Casein as protein and hydrolysate: Biostimulant or nitrogen source for Nicotiana tabacum plants grown in vitro?

In contrast to inorganic nitrogen (N) assimilation, the role of organic N forms, such as proteins and peptides, as sources of N and their impact on plant metabolism remains unclear. Simultaneously, organic biostimulants are used as priming agents to improve plant defense response. Here, we analysed the metabolic response of tobacco plants grown in vitro with casein hydrolysate or protein. As the sole source of N, casein hydrolysate enabled tobacco growth, while protein casein was used only to a limited extent. Free amino acids were detected in the roots of tobacco plants grown with protein casein but not in the plants grown with no source of N. Combining hydrolysate with inorganic N had beneficial effects on growth, root N uptake and protein content. The metabolism of casein-supplemented plants shifted to aromatic (Trp), branched-chain (Ile, Leu, Val) and basic (Arg, His, Lys) amino acids, suggesting their preferential uptake and/or alterations in their metabolic pathways. Complementarily, proteomic analysis of tobacco roots identified peptidase C1A and peptidase S10 families as potential key players in casein degradation and response to N starvation. Moreover, amidases were significantly upregulated, most likely for their role in ammonia release and impact on auxin synthesis. In phytohormonal analysis, both forms of casein influenced phenylacetic acid and cytokinin contents, suggesting a root system response to scarce N availability. In turn, metabolomics highlighted the stimulation of some plant defense mechanisms under such growth conditions, that is, the high concentrations of secondary metabolites (e.g., ferulic acid) and heat shock proteins.

Effects of chilling on the photosynthetic performance of the CAM orchid Phalaenopsis

INTRODUCTION

Crassulacean acid metabolism (CAM) is one of the three main metabolic adaptations for CO₂ fixation found in plants. A striking feature for these plants is nocturnal carbon fixation and diurnal decarboxylation of malic acid to feed Rubisco with CO₂ behind closed stomata, thereby saving considerable amounts of water. Compared to the effects of high temperatures, drought, and light, much less information is available about the effects of chilling temperatures on CAM plants. In addition a lot of CAM ornamentals are grown in heated greenhouses, urging for a deeper understanding about the physiological responses to chilling in order to increase sustainability in the horticultural sector.

METHODS

The present study focuses on the impact of chilling temperatures (10°C) for 3 weeks on the photosynthetic performance of the obligate CAM orchid Phalaenopsis 'Edessa'. Detailed assessments of the light reactions were performed by analyzing chlorophyll a fluorescence induction (OJIP) parameters and the carbon fixation reactions by measuring diel leaf gas exchange and diel metabolite patterns.

RESULTS AND DISCUSSION

Results showed that chilling already affected the light reactions after 24h. Whilst the potential efficiency of photosystem II (PSII) (F_v/F_m) was not yet influenced, a massive decrease in the performance index (PI_abs) was noticed. This decrease did not depict an overall downregulation of PSII related energy fluxes since energy absorption and dissipation remained uninfluenced whilst the trapped energy and reduction flux were upregulated. This might point to the presence of short-term adaptation mechanisms to chilling stress. However, in the longer term the electron transport chain from PSII to PSI was affected, impacting both ATP and NADPH provision. To avoid over-excitation and photodamage plants showed a massive increase in thermal dissipation. These considerations are also in line with carbon fixation data showing initial signs of cold adaptation by achieving comparable Rubisco activity compared to unstressed plants but increasing daytime stomatal opening in order to capture a higher proportion of CO₂ during daytime. However, in accordance with the light reactions data, Rubisco activity declined and stomatal conductance and CO₂ uptake diminished to near zero levels after 3 weeks, indicating that plants were not successful in cold acclimation on the longer term.

