Prolonged root hypoxia induces ammonium accumulation and decreases the nutritional quality of tomato fruits

Silicon nanoparticles confer hypoxia tolerance in citrus rootstocks by modulating antioxidant activities and carbohydrate metabolism

Citrus is a remarkable fruit crop, extremely sensitive to flooding conditions, which frequently trigger hypoxia stress and cause severe damage to citrus plants. Silicon nanoparticles (SiNPs) are beneficial and have the potential to overcome this problem. Therefore, the present study aimed to investigate the effect of silicon nanoparticles to overcome hypoxia stress through modulating antioxidant enzyme activity and carbohydrate metabolism. Three citrus rootstocks (Carrizo citrange, Roubidoux, and Rich 16-6) were exposed to flooding (with and without oxygen) through different SiNP treatments via foliar and root zone. SiNPs applied treatment plants showed a significant increase in photosynthesis, leaf greenness, antioxidant enzymes, and carbohydrate metabolic activities, besides the higher accumulation of proline and glycine betaine. The rate of lipid peroxidation was drastically higher in flooded plants; however, SiNPs application reduced it significantly, ultimately reducing oxidative damage. Overall, Rich16-6 rootstock showed good performance via root zone application compared to other rootstocks, possibly due to genotypical variation in silicon uptake. Our outcomes demonstrate that SiNPs significantly affect plant growth during hypoxia stress conditions, and their use is an optimal strategy to overcome this issue. This study laid the foundation for future research to use at the commercial level to overcome hypoxia stress and a potential platform for future research.

Micro-nano oxygenated irrigation improves the yield and quality of greenhouse cucumbers under-film drip irrigation

To study the influence mechanism of micro-nano oxygenated irrigation (MNOI) on greenhouse fruit cucumber in arid and semi-arid cold regions, the yield and quality of greenhouse fruit cucumber were evaluated and verified based on 2 years of observation data. Taking fruit cucumber in Ningxia solar greenhouse as the research object, three dissolved oxygen (DO) levels of MNOI (DO; 6, 7.5, and 9 mg L-1, O1, O2, and O3, respectively) and non-oxygenated irrigation (CK, 4 mg L-1) were set up as the control treatment. Through comparative design, the influence mechanism of different levels of aerobic irrigation on the yield and quality of greenhouse fruit cucumber was studied. The main indicators of fruit cucumber yield and quality increased with dissolved oxygen in irrigation water from 4 to 9 mg L-1. In spring-summer (autumn-winter), compared with CK, the leaf area index (LAI) and net photosynthetic rate (A) increased by 28.83% (28.77%) and 44.90% (35.00%), respectively, and Vitamin C, soluble protein, soluble sugar, soluble solids and total acid content increased by 100.00% (51.88%), 37.78% (61.11%), 34.17% (54.17%), 37.07% (78.72%) and 26.92% (30.67%) respectively, while nitrate content decreased by 44.88% (51.15%), and dry matter accumulation (DMA), soil respiration rate (SRR), microbial carbon (MC), and microbial nitrogen (MN) increased by 49.81% (127.25%), 55.22% (110.34%), 117.50% (90.91%) and 70.37% (74.42%) respectively, and yield, irrigation water use efficiency (IWUE) and soil oxygen content (SO) increased by 22.47% (28.04%), 22.39% (28.05%) and 33.21% (35.33%) respectively. A model of DO in irrigation water and SO was established and the applicability of the model was verified with an average relative error of 2% (less than 5%). MNOI increased SO and soil enzyme activity, enriched soil microorganisms, improved soil microenvironment, promoted water nutrient uptake and growth of root system, increased chlorophyll, photosynthesis and DMA, which improved fruit cucumber yield and quality, and the better DO concentration in irrigation water is 9 mg L-1. The research results provide theoretical support for regulating soil water, fertilizer and air environment, and at the same time, provide feasible ways to improve the quality and efficiency of crops in arid and semi-arid cold regions.

