Interest in plant asparagine has rapidly taken off over the past 5 years following the report that acrylamide, a neurotoxin and potential carcinogen, is present in cooked foods, particularly carbohydrate-rich foods such as wheat and potatoes which are subjected to roasting, baking or frying at high temperatures. Subsequent studies showed that acrylamide could be formed in foods by the thermal degradation of free asparagine in the presence of sugars in the Maillard reaction. In this article, our current knowledge of asparagine in plants and in particular its occurrence in cereal seeds and potatoes is reviewed and discussed in relation to acrylamide formation. There is now clear evidence that soluble asparagine accumulates in most if not all plant organs during periods of low rates of protein synthesis and a plentiful supply of reduced nitrogen. The accumulation of asparagine occurs during normal physiological processes such as seed germination and nitrogen transport. However, in addition, stress-induced asparagine accumulation can be caused by mineral deficiencies, drought, salt, toxic metals and pathogen attack. The properties and gene regulation of the enzymes involved in asparagine synthesis and breakdown in plants are discussed in detail.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1744-7348.2006.00104.x

Asparagine in plants

The sulfonylurea herbicides are characterized by broad-spectrum weed control at very low use rates (c. 2–75 g ha−1), good crop selectivity, and very low acute and chronic animal toxicity. This class of herbicides acts through inhibition of acetolactate synthase (EC 4.1.3.18; also known as acetohydroxyacid synthase), thereby blocking the biosynthesis of the branched-chain amino acids valine, leucine and isoleucine. This inhibition leads to the rapid cessation of plant cell division and growth. Crop-selective sulfonylurea herbicides have been commercialized for use in wheat, barley, rice, corn, soybeans and oilseed rape, with additional crop-selective compounds in cotton, potatoes, and sugarbeet having been noted. Crop selectivity results from rapid metabolic inactivation of the herbicide in the tolerant crop. Under growth-room conditions, metabolic half-lives in tolerant crop plants range from 1–5 h, while sensitive plant species metabolize these herbicides much more slowly, with half-lives > 20 h. Pathways by which sulfonylurea herbicides are inactivated among these plants include aryl and aliphatic hydroxylation followed by glucose conjugation, sulfonylurea bridge hydrolysis and sulfonamide bond cleavage, oxidative O-demethylation and direct conjugation with (homo)glutathione. Sulfonylurea herbicides degrade in soil through a combination of bridge hydrolysis and microbial degradation. Hydrolysis is significantly faster under acidic (pH 5) than alkaline (pH 8) conditions, allowing the use of soil pH as a predictor of soil residual activity. Chemical and microbial processes combine to give typical field dissipation half-lives of 1–6 weeks, depending on the soil type, location and compound. Very short residual sulfonylurea herbicides with enhanced susceptibility to hydrolysis (DPX-L5300) and microbial degradation (thifensulfuron-methyl) have been developed.

Mode of action, crop selectivity, and soil relations of the sulfonylurea herbicides†

Symbiotic nitrogen fixation is highly sensitive to drought, which results in decreased N accumulation and yield of legume crops. The effects of drought stress on N 2 fixation usually have been perceived as a consequence of straightforward physiological responses acting on nitrogenase activity and involving exclusively one of three mechanisms: carbon shortage, oxygen limitation, or feedback regulation by nitrogen accumulation. The sensitivity of the nodule water economy to the volumetric flow rate of the phloem into the nodule offers a common framework to understand each of these mechanisms. As these processes are sensitive to volumetric phloem flow into the nodules, variations in phloem flow as a result of changes in turgor pressure in the leaves are likely to cause rapid changes in nodule activity. This could explain the special sensitivity of N 2 fixation to drying soils. It seems likely that N feedback may be especially important in explaining the response mechanism in nodules. A number of studies have indicated that a nitrogenous signal(s), associated with N accumulation in the shoot and nodule, exists in legume plants so that N 2 fixation is inhibited early in soil drying. The existence of genetic variation in N 2 fixation response to water deficits among legume cultivars opens the possibility for enhancing N 2 fixation tolerance to drought through selection and breeding.