Enhanced Laterally Resolved ToF-SIMS and AFM Imaging of the Electrically Conductive Structures in Cable Bacteria

Cable bacteria are electroactive bacteria that form a long, linear chain of ridged cylindrical cells. These filamentous bacteria conduct centimeter-scale long-range electron transport through parallel, interconnected conductive pathways of which the detailed chemical and electrical properties are still unclear. Here, we combine time-of-flight secondary-ion mass spectrometry (ToF-SIMS) and atomic force microscopy (AFM) to investigate the structure and composition of this naturally occurring electrical network. The enhanced lateral resolution achieved allows differentiation between the cell body and the cell-cell junctions that contain a conspicuous cartwheel structure. Three ToF-SIMS modes were compared in the study of so-called fiber sheaths (i.e., the cell material that remains after the removal of cytoplasm and membranes, and which embeds the electrical network). Among these, fast imaging delayed extraction (FI-DE) was found to balance lateral and mass resolution, thus yielding the following multiple benefits in the study of structure-composition relations in cable bacteria: (i) it enables the separate study of the cell body and cell-cell junctions; (ii) by combining FI-DE with in situ AFM, the depth of Ni-containing protein-key in the electrical transport-is determined with greater precision; and (iii) this combination prevents contamination, which is possible when using an ex situ AFM. Our results imply that the interconnects in extracted fiber sheaths are either damaged during extraction, or that their composition is different from fibers, or both. From a more general analytical perspective, the proposed methodology of ToF-SIMS in the FI-DE mode combined with in situ AFM holds great promise for studying the chemical structure of other biological systems.

A highly conductive fibre network enables centimetre-scale electron transport in multicellular cable bacteria

Biological electron transport is classically thought to occur over nanometre distances, yet recent studies suggest that electrical currents can run along centimetre-long cable bacteria. The phenomenon remains elusive, however, as currents have not been directly measured, nor have the conductive structures been identified. Here we demonstrate that cable bacteria conduct electrons over centimetre distances via highly conductive fibres embedded in the cell envelope. Direct electrode measurements reveal nanoampere currents in intact filaments up to 10.1 mm long (>2000 adjacent cells). A network of parallel periplasmic fibres displays a high conductivity (up to 79 S cm⁻¹), explaining currents measured through intact filaments. Conductance rapidly declines upon exposure to air, but remains stable under vacuum, demonstrating that charge transfer is electronic rather than ionic. Our finding of a biological structure that efficiently guides electrical currents over long distances greatly expands the paradigm of biological charge transport and could enable new bio-electronic applications.

Cable bacteria’ form long multicellular filaments that can transfer electrical currents over centimetre-long distances. Here, Meysman et al. show that the electrical currents run along highly conductive fibres embedded in the cell envelope, and charge transfer is electronic rather than ionic.

Performance Index and PSII Connectivity Under Drought and Contrasting Light Regimes in the CAM Orchid Phalaenopsis

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis characterized by improved water use efficiency mediated by major nocturnal CO2 fixation. Due to its inherent metabolic plasticity CAM represents a successful physiological strategy for plant adaptation to abiotic stress. The present study reports on the impact of drought stress and different light intensities (PPFD 50 and 200 μmol m-2 s-1) on the photosynthetic performance of the obligate CAM orchid Phalaenopsis "Edessa" by integrating diel gas exchange patterns with assessments of the light reactions by analyzing fast chlorophyll a fluorescence induction. Parameters such as PIabs (performance index), different energy fluxes per active reaction centre (RC) reflecting the electron flow from photosystem II to photosystem I and the energetic communication between PSII complexes defined as connectivity were considered for the first time in a CAM plant. A higher PS II connectivity for plants grown under low light (p ∼ 0.51) compared to plants grown under high light (p ∼ 0.31) brought about similar specific energy fluxes of light absorbance, dissipation and processing through the electron transport chain, irrespective of the light treatment. With a 25% higher maximum quantum yield and comparable biomass formation, low light grown plants indeed proved to process light energy more efficiently compared to high light grown plants. The performance index was identified as a very reliable and sensitive parameter to indicate the onset and progress of drought stress. Under restricted CO2 availability (due to closed stomata) leaves showed higher energy dissipation and partial inactivation of PSII reaction centres to reduce the energy input to the electron transport chain and as such aid in avoiding overexcitation and photodamage. Especially during CAM idling there is a discrepancy between continuous input of light energy but severely reduced availability of both water and CO2, which represents the ultimate electron acceptor. Taken together, our results show a unique flexibility of CAM plants to optimize the light reactions under different environmental conditions in a dual way by either attenuating or increasing energy flux.