Key molecular events involved in root exudates-mediated replanted disease of Rehmannia glutinosa

The perennial herbaceous plant, Rehmannia glutinosa Libosch, is one of traditional Chinese medicines with a long history of cultivation. However, replanted disease severely affects its yield and quality in production. In this study, a specific culture device was designed to accurately isolate the root exudates of R. glutinosa. In addition, the formation mechanism of replanted diseases mediated by root exudates was deeply studied in R. glutinosa. The results indicated that root exudates have obvious allelopathic activity, furthermore, metagenomics analysis found that the exudates were found to significantly induce the proliferation of harmful pathogenic fungal and the reduction of probiotics in rhizosphere of R. glutinosa. Further analysis found that, 8,758 genes were differentially expressed in root exudate-treated R. glutinosa plants. These genes mainly involved in critical cellular processes including immune response, hormone metabolism, signaling transduction and cell membrane transport. Of which, numerous genes were found to involve in immune response, such as PR (Pathogenesis-related protein), were highly expressed in root exudate-treated plants. Transiently overexpression experiments found that a PR1 could enhance the resistance of R. glutinosa to root exudates treatment. These results indicated that the interaction between root exudates and microbes altered the expression pattern of the genes related to immune pathway and signaling transduction mediated by it. These disordered genes finally severely affected the growth and development of R. glutinosa, and eventually formed the replanted disease. This study provides a novel approach to collect root exudates and a new data basis for revealing the molecular events occurring in replanted plants.

Gluconeogenesis in Plants: A Key Interface between Organic Acid/Amino Acid/Lipid and Sugar Metabolism

Gluconeogenesis is a key interface between organic acid/amino acid/lipid and sugar metabolism. The aims of this article are four-fold. First, to provide a concise overview of plant gluconeogenesis. Second, to emphasise the widespread occurrence of gluconeogenesis and its utilisation in diverse processes. Third, to stress the importance of the vacuolar storage and release of Krebs cycle acids/nitrogenous compounds, and of the role of gluconeogenesis and malic enzyme in this process. Fourth, to outline the contribution of fine control of enzyme activity to the coordinate-regulation of gluconeogenesis and malate metabolism, and the importance of cytosolic pH in this.

DNRA: A short-circuit in biological N-cycling to conserve nitrogen in terrestrial ecosystems

This paper reviews dissimilatory nitrate reduction to ammonium (DNRA) in soils - a newly appreciated pathway of nitrogen (N) cycling in the terrestrial ecosystems. The reduction of NO₃^- occurs in two steps; in the first step, NO₃^- is reduced to NO₂^-; and in the second, unlike denitrification, NO₂^- is reduced to NH₄⁺ without intermediates. There are two sets of NO₃^-/NO₂^- reductase enzymes, i.e., Nap/Nrf and Nar/Nir; the former occurs on the periplasmic-membrane and energy conservation is respiratory via electron-transport-chain, whereas the latter is cytoplasmic and energy conservation is both respiratory and fermentative (Nir, substrate-phosphorylation). Since, Nir catalyzes both assimilatory- and dissimilatory-nitrate reduction, the nrfA gene, which transcribes the NrfA protein, is treated as a molecular-marker of DNRA; and a high nrfA/nosZ (N₂O-reductase) ratio favours DNRA. Recently, several crystal structures of NrfA have been presumed to producee N₂O as a byproduct of DNRA via the NO (nitric-oxide) pathway. Meta-analyses of about 200 publications have revealed that DNRA is regulated by oxidation state of soils and sediments, carbon (C)/N and NO₂^-/NO₃^- ratio, and concentrations of ferrous iron (Fe²⁺) and sulfide (S^2-). Under low-redox conditions, a high C/NO₃^- ratio selects for DNRA while a low ratio selects for denitrification. When the proportion of both C and NO₃^- are equal, the NO₂^-/NO₃^- ratio modulates partitioning of NO₃^-, and a high NO₂^-/NO₃^- ratio favours DNRA. A high S^2-/NO₃^- ratio also promotes DNRA in coastal-ecosystems and saline sediments. Soil pH, temperature, and fine soil particles are other factors known to influence DNRA. Since, DNRA reduces NO₃^- to NH₄⁺, it is essential for protecting NO₃^- from leaching and gaseous (N₂O) losses and enriches soils with readily available NH₄⁺-N to primary producers and heterotrophic microorganisms. Therefore, DNRA may be treated as a tool to reduce ground-water NO₃^- pollution, enhance soil health and improve environmental quality.

