-caffeoylquinic acids were identified. In vivo treatments of tobacco BY-2 cells with ultrasonic extracts consistently enhanced their antioxidant power, making the cells more resistant to heat stress. UAE applied to artichoke by-products, using water as a solvent, appears to be a powerful eco-friendly technique that can provide extracts rich in nutraceuticals and turn waste products into resources. The extracts could be advantageously utilized in the food industry to produce functional foods.

Phenols and antioxidant activity in vitro and in vivo of aqueous extracts obtained by ultrasound-assisted extraction from artichoke by-products

Artichoke by-products are rich in phenolic compounds although they represent a waste for the food industry. This paper examines the application of ultrasound-assisted extraction (UAE) for obtaining organic solvent-free extracts rich in nutraceuticals from artichoke scraps. Application of ultrasounds for 60 minutes on test samples, using water as a solvent, improved recovery of phenolic substances compared with untreated samples. Among the phenols detected by high performance liquid chromatography, 5-O-caffeoylquinic and 1,5-di-O-caffeoylquinic acids were identified. In vivo treatments of tobacco BY-2 cells with ultrasonic extracts consistently enhanced their antioxidant power, making the cells more resistant to heat stress. UAE applied to artichoke by-products, using water as a solvent, appears to be a powerful eco-friendly technique that can provide extracts rich in nutraceuticals and turn waste products into resources. The extracts could be advantageously utilized in the food industry to produce functional foods.

S-nitrosylation of ascorbate peroxidase is part of programmed cell death signaling in tobacco Bright Yellow-2 cells

Nitric oxide (NO) is a small redox molecule that acts as a signal in different physiological and stress-related processes in plants. Recent evidence suggests that the biological activity of NO is also mediated by S-nitrosylation, a well-known redox-based posttranslational protein modification. Here, we show that during programmed cell death (PCD), induced by both heat shock (HS) or hydrogen peroxide (H2O2) in tobacco (Nicotiana tabacum) Bright Yellow-2 cells, an increase in S-nitrosylating agents occurred. NO increased in both experimentally induced PCDs, although with different intensities. In H2O2-treated cells, the increase in NO was lower than in cells exposed to HS. However, a simultaneous increase in S-nitrosoglutathione (GSNO), another NO source for S-nitrosylation, occurred in H2O2-treated cells, while a decrease in this metabolite was evident after HS. Consistently, different levels of activity and expression of GSNO reductase, the enzyme responsible for GSNO removal, were found in cells subjected to the two different PCD-inducing stimuli: low in H2O2-treated cells and high in the heat-shocked ones. Irrespective of the type of S-nitrosylating agent, S-nitrosylated proteins formed upon exposure to both of the PCD-inducing stimuli. Interestingly, cytosolic ascorbate peroxidase (cAPX), a key enzyme controlling H2O2 levels in plants, was found to be S-nitrosylated at the onset of both PCDs. In vivo and in vitro experiments showed that S-nitrosylation of cAPX was responsible for the rapid decrease in its activity. The possibility that S-nitrosylation induces cAPX ubiquitination and degradation and acts as part of the signaling pathway leading to PCD is discussed.

Effect of some light rare earth elements on seed germination, seedling growth and antioxidant metabolism in Triticum durum

Rare earth elements (REEs) enriched fertilizers have been commonly used in China since the 1980s, thus inducing a growing concern about their environmental impact in agriculture. In this work, the effect of some light REEs nitrate mixture and La(3+) nitrate on seed germination, seedling growth and antioxidant metabolism in Triticum durum was investigated with the aim of clarifying the potential benefits or damages of REEs on plants. Seed pre-soaking for 8 h with La(3+) and REEs nitrate inhibited seed germination at low concentrations (0.01 mM and 0.1 mM), while pre-soaking for 2 and 4 h already inhibited seed germination when higher concentrations (1 mM and 10 mM) of La(3+) and REEs nitrate were used. La(3+) and REEs nitrate treatment also affected seedling growth. Root growth was enhanced and inhibited at low and high concentrations, respectively. Shoot growth was inhibited by La(3+) and REEs nitrate at all tested concentrations after 12 d of treatments. Enzymatic and non enzymatic antioxidants were differently affected by La(3+) and REEs nitrate and their behaviour changed also depending on the plant organ. In roots La(3+) and REEs nitrate treatments induced an increase in ascorbate (ASC) and glutathione (GSH) contents. In shoots only La(3+) nitrate induced an increase in the ASC content whereas GSH decreased following both La(3+) and REEs nitrate treatments. An increase in ASC peroxidase activity was observed in shoots and roots, while catalase did not change in roots and slightly decreased in shoots. The possible role of the increase in some antioxidants as indicators of stress caused by lanthanide treatments is discussed.

