Moving through the Stressed Genome: Emerging Regulatory Roles for Transposons in Plant Stress Response

The recognition of a positive correlation between organism genome size with its transposable element (TE) content, represents a key discovery of the field of genome biology. Considerable evidence accumulated since then suggests the involvement of TEs in genome structure, evolution and function. The global genome reorganization brought about by transposon activity might play an adaptive/regulatory role in the host response to environmental challenges, reminiscent of McClintock's original 'Controlling Element' hypothesis. This regulatory aspect of TEs is also garnering support in light of the recent evidences, which project TEs as "distributed genomic control modules." According to this view, TEs are capable of actively reprogramming host genes circuits and ultimately fine-tuning the host response to specific environmental stimuli. Moreover, the stress-induced changes in epigenetic status of TE activity may allow TEs to propagate their stress responsive elements to host genes; the resulting genome fluidity can permit phenotypic plasticity and adaptation to stress. Given their predominating presence in the plant genomes, nested organization in the genic regions and potential regulatory role in stress response, TEs hold unexplored potential for crop improvement programs. This review intends to present the current information about the roles played by TEs in plant genome organization, evolution, and function and highlight the regulatory mechanisms in plant stress responses. We will also briefly discuss the connection between TE activity, host epigenetic response and phenotypic plasticity as a critical link for traversing the translational bridge from a purely basic study of TEs, to the applied field of stress adaptation and crop improvement.

Brassica RNA binding protein ERD4 is involved in conferring salt, drought tolerance and enhancing plant growth in Arabidopsis

'Early responsive to dehydration' (ERD) genes are a group of plant genes having functional roles in plant stress tolerance and development. In this study, we have isolated and characterized a Brassica juncea 'ERD' gene (BjERD4) which encodes a novel RNA binding protein. The expression pattern of ERD4 analyzed under different stress conditions showed that transcript levels were increased with dehydration, sodium chloride, low temperature, heat, abscisic acid and salicylic acid treatments. The BjERD4 was found to be localized in the chloroplasts as revealed by Confocal microscopy studies. To study the function, transgenic Arabidopsis plants were generated and analyzed for various morphological and physiological parameters. The overexpressing transgenic lines showed significant increase in number of leaves with more leaf area and larger siliques as compared to wild type plants, whereas RNAi:ERD4 transgenic lines showed reduced leaf number, leaf area, dwarf phenotype and delayed seed germination. Transgenic Arabidopsis plants overexpressing BjERD4 gene also exhibited enhanced tolerance to dehydration and salt stresses, while the knockdown lines were susceptible as compared to wild type plants under similar stress conditions. It was observed that BjERD4 protein could bind RNA as evidenced by the gel-shift assay. The overall results of transcript analysis, RNA gel-shift assay, and transgenic expression, for the first time, show that the BjERD4 is involved in abiotic stress tolerance besides offering new clues about the possible roles of BjERD4 in plant growth and development.

Colonization of roots of rice (Oryza sativa) by symbiotic Nostoc strains

•   The lack of nitrogen in agriculture, and negative environmental effects of fertilizers, have stimulated interest in creating artificial associations between N₂ -fixing cyanobacteria and rice (Oryza sativa). •   For the first time, numerous (57) Nostoc isolates from natural symbioses were screened for their ability to associate with rice. Successful colonizers were tested for N₂ -fixation by acetylene reduction_, and for their ability to adsorb to roots by chlorophyll a measurements. Paranodules were induced by 2,4-dichlorophenoxyacetic acid. And genetic fingerprints of the cyanobacteria were obtained for identification. Ultrastructural investigations were made by light and scanning electron microscopy. •   Twenty-one symbiotic Nostoc isolates associated with rice roots, colonizing surfaces and intercellular spaces. Adsorption was high and appeared biphasic. The rates of N₂ fixation by associated cyanobacteria were higher compared with those in free-living cyanobacteria. Paranodules were formed and colonized, but root growth was adversely affected. •   Under laboratory conditions, artificial associations were created between one-third of the screened symbiotic cyanobacteria and rice. The agricultural potential for the association appears high since the cyanobacteria adsorb tightly and fix more N₂ than when free-living.