Characteristics of the root exudate release system of typical plants in plateau lakeside wetland under phosphorus stress conditions

In this study, the root exudates of wetland plants, Pistia stratiotes, black algae, and Cyperus alternifolius, exposed to six phosphorus concentration gradients (0, 0.2, 1, 5, 10, and 20 mg/L) were characterized. The experimental seedlings were cultivated in Hoagland solutions, which were then extracted, decompressed, and concentrated with CH2Cl2; subsequently, a gas chromatography-mass spectrometry (GC-MS) analysis was performed to study the root exudates effects under different phosphorus concentrations. Results showed the existence of several organic compounds, such as alkanes, esters, alcohols, amines, benzene, and acids (phthalic acid, cycloheptasiloxane, benzoic acid, and cyclopentasiloxane) in the root exudates of the wetland plants. The relative contents of phthalate, benzene dicarboxylic acid, and cyclohexasiloxane in the root exudates first increased, and then decreased, with the change in phosphorus concentration. The relative contents of three compounds in Pistia were the highest at 1 mg/L of phosphorus, and the lowest relative contents of phthalic acid and benzene dicarboxylic acid were observed at 20 mg/L of phosphorus. However, the relative content of cyclohexasiloxane was the lowest in the absence of P stress. In black algae, the relative contents of the three compounds were 36.66, 16.24, and 14.61%, respectively. The relative content of cyclohexasiloxane in the black algae first decreased and then increased, with its lowest relative content occurring at 5 mg/L of phosphorus and the highest at 10 mg/L of phosphorus. In Cyperus alternifolius, the highest relative concentrations of the four compounds: phthalic acid, dimethyl phthalate, octadecane, and diphenyl sulfone in Cyperus were observed at 5 mg/L phosphorus and the lowest at 10 mg/L phosphorus.

Aerated Irrigation of Different Irrigation Levels and Subsurface Dripper Depths Affects Fruit Yield, Quality and Water Use Efficiency of Greenhouse Tomato

Aerated irrigation (AI) is a method to mitigate rhizosphere hypoxia caused by the wetting front from subsurface drip irrigation (SDI). This study evaluated the impacts of AI on soil aeration, plant growth performance, fruit yield (tomato), irrigation water use efficiency (IWUE), fruit nutrition (lycopene and Vitamin C (VC)) and taste (soluble sugar, organic acid and sugar–acid ratio) quality. A three-factorial experiment including AI and SDI at three irrigation levels (W0.6, W0.8 and W1.0, corresponding with crop-pan coefficients of 0.6, 0.8 and 1.0) and two dripper depths (D15 and D25, burial at 15 and 25 cm, respectively), totaling 12 treatments overall, was conducted in a greenhouse during the tomato-growing season (April–July) in 2016. The AI improved soil aeration conditions, with significantly increased soil oxygen concentration and air-filled porosity relative to SDI. Moreover, the AI improved crop growth performance, with increased root morphology (diameter, length density, surface area and volume density), delayed flowering time, prolonged flowering duration and increased shoot (leaf, stem and fruit) dry weight, and harvest index. Fruit yield per plant, fruit weight, IWUE, the contents of lycopene, VC and soluble sugar, and sugar–acid ratio significantly increased under AI treatments (P < 0.05). As the irrigation level increased, fruit yield, number, and weight increased (P < 0.05), but IWUE and fruit lycopene, soluble sugar, and organic acid content decreased (P < 0.05). The dripper depth had no significant impact on fruit yield, nutrition and taste quality. Principal component analysis revealed that the optimal three treatments in terms of fruit yield, IWUE, and nutrition and taste quality were the treatments W0.6D25AI, W1.0D25AI and W1.0D15AI. These results suggest that AI can improve tomato growth performance and increase fruit yield, nutrition and taste quality, and IWUE through enhancing soil aeration conditions.