Different involvement of the mitochondrial, plastidial and cytosolic ascorbate-glutathione redox enzymes in heat shock responses

Plant survival under heat stress requires the activation of proper defence mechanisms to avoid the impairment of metabolic functions. Heat stress leads to the overproduction of reactive oxygen species (ROS) in the cell. In plants, the ascorbate (ASC)-GSH cycle plays a pivotal role in controlling ROS levels and cellular redox homeostasis. Ascorbate peroxidase (APX) is the enzyme of this cycle mainly involved in ROS detoxification. In this study, the ASC-GSH cycle enzymes were analysed in the cytosol, mitochondria and plastids of tobacco Bright Yellow-2 cultured cells. The cells were also subjected to two different heat shocks (HSs; 35 or 55 degrees C for 10 min) and the cell compartments were isolated in both conditions. The results reported here indicate that moderate HS (35 degrees C) does not affect cell viability, whereas cell exposure to 55 degrees C HS induces programmed cell death (PCD). In relation to ASC-GSH cycle, the three analysed compartments have specific enzymatic profiles that are diversely altered by the HS treatments. The cytosol contains the highest activity of all ASC-GSH cycle enzymes and the data reported here suggest that it acts as a redox buffer for the whole cells. In particular, the cytosolic APX seems to be the most versatile enzyme, being its activity enhanced after moderate HS and reduced during PCD induction, whereas the other APX isoenzymes are only affected in the cells undergoing PCD. The relevance of the changes in the different ASC-GSH cycle isoenzymes in allowing cell survival or promoting PCD is discussed.

In the early phase of programmed cell death in Tobacco Bright Yellow 2 cells the mitochondrial adenine nucleotide translocator, adenylate kinase and nucleoside diphosphate kinase are impaired in a reactive oxygen species-dependent manner

To investigate whether and how mitochondria can change in plant programmed cell death (PCD), we used the non-photosynthetic Tobacco Bright Yellow 2 (TBY-2) cells. These can be synchronized to high levels, stand out in terms of growth rate and homogeneity and undergo PCD as a result of heat shock. Using these cells we investigated the activity of certain mitochondrial proteins that have a role in providing ATP and/or other nucleoside triphosphates (NTPs). We show that, already after 2 h from the heat shock, when cell viability remains unaffected, the rate of ADP/ATP exchange due to adenine nucleotide translocator (ANT) activity, and the rate of the reactions catalysed by adenylate kinase (ADK; EC 2.7.4.3) and nucleoside diphosphate kinase (NDPK; EC 2.7.4.6) are inhibited in a non-competitive-like manner. In all cases, externally added ascorbate partially prevented the inhibition. These effects occurred in spite of minor (for ANT) or no changes in the mitochondrial protein levels as immunologically investigated. Interestingly, a decrease of both the steady state level of the ascorbate pool and of the activity of l-galactono-gamma-lactone dehydrogenase (GLDH) (EC 1.3.2.3), the mitochondrial enzyme catalysing the last step of ascorbate biosynthesis, were also found.

Hydrogen peroxide, nitric oxide and cytosolic ascorbate peroxidase at the crossroad between defence and cell death

An increase in the production of reactive oxygen species (ROS) is a typical event occurring during different stress conditions and activating conflicting responses in plants. In order to investigate the relevance of different timing and amounts of ROS production, tobacco (Nicotiana tabacum) Bright Yellow-2 (TBY-2) cells were incubated with different amounts of glucose plus glucose oxidase, for generating H(2)O(2) during time, or directly with known amounts of H(2)O(2). Data presented here indicate that, in TBY-2 cells, a difference in H(2)O(2) level is a critical point for shifting metabolic responses towards strengthening of antioxidant defences, or their depletion with consequent cell death. Timing of ROS production is also critical because it can determine programmed cell death (PCD) or necrosis. Depending on the different kinds of activated cell death, ascorbate (ASC) and glutathione (GSH) pools are altered differently. Moreover, an H(2)O(2)-dependent activation of nitric oxide synthesis is triggered only in the conditions inducing PCD. Ascorbate peroxidase (APX) has been analysed under different conditions of H(2)O(2) generation. Under a threshold value of H(2)O(2) overproduction, a transient increase in APX occurs, whereas under conditions inducing cell necrosis, the activity of APX decreases in proportion to cell death without any evident alteration in APX gene expression. Under conditions triggering PCD, the suppression of APX involves both gene expression and alteration of the kinetic characteristics of the enzyme. The changes in ASC, GSH and APX are involved in the signalling pathway leading to PCD, probably contributing to guaranteeing the cellular redox conditions required for successful PCD.

