Influence of mitochondrial genome rearrangement on cucumber leaf carbon and nitrogen metabolism

The MSC16 cucumber (Cucumis sativus L.) mitochondrial mutant was used to study the effect of mitochondrial dysfunction and disturbed subcellular redox state on leaf day/night carbon and nitrogen metabolism. We have shown that the mitochondrial dysfunction in MSC16 plants had no effect on photosynthetic CO2 assimilation, but the concentration of soluble carbohydrates and starch was higher in leaves of MSC16 plants. Impaired mitochondrial respiratory chain activity was associated with the perturbation of mitochondrial TCA cycle manifested, e.g., by lowered decarboxylation rate. Mitochondrial dysfunction in MSC16 plants had different influence on leaf cell metabolism under dark or light conditions. In the dark, when the main mitochondrial function is the energy production, the altered activity of TCA cycle in mutated plants was connected with the accumulation of pyruvate and TCA cycle intermediates (citrate and 2-OG). In the light, when TCA activity is needed for synthesis of carbon skeletons required as the acceptors for NH4+ assimilation, the concentration of pyruvate and TCA intermediates was tightly coupled with nitrate metabolism. Enhanced incorporation of ammonium group into amino acids structures in mutated plants has resulted in decreased concentration of organic acids and accumulation of Glu

Extra-Cellular But Extra-Ordinarily Important for Cells: Apoplastic Reactive Oxygen Species Metabolism

Reactive oxygen species (ROS), by their very nature, are highly reactive, and it is no surprise that they can cause damage to organic molecules. In cells, ROS are produced as byproducts of many metabolic reactions, but plants are prepared for this ROS output. Even though extracellular ROS generation constitutes only a minor part of a cell’s total ROS level, this fraction is of extraordinary importance. In an active apoplastic ROS burst, it is mainly the respiratory burst oxidases and peroxidases that are engaged, and defects of these enzymes can affect plant development and stress responses. It must be highlighted that there are also other less well-known enzymatic or non-enzymatic ROS sources. There is a need for ROS detoxification in the apoplast, and almost all cellular antioxidants are present in this space, but the activity of antioxidant enzymes and the concentration of low-mass antioxidants is very low. The low antioxidant efficiency in the apoplast allows ROS to accumulate easily, which is a condition for ROS signaling. Therefore, the apoplastic ROS/antioxidant homeostasis is actively engaged in the reception and reaction to many biotic and abiotic stresses

Plant mitochondrial respiratory chain

Roślinny mitochondrialny łańcuch oddechowy, oprócz dużych kompleksów białkowych (Kompleksów I-IV) zawiera dodatkowe elementy: wewnętrzne i zewnętrzne dehydrogenazy typu II (NDin/NDex) utleniające NAD(P)H pochodzące odpowiednio z macierzy mitochondrialnej lub cytozolu oraz dodatkową terminalną oksydazę nazwaną oksydazę alternatywną (AOX). Udział szlaków alternatywnych w oddychaniu musi być ściśle regulowany ponieważ aktywność tych enzymów nie jest związana z translokacją protonów w poprzek błony i w konsekwencji nie prowadzi do produkcji ATP. Aktywność ta pozwala natomiast rozproszyć nadmiar siły redukcyjnej, co jest szczególnie ważne w warunkach stresowych. W tym artykule przeglądowym omówiono budowę szlaków alternatywnych roślinnego mtETC, regulację ich aktywności na różnych poziomach oraz opisano role jakie pełnią NDin/ex i AOX w metabolizmie komórek roślinnych.Plant mitochondrial electron transport chain (mtETC), besides large complexes (Complex I-IV), consists of additional elements: internal and external type II NAD(P)H dehydrogenases (NDin/NDex) and an additional terminal oxidase, named alternative oxidase (AOX). The engagement of alternative pathways in respiration must be tightly regulated since their activity is not linked to pumping protons across membrane and, as a consequence, is not associated with ATP synthesis. The activity of plant-specific components in mtETC allows to dissipate the excess of reducing power and may be especially important under stress conditions. In this review the structure, the regulation of activities and the role of NDin/NDex and AOX in metabolism of plant cells is described