The Cell Envelope Structure of Cable Bacteria

Cable bacteria are long, multicellular micro-organisms that are capable of transporting electrons from cell to cell along the longitudinal axis of their centimeter-long filaments. The conductive structures that mediate this long-distance electron transport are thought to be located in the cell envelope. Therefore, this study examines in detail the architecture of the cell envelope of cable bacterium filaments by combining different sample preparation methods (chemical fixation, resin-embedding, and cryo-fixation) with a portfolio of imaging techniques (scanning electron microscopy, transmission electron microscopy and tomography, focused ion beam scanning electron microscopy, and atomic force microscopy). We systematically imaged intact filaments with varying diameters. In addition, we investigated the periplasmic fiber sheath that remains after the cytoplasm and membranes were removed by chemical extraction. Based on these investigations, we present a quantitative structural model of a cable bacterium. Cable bacteria build their cell envelope by a parallel concatenation of ridge compartments that have a standard size. Larger diameter filaments simply incorporate more parallel ridge compartments. Each ridge compartment contains a ~50 nm diameter fiber in the periplasmic space. These fibers are continuous across cell-to-cell junctions, which display a conspicuous cartwheel structure that is likely made by invaginations of the outer cell membrane around the periplasmic fibers. The continuity of the periplasmic fibers across cells makes them a prime candidate for the sought-after electron conducting structure in cable bacteria.

Virulence of Erwinia amylovora, a prevalent apple pathogen: Outer membrane proteins and type III secreted effectors increase fitness and compromise plant defenses

Until now, no data are available on the outer membrane (OM) proteome of Erwinia amylovora, a Gram-negative plant pathogen, causing fire blight in most of the members of the Rosaceae family. Since the OM forms the interface between the bacterial cell and its environment it is in direct contact with the host. Additionally, the type III secretion system, embedded in the OM, is a pathogenicity factor of E. amylovora. To assess the influence of the OM composition and the secretion behavior on virulence, a 2D-DIGE analysis and gene expression profiling were performed on a high and lower virulent strain, both in vitro and in planta. Proteome data showed an increase in flagellin for the lower virulent strain in vitro, whereas, in planta several interesting proteins were identified as being differently expressed between both the strains. Further, gene expression of nearly all type III secreted effectors was elevated for the higher virulent strain, both in vitro and in planta. As a first, we report that several characteristics of virulence can be assigned to the OM proteome. Moreover, we demonstrate that secreted proteins prove to be the important factors determining differences in virulence between the strains, otherwise regarded as homogeneous on a genome level.

A comparative proteome analysis reveals flagellin, chemotaxis regulated proteins and amylovoran to be involved in virulence differences between Erwinia amylovora strains

Erwinia amylovora is a Gram-negative bacterium that causes the destructive disease fire blight affecting most members of the Rosaceae family, of which apple and pear are economically the most important hosts. E. amylovora has been considered as a homogeneous species in whole, although significant differences in virulence patterns have been observed. However, the underlying causes of the differences in virulence remain to be discovered. In a first-time comparative proteomic approach using E. amylovora, 2D differential in-gel electrophoresis (DIGE) was used to identify proteins that could explain the gradual difference in virulence between four different strains. Two important proteins were identified, FliC and CheY, both involved in flagella structure, motility and chemotaxis, which were more abundant in the least virulent strain. In the highly virulent strains the protein GalF, involved in amylovoran production, was more abundant, which was consistent with the higher expression of the gene and the higher amylovoran content in this strain in vitro. Together, these results confirm the involvement of amylovoran in virulence, but also imply an indirect role of flagellin in virulence as elicitor of plant defence.

