Anaerobic Metabolism in the N-Limited Green Alga Selenastrum minutum: II. Assimilation of Ammonium by Anaerobic Cells

Nitric Oxide Turnover Under Hypoxia Results in the Rapid Increased Expression of the Plastid-Localized Phosphorylated Pathway of Serine Biosynthesis

The plant mitochondrial electron transport chain influences carbon and nitrogen metabolism under near anoxic conditions through its involvement in the phytoglobin-nitric oxide cycle, where the respiratory chain reduces nitrite to nitric oxide (NO), followed by NO conversion to nitrate by class 1 phytoglobin. Wild type (WT) and transgenic tobacco (Nicotiana tabacum L.) with differing amounts of alternative oxidase (AOX) were used to manipulate NO generation under hypoxia, and to examine whether this in turn influenced the gene expression of two stress-related amino acid biosynthetic pathways, the plastid-localized phosphorylated pathway of serine biosynthesis (PPSB), and the γ-aminobutyric acid (GABA) shunt. Under hypoxia, leaf NO emission rate was highest in AOX overexpressors and lowest in AOX knockdowns, with WT showing an intermediate rate. In turn, the rate of NO emission correlated with the degree to which amino acids accumulated. This amino acid accumulation was associated with the increased expression of the enzymes of the stress-related amino acid biosynthetic pathways. However, induction of the PPSB occurred much earlier than the GABA shunt. This work shows that high rates of NO turnover associate with rapid gene induction of the PPSB, establishing a clear link between this pathway and the maintenance of carbon, nitrogen and energy metabolism under hypoxia.

Products of anaerobic metabolism in waterlogged roots of soybean are exported in the xylem

Waterlogging leads to hypoxia of the root system. Metabolic changes occur that enable the plant to tolerate the hypoxic stress. We investigated the export of organic acids, products of anaerobic metabolism, via xylem of waterlogged soybean (Glycine max) plants. Organic acids were quantified by GC-MS and their formation via aspartate metabolism investigated using [4-13C]aspartate. Elevated levels of malate were found together with variable amounts of other organic acids, notably lactate and succinate. Addition of [4-13C]aspartate to the medium led to isotopic enrichment of several organic acids in the xylem sap. Quantitatively, malate carried the highest amount of label among the organic acids. Labelling of succinate indicates its formation by reversal of the TCA-cycle from oxaloacetate. Since aspartate was a prominent amino acid of the phloem sap, it is suggested that this is an important source of malate exported in the xylem. The export of these organic acids will play the role of removing electrons from the hypoxic roots, representing an additional mechanism in the metabolic response to root hypoxia. Malate, normally considered an intermediate in succinate formation, is definitively a product of anaerobic metabolism.

Regulation of Primary Metabolism in Response to Low Oxygen Availability as Revealed by Carbon and Nitrogen Isotope Redistribution

Based on enzyme activity assays and metabolic responses to waterlogging of the legume Lotus japonicus, it was previously suggested that, during hypoxia, the tricarboxylic acid cycle switches to a noncyclic operation mode. Hypotheses were postulated to explain the alternative metabolic pathways involved, but as yet, a direct analysis of the relative redistribution of label through the corresponding pathways was not made. Here, we describe the use of stable isotope-labeling experiments for studying metabolism under hypoxia using wild-type roots of the crop legume soybean (Glycine max). [(13)C]Pyruvate labeling was performed to compare metabolism through the tricarboxylic acid cycle, fermentation, alanine metabolism, and the γ-aminobutyric acid shunt, while [(13)C]glutamate and [(15)N]ammonium labeling were performed to address the metabolism via glutamate to succinate. Following these labelings, the time course for the redistribution of the (13)C/(15)N label throughout the metabolic network was evaluated with gas chromatography-time of flight-mass spectrometry. Our combined labeling data suggest the inhibition of the tricarboxylic acid cycle enzyme succinate dehydrogenase, also known as complex II of the mitochondrial electron transport chain, providing support for the bifurcation of the cycle and the down-regulation of the rate of respiration measured during hypoxic stress. Moreover, up-regulation of the γ-aminobutyric acid shunt and alanine metabolism explained the accumulation of succinate and alanine during hypoxia.

