https://www.researchgate.net/profile/Rupam_Kapoor/publication/26881549_Arbuscular_mycorrhizal_fungi_in_alleviation_of_salt_stress_A_review/links/0c960523fd2cc5ec7f000000.pdf

Arbuscular mycorrhizal fungi in alleviation of salt stress: a review.

Cadmium (Cd) is a highly toxic heavy metal for both plants and animals. The presence of Cd in agricultural soils is of great concern regarding its entry into the food chain. Cadmium enters into the soil-plant environment mainly through anthropogenic activities. Compounds of Cd are more soluble than other heavy metals, so it is more available and readily taken up by plants and accumulates in different edible plant parts through which it enters the food chain. A number of approaches are being used to minimize the entry of Cd into the food chain. Proper plant nutrition is one of the good strategies to alleviate the damaging effects of Cd on plants and to avoid its entry into the food chain. Plant nutrients play a very important role in developing plant tolerance to Cd toxicity and thus, low Cd accumulation in different plant parts. In this report, the role of some macronutrients (nitrogen, phosphorus, sulfur and calcium), micronutrients (zinc, iron and manganese), and silicon (a beneficial nutrient) has been discussed in detail as to how these nutrients play their role in decreasing Cd uptake and accumulation in crop plants.

Role of mineral nutrition in minimizing cadmium accumulation by plants

In green parts of the plant, during illumination ATP and NAD(P)H act as energy sources that are generated mainly in photosynthesis and respiration, whereas in darkness, glycolysis, respiration and the oxidative pentose-phosphate pathway (OPP) generate the required energy forms. In non-green parts, sugar oxidation in glycolysis, respiration and OPP are the only means of producing energy. For energy-consuming reactions, the delivery of NADPH, NADH, reduced ferredoxin and ATP has to take place at the required rates and in the specific compartments, since the pool sizes of these energy carriers are rather limited and, in general, they are not directly transported across biomembranes. Indirect transport of reducing equivalents can be achieved by malateoxaloacetate shuttles, involving malate dehydrogenase (MDH) for the interconversion. Isoenzymes of MDH are present in each cellular compartment. Chloroplasts contain the redox-controlled NADP-MDH that is only active in the light. In addition, a plastid NAD-MDH that is permanently active and is present in all plastid types has been found. Export of excess NAD(P)H through the malate valves will allow for the continued production of ATP (1) in photosynthesis, and (2) in oxidative phosphorylation. In the latter case, the coupled production of NADH is catalysed by the bispecific NAD(P)-GAPDH (GapAB) in chloroplasts that is active with NAD even in darkness, or by the specific plastid NAD-GAPDH (GapCp) in non-green tissues. When plants are subjected to conditions such as high light, high CO(2), NH(4) (+) nutrition, cold stress, which require changed activities of the enzymes of the malate valves, changed expression levels of the MDH isoforms can be observed. In nodules, the induction of a nodule-specific plastid NAD-MDH indicates the changed requirements for energy supply during N(2) fixation. Furthermore, the induction of glucose 6-phosphate dehydrogenase isoforms by ammonium and of ferredoxin and ferredoxin-NADP reductase by nitrate has been described. All these findings are in line with the assumption that a changed redox state caused by metabolic variability leads to the induction of enzymes involved in redox poise.

Malate valves to balance cellular energy supply.

Glutamine synthetase assimilates ammonium into amino acids, thus it is a key enzyme for nitrogen metabolism. The cytosolic isoenzymes of glutamine synthetase assimilate ammonium derived from primary nitrogen uptake and from various internal nitrogen recycling pathways. In this way, cytosolic glutamine synthetase is crucial for the remobilization of protein-derived nitrogen. Cytosolic glutamine synthetase is encoded by a small family of genes that are well conserved across plant species. Members of the cytosolic glutamine synthetase gene family are regulated in response to plant nitrogen status, as well as to environmental cues, such as nitrogen availability and biotic/abiotic stresses. The complex regulation of cytosolic glutamine synthetase at the transcriptional to post-translational levels is key to the establishment of a specific physiological role for each isoenzyme. The diverse physiological roles of cytosolic glutamine synthetase isoenzymes are important in relation to current agricultural and ecological issues.