BIOLOGICAL SIGNIFICANCE

This research provides new insights into our current understanding of the virulence of Erwinia amylovora. This plant-pathogen is considered a homogeneous species although different strains show differences in virulence. Despite the efforts made on the genomic level which resulted in the discovery of virulence factors, the reason for the different virulence patterns between strains has not yet been identified. In our lab we used a comparative proteomic approach, which has never been published before, to identify proteins involved in these differences between strains and hereby possibly involved in virulence. Our results provide interesting insights in virulence and present us with the opportunity to glance into the proteome of E. amylovora.

The Arabidopsis thaliana RNA editing factor SLO2, which affects the mitochondrial electron transport chain, participates in multiple stress and hormone responses

Recently, we reported that the novel mitochondrial RNA editing factor SLO2 is essential for mitochondrial electron transport, and vital for plant growth through regulation of carbon and energy metabolism. Here, we show that mutation in SLO2 causes hypersensitivity to ABA and insensitivity to ethylene, suggesting a link with stress responses. Indeed, slo2 mutants are hypersensitive to salt and osmotic stress during the germination stage, while adult plants show increased drought and salt tolerance. Moreover, slo2 mutants are more susceptible to Botrytis cinerea infection. An increased expression of nuclear-encoded stress-responsive genes, as well as mitochondrial-encoded NAD genes of complex I and genes of the alternative respiratory pathway, was observed in slo2 mutants, further enhanced by ABA treatment. In addition, H2O2 accumulation and altered amino acid levels were recorded in slo2 mutants. We conclude that SLO2 is required for plant sensitivity to ABA, ethylene, biotic, and abiotic stress. Although two stress-related RNA editing factors were reported very recently, this study demonstrates a unique role of SLO2, and further supports a link between mitochondrial RNA editing events and stress response.

A field study on solar-induced chlorophyll fluorescence and pigment parameters along a vertical canopy gradient of four tree species in an urban environment

To better understand the potential uses of vegetation indices based on the sun-induced upward and downward chlorophyll fluorescence at leaf and at canopy scales, a field study was carried out in the city of Valencia (Spain). Fluorescence yield (FY) indices were derived for trees at different traffic intensity locations and at three canopy heights. This allowed investigating within-tree and between-tree variations of FY indices for four tree species. Several FY indices showed a significant (p < 0.05) and important effect of tree location for the species Morus alba (white mulberry) and Phoenix canariensis (Canary Island date palm). The upward FY parameters of M. alba, and the upward to downward ratios at 687 and 741 nm for both species, were significantly related to tree location. It was found that not the total chlorophyll (Chl) content, but rather the Chl a/b ratio showed the strongest correlations with several of the indices applied. Chl a/b was lowest at the bottom level of the highest traffic intensity location for both species due to an increased Chl b, indicating a larger light harvesting complex related to Photosystem II (LHCII) as a response to limiting light. The leaf deposits from traffic observed at this sampling location possibly led to a shading effect, resulting further in an adaptive response of the photosynthetic system and subsequent difference of FY indices. This study therefore indicated the importance of the size of LHCII on the fluorescence emission, observed under different traffic generated pollution conditions.

Erwinia amylovora affects the phenylpropanoid-flavonoid pathway in mature leaves of Pyrus communis cv. Conférence

Flavonoids, which are synthesized by the phenylpropanoid-flavonoid pathway, not only contribute to fruit colour and photoprotection, they also may provide antimicrobial and structural components during interaction with micro-organisms. A possible response of this pathway was assessed in both mature and immature leaves of shoots of 2-year-old pear trees cv. Conférence, which were inoculated with the gram-negative bacterium Erwinia amylovora strain SGB 225/12, were mock-inoculated or were left untreated. The phenylpropanoid-flavonoid pathway was analysed by histological studies, by gene expression using RT-qPCR and by HPLC analyses of the metabolites at different time intervals after infection. Transcription patterns of two key genes anthocyanidin reductase (ANR) and chalcone synthase (CHS) related to the phenylpropanoid-flavonoid pathway showed differences between control, mock-inoculated and E. amylovora-inoculated mature leaves, with the strongest reaction 48 h after inoculation. The impact of E. amylovora was also visualised in histological sections, and confirmed by HPLC, as epicatechin -which is produced via ANR- augmented 72 h after inoculation in infected leaf tissue. Besides the effect of treatments, ontogenesis-related differences were found as well. The increase of certain key genes, the rise in epicatechin and the visualisation in several histological sections in this study suggest a non-negligible impact on the phenylpropanoid-flavonoid pathway in Pyrus communis due to inoculation with E. amylovora. In this study, we propose a potential role of this pathway in defence mechanisms, providing a detailed analysis of the response of this system attributable to inoculation with E. amylovora.