High irradiance improves ammonium tolerance in wheat plants by increasing N assimilation

Ammonium is a paradoxical nutrient ion. Despite being a common intermediate in plant metabolism whose oxidation state eliminates the need for its reduction in the plant cell, as occurs with nitrate, it can also result in toxicity symptoms. Several authors have reported that carbon enrichment in the root zone enhances the synthesis of carbon skeletons and, accordingly, increases the capacity for ammonium assimilation. In this work, we examined the hypothesis that increasing the photosynthetic photon flux density is a way to increase plant ammonium tolerance. Wheat plants were grown in a hydroponic system with two different N sources (10mM nitrate or 10mM ammonium) and with two different light intensity conditions (300 μmol photon m(-2)s(-1) and 700 μmol photon m(-2)s(-1)). The results show that, with respect to biomass yield, photosynthetic rate, shoot:root ratio and the root N isotopic signature, wheat behaves as a sensitive species to ammonium nutrition at the low light intensity, while at the high intensity, its tolerance is improved. This improvement is a consequence of a higher ammonium assimilation rate, as reflected by the higher amounts of amino acids and protein accumulated mainly in the roots, which was supported by higher tricarboxylic acid cycle activity. Glutamate dehydrogenase was a key root enzyme involved in the tolerance to ammonium, while glutamine synthetase activity was low and might not be enough for its assimilation.

Analysis of alanine aminotransferase in various organs of soybean (Glycine max) and in dependence of different nitrogen fertilisers during hypoxic stress

Alanine aminotransferase (AlaAT) catalyses the reversible conversion of pyruvate and glutamate into alanine and oxoglutarate. In soybean, two subclasses were identified, each represented by two highly similar members. To investigate the role of AlaAT during hypoxic stress in soybean, changes in transcript level of both subclasses were analysed together with the enzyme activity and alanine content of the tissue. Moreover, the dependency of AlaAT activity and gene expression was investigated in relation to the source of nitrogen supplied to the plants. Using semi-quantitative PCR, GmAlaAT genes were determined to be highest expressed in roots and nodules. Under normal growth conditions, enzyme activity of AlaAT was detected in all organs tested, with lowest activity in the roots. Upon waterlogging-induced hypoxia, AlaAT activity increased strongly. Concomitantly, alanine accumulated. During re-oxygenation, AlaAT activity remained high, but the transcript level and the alanine content decreased. Our results show a role for AlaAT in the catabolism of alanine during the initial period of re-oxygenation following hypoxia. GmAlaAT also responded to nitrogen availability in the solution during waterlogging. Ammonium as nitrogen source induced both gene expression and enzyme activity of AlaAT more than when nitrate was supplied in the nutrient solution. The work presented here indicates that AlaAT might not only be important during hypoxia, but also during the recovery phase after waterlogging, when oxygen is available to the tissue again.

Genetic engineering of improved nitrogen use efficiency in rice by the tissue-specific expression of alanine aminotransferase

Summary Nitrogen is quantitatively the most essential nutrient for plants and a major factor limiting crop productivity. One of the critical steps limiting the efficient use of nitrogen is the ability of plants to acquire it from applied fertilizer. Therefore, the development of crop plants that absorb and use nitrogen more efficiently has been a long-term goal of agricultural research. In an attempt to develop nitrogen-efficient plants, rice (Oryza sativa L.) was genetically engineered by introducing a barley AlaAT (alanine aminotransferase) cDNA driven by a rice tissue-specific promoter (OsAnt1). This modification increased the biomass and grain yield significantly in comparison with control plants when plants were well supplied with nitrogen. Compared with controls, transgenic rice plants also demonstrated significant changes in key metabolites and total nitrogen content, indicating increased nitrogen uptake efficiency. The development of crop plants that take up and assimilate nitrogen more efficiently would not only improve the use of nitrogen fertilizers, resulting in lower production costs, but would also have significant environmental benefits. These results are discussed in terms of their relevance to the development of strategies to engineer enhanced nitrogen use efficiency in crop plants.

Engineering nitrogen use efficiency with alanine aminotransferase

Nitrogen (N) is the most important factor limiting crop productivity worldwide. The ability of plants to acquire N from applied fertilizers is one of the critical steps limiting the efficient use of nitrogen. To improve N use efficiency, genetically modified plants that overexpress alanine aminotransferase (AlaAT) were engineered by introducing a barley AlaAT cDNA driven by a canola root specific promoter (btg26). Compared with wild-type canola, transgenic plants had increased biomass and seed yield both in the laboratory and field under low N conditions, whereas no differences were observed under high N. The transgenics also had increased nitrate influx. These changes resulted in a 40% decrease in the amount of applied nitrogen fertilizer required under field conditions to achieve yields equivalent to wild-type plants.

