Response of rice seedlings to aluminium stress with varying phosphate supplies

In this study the effects of Al and phosphate (P) treatments on the growth and potassium influx in roots and transport toward the shoots of rice (Oryza sativa L. cv. Oryzella) was examined. Seedlings were grown hydroponically at pH 4.1 with different levels of Al and P. Increased levels of Al in the growth solution reduced root growth in accordance of the Al-concentration. Shoot growth was only moderately influenced in 14 day experiment. P (as phosphate) supply in the growth solution enabled rice plants to overcome the Al toxicity symptoms, however, the available P concentrations must exceed the Al concentration in the external medium. In short-term (6h) uptake experiments, P reduced the Al-stimulated K+(86Rb) influx anomaly in roots, which indicates a definite Al-P interaction at the plant level too. Stimulation of glutamine synthetase activity was confirmed by in vitro tests and western blot detection after native PAGE separation of enzyme isoforms

A nitrogén metabolizmusban kulcsszerepet betöltő glutamin szintetáz enzim indikátorszerepének vizsgálata abiotikus stressznek kitett búza növényekben = Study of the indicatory role of the nitrogen metabolism enzyme glutamine synthetase in wheat exposed to abiotic stress

A szárazságstressz és a savas talajokon fellépő alumínium toxicitás élettani hátterét vizsgáltuk. A gabonafélék fakultatív metabolikus változásokkal reagálnak a kedvezőtlen körülményekre, ennek hatása a C/N anyagcserében is megjelenik. A glutamin szintetáz (GS, EC 6.3.1.2) izoenzimeinek arány és aktivitás változását megmérve igazoltuk feltételezett stressz-indikátor szerepét. Az izoenzimek aktivitásváltozása meghatározott élettani folyamatokat tükröz. A C3-as növények levelében a legnagyobb mennyiségben levő fehérjét, a Rubisco-t párhuzamosan mérve kimutattuk ennek a „N raktárnak” a lebontását, a N-remobilizálódást. Az enzim jól jelezte, hogy a magfeltöltődés idején alkalmazott szárazságstressz felgyorsítja a szeneszcenciát a fiatalabb levelekben ill. a zászlóslevélben a kevésbé toleráns fajtákban: az asszimiláló „source” jelleg átalakul degradatív „source”-szá. Az alumínium stressz tolerancia egyik növényekben leírt módja az Al szerves ligandumokhoz kapcsolása, komplexálása. A glutamin szintetáz szerkezetében két eltérő affinitású fémkötő helyet találunk, melyek az enzim katalitikus és strukturális tulajdonságait határozzák meg. A szerves Al(III)-komplexek jelenlétében felvett kinetikai görbék értékelésével az Al toxikus hatásának a mértéke meghatározható. Kimutattuk, hogy alumíniummal kezelt és ezt követően izolált GS enzim specifikus fémkötő helyeinek egyikét az Al elfoglalja, és serkenti a GS aktivitását. | The project dealt with drought and soil-borne aluminium toxicity. Cereals respond with facultative metabolic changes to these unfavourable conditions. Measuring the ratio and activity of isoforms of glutamine synthetase (GS, EC 6.3.1.2) the supposed indicatory role of this enzyme was proved. Changes in activity of GS isoforms reflect well-defined physiological processes. Rubisco is the major protein of leaves of C3 plants. The breakdown of this protein pool, the mobilization of N was detected too. The drought stress during the grain filling period initiated senescence in the younger leaves (and in flag leaf) and accelerated it in the older ones as GS indicated: the leaves converted from assimilatory source into degradative source in case of less tolerant wheat cultivars. There is an increasing gradient of GS activity and of the protein content in the series of leaves from the older towards the younger ones. In case of tolerance only the slope of this gradient increases, but in case of sensitivity the gradient breaks. Al resistance can be achieved by mechanisms that facilitate Al exclusion from cells and mechanisms that confer the ability of plants to tolerate Al in the symplast. Eukaryotic GS requires two divalent metal ions per subunit for activity, which makes GS a potential target of metal stress. The Al from an organic metal complex is able to activate GS, Al becomes bound to the polypeptide structure of the GS molecule, occupying one of the specific metal binding sites