Respiratory Burst Oxidase Homolog D as a Modulating Component of Oxidative Response under Ammonium Toxicity

Delayed growth, a visible phenotypic component of the so-called ammonium syndrome, occurs when ammonium is the sole inorganic nitrogen source. Previously, we have shown that modification of apoplastic reactive oxygen species (apROS) metabolism is a key factor contributing to plant growth retardation under ammonium nutrition. Here, we further analyzed the changes in apROS metabolism in transgenic plants with disruption of the D isoform of the respiratory burst oxidase homolog (RBOH) that is responsible for apROS production. Ammonium-grown Arabidopsisrbohd plants are characterized by up to 50% lower contents of apoplastic superoxide and hydrogen peroxide. apROS sensing markers such as OZF1 and AIR12 were downregulated, and the ROS-responsive signaling pathway, including MPK3, was also downregulated in rbohd plants cultivated using ammonium as the sole nitrogen source. Additionally, the expression of the cell-wall-integrity marker FER and peroxidases 33 and 34 was decreased. These modifications may contribute to phenomenon wherein ammonium inhibited the growth of transgenic plants to a greater extent than that of wild-type plants. Overall, this study indicated that due to disruption of apROS metabolism, rbohd plants cannot adjust to ammonium toxicity and are more sensitive to these conditions

Respiratory burst oxidases and apoplastic peroxidases facilitate ammonium syndrome development in Arabidopsis

Ammonium-nitrogen (NH4+) nutrition is linked to metabolic over-reduction for plants. The characteristic symptom of sole NH4+ nutrition is growth suppression, signifying this condition as the ammonium syndrome. In the present study, we investigated the mechanism of perception of high NH4+ conditions in Arabidopsis thaliana plants by examining apoplastic reactive oxygen species (ROS) metabolism. Major enzyme activity and a special pattern of expression of NADPH-dependent respiratory burst oxidases (RBOH) was found in Arabidopsis individuals cultured under NH4+ as the sole nitrogen source. This oxidative burst is independent of RBOHD/F expression and does not activate typical intracellular signalling pathways. In addition, elevated superoxide dismutase and apoplastic secretory peroxidase activities contributed to hydrogen peroxide (H2O2) accumulation in plants exposed to NH4+ nutrition. Consequently, higher H2O2 contents were determined in the extracellular space and were localised cytochemically. H2O2 is a substrate for cell wall cross-linking peroxidases, which showed enhanced activity in the presence of NH4+. Increase of cell wall polymerisation, could in turn inhibit cell elongation and slow down growth, as observed under NH4+ toxicity

Glyoxalase I activity affects Arabidopsis sensitivity to ammonium nutrition

Key message: Elevated methylglyoxal levels contribute to ammonium-induced growth disorders in Arabidopsis thaliana. Methylglyoxal detoxification pathway limitation, mainly the glyoxalase I activity, leads to enhanced sensitivity of plants to ammonium nutrition. Abstract: Ammonium applied to plants as the exclusive source of nitrogen often triggers multiple phenotypic effects, with severe growth inhibition being the most prominent symptom. Glycolytic flux increase, leading to overproduction of its toxic by-product methylglyoxal (MG), is one of the major metabolic consequences of long-term ammonium nutrition. This study aimed to evaluate the influence of MG metabolism on ammonium-dependent growth restriction in Arabidopsis thaliana plants. As the level of MG in plant cells is maintained by the glyoxalase (GLX) system, we analyzed MG-related metabolism in plants with a dysfunctional glyoxalase pathway. We report that MG detoxification, based on glutathione-dependent glyoxalases, is crucial for plants exposed to ammonium nutrition, and its essential role in ammonium sensitivity relays on glyoxalase I (GLXI) activity. Our results indicated that the accumulation of MG-derived advanced glycation end products significantly contributes to the incidence of ammonium toxicity symptoms. Using A. thaliana frostbite1 as a model plant that overcomes growth repression on ammonium, we have shown that its resistance to enhanced MG levels is based on increased GLXI activity and tolerance to elevated MG-derived advanced glycation end-product (MAGE) levels. Furthermore, our results show that glyoxalase pathway activity strongly affects cellular antioxidative systems. Under stress conditions, the disruption of the MG detoxification pathway limits the functioning of antioxidant defense. However, under optimal growth conditions, a defect in the MG detoxification route results in the activation of antioxidative systems