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The importance of cytosolic glutamine synthetase in nitrogen assimilation and recycling

A review on the main challenges in automatic plant disease identification based on visible range images

The wide range of plant responses to ammonium nutrition can be used to study the way ammonium interferes with plant metabolism and to assess some characteristics related with ammonium tolerance by plants. In this work we investigated the hypothesis of plant tolerance to ammonium being related with the plants' capacity to maintain high levels of inorganic nitrogen assimilation in the roots. Plants of several species (Spinacia oleracea L., Lycopersicon esculentum L., Lactuca sativa L., Pisum sativum L. and Lupinus albus L.) were grown in the presence of distinct concentrations (0.5, 1.5, 3 and 6 mM) of nitrate and ammonium. The relative contributions of the activity of the key enzymes glutamine synthetase (GS; under light and dark conditions) and glutamate dehydrogenase (GDH) were determined. The main plant organs of nitrogen assimilation (root or shoot) to plant tolerance to ammonium were assessed. The results show that only plants that are able to maintain high levels of GS activity in the dark (either in leaves or in roots) and high root GDH activities accumulate equal amounts of biomass independently of the nitrogen source available to the root medium and thus are ammonium tolerant. Plant species with high GS activities in the dark coincide with those displaying a high capacity for nitrogen metabolism in the roots. Therefore, the main location of nitrogen metabolism (shoots or roots) and the levels of GS activity in the dark are an important strategy for plant ammonium tolerance. The relative contribution of each of these parameters to species tolerance to ammonium is assessed. The efficient sequestration of ammonium in roots, presumably in the vacuoles, is considered as an additional mechanism contributing to plant tolerance to ammonium nutrition.

How does glutamine synthetase activity determine plant tolerance to ammonium

New method to assess barley nitrogen nutrition status based on image colour analysis

The effect of the source of nitrogen nutrition (ammonium or nitrate), onthe response of pea plants to a moderate saline stress (30 mMNaCl)was studied. Growth declined under saline stress but nitrate-fed plants wereless sensitive to salinity than ammonium-fed plants. This different sensitivitywas due mainly to a better maintenance of root growth in nitrate-fed plants.Organic nitrogen content decreased significantly in roots of ammonium-fedplants. Water relations changed slightly under saline stress leading to adecrease in stomatal conductance, which was correlated to a decline in carbonassimilation rates regardless of nitrogen source. Salinity affects the uptakeofseveral nutrients in a different way, depending on the nitrogen source. Thus,chloride was accumulated mainly in nitrate-fed plants, displacing nitrate,whereas sodium was accumulated mainly in ammonium-fed plants, especially inroots, displacing other cations such as ammonium and potassium. It is concludedthat the nitrogen source (ammonium or nitrate) is a major factor affecting pearesponses to saline stress, plants being more sensitive when ammonium is thesource used. The different sensitivity is discussed in terms of a competitionfor energy between nitrogen assimilation and sodium exclusion processes.

Pea responses to saline stress is affected by the source of nitrogen nutrition (ammonium or nitrate)

Ammonium nutrition is of interest as an alternative to that of using nitrate. However, the former has been reported as stressful to many plant species especially to some important crops, as most abiotic stresses may trigger oxidative imbalances in plants. In this work, we investigate the response of oxidative metabolism of two plant species, spinach (Spinacia oleracea L. cv. Gigante de invierno) and pea (Pisum sativum L. cv. Rondo), which have distinct tolerance to ammonium. Plants were grown in the presence of 1.5 and 3.0 mM N as ammonium and compared with equivalent nitrate nutrition. The antioxidant enzymes and metabolites as well as oxidative damage to proteins were determined. Protein and amino acid contents in both types of plants were also analysed. Ammonium nutrition in sensitive spinach or in the tolerant pea plants does not alter the redox status of ascorbate and glutathione or the phenolic contents, while no clear effect is seen in the antioxidant enzymes. The results showed that the stress originated from applying ammonium as the only N source is not an oxidative stress, independent of the ammonium tolerance of the plant species studied. Moreover, ammonium stress diminishes oxidative damage to proteins in the spinach plants. The data of the protein oxidation together with those from N metabolism highlight the relation between the stress induced by ammonium and an increased protein turnover.

Carmen Lamsfus

Papers

How does glutamine synthetase activity determine plant tolerance to ammonium

New method to assess barley nitrogen nutrition status based on image colour analysis

Pea responses to saline stress is affected by the source of nitrogen nutrition (ammonium or nitrate)

Nitrogen nutrition and antioxidant metabolism in ammonium-tolerant and -sensitive plants

Role of glutamate dehydrogenase and phosphoenolpyruvate carboxylase activity in ammonium nutrition tolerance in roots