Seasonal, diurnal and vertical variation of chlorophyll fluorescence on Phyllostachys humilis in Ireland

In recent years, temperate bamboo species have been introduced in Europe not only as an ornamental plant, but also as a new biomass crop. To measure adaptation stress of bamboo to the climate of Western Europe, chlorophyll fluorescence was measured on a diurnal and seasonal basis in Ballyboughal, Co. Dublin, Ireland. Measurements were attained on the leaves of each node of Phyllostachys humilis. The most frequently used parameter in chlorophyll fluorescence is the photosynthetic efficiency (Fv/Fm). A seasonal dip - as well as a larger variation - of Fv/Fm in spring compared to the rest of the year was observed. Over the year, the upper leaves of the plant perform better than the bottom leaves. These findings were linked to environmental factors such as light intensity, air temperature and precipitation, as increased light intensities, decreasing air temperatures and their interactions, also with precipitation levels have an effect on the photosynthetic efficiency (Fv/Fm) in these plants.

Evaluation of the photosynthetic activity in transgenic tobacco plants with altered endogenous cytokinin content: lessons from cytokinin

Cytokinin is known to be involved in many processes related to plastid development and function but the exact role of cytokinin in photosynthesis remains elusive. To investigate more profoundly the effects of cytokinin in this process, the photosynthetic activity of transgenic Pssuipt and 35S:CKX1 tobacco (Nicotiana tabacum) plants with respectively elevated and reduced endogenous cytokinin content was evaluated. Pigment analysis indicated that elevated endogenous cytokinin content resulted in increased pigment content. Functional analysis of the photosynthetic apparatus by chlorophyll a fluorescence and in vitro electron transport measurements clearly showed that changing the endogenous cytokinin content affects the activity of the photosynthetic apparatus. Surprisingly, both an increase as well as a decrease in cytokinin content results in a better photosynthetic performance. Quenching analysis revealed that the initial responses of the photosynthetic apparatus on a dark-light transition are not affected by changed cytokinin content. However, it has an effect on the further kinetic behavior. Taken together, we suggest that cytokinins can induce structural changes in the different parts of the electron transport chain as also demonstrated by the in vitro electron transport measurements.

Analysis of the photosynthetic apparatus in transgenic tobacco plants with altered endogenous cytokinin content: a proteomic study

Background

Cytokinin is a plant hormone that plays a crucial role in several processes of plant growth and development. In recent years, major breakthroughs have been achieved in the elucidation of the metabolism, the signal perception and transduction, as well as the biological functions of cytokinin. An important activity of cytokinin is the involvement in chloroplast development and function. Although this biological function has already been known for 50 years, the exact mechanisms remain elusive.

Results

To elucidate the effects of altered endogenous cytokinin content on the structure and function of the chloroplasts, chloroplast subfractions (stroma and thylakoids) from transgenic Pssu-ipt and 35S:CKX1 tobacco (Nicotiana tabacum) plants with, respectively, elevated and reduced endogenous cytokinin content were analysed using two different 2-DE approaches. Firstly, thykaloids were analysed by blue-native polyacrylamide gel electrophoresis followed by SDS-PAGE (BN/SDS-PAGE). Image analysis of the gel spot pattern thus obtained from thylakoids showed no substantial differences between wild-type and transgenic tobacco plants. Secondly, a quantitative DIGE analysis of CHAPS soluble proteins derived from chloroplast subfractions indicated significant gel spot abundance differences in the stroma fraction. Upon identification by MALDI-TOF/TOF mass spectrometry, these proteins could be assigned to the Calvin-Benson cycle and photoprotective mechanisms.