Tansley Review No. 116: Cyanobacterium-plant symbioses

Cyanobacteria are an ancient, morphologically diverse group of prokaryotes with an oxygenic photosynthesis. Many cyanobacteria also possess the ability to fix N₂ . Although well suited to an independent existence in nature, some cyanobacteria occur in symbiosis with a wide range of hosts (protists, animals and plants). Among plants, such symbioses have independently evolved in phylogenetically diverse genera belonging to the algae, fungi, bryophytes, pteridophytes, gymnosperms and angiosperms. These are N₂ -fixing symbioses involving heterocystous cyanobacteria, particularly Nostoc, as cyanobionts (cyanobacterial partners). A given host species associates with only a particular cyanobiont genus but such specificity does not extend to the strain level. The cyanobiont is located under a microaerobic environment in a variety of host organs and tissues (bladder, thalli and cephalodia in fungi; cavities in gametophytes of hornworts and liverworts or fronds of the Azolla sporophyte; coralloid roots in cycads; stem glands in Gunnera). Except for fungi, the hosts form these structures ahead of the cyanobiont infection. The symbiosis lasts for one generation except in Azolla and diatoms, in which it is perpetuated from generation to generation. Within each generation, multiple fresh infections occur as new symbiotic tissues and organs develop. The symbioses are stable over a wide range of environmental conditions, and sensing-signalling between partners ensures their synchronized growth and development. The cyanobiont population is kept constant in relation to the host biomass through controlled initiation and infection, nutrient supply and cell division. In most cases, the partners have remained facultative, with the cyanobiont residing extracellularly in the host. However, in the water-fern Azolla and the freshwater diatom Rhopalodia the association is obligate. The cyanobionts occur intracellularly in diatoms, the fungus Geosiphon and the angiosperm Gunner a. Close cell-cell contact and the development of special structures ensure efficient nutrient exchange between the partners. The mobile nutrients are normal products of the donor cells, although their production is increased in symbiosis. Establishment of cyanobacterial-plant symbioses differs from chloroplast evolution. In these symbioses, the cyanobiont undergoes structural-functional changes suited to its role as provider of fixed N rather than fixed C, and the level of intimacy is far less than that of an organelle. This review provides an updated account of cyanobacterial-plant symbioses, particularly concerning developments during the past 10 yr. Various aspects of these symbioses such as initiation and development, symbiont diversity, recognition and signalling, structural-functional modifications, integration, and nutrient exchange are reviewed and discussed, as are evolutionary aspects and the potential uses of cyanobacterial-plant symbioses. Finally we outline areas that require special attention for future research. Not only will these provide information of academic interest but they will also help to improve the use of Azolla as green manure, to enable us to establish artificial N₂ -fixing associations with cereals such as rice, and to allow the manipulation of free-living cyanobacteria for photobiological ammonia or hydrogen production or for use as biofertilizers. contents Summary 449 I. introduction 450 II. the partners 451 III. initiation and development of symbioses 458 IV. the symbioses 462 V. evolutionary aspects 472 VI. artificial symbioses 474 VII. future outlook and perspectives 475 Acknowledgements 477 References 477.

Mercury intoxication and arterial hypertension: report of two patients and review of the literature

Two children in the same household with symptomatic arterial hypertension simulating pheochromocytoma were found to be intoxicated with elemental mercury. The first child was a 4-year-old boy who presented with new-onset seizures, rash, and painful extremities, who was found to have a blood pressure of 171/123 mm Hg. An extensive investigation ensued. Elevated catecholamines were demonstrated in plasma and urine; studies did not confirm pheochromocytoma. Mercury levels were elevated. These findings prompted an evaluation of the family. A foster sister had similar findings of rash and hypertension. Both had been exposed to elemental mercury in the home. The family was temporarily relocated and chelation therapy was started. A Medline search for mercury intoxication with hypertension found 6 reports of patients ranging from 11 months to 17 years old. All patients showed symptoms of acrodynia. Because of the clinical presentation and the finding of elevated catecholamines, most of the patients were first studied for possible pheochromocytoma. Subsequently, elevated levels of mercury were found. Three children had contact with elemental mercury from a broken thermometer, 2 had played with metallic mercury and 1 had poorly protected occupational exposure. All responded to chelation therapy. Severe systemic arterial hypertension in infants and children is usually secondary to an underlying disease process. The most frequent causes of hypertension in this group include renal parenchymal disease, obstructive uropathy, and chronic pyelonephritis associated with reflux and renal artery stenosis. Less frequent causes include adrenal tumors, pheochromocytomas, neurofibromas, and a number of familial forms of hypertension. Other causes include therapeutic and recreational drugs, notably sympathomimetics and cocaine, and rarely, heavy metals. In children with severe hypertension and elevated catecholamines, the physician should consider mercury intoxication as well as pheochromocytoma. The health hazards of heavy metals need to be reinforced to the medical profession and the general public.