Effect of soil aeration on root morphology and photosynthetic characteristics of potted tomato plants (Solanum lycopersicum) at different NaCl salinity levels

BACKGROUND

Salt stress is one of the environmental factors that greatly limits crop production worldwide because high salt concentrations in the soil affect morphological responses and physiological and metabolic processes, including root morphology and photosynthetic characteristics. Soil aeration has been reported to accelerate the growth of plants and increase crop yield. The objective of this study was to examine the effects of 3 NaCl salinity levels (28, 74 and 120 mM) and 3 aeration volume levels (2.3, 4.6 and 7.0 L/pot) versus non-aeration and salinity treatments on the root morphology, photosynthetic characteristics and chlorophyll content of potted tomato plants.

RESULTS

The results showed that both aeration volume and salinity level affected the root parameters, photosynthetic characteristics and chlorophyll content of potted tomato plants. The total length, surface area and volume of roots increased with the increase in aeration volume under each NaCl stress level. The effect was more marked in the fine roots (especially in ≤1 mm diameter roots). Under each NaCl stress level, the photosynthetic rate and chlorophyll content of tomato significantly increased in response to the aeration treatments. The net photosynthetic rate and chlorophyll a and t content increased by 39.6, 26.9, and 17.9%, respectively, at 7.0 L/pot aeration volume compared with no aeration in the 28 mM NaCl treatment. We also found that aeration could reduce the death rate of potted tomato plants under high salinity stress conditions (120 mM NaCl).

CONCLUSIONS

The results suggest that the negative effect of NaCl stress can be offset by soil aeration. Soil aeration can promote root growth and increase the photosynthetic rate and chlorophyll content, thus promoting plant growth and reducing the plant death rate under NaCl stress conditions.

The role of the NADH-dependent nitrite reductase, Nir, from Escherichia coli in fermentative ammonification

Nitrate and nitrite reduction are of paramount importance for nitrogen assimilation and anaerobic metabolism, and understanding the specific roles of each participating reductase is necessary to describe the biochemical balance that dictates cellular responses to their environments. The soluble, cytoplasmic siroheme NADH-nitrite reductase (Nir) in Escherichia coli is necessary for nitrate/nitrite assimilation but has also been reported to either "detoxify" nitrite, or to carry out fermentative ammonification in support of anaerobic catabolism. Theoretically, nitrite detoxification would be important for anaerobic growth on nitrate, during which excess nitrite would be reduced to ammonium. Fermentative ammonification by Nir would be important for maximization of non-respiratory ATP production during anaerobic growth in the presence of nitrite. Experiments reported here were designed to test the potential role of Nir in fermentative ammonification directly by growing E. coli along with mutant strains lacking Nir or the respiratory nitrite reductase (Nrf) under anaerobic conditions in defined media while monitoring nitrogen utilization and fermentation metabolites. To focus on the role of Nir in fermentative ammonification, pH control was used in most experiments to eliminate nitrite toxicity due to nitric acid formation. Our results demonstrate that Nir confers a significant benefit during fermentative growth that reflects fermentative ammonification rather than detoxification. We conclude that fermentative ammonification by Nir allows for the energetically favorable fermentation of glucose to formate and acetate. These results and conclusions are discussed in light of the roles of Nir in other bacteria and in plants.

Putting primary metabolism into perspective to obtain better fruits

Background

One of the key goals of fruit biology is to understand the factors that influence fruit growth and quality, ultimately with a view to manipulating them for improvement of fruit traits.

Scope

Primary metabolism, which is not only essential for growth but is also a major component of fruit quality, is an obvious target for improvement. However, metabolism is a moving target that undergoes marked changes throughout fruit growth and ripening.