Conclusion

Taken together, presented proteomic data reveal that the constitutively altered cytokinin status of transgenic plants does not result in any qualitative changes in either stroma proteins or protein complexes of thylakoid membranes of fully developed chloroplasts, while few but significant quantitative differences are observed in stroma proteins.

Selection of plastid- and nuclear-encoded reference genes to study the effect of altered endogenous cytokinin content on photosynthesis genes in Nicotiana tabacum

Selection and use of appropriate reference genes as internal controls in real-time reverse transcription PCR (RT-PCR) assays is highly important for accurate quantification of gene expression levels. Since some photosynthetic genes are encoded in the nuclear genome and others in the chloroplast genome, we evaluated both nuclear- and plastid-encoded candidate reference genes. Six plastid-encoded candidate reference genes were derived from Arabidopsis microarray data and three plastid- and five nuclear-encoded reference genes were derived from literature. Cytokinins influence photosynthetic gene expression, so we evaluated the expression stability of the candidate reference genes in transgenic Nicotiana tabacum plants with elevated or diminished cytokinin content. We found that the most reliable strategy makes use of plastid-encoded genes for normalizing plastid photosynthetic genes and nuclear-encoded reference genes for normalizing nuclear photosynthetic genes. Compared to the use of nuclear reference genes only, this approach assimilates any effects on transcriptional activity of chloroplasts or number of chloroplast. The best expression stabilities in Nicotiana tabacum were observed for the plastid-encoded references genes Nt-RPS3, Nt-NDHI and Nt-IN1 and for the nuclear-encoded genes Nt-ACT9, Nt-alphaTUB and Nt-SSU. These genes may be suitable for normalization of photosynthetic genes under other experimental conditions in Nicotiana tabacum, and orthologues of these genes may be suitable candidates for normalizing photosynthetic gene expression in other species.

Effects of climate warming and species richness on photochemistry of grasslands

In view of the projected climatic changes and the global decrease in plant species diversity, it is critical to understand the effects of elevated air temperature (T(air)) and species richness (S) on physiological processes in plant communities. Therefore, an experiment of artificially assembled grassland ecosystems, with different S (one, three or nine species), growing in sunlit climate-controlled chambers at ambient T(air) and ambient T(air) + 3 degrees C was established. We investigated whether grassland species would be more affected by midday high-temperature stress during summer in a warmer climate scenario. The effect of elevated T(air) was expected to differ with S. This was tested in the second and third experimental years by means of chlorophyll a fluorescence. Because acclimation to elevated T(air) would affect the plant's stress response, the hypothesis of photosynthetic acclimation to elevated T(air) was tested in the third year by gas exchange measurements in the monocultures. Plants in the elevated T(air) chambers suffered more from midday stress on warm summer days than those in ambient chambers. In absence of severe drought, the quantum yield of PSII was not affected by elevated T(air). Our results further indicate that species had not photosynthetically acclimated to a temperature increase of 3 degrees C after 3 years exposure to a warmer climate. Although effects of S and T(air) x S interactions were mostly not significant in our study, we expect that combined effects of T(air) and S would be important in conditions of severe drought events.