Evidence for a role of glutamine synthetase in assimilation of amino acids as nitrogen source in the cyanobacterium Nostoc muscorum

Methylammonium/ammonium ion, glutamine, glutamate, arginine and proline uptake, and their assimilation as nitrogen sources, was studied in Nostoc muscorum and its glutamine synthetase-deficient mutant. Glutamine served as nitrogen source independent of glutamine synthetase activity. Glutamate was not metabolised as a nitrogen source but still inhibited nitrogenase activity and diazotrophic growth. Glutamine synthetase activity was essential for the assimilation of N2, ammonia, arginine and proline as nitrogen sources but not for the control of their transport, heterocyst formation, and production of ammonia or aminoacid dependent repressor signal for N2-fixing heterocysts. These results also suggest that glutamine synthetase serves as the sole route of ammonia assimilation and glutamine synthesis, and ammonia per se as the repressor signal for N2-fixing heterocysts and methylammonium (ammonium) transport.

Influence of different forms of nitrogen on uptake of ammonium, glutamate and glutamine in the cyanobacterium Nostoc muscorum

Effect of various types of nitrogen nutrition was studied on the uptake of ammonium, glutamate and glutamine by Nostoc muscorum and its Het-Nif- mutant. Ammonium nitrogen acted as a potent inhibitor/repressor of ammonium, glutamate and glutamine transport. Nitrate nitrogen was found to be a strong inhibitor/repressor of ammonium transport, a partial inhibitor/repressor of glutamate transport but, caused a partial stimulation of glutamine transport.

Characteristics of the methylammonium/ammonium transport systems of the nitrogen-fixing cyanobacterium Anabaena 7120 (ATCC 27893)

NH4(+)-transport in Anabaena 7120 was studied using the NH4+ analogue, 14CH3NH3+. At pH 7, two energy-dependent NH4(+)-transport systems were detected in both N2- and NO3(-)-grown cells, but none in NH4(+)-grown cells. Both transport systems showed a low and a high affinity mode of operation depending on the substrate concentration. One of the transport systems showed Km values of 8 microM (Vmax = 1 nmole min-1mg-1protein) and 80 microM (Vmax = 7 nmole min-1mg-1protein), and was insensitive to L-methionine-DL-sulphoximine, a glutamate analogue and irreversible inhibitor of glutamine synthetase. The other transport system showed Km values of 2.5 microM (Vmax = 0.1 nmole min-1mg-1protein) and 70 microM (Vmax = 0.7 nmole min-1mg-1protein), and was sensitive to L-methionine-DL-sulphoximine. Intracellular accumulation of free 14CH3NH3+ showed a biphasic pattern in response to variation in external 14CH3NH3+ concentrations. A maximum intracellular concentration of 2.5 mM and 7.5 mM was reached in the external 14CH3NH3+ concentration range of 1-50 microM and 1-500 microM, respectively. At pH 9, an energy-independent diffusion of 14CH3NH2 leading to a higher intracellular accumulation and assimilation rate, than that at pH 7, was observed.

Absence of the glutamine-synthetase-linked methylammonium (ammonium)-transport system in the cyanobiont of Cycas-cyanobacterial symbiosis

Using the ammonium analogue (14)CH3NH 3 (+) , ammonium transport was studied in the cyanobiont cells freshly isolated from the root nodules of Cycas revoluta. An L-methionine-DL-sulphoximine (MSX)-insensitive ammonium-transport system, which was dependent on membrane potential (ΔΨ), was found in the cyanobiont. However, the cyanobiont was incapable of metabolizing exogenous (14)CH3NH 3 (+) or NH 4 (+) because of the absence of another ammonium-transport system responsible for the uptake of ammonium for assimilation via glutamine synthetase (EC 6.3.1.2). Such a modification seems to be the result of symbiosis because the free-living cultured isolate, Anabaena cycadeae, has been shown to possess both the ammonium-transport systems.

Cyanobacteria-eukaryotic plant symbioses

N2-fixing heterocystous cyanobacteria develop in symbiotic association with a small number of eukaryotic plant species belonging to the algae, fungi, liverworts, ferns, gymnosperms and angiosperm. When the free-living cyanobacteria develop in symbiosis, they become modified morphologically, physiologically and biochemically. The symbiosis are relatively specific, and among the changes which occur in the endophytic cyanobacteria are increases in the size of the vegetative cells, changes in the ultrastructure of the vegetative cells, a tendency for a reduction in the filamentous habit, an increased heterocyst frequency when another photosynthetic partner is present, reduced activities of glutamine synthetase and glutamate synthase, and NH4+ release by; the endophytic cyanobacteria. These and other aspects are considered, emphasizing, in particular, work carried out in the authors' laboratory.