Effects of storage temperature on viability, germination and antioxidant metabolism in Ginkgo biloba L. seeds

The behaviour of the Ginkgo biloba L. seeds was studied during storage at 4 and 25 degrees C. When stored at 25 degrees C, all the seeds died in 6 months. Cold temperatures preserved seed tissue viability for 1 year but did not preserve their capability to germinate, since such capability decreased after 6 months. A significant increase in lipid peroxidation occurred in the seed both in the embryo and in the endosperm. During storage a progressive deterioration of the endosperm tissues was evident. The two major water soluble antioxidants, ascorbate (ASC) and glutathione (GSH), showed different behaviour in the two conditions of storage and in the two main structures of the seed, the embryo and the endosperm. The ASC content of embryos and endosperms remained quite unchanged in the first 9 months at 4 degrees C, then increased. At 25 degrees C a significant decrease in the ASC content in the embryos was evident, whereas it remained more stable in the endosperm. The GSH pool decreased at both storage temperatures in the embryos. As far as the ASC-GSH redox enzymes are concerned, their activities decreased with storage, but changes appeared to be time-dependent more than temperature-dependent, with the exception of the endosperm ascorbate free radical (AFR) reductase (EC 1.6.5.4), the activity of which rapidly decreased at 25 degrees C. Therefore overall the antioxidant enzymes were scarcely regulated and unable to counteract oxidative stress occurring during the long-term storage.

Changes in the ascorbate metabolism of apoplastic and symplastic spaces are associated with cell differentiation

Ascorbate levels and redox state, as well as the activities of the ascorbate related enzymes, have been analysed both in the apoplastic and symplastic spaces of etiolated pea (Pisum sativum L.) shoots during cellular differentiation. The ascorbate pool and the ascorbate oxidizing enzymes, namely ascorbate oxidase and ascorbate peroxidase, were present in both pea apoplast and symplast, whereas ascorbate free radical reductase and dehydroascorbate reductase were only present in the symplastic fractions. During cell differentiation the ascorbate redox enzymes changed in different ways, since a decrease in ascorbate levels, ascorbate peroxidase and ascorbate free radical reductase occurred from meristematic to differentiated cells, whereas ascorbate oxidase and dehydroascorbate reductase increased. The activity of secretory peroxidases has also been followed in the apoplast of meristematic and differentiating cells. These peroxidases increased their activity during differentiation. This behaviour was accompanied by changes in their isoenzymatic profiles. The analysis of the kinetic characteristics of the different peroxidases present in the apoplast suggests that the presence of ascorbate and ascorbate peroxidase in the cell wall could play a critical role in regulating the wall stiffening process during cell differentiation by interfering with the activity of secretory peroxidases.

Comparative effects of various nitric oxide donors on ferritin regulation, programmed cell death, and cell redox state in plant cells

Past studies investigating the regulatory functions of nitric oxide (NO) in plant cells have utilized various NO-donors that release NO in different redox forms, which has lead to problems in the interpretation of data. In the present study, the effects of different NO-donors releasing NO with either NO+ (SNP) or NO' (SNAP, GSNO, NOC-18) character have been compared in plant cells. In particular, ferritin regulation, programmed cell death, cellular redox state, and ROS-scavenging enzymes in Arabidopsis thaliana and Nicotiana tabacum cells were examined. The results show that SNP behaves differently than the other NO-donors tested; indeed, SNP induces accumulation of ferritin transcripts in Arabidopsis, whereas SNAP inhibits its accumulation. Moreover, among the assortment of donors tested, only SNP caused programmed cell death and suppression of ROS-scavenging systems.