Subcellular localization of cadmium in roots and leaves of Arabidopsis thaliana

We examined the subcellular cadmium (Cd) localization in roots and leaves of wild-type Arabidopsis thaliana (ecotype Columbia) exposed to environmentally relevant Cd concentrations. Energy-dispersive X-ray microanalysis (EDXMA) was performed on high-pressure frozen and freeze-substituted tissues. In the root cortex, Cd was associated with phosphorus (Cd/P) in the apoplast and sulfur (Cd/S) in the symplast, suggesting phosphate and phytochelatin sequestration, respectively. In the endodermis, sequestration of Cd/S was present as fine granular deposits in the vacuole and as large granular deposits in the cytoplasm. In the central cylinder, symplastic accumulation followed a distinct pattern illustrating the importance of passage cells for the uptake of Cd. In the apoplast, a shift of Cd/S granular deposits from the middle lamella towards the plasmalemma was observed. Large amounts of precipitated Cd in the phloem suggest retranslocation from the shoot. In leaves, Cd was detected in tracheids but not in the mesophyll tissue. Extensive symplastic and apoplastic sequestration in the root parenchyma combined with retranslocation via the phloem confirms the excluder strategy of Arabidopsis thaliana.

Chronic ozone exposure affects leaf senescence of adult beech trees: a chlorophyll fluorescence approach

Accelerated leaf senescence is one of the harmful effects of elevated tropospheric ozone concentrations ([O(3)]) on plants. The number of studies dealing with mature forest trees is scarce however. Therefore, five 66-year-old beech trees (Fagus sylvatica L.) have been exposed to twice-ambient (2xambient) [O(3)] levels by means of a free-air canopy O(3) exposure system. During the sixth year of exposure, the hypothesis of accelerated leaf senescence in 2xambient [O(3)] compared with ambient [O(3)] trees was tested for both sun and shade leaves. Chlorophyll (chl) fluorescence was used to assess the photosynthetic quantum yield, and chl fluorescence images were processed to compare functional leaf homogeneity and the proportion of O(3)-injured leaf area (stipples) under ambient and 2xambient [O(3)] regimes. Based on the analysis of chl fluorescence images, sun leaves of both ambient and 2xambient [O(3)] trees had apparently developed typical necrotic O(3) stipples during high O(3) episodes in summer, while accelerated senescence was only observed with sun leaves of 2xambient [O(3)] trees. This latter effect was indicated along with a faster decrease of photosynthetic quantum yield, but without evidence of changes in non-photochemical quenching. Overall, treatment effects were small and varied among trees. Therefore, compared with ambient [O(3)], the consequence of the observed O(3)-induced accelerated leaf senescence for the carbon budget is likely limited.

Imaging viral infection: studies on Nicotiana benthamiana plants infected with the pepper mild mottle tobamovirus

We have studied by kinetic Chl-fluorescence imaging (Chl-FI) Nicotiana benthamiana plants infected with the Italian strain of the pepper mild mottle tobamovirus (PMMoV-I). We have mapped leaf photosynthesis at different points of the fluorescence induction curve as well as at different post-infection times. Images of different fluorescence parameters were obtained to investigate which one could discriminate control from infected leaves in the absence of symptoms. The non-photochemical quenching (NPQ) of excess energy in photosystem II (PSII) seems to be the most adequate chlorophyll fluorescence parameter to assess the effect of tobamoviral infection on the chloroplast. Non-symptomatic mature leaves from inoculated plants displayed a very characteristic time-varying NPQ pattern. In addition, a correlation between NPQ amplification and virus localization by tissue-print was found, suggesting that an increase in the local NPQ values is associated with the areas invaded by the pathogen. Changes in chloroplast ultrastructure in non-symptomatic leaf areas showing different NPQ levels were also investigated. A gradient of ultrastructural modifications was observed among the different areas.

Chlorophyll a fluorescence imaging of ozone-stressed Brassica napus L. plants differing in glucosinolate concentrations

Brassicaceae are characterised by glucosinolates (GS), which appear to be involved not only in biotic but also in abiotic stress responses of plants. We investigated the effect of O (3) stress on leaf GS concentrations in two lines of BRASSICA NAPUS L., differing in GS content. Ozone fumigation decreased GS concentrations in leaves of B. NAPUS of one line. In control conditions, chlorophyll content, rates of saturating photosynthesis, and quantum yield of photosystem 2 differed between the two BRASSICA lines, but differences were smaller in O (3)-stress conditions, suggesting that the relationship between leaf GS concentration and sensitivity to abiotic stress merits further research. In agreement with other ecophysiological measurements, chlorophyll fluorescence imaging clearly distinguished both lines and in some cases also treatments. A method for analysis of fluorescence images accounting for the two-dimensional leaf heterogeneity is presented.