Mitochondrial NAD(P)H oxidation pathways and nitrate/ammonium redox balancing in plants

Plant mitochondrial oxidative phosphorylation is characterised by alternative electron transport pathways with different energetic efficiencies, allowing turnover of cellular redox compounds like NAD(P)H. These electron transport chain pathways are profoundly affected by soil nitrogen availability, most commonly as oxidized nitrate (NO3 −) and/or reduced ammonium (NH4 +). The bioenergetic strategies involved in assimilating different N sources can alter redox homeostasis and antioxidant systems in different cellular compartments, including the mitochondria and the cell wall. Conversely, changes in mitochondrial redox systems can affect plant responses to N. This review explores the integration between N assimilation, mitochondrial redox metabolism, and apoplast metabolism

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Altered Cell Wall Plasticity Can Restrict Plant Growth under Ammonium Nutrition

Plants mainly utilize inorganic forms of nitrogen (N), such as nitrate (NO3–) and ammonium (NH4+). However, the composition of the N source is important, because excess of NH4+ promotes morphological disorders. Plants cultured on NH4+ as the sole N source exhibit serious growth inhibition, commonly referred to as “ammonium toxicity syndrome.” NH4+-mediated suppression of growth may be attributable to both repression of cell elongation and reduction of cell division. The precondition for cell enlargement is the expansion of the cell wall, which requires the loosening of the cell wall polymers. Therefore, to understand how NH4+ nutrition may trigger growth retardation in plants, properties of their cell walls were analyzed. We found that Arabidopsis thaliana using NH4+ as the sole N source has smaller cells with relatively thicker cell walls. Moreover, cellulose, which is the main load-bearing polysaccharide revealed a denser assembly of microfibrils. Consequently, the leaf blade tissue showed elevated tensile strength and indicated higher cell wall stiffness. These changes might be related to changes in polysaccharide and ion content of cell walls. Further, NH4+ toxicity was associated with altered activities of cell wall modifying proteins. The lower activity and/or expression of pectin hydrolyzing enzymes and expansins might limit cell wall expansion. Additionally, the higher activity of cell wall peroxidases can lead to higher cross-linking of cell wall polymers. Overall, the NH4+-mediated inhibition of growth is related to a more rigid cell wall structure, which limits expansion of cells. The changes in cell wall composition were also indicated by decreased expression of Feronia, a receptor-like kinase involved in the control of cell wall extension

Suppression of external NADPH dehydrogenase—NDB1 in arabidopsis thaliana confers improved tolerance to ammonium toxicity via efficient Glutathione/Redox metabolism

Environmental stresses, including ammonium (NH4 +) nourishment, can damage key mitochondrial components through the production of surplus reactive oxygen species (ROS) in the mitochondrial electron transport chain. However, alternative electron pathways are significant for efficient reductant dissipation in mitochondria during ammonium nutrition. The aim of this study was to define the role of external NADPH-dehydrogenase (NDB1) during oxidative metabolism of NH4 +-fed plants. Most plant species grown with NH4 + as the sole nitrogen source experience a condition known as “ammonium toxicity syndrome”. Surprisingly, transgenic Arabidopsis thaliana plants suppressing NDB1 were more resistant to NH4 + treatment. The NDB1 knock-down line was characterized by milder oxidative stress symptoms in plant tissues when supplied with NH4 +. Mitochondrial ROS accumulation, in particular, was attenuated in the NDB1 knock-down plants during NH4 + treatment. Enhanced antioxidant defense, primarily concerning the glutathione pool, may prevent ROS accumulation in NH4 +-grown NDB1-suppressing plants. We found that induction of glutathione peroxidase-like enzymes and peroxiredoxins in the NDB1-surpressing line contributed to lower ammonium-toxicity stress. The major conclusion of this study was that NDB1 suppression in plants confers tolerance to changes in redox homeostasis that occur in response to prolonged ammonium nutrition, causing cross tolerance among plants