Symbiotic N2 fixation response to drought

INTRODUCTION 539 THE EXCHANGE OF REDUCED CARBON AND NITROGEN: AN OVERVIEW. ... 540 MAJOR PRODUCTS OF RECENTLY FIXED N2 EXPORTED FROM NODULES.... 542 SYNTHESIS OF AMIDES AND UREIDES 546 Ammonium Assimilation .... 546 Asparagine Synthesis . 547 Synthesis of 4-Methyleneglutamine 548 Synthesis of Citrulline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549 Biogenesis of Allantoin and Allantoic Acid 551 METABOLISM OF AMIDES AND UREIDES . ......... . . ... . . ........ .... . . .. ..... . 558 ENERGETICS OF SYNTHESIS AND METABOLISM OF THE PRODUCTS OF Nz FIXATION 564 SUMMARy......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567

Products of Biological Nitrogen Fixation in Higher Plants: Synthesis, Transport, and Metabolism

Developing seeds are net importers of organic and inorganic nutrients. Nutrients enter seeds through the maternal vascular system at relatively high concentrations in the phloem. They exit importing sieve elements via interconnecting plasmodesmata and, during subsequent symplasmic passage, are sequestered into labile storage pools (vacuoles; starch). Transporters function to retrieve nutrients leaked to the seed apoplasm during symplasmic passage. Maternal cells responsible for nutrient release to the seed apoplasm are characteristically located at the maternal/filial interface. Their plasma membranes are enriched in transport proteins and, in some species, these cells are modified to a transfer cell morphology. Apoplasmic volumes of seeds are relatively small, but contain high concentrations of sugars, potassium and a range of amino acids. Sucrose and amino acids are taken up from the seed apoplasm by one to two cell layers of filial tissues that juxtapose the maternal tissues. The plasma membranes of the uptake cells are enriched in sucrose and amino acid/H(+) transporters which co-localize with H(+)-ATPASES: In some species, these cells are modified to a transfer cell morphology. High densities of plasmodesmata support symplasmic delivery of accumulated nutrients to underlying storage cells where polymer formation (starch, protein) takes place. Hexoses, resulting from sucrose hydrolysis and leakage to the seed apoplasm, are retrieved by hexose/H(+) symporters.

Compartmentation of transport and transfer events in developing seeds

The various nitrogenous solutes important to embryo development in symbiotic soybean plants were determined during the midpodfilling stage. Glutamine was the principal form of nitrogen, contributing 55% of the embryo nitrogen requirement. Asparagine was the second most important, contributing 20%. The ureides allantoin and allantoic acid directly contributed only insignificantly to the total nitrogen requirement of the embryo. These conclusions were based upon analyses of tissue extracts, translocation studies of radiolabeled solutes, analysis of in vivo seed coat exudate collected from the freespace of attached, surgically altered seeds, and the in vitro culture of isolated immature soybean embryos.

/pdf/role-of-amides-amino-acids-and-ureides-in-the-nutrition-of-2ud6uwbbw2.pdf

Role of Amides, Amino Acids, and Ureides in the Nutrition of Developing Soybean Seeds

The total metabolic cost of soybean (Glycine max L. Mer Clark) nodule nitrogen fixation was empirically separated into respiration associated with electron flow through nitrogenase and respiration associated with maintenance of nodule function.Rates of CO(2) evolution and H(2) evolution from intact, nodulated root systems under Ar:O(2) atmospheres decreased in parallel when plants were maintained in an extended dark period. While H(2) evolution approached zero after 36 hours of darkness at 22 degrees C, CO(2) evolution rate remained at 38 degrees of the rate measured in light. Of the remaining CO(2) evolution, 62% was estimated to originate from the nodules and represents a measure of nodule maintenance respiration. The nodule maintenance requirement was temperature dependent and was estimated at 79 and 137 micromoles CO(2) (per gram dry weight nodule) per hour at 22 degrees C and 30 degrees C, respectively.The cost of N(2) fixation in terms of CO(2) evolved per electron pair utilized by nitrogenase was estimated from the slope of H(2) evolution rate versus CO(2) evolution rate. The cost was 2 moles CO(2) evolved per mole H(2) evolved and was independent of temperature.In this symbiosis, nodule maintenance consumed 22% of total respiratory energy while the functioning of nitrogenase consumed a further 52%. The remaining respiratory energy was calculated to be associated with ammonia assimilation, transport of reduced N, and H(2) evolution.

Experimental determination of the respiration associated with soybean/rhizobium nitrogenase function, nodule maintenance, and total nodule nitrogen fixation.

Ross M. Rainbird

Papers

Role of Amides, Amino Acids, and Ureides in the Nutrition of Developing Soybean Seeds

Experimental determination of the respiration associated with soybean/rhizobium nitrogenase function, nodule maintenance, and total nodule nitrogen fixation.

Status of New Nitrogen Inputs for Crops