The presence of amino acids affects inorganic N uptake in non-mycorrhizal seedlings of European beech (Fagus sylvatica).

To investigate the impact of organic N compounds for inorganic nitrogen uptake in the rhizosphere, we fed ammonium nitrate with or without amino acids (i.e., glutamine or arginine) to the roots of non-mycorrhizal beech (Fagus sylvatica L.) seedlings under controlled conditions at different levels of N availability. Uptake of individual N sources was determined from ¹5N (inorganic N) and ¹5N ¹³C (organic N) accumulation in the roots. In addition, gene fragments encoding proteins involved in N uptake and metabolism were cloned from beech for gene expression analyses by quantitative real-time PCR in the roots. Generally, ammonium was preferred over nitrate as N source. Organic N sources were taken up by beech roots as intact molecules. Uptake of organic N was significantly higher than inorganic N uptake, thus contributing significantly to N nutrition of beech. Depending on the level of N availability, inorganic N uptake was negatively affected by the presence of organic N sources. This result indicates an overestimation of the contribution of inorganic N uptake to N nutrition of beech in previous studies. Apparently, association with mycorrhizal fungi is not essential for organic N uptake by beech roots. Gene expression analyses showed that transcriptional regulation of the amino acid transporters FsCAT3, FsCAT5, FsAAT and FsAAP and the ammonium transporter FsAMT1.2 in the roots is involved in N nutrition of beech.

RNAi-mediated suppression of isoprene emission in poplar transiently impacts phenolic metabolism under high temperature and high light intensities: a transcriptomic and metabolomic analysis.

In plants, isoprene plays a dual role: (a) as thermo-protective agent proposed to prevent degradation of enzymes/membrane structures involved in photosynthesis, and (b) as reactive molecule reducing abiotic oxidative stress. The present work addresses the question whether suppression of isoprene emission interferes with genome wide transcription rates and metabolite fluxes in grey poplar (Populus x canescens) throughout the growing season. Gene expression and metabolite profiles of isoprene emitting wild type plants and RNAi-mediated non-isoprene emitting poplars were compared by using poplar Affymetrix microarrays and non-targeted FT-ICR-MS (Fourier transform ion cyclotron resonance mass spectrometry). We observed a transcriptional down-regulation of genes encoding enzymes of phenylpropanoid regulatory and biosynthetic pathways, as well as distinct metabolic down-regulation of condensed tannins and anthocyanins, in non-isoprene emitting genotypes during July, when high temperature and light intensities possibly caused transient drought stress, as indicated by stomatal closure. Under these conditions leaves of non-isoprene emitting plants accumulated hydrogen peroxide (H(2)O(2)), a signaling molecule in stress response and negative regulator of anthocyanin biosynthesis. The absence of isoprene emission under high temperature and light stress resulted transiently in a new chemo(pheno)type with suppressed production of phenolic compounds. This may compromise inducible defenses and may render non-isoprene emitting poplars more susceptible to environmental stress.

Changes in sulphur metabolism of grey poplar (Populus x canescens) leaves during salt stress: a metabolic link to photorespiration.

The poplar hybrid Populus x canescens (syn. Populus tremula x Populus alba) was subjected to salt stress by applying 75 mM NaCl for 2 weeks in hydroponic cultures. Decreasing maximum quantum yield (Fv/Fm) indicated damage of photosystem II (PS II), which was more pronounced under nitrate compared with ammonium nutrition. In vivo staining with diaminobenzidine showed no accumulation of H(2)O(2) in the leaf lamina; moreover, staining intensity even decreased. But at the leaf margins, development of necrotic tissue was associated with a strong accumulation of H(2)O(2). Glutathione (GSH) contents increased in response to NaCl stress in leaves but not in roots, the primary site of salt exposure. The increasing leaf GSH concentrations correlated with stress-induced decreases in transpiration and net CO(2) assimilation rates at light saturation. Enhanced rates of photorespiration could also be involved in preventing reactive oxygen species formation in chloroplasts and, thus, in protecting PS II from damage. Accumulation of Gly and Ser in leaves indeed indicates increasing rates of photorespiration. Since Ser and Gly are both immediate precursors of GSH that can limit GSH synthesis, it is concluded that the salt-induced accumulation of leaf GSH results from enhanced photorespiration and is thus probably restricted to the cytosol.

Seasonal nitrogen cycling in the bark of field-grown Grey poplar is correlated with meteorological factors and gene expression of bark storage proteins.

Seasonal tree-internal nitrogen cycling is an important strategy for trees to achieve high efficiency in the use of nitrogen (N). Key processes of this N redistribution are autumnal leaf senescence and storage of released N as bark storage proteins (BSP) in perennial tissues. While the regulation of leaf senescence has been intensively analysed in trees, the coordination of the complementary storage processes is still poorly understood. Therefore, we ascertained relationships between physiological-level and molecular-level processes and environmental factors under natural conditions in the bark of Populus x canescens. We analysed amino-N concentrations, total soluble protein concentration and transcript abundances of BSP genes in the bark of field-grown P. x canescens harvested during two annual growth cycles. By correlation analysis and linear modelling, we assessed interactions between biological data and meteorological conditions. Day length correlated with BSP expression, and air temperature correlated strongly with total protein concentration (r = -0.92), gamma-aminobutyric acid (GABA; r = 0.76) and arginine (r = -0.70). GABA and arginine also correlated significantly with total protein concentration and transcript abundances of BSP genes. We conclude that GABA and arginine potentially contribute to adjust storage processes in the bark of poplar trees to seasonal changes in environmental conditions.

Expression of the Arabidopsis mutant ABI1 gene alters abscisic acid sensitivity, stomatal development, and growth morphology in gray poplars.