(15)N 2 Incorporation and metabolism in the lichen Peltigera aphthosa Willd

The Nostoc in the cephalodia of the lichen Peltigera aphthosa Willd. fixed (15)N2 and the bulk of the nitrogen fixed was continuously transferred from it to its eukaryotic partners (a fungus and a green alga, Coccomyxa sp.). Kinetic studies carried out over the first 30 min, after exposure of isolated cephalodia to (15)N2, showed that highest initial (15)N2-labelling was into NH 4 (+) . After 12 min little further increase in the NH 4 (+) label occurred while that in the amide group of glutamine and in glutamate continued to increase. The (15)N-labelling of the amino group of glutamine and of aspartate increased more slowly, followed by an increase in the labelling of alanine. When total incorporation of (15)N-label was calculated, the overall pattern was found to be rather similar except that, throughout the experiment, the total (15)N incorporated into glutamate was about six times greater than that into the amide group of glutamine. Pulse chase experiments, in which (14)N2 was added to cephalodia previously exposed to (15)N2, showed that the NH 4 (+) pool rapidly became depleted of (15)N-label, followed by decreases in the labelling of glutamate, the amide group of glutamine and aspartate. The (15)N-labelling of alanine, however, continued to increase for a period. When isolated cephalodia were treated with L-methionine-SR-sulphoximine, an inhibitor of glutamine synthetase (EC 6.3.1.2), and azaserine, an inhibitor of glutamate synthase (EC 2.6.1.53), there was no detectable labelling in glutamine although the (15)N-labelling of glutamate increased unimpaired. On treating the cephalodia with amino-oxyacetate, an inhibitor of aminotransferase activity, the alanine pool decreased. Evidence was obtained that glutamine synthetase and glutamate synthase were located in the Nostoc, and that glutamate dehydrogenase (EC 1.4.1.4) and various amino-transferases were located in the cephalodial fungus. Possible implications of these findings are discussed.

Nitrogenase activity and dark CO2 fixation in the lichen Peltigera aphthosa Willd

The lichen Peltigera aphthosa consists of a fungus and green alga (Coccomyxa) in the main thallus and of a Nostoc located in superficial packets, intermixed with fungus, called cephalodia. Dark nitrogenase activity (acetylene reduction) of lichen discs (of alga, fungus and Nostoc) and of excised cephalodia was sustained at higher rates and for longer than was the dark nitrogenase activity of the isolated Nostoc growing exponentially. Dark nitrogenase activity of the symbiotic Nostoc was supported by the catabolism of polyglucose accumulated in the ligh and which in darkness served to supply ATP and reductant. The decrease in glucose content of the cephalodia paralleled the decline in dark nitrogenase activity in the presence of CO2; in the absence of CO2 dark nitrogenase activity declined faster although the rate of glucose loss was similar in the presence and absence of CO2. Dark CO2 fixation, which after 30 min in darkness represented 17 and 20% of the light rates of discs and cephalodia, respectively, also facilitated dark nitrogenase activity. The isolated Nostoc, the Coccomyxa and the excised fungus all fixed CO2 in the dark; in the lichen most dark CO2 fixation was probably due to the fungus. Kinetic studies using discs or cephalodia showed highest initial incorporation of (14)CO2 in the dark in to oxaloacetate, aspartate, malate and fumarate; incorporation in to alanine and citrulline was low; incorporation in to sugar phosphates, phosphoglyceric acid and sugar alcohols was not significant. Substantial activities of the enzymes phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and carbamoyl-phosphate synthase (EC 2.7.2.5 and 2.7.2.9) were detected but the activities of PEP carboxykinase (EC 4.1.1.49) and PEP carboxyphosphotransferase (EC 4.1.1.38) were negligible. In the dark nitrogenase activity by the cephalodia, but not by the free-living Nostoc, declined more rapidly in the absence than in the presence of CO2 in the gas phase. Exogenous NH 4 (+) inhibited nitrogenase activity by cephalodia in the dark especially in the absence of CO2 but had no effect in the light. The overall data suggest that in the lichen dark CO2 fixation by the fungus may provide carbon skeletons which accept NH 4 (+) released by the cyanobacterium and that in the absence of CO2, NH 4 (+) directly, or indirectly via a mechanism which involves glutamine synthetase, inhibits nitrogenase activity.

Radioimmunoassay of serum digoxin in relation to digoxin intoxication

Serum digoxin estimations were done in 98 patients receiving digoxin for heart failure of varied aetiology. Digoxin toxicity or the lack of it was determined on the basin of established electrocardiographic criteria. Fifty-two patients were classified as 'toxic' and 46 as 'non-toxic'. The difference is the mean digoxin levels between the two groups was highly significant (P less than 0.001). The mean serum digoxin level in 'non-toxic' patients was slightly higher than that found by other investigators. Fairly good correlations have been noted between different dosage schedules and various rhythm disturbances. Death was attributed to digoxin toxicity in only 2 patients who showed electrocardiographic evidence of intoxication at the time of death.