SummaryThe 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 (Populus · canescens) 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 posttranscriptional 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°C, each followed by phases of recovery at 30°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). Isoprenerepressed 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.

Many forest ecosystems have evolved at sites with growth-limiting nitrogen (N) availability, low N input from external sources and high ecosystem internal cycling of N. By contrast, many poplar species are frequent constituents of floodplain forests where they are exposed to a significant ecosystem external supply of N, mainly nitrate, in the moving water table. Therefore, nitrate is much more important for N nutrition of these poplar species than for many other tree species. We summarise current knowledge of nitrate uptake and its regulation by tree internal signals, as well as acquisition of ammonium and organic N from the soil. Unlike herbaceous plants, N nutrition of trees is sustained by seasonal, tree internal cycling. Recent advances in the understanding of seasonal storage and mobilisation in poplar bark and regulation of these processes by temperature and daylength are addressed. To explore consequences of global climate change on N nutrition of poplar trees, responses of N uptake and metabolism to increased atmospheric CO(2) and O(3) concentrations, increased air and soil temperatures, drought and salt stress are highlighted.

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 (Populusxcanescens) 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 (H2O2), 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.Electronic supplementary materialThe online version of this article (doi:10.1007/s11103-010-9654-z) contains supplementary material, which is available to authorized users.

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.