The consequences of altered abscisic acid (ABA) sensitivity in gray poplar (Populus x canescens [Ait.] Sm.) development were examined by ectopic expression of the Arabidopsis (Arabidopsis thaliana) mutant abi1 (for abscisic acid insensitive1) gene. The expression resulted in an ABA-insensitive phenotype revealed by a strong tendency of abi1 poplars to wilt, impaired responsiveness of their stomata to ABA, and an ABA-resistant bud outgrowth. These plants therefore required cultivation under very humid conditions to prevent drought stress symptoms. Morphological alterations became evident when comparing abi1 poplars with poplars expressing Arabidopsis nonmutant ABI1 or wild-type plants. abi1 poplars showed increased stomatal size, enhanced shoot growth, and retarded leaf and root development. The increased stomatal size and its reversion to the size of wild-type plants by exogenous ABA indicate a role for ABA in regulating stomatal development. Enhanced shoot growth and retarded leaf and root development support the hypothesis that ABA acts independently from drought stress as a negative regulator of growth in shoots and as a positive regulator of growth in leaves and roots. In shoots, we observed an interaction of ABA with ethylene: abi1 poplars exhibited elevated ethylene production, and the ethylene perception inhibitor Ag(+) antagonized the enhanced shoot growth. Thus, we provide evidence that ABA acts as negative regulator of shoot growth in nonstressed poplars by restricting ethylene production. Furthermore, we show that ABA has a role in regulating shoot branching by inhibiting lateral bud outgrowth.

Natural variation in Arabidopsis adaptation to growth at low nitrogen conditions.

Improving nutrient use efficiency of crop plants, especially at low input, is essential to ensure sustainable food production in the future. In order to address the genetic basis of nutrient use efficiency in a model system, growth of Arabidopsis ecotypes at normal and low nitrogen (N) supply was compared. The ecotypes differed significantly in the extent of growth reduction in limiting conditions. The fresh weight of Shahdara and Ws grown at 1mM nitrate was reduced by 30% compared to control, whereas Col-0 and Ga-0 were almost unaffected. Total N content was reduced in all ecotypes by 10-30%. The capacity to store nitrate correlated with the tolerance to low N; in Shahdara and Ws, but not in Col-0 and Ga-0, nitrate content on low N was significantly reduced compared to control nutrition. The mRNA levels for genes of nitrate uptake and assimilation were only moderately affected by the treatment. The transcript levels of nitrate reductase NIA1 and nitrite reductase were higher in the ecotypes tolerant to low N (Col-0 and Ga-0) with normal N nutrition but on low N they were reduced to a much higher extent than the sensitive ecotypes (Shahdara and Ws). It seems that a higher capacity to keep nitrate reserves at low N, perhaps due to the ability to turn down nitrate reduction rate, is responsible for a better tolerance of Col-0 and Ga-0 to low N supply.

Transgenic, non-isoprene emitting poplars don't like it hot.

The physiological role of isoprene emission in plants is a matter of much debate. One of the most widely propagated hypotheses suggests a function of isoprene in the protection of leaf physiological processes against thermal and oxidative stress. To test this hypothesis, we developed transgenic Grey poplar (Populusxcanescens) plants in which gene expression of isoprene synthase (ISPS) was either silenced by RNA interference (RNAi) or upregulated by over-expression of the ISPS gene. Despite increased ISPS mRNA levels, we did not observe consistent increases in isoprene emission in the over-expressing lines, indicating post-transcriptional control of ISPS by co-suppression. In the RNAi lines, levels of isoprene emission were effectively suppressed to virtually zero. Transgenic plants were subjected to temperature stress with three transient heat phases of 38-40 degrees C, each followed by phases of recovery at 30 degrees C. Parallel measurements of gas exchange, chlorophyll fluorescence and isoprene emission provided new insights into the physiological link between isoprene and enhanced temperature tolerance. Transgenic non-isoprene-emitting poplars showed reduced rates of net assimilation and photosynthetic electron transport during heat stress, but not in the absence of stress. The decrease in the efficiency of photochemistry was inversely correlated with the increase in heat dissipation of absorbed light energy, measured as NPQ (non-photochemical quenching). Isoprene-repressed poplars also displayed an increased formation of the xanthophyll cycle pigment zeaxanthin in the absence of stress, which can cause increased NPQ or may indicate an increased requirement for antioxidants. In conclusion, using a molecular genetic approach, we show that down-regulation of isoprene emission affects thermotolerance of photosynthesis and induces increased energy dissipation by NPQ pathways.

Calcium nutrition has a significant influence on wood formation in poplar.

To test the effects of calcium on wood formation, Populus tremula x Populus tremuloides clones were supplied with Hoagland solution modified in its calcium contents. Energy-dispersive X-ray analysis (EDXA) revealed an increase in calcium in the phloem, the cambium and the xylem elongation zone with increasing Ca(2+) supply in the nutrient solution. Using light and electron microscopy, a strong impact was shown on the cambial and the elongation zones under calcium starvation. Using Fourier transform infrared (FTIR) spectroscopy on wood and bark cells formed under calcium starvation, we detected a reduction of some absorptions, such as carbonyl and methoxy groups from S-lignin. Also, a significant reduction in fiber length was detected with decreasing calcium supply in the nutrient solution. High-performance liquid chromatography (HPLC) analysis revealed a large increase in sugar concentrations in the leaves, but reduced concentrations in the bark under Ca(2+) deficiency. In conclusion, our results show a significant influence of calcium on the structure, chemistry and physiology of wood formation. Thus, efficient Ca(2+) supply has to be considered a decisive factor in wood formation.