Amino acid and ureide transport in the xylem of symbiotic soybean plants during short-term flooding of the root system in the presence of different sources of nitrogen

The transport of organic N compounds to the shoot in the xylem sap of nodulated soybean plants was investigated in an attempt to better understand the changes in N metabolism under root hypoxia (first 5 days of flooding), with different sources of N in the medium. NO3- is beneficial for tolerance of plants to waterlogging, whereas other N sources such as NH4+ and NH4NO3, are not. Nevertheless, in the presence of NH4+ high levels of amino acids were transported in the xylem, consistent with its assimilation. Some increase in the transport of amino acids was also seen with NO3- nutrition during waterlogging, but not with N-free medium. Ureide transport in the xylem was severely reduced during waterlogging, consistent with impaired N2 fixation under these conditions. The relative proportions of some amino acids in the xylem showed dramatic changes during treatment. Alanine increased tremendously under root hypoxia, especially with NH4+ as N source, where it reached near 70 % of the total amino acids present. Aspartic acid, on the other hand, dropped to very low levels and was inversely related to alanine levels, consistent with this amino acid being the immediate source of N for alanine synthesis. Glutamine levels also fell to a larger or lesser extent, depending on the N source present. The changes in asparagine, one of the prominent amino acids of the xylem sap, were most outstanding in the treatment with NO3-, where they showed a large increase, characteristic of plants switching from dependence on N2 fixation to NO3- assimilation. The data indicate that the lesser effectiveness of NH4+ during waterlogging, in contrast to NO3-, involves restricted amino acids metabolism, and may result from energy metabolism being directed towards NH4+ detoxification.

Development of the nodulated soybean plant after flooding of the root system with different sources of nitrogen

Flooding leads to hypoxia, a stress to which symbiotic N2 fixation is especially sensitive. The response of fully nodulated soybean plants to a 21-day period of flooding was studied by measurements of growth parameters and xylem transport of organic nitrogenous components to the shoot, in the presence and absence of NO3- and NH4+ in the medium. Flooding was found to seriously impair N2 fixation, irrespective of the N source, as indicated by strongly reduced xylem ureide levels. In the absence of a source of N, growth was strongly reduced during flooding while accumulation of N in the shoot was virtually abolished. Flooding in the presence of 5 mM NO3- or NH4+ led to the accumulation of total N in the shoot but only NO3- promoted increases in total dry matter, plant height and leaf area above that found in the absence of N. The accumulation of N, however, was lower than that of the non-flooded control for both NO3- and NH4+. The increases in total dry matter, plant height and leaf area with NO3- was as high as those of the non-flooded control. These data clearly show the beneficial effects of NO3- during a prolonged period of flooding of the nodulated root system of soybean.

The metabolic response of plants to oxygen deficiency

Plants, under natural or experimental conditions, can be subject to a range of O2 concentrations from normal (normoxia) through deficient (hypoxia) to total absence (anoxia). Many metabolic processes are affected by O2 deficiency but the most studied events are those related to respiration and metabolism of N. In the absence of a terminal electron acceptor for the electron transport chain, the tricarboxylic acid cycle functions only partially and in both directions. Acidification of the cytosol occurs and pyruvate, the product of glycolysis, is transformed to lactate and ethanol, which represent the main fermentation reactions in plants. Alanine is the third most important product of anaerobic metabolism, resulting from high rates of amino acid interconversion in which transaminases such as alanine aminotransferase play an important role. The role of alanine accumulation under anaerobiosis is not clear and appears to be independent of the source of N whether NO3-, NH4+ or N2. How nitrate exerts its beneficial effect on tolerance of root hypoxia in waterlogged plants is still not clearly understood. Such aspects of N metabolism pose interesting challenges for future research on metabolic responses of plants to oxygen deficiency.

Anaerobic Metabolism in the N-Limited Green Alga Selenastrum minutum: III. Alanine Is the Product of Anaerobic Ammonium Assimilation

We have determined the flow of (15)N into free amino acids of the N-limited green alga Selenastrum minutum (Naeg.) Collins after addition of (15)NH(4) (+) to aerobic or anaerobic cells. Under aerobic conditions, only a small proportion of the N assimilated was retained in the free amino acid pool. However, under anaerobic conditions almost all assimilated NH(4) (+) accumulates in alanine. This is a unique feature of anaerobic NH(4) (+) assimilation. The pathway of carbon flow to alanine results in the production of ATP and reductant which matches exactly the requirements of NH(4) (+) assimilation. Alanine synthesis is therefore an excellent strategy to maintain energy and redox balance during anaerobic NH(4) (+) assimilation.