Cytoenzymological analysis of adenylyl cyclase activity and 3':5'-cAMP immunolocalization in chloroplasts of Nicotiana tabacum

In this study a combination of cytoenzymological and immunocytochemical techniques was used in order to demonstrate the presence of cyclic nucleotide metabolism in chloroplasts of higher plants. Catalytic cytochemistry was used to localize adenylyl cyclase activity by means of electron microscope investigation on Nicotiana tabacum cv. Petit Havana leaf fragments. Various immunocytochemical techniques were explored to visualize the presence of the second messenger adenosine 3':5'-cyclic monophosphate. Making use of adenylyl imidodiphosphate as a substrate, the enzyme activity was predominantly located at the intermembrane space of the chloroplast envelope. In order to provide further topographical information, intact, isolated chloroplasts were submitted to the same cytoenzymological procedure and revealed stromal adenylyl cyclase activity. Using high-pressure freezing as a physical fixative to obtain an instantaneous metabolic arrest the cellular vitrified water phase was sublimed under ultra-high vacuum by means of molecular distillation drying, avoiding recrystallization and hence redistribution of small highly diffusible molecules. This sequential combination preserved 3':5'-cAMP epitope retention in chloroplasts as was demonstrated by immunogold labelling. These results further substantiate in a unique way the growing evidence of the presence of an organelle-specific cAMP metabolism in higher plants. Furthermore the data presented support the status of chloroplasts as an excellent model to further investigate cAMP metabolism and to correlate it with a variety of physiological functions.

Thermal and chlorophyll-fluorescence imaging distinguish plant-pathogen interactions at an early stage

Different biotic stresses yield specific symptoms, owing to their distinct influence on a plant's physiological status. To monitor early changes in a plant's physiological status upon pathogen attack, chlorophyll fluorescence imaging (Chl-FI) and thermography, which respectively visualize photosynthetic efficiency and transpiration, were carried out in parallel for two fundamentally different plant-pathogen interactions. These non-destructive imaging techniques were able to visualize infections at an early stage, before damage appeared. Under growth-room conditions, a robotized set-up captured time series of visual, thermal and chlorophyll fluorescence images from infected regions on attached leaves. As a first symptom of the plant-virus interaction between resistant tobacco and tobacco mosaic virus (TMV), thermal imaging detected a local rise in temperature while Chl-FI monitored a co-localized increase in fluorescence intensity. Chl-FI also revealed pre-symptomatic high-intensity spots for the plant-fungus system sugar beet-Cercospora beticola. Concomitantly, spots of lower temperature were monitored with thermography, in marked contrast with our observations on TMV-infection in tobacco. Knowledge of disease signatures for different plant-pathogen interactions could allow early identification of emerging biotic stresses in crops, facilitating the containment of disease outbreaks. Presymptomatic monitoring clearly opens perspectives for quantitative screening for disease resistance, either on excised leaf pieces or attached leaves.

Product identification and adenylyl cyclase activity in chloroplasts of Nicotiana tabacum

In view of the ongoing debate on plant cyclic nucleotide metabolism, especially the functional presence of adenylyl cyclase, a novel detection method has been worked out to quantify the reaction product. Using uniformly labelled (15)N-ATP as a substrate for adenylyl cyclase, a qualitative and quantitative liquid chromatography/electrospray ionisation tandem mass spectrometry (LC/ESI-MS/MS) method was developed to measure de novo formed (15)N-adenosine 3',5'-cyclic monophosphate. Adenylyl cyclase activity was observed in chloroplasts obtained from Nicotiana tabacum cv. Petit Havana and the kinetic parameters and influence of various metabolic effectors are discussed in their context.

Apical dominance in Pssu-ipt-transformed tobacco

In Pssu-ipt-transformed tobacco, apical dominance was released by defoliation of the upper nodes, while the apex remained intact. After defoliation, the concentration of cