(1988). Inhibition of legume nodule formation and N2 fixation by nitrate. Critical Reviews in Plant Sciences: Vol. 7, No. 1, pp. 1-23.

Inhibition of legume nodule formation and N2 fixation by nitrate

In mammals, cadmium is widely considered as a non-genotoxic carcinogen acting through a methylation-dependent epigenetic mechanism. Here, the effects of Cd treatment on the DNA methylation patten are examined together with its effect on chromatin reconfiguration in Posidonia oceanica. DNA methylation level and pattern were analysed in actively growing organs, under short- (6 h) and long- (2 d or 4 d) term and low (10 mM) and high (50 mM) doses of Cd, through a Methylation-Sensitive Amplification Polymorphism technique and an immunocytological approach, respectively. The expression of one member of the CHROMOMETHYLASE (CMT) family, a DNA methyltransferase, was also assessed by qRT-PCR. Nuclear chromatin ultrastructure was investigated by transmission electron microscopy. Cd treatment induced a DNA hypermethylation, as well as an up-regulation of CMT, indicating that de novo methylation did indeed occur. Moreover, a high dose of Cd led to a progressive heterochromatinization of interphase nuclei and apoptotic figures were also observed after long-term treatment. The data demonstrate that Cd perturbs the DNA methylation status through the involvement of a specific methyltransferase. Such changes are linked to nuclear chromatin reconfiguration likely to establish a new balance of expressed/repressed chromatin. Overall, the data show an epigenetic basis to the mechanism underlying Cd toxicity in plants.

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In Posidonia oceanica cadmium induces changes in DNA methylation and chromatin patterning

Ecophysiology of Crassulacean Acid Metabolism (CAM)

Contents Summary 35 I. Introduction 35 II. Nitrogen acquisition and assimilation 36 III. Root-to-shoot transport of nitrogen 38 IV. Nitrogen storage pools in vegetative tissues 39 V. Nitrogen transport from source leaf to sink 40 VI. Nitrogen import into sinks 42 VII. Relationship between source and sink nitrogen transport processes and metabolism 43 VIII. Regulation of nitrogen transport 43 IX. Strategies for crop improvement 44 X. Conclusions 46 Acknowledgements 47 References 47 SUMMARY: Nitrogen is an essential nutrient for plant growth. World-wide, large quantities of nitrogenous fertilizer are applied to ensure maximum crop productivity. However, nitrogen fertilizer application is expensive and negatively affects the environment, and subsequently human health. A strategy to address this problem is the development of crops that are efficient in acquiring and using nitrogen and that can achieve high seed yields with reduced nitrogen input. This review integrates the current knowledge regarding inorganic and organic nitrogen management at the whole-plant level, spanning from nitrogen uptake to remobilization and utilization in source and sink organs. Plant partitioning and transient storage of inorganic and organic nitrogen forms are evaluated, as is how they affect nitrogen availability, metabolism and mobilization. Essential functions of nitrogen transporters in source and sink organs and their importance in regulating nitrogen movement in support of metabolism, and vegetative and reproductive growth are assessed. Finally, we discuss recent advances in plant engineering, demonstrating that nitrogen transporters are effective targets to improve crop productivity and nitrogen use efficiency. While inorganic and organic nitrogen transporters were examined separately in these studies, they provide valuable clues about how to successfully combine approaches for future crop engineering.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.14876

Source and sink mechanisms of nitrogen transport and use

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

Ureide metabolism in higher plants

The leaf anatomy and certain photosynthetic properties of nitrate- and ammonia-grown plants of Moricandia arvensis (L.) DC., a species previously reported to be a C 3 -C 4 intermediate, were investigated. Nitrate-grown plants had a high level of malate in the leaves while ammonia-grown plants had low levels of malate. In young leaves of nitrate-grown plants, there was a diurnal fluctuation of malate content, increasing during the day and decreasing during the night. Titratable acidity remained low in leaves of both nitrate- and ammonia-grown plants. In nitrate-grown plants, the activity of phosphoenolpyruvate (PEP) carboxylase was about 2-fold higher than in ammonia-grown plants, the latter having activity typical of C 3 species. Also, in nitrate-grown plants, the ratio of activities of ribulose 1,5-bisphosphate (RuBP) carboxylase/PEP carboxylase was lower than in ammonia-grown plants. Nitrate reductase activities were higher in nitrate- than in ammonia-grown plants and the greatest activity was found in younger leaves. With nitrate-grown plants, during a pulse-chase experiment the label in malate, as a percentage of the total labeled products, increased from about 7% after a 10-second pulse with 14 CO 2 up to 17% during a 5-minute chase with 12 CO 2 . The pattern of 14 C labeling in various metabolites suggests the primary carboxylation is through RuBP carboxylase with a secondary carboxylation through PEP carboxylase. In similar experiments, with ammonia-grown plants, the percentage label in malate was only 0% to 4% with no increase in malate labeling during the chase period. The CO 2 compensation point was lower in nitrate-grown than ammonia-grown plants. There was no evidence of Kranz-like anatomy in either the nitrate or ammonia-grown plants. Mitochondria of bundle-sheath cells were strikingly positioned along the inner tangential wall. This might allow the chloroplasts of these cells to fix the mitochondrial photorespired CO 2 more effectively and contribute to the low CO 2 compensation point in the species. Chloroplasts of bundle-sheath cells and contiguous mesophyll cells were similar in size and structure in plants grown on different media, although chloroplast thylakoids and stromata of the ammonia-grown plants stained more intensely than those of nitrate-grown plants. In addition, irregular clusters of phytoferritin particles occurred in the chloroplasts of the ammonia-grown plants. The results indicate that the substantial activity of PEP carboxylase, incorporation of CO 2 into malate, the high malate content, and in part the relatively low CO 2 compensation point in Moricandia arvensis may be accounted for by metabolism of nitrate rather than by a state of C 3 -C 4 intermediacy.

Influence of Nitrate and Ammonia on Photosynthetic Characteristics and Leaf Anatomy of Moricandia arvensis

The ureides, allantoin and allantoic acid, are major forms of N transported from nodules to shoots in soybeans (Merr.). Little is known about the occurrence, localization, or properties of the enzymes involved in the assimilation of ureides in shoot tissues. We have examined the capacity of the shoot tissues to assimilate allantoin via allantoinase (EC 3.5.2.5) during leaf and fruit development in nodulated soybeans. Specific activity of allantoinase in leaves peaked during pod formation and early seed filling. In developing fruits allantoinase activity in the seeds was 2 to 4 times that in the pods when expressed on a fresh weight or organ basis. In seeds, the embryos contained the highest specific allantoinase activity. Stems and petioles also had appreciable allantoinase activity. With development, peaks in the amounts of allantoic acid, but not allantoin, were measured in both leaves and fruits suggesting that the assimilation of allantoic acid may be a limiting factor in ureide assimilation. Highest amounts of ureides were measured in the pith and xylem of stem tissues and in developing pod walls.

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Richard J. Thomas

Papers

Ureide metabolism in higher plants

Influence of Nitrate and Ammonia on Photosynthetic Characteristics and Leaf Anatomy of Moricandia arvensis

The Assimilation of Ureides in Shoot Tissues of Soybeans: 1. CHANGES IN ALLANTOINASE ACTIVITY AND UREIDE CONTENTS OF LEAVES AND FRUITS

Allantoinase from shoot tissues of soybeans

Effect of Rhizobium japonicum mutants with enhanced N2 fixation activity on N transport and photosynthesis of soybeans during vegetative growth