Conclusions

Agricultural practice and breeding have successfully improved fruit metabolic traits, but both face the complexity of the interplay between development, metabolism and the environment. Thus, more fundamental knowledge is needed to identify further strategies for the manipulation of fruit metabolism. Nearly two decades of post-genomics approaches involving transcriptomics, proteomics and/or metabolomics have generated a lot of information about the behaviour of fruit metabolic networks. Today, the emergence of modelling tools is providing the opportunity to turn this information into a mechanistic understanding of fruits, and ultimately to design better fruits. Since high-quality data are a key requirement in modelling, a range of must-have parameters and variables is proposed.

Effects of flooding stress in ‘Micro-Tom’ tomato plants transformed with different levels of mitochondrial sHSP23.6

Abstract Soil flooding is an environmental stressor for crops that can affect physiological performance and reduce crop yields. Abiotic stressors cause changes in protein synthesis, modifying the levels of a series of proteins, especially the heat shock proteins (HSP), and these proteins can help protect the plants against abiotic stress. The objective of this study was to verify if tomato plants cv. Micro-Tom from different genotypes with varying expression levels of MT-sHSP23.6 (mitochondrial small heat shock proteins) have different responses physiological to flooding. Plants from three genotypes (untransformed, MT-sHSP23.6 sense expression levels and MT-sHSP23.6 antisense expression levels) were cultivated under controlled conditions. After 50 days, the plants were flooded for 14 days. After this period half of the plants from each genotype were allowed to recover. Chlorophyll fluorescence, gas exchange, chlorophyll index, leaf area and dry matter were evaluated. Flood stress affected the photosynthetic electron transport chain, which is related to inactivation of the oxygen-evolving complex, loss of connectivity among units in photosystem II, oxidation-reduction of the plastoquinone pool and activity of photosystem I. The genotype with MT-sHSP23.6 sense expression levels was less sensitive to stress from flooding.

Yields and Nutritional of Greenhouse Tomato in Response to Different Soil Aeration Volume at two depths of Subsurface drip irrigation

This study investigated the effects of 4 aeration levels (varied by injection of air to the soil through subsurface irrigation lines) at two subsurface irrigation line depths (15 and 40 cm) on plant growth, yield and nutritional quality of greenhouse tomato. In all experiments, fruit number, width and length, yield, vitamin C, lycopene and sugar/acid ratio of tomato markedly increased in response to the aeration treatments. Vitamin C, lycopene, and sugar/acid ratio increased by 41%, 2%, and 43%, respectively, in the 1.5 times standard aeration volume compared with the no-aeration treatment. An interaction between aeration level and depth of irrigation line was also observed with yield, fruit number, fruit length, vitamin C and sugar/acid ratio of greenhouse tomato increasing at each aeration level when irrigation lines were placed at 40 cm depth. However, when the irrigation lines were 15 cm deep, the trend of total fruit yields, fruit width, fruit length and sugar/acid ratio first increased and then decreased with increasing aeration level. Total soluble solids and titrable acid decreased with increasing aeration level both at 15 and 40 cm irrigation line placement. When all of the quality factors, yields and economic benefit are considered together, the combination of 40 cm line depth and "standard" aeration level was the optimum combination.

Prolonged root hypoxia effects on ethylene biosynthesis and perception in tomato fruit

The effects of root hypoxia on ethylene biosynthesis and perception have been documented in many vegetative organs, but not extensively in fruit. Therefore, in the present study, the effects of root hypoxia on ethylene biosynthesis and perception were investigated in tomato (Solanum lycopersicum L.) fruit at five stages of the maturation phase. Our results showed that root hypoxia do not affect ethylene biosynthesis in fruit, but stimulates its reception from other plant part, as indicated by the increase in the expression of ethylene receptors ETR 1 and 3.