Effect of salinity stress on ion accumulation and on the photosynthetic activity of a new energy plant, phalaris arundinacea cultivars

Reed canarygrass (Phalaris arundinacea) is a good candidate for bioenergy production in Northern and Middle Europe. The crop is well-adapted to cold and drought stress but its resistance to high salinity has not been revealed in details. In this study the effects of 75 and 150 mM NaCl treatments were investigated on the ion accumulation, water potential changes and photosynthetic activity of three Romanian reed canarygrass genotypes, Tardin, Romanesti diverse and Timpuriu. Since cv. Tardin was able to maintain high K+ level and relatively low Na+ concentration in leaf tissues, high stomatal conductance and net CO2 fixation rate under salt stress and as it could maintain the water potential of tissues at control level, this genotype can be defined as salt tolerant. Salt stress induced significant Na+ accumulation, very low K /Na+ ratio, and severe reduction in stomatal conductance and photosynthetic activity in the leaf tissues of Timpuriu cultivar, which proved to be sensitive to high salinity

Chloroplast Glutamine Synthetase, the Key Regulator of Nitrogen Metabolism in Wheat, Performs Its Role by Fine Regulation of Enzyme Activity via Negative Cooperativity of Its Subunits

Glutamine synthetase (GS) is of central interest as the main route of ammonia assimilation in plants, and as a connection point between the organic and inorganic worlds. Even though GS activity is critical for producing high yields of crop plants, the autoregulation of substrate consumption of wheat GS remained unknown until now. Here we show kinetic evidence, that the chloroplast localized GS isoform (GS2) of wheat (Triticum aestivum L. cv. Jubilejnaja-50) takes place at the carbon-nitrogen metabolic branch point, where it is a mediator, and its enzymatic activity is regulated in a negatively cooperative allosteric manner. We have discovered that GS2 activity is described by a tetraphasic kinetic curve in response to increasing levels of glutamate supply. We constructed a model that explains the kinetic properties of glutamate consumption and this unique allosteric behavior. We also studied the subunit composition of both wheat leaf GS isoenzymes by a combination of two dimensional gel electrophoresis and protein blotting. Both leaf isozymes have homogeneous subunit composition. Glutamate is both a substrate, and an allosteric regulator of the biosynthetic reaction. We have concluded on the basis of our results and previous reports, that wheat GS2 is probably a homooctamer, and that it processes its substrate in a well-regulated, concentration dependent way, as a result of its negatively cooperative, allosteric activity. Thus, GS2 has a central role as a regulator between the nitrogen and the carbon cycles via maintaining glutamine-glutamate pool in the chloroplast on the level of substrates, in addition to its function in ammonia assimilation

Metabolic indicators of drought stress tolerance in wheat: Glutamine synthetase isoenzymes and Rubisco

Drought stress has a considerable impact on the ecosystem and agriculture. Continuous water deficit induces early leaf senescence in plants. During this process, chloroplasts are degraded and photosynthesis drastically drops. The objective of this investigation was to look into the regulation of nitrogen and carbon metabolism during water deficit. Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase; EC 4.1.1.39) and the total protein contents inform us of the sink-source relation in plants. Glutamine synthetase (GS, EC 6.3.1.2) isoenzymes are good markers of plastid status (GS2) and the nitrogen metabolism (GS1). Tolerant and sensitive wheat (Triticum aestivum L.) genotypes were tested, which are widely used in agriculture. The amount of protein, Rubisco and GS isoforms in leaves were measured during the grain filling period, as indicative traits that ultimately determine the onset and stage of senescence. The symptoms of senescence first appeared on the oldest and finally on the youngest leaves. Drought stress disrupted the sequentiality of senescence in the sensitive varieties. An untimely senescence appeared in flag leaves, earlier than in the older leaves. Total protein and Rubisco contents decreased and the GS2 isoenzyme declined considerably in the youngest leaves. In the tolerant varieties, however, these physiological parameters did not change under drought, only the sequential senescence of leaf levels accelerated in some cases compared to the control, well-watered plants. Our results revealed that GS is a good indicator of drought stress, which can be applied for the characterization of wheat cultivars in terms of drought stress tolerance