Ectopic expression of maize polyamine oxidase and pea copper amine oxidase in the cell wall of tobacco plants

To test the feasibility of altering polyamine levels by influencing their catabolic pathway, we obtained transgenic tobacco (Nicotiana tabacum) plants constitutively expressing either maize (Zea mays) polyamine oxidase (MPAO) or pea (Pisum sativum) copper amine oxidase (PCuAO), two extracellular and H(2)O(2)-producing enzymes. Despite the high expression levels of the transgenes in the extracellular space, the amount of free polyamines in the homozygous transgenic plants was similar to that in the wild-type ones, suggesting either a tight regulation of polyamine levels or a different compartmentalization of the two recombinant proteins and the bulk amount of endogenous polyamines. Furthermore, no change in lignification levels and plant morphology was observed in the transgenic plants compared to untransformed plants, while a small but significant change in reactive oxygen species-scavenging capacity was verified. Both the MPAO and the PCuAO tobacco transgenic plants produced high amounts of H(2)O(2) only in the presence of exogenously added enzyme substrates. These observations provided evidence for the limiting amount of freely available polyamines in the extracellular space in tobacco plants under physiological conditions, which was further confirmed for untransformed maize and pea plants. The amount of H(2)O(2) produced by exogenously added polyamines in cell suspensions from the MPAO transgenic plants was sufficient to induce programmed cell death, which was sensitive to catalase treatment and required gene expression and caspase-like activity. The MPAO and PCuAO transgenic plants represent excellent tools to study polyamine secretion and conjugation in the extracellular space, as well as to determine when and how polyamine catabolism actually intervenes both in cell wall development and in response to stress.

Production of reactive oxygen species, alteration of cytosolic ascorbate peroxidase, and impairment of mitochondrial metabolism are early events in heat shock-induced programmed cell death in tobacco Bright-Yellow 2 cells

To gain some insight into the mechanisms by which plant cells die as a result of abiotic stress, we exposed tobacco (Nicotiana tabacum) Bright-Yellow 2 cells to heat shock and investigated cell survival as a function of time after heat shock induction. Heat treatment at 55 degrees C triggered processes leading to programmed cell death (PCD) that was complete after 72 h. In the early phase, cells undergoing PCD showed an immediate burst in hydrogen peroxide (H2O2) and superoxide (O2*-) anion production. Consistently, death was prevented by the antioxidants ascorbate (ASC) and superoxide dismutase (SOD). Actinomycin D and cycloheximide, inhibitors of transcription and translation, respectively, also prevented cell death, but with a lower efficiency. Induction of PCD resulted in gradual oxidation of endogenous ASC; this was accompanied by a decrease in both the amount and the specific activity of the cytosolic ASC peroxidase (cAPX). A reduction in cAPX gene expression was also found in the late PCD phase. Moreover, changes of cAPX kinetic properties were found in PCD cells. Production of ROS in PCD cells was accompanied by early inhibition of glucose (Glc) oxidation, with a strong impairment of mitochondrial function as shown by an increase in cellular NAD(P)H fluorescence, and by failure of mitochondria isolated from cells undergoing PCD to generate membrane potential and to oxidize succinate in a manner controlled by ADP. Thus, we propose that in the early phase of tobacco Bright-Yellow 2 cell PCD, ROS production occurs, perhaps because of damage of the cell antioxidant system, with impairment of the mitochondrial oxidative phosphorylation.

Changes in the antioxidant systems as part of the signaling pathway responsible for the programmed cell death activated by nitric oxide and reactive oxygen species in tobacco Bright-Yellow 2 cells

Nitric oxide (NO) has been postulated to be required, together with reactive oxygen species (ROS), for the activation of the hypersensitive reaction, a defense response induced in the noncompatible plant-pathogen interaction. However, its involvement in activating programmed cell death (PCD) in plant cells has been questioned. In this paper, the involvement of the cellular antioxidant metabolism in the signal transduction triggered by these bioactive molecules has been investigated. NO and ROS levels were singularly or simultaneously increased in tobacco (Nicotiana tabacum cv Bright-Yellow 2) cells by the addition to the culture medium of NO and/or ROS generators. The individual increase in NO or ROS had different effects on the studied parameters than the simultaneous increase in the two reactive species. NO generation did not cause an increase in phenylalanine ammonia-lyase (PAL) activity or induction of cellular death. It only induced minor changes in ascorbate (ASC) and glutathione (GSH) metabolisms. An increase in ROS induced oxidative stress in the cells, causing an oxidation of the ASC and GSH redox pairs; however, it had no effect on PAL activity and did not induce cell death when it was generated at low concentrations. In contrast, the simultaneous increase of NO and ROS activated a process of death with the typical cytological and biochemical features of hypersensitive PCD and a remarkable rise in PAL activity. Under the simultaneous generation of NO and ROS, the cellular antioxidant capabilities were also suppressed. The involvement of ASC and GSH as part of the transduction pathway leading to PCD is discussed.