Prolonged root hypoxia effects on enzymes involved in nitrogen assimilation pathway in tomato plants

In order to investigate the effects of root hypoxia (1-2 % oxygen) on the nitrogen (N) metabolism of tomato plants (Solanum lycopersicum L. cv. Micro-Tom), a range of N compounds and N-assimilating enzymes were performed on roots and leaves of plants submitted to root hypoxia at the second leaf stage for three weeks. Obtained results showed that root hypoxia led to a significant decrease in dry weight (DW) production and nitrate content in roots and leaves. Conversely, shoot to root DW ratio and nitrite content were significantly increased. Contrary to that in leaves, glutamine synthetase activity was significantly enhanced in roots. The activities of nitrate and nitrite reductase were enhanced in roots as well as leaves. The higher increase in the NH(4)(+) content and in the protease activities in roots and leaves of hypoxically treated plants coincide with a greater decrease in soluble protein contents. Taken together, these results suggest that root hypoxia leaded to higher protein degradation. The hypoxia-induced increase in the aminating glutamate dehydrogenase activity may be considered as an alternative N assimilation pathway involved in detoxifying the NH(4)(+), accumulated under hypoxic conditions. With respect to hypoxic stress, the distinct sensitivity of the enzymes involved in N assimilation is discussed.

Upregulation of pyrophosphate: fructose 6-phosphate 1-phosphotransferase (PFP) activity in strawberry

Pyrophosphate: fructose 6-phosphate 1-phosphotransferase (PFP) is a cytosolic enzyme catalyzing the first committed step in glycolysis by reversibly phosphorylating fructose-6-phosphate to fructose-1,6-bisphosphate. The position of PFP in glycolytic and gluconeogenic metabolism, as well as activity patterns in ripening strawberry, suggest that the enzyme may influence carbohydrate allocation to sugars and organic acids. Fructose-2,6-bisphosphate activates and tightly regulates PFP activity in plants and has hampered attempts to increase PFP activity through overexpression. Heterologous expression of a homodimeric isoform from Giardia lamblia, not regulated by fructose-2,6-bisphosphate, was therefore employed to ensure in vivo increases in PFP activity. The coding sequence was placed into a constitutive expression cassette under control of the cauliflower mosaic virus 35S promoter and introduced into strawberry by Agrobacterium tumefaciens-mediated transformation. Heterologous expression of PFP resulted in an up to eightfold increase in total activity in ripe berries collected over two consecutive growing seasons. Total sugar and organic acid content of transgenic berries harvested during the first season were not affected when compared to the wild type, however, fructose content increased at the expense of sucrose. In the second season, total sugar content and composition remained unchanged while the citrate content increased slightly. Considering that PFP catalyses a reversible reaction, PFP activity appears to shift between gluconeogenic and glycolytic metabolism, depending on the metabolic status of the cell.

Global gene expression responses to waterlogging in roots and leaves of cotton (Gossypium hirsutum L.)

Waterlogging stress causes yield reduction in cotton (Gossypium hirsutum L.). A major component of waterlogging stress is the lack of oxygen available to submerged tissues. While changes in expressed protein, gene transcription and metabolite levels have been studied in response to low oxygen stress, little research has been done on molecular responses to waterlogging in cotton. We assessed cotton growth responses to waterlogging and assayed global gene transcription responses in root and leaf cotton tissues of partially submerged plants. Waterlogging caused significant reductions in stem elongation, shoot mass, root mass and leaf number, and altered the expression of 1,012 genes (4% of genes assayed) in root tissue as early as 4 h after flooding. Many of these genes were associated with cell wall modification and growth pathways, glycolysis, fermentation, mitochondrial electron transport and nitrogen metabolism. Waterlogging of plant roots also altered global gene expression in leaf tissues, significantly changing the expression of 1,305 genes (5% of genes assayed) after 24 h of flooding. Genes affected were associated with cell wall growth and modification, tetrapyrrole synthesis, hormone response, starch metabolism and nitrogen metabolism The implications of these results for the development of waterlogging-tolerant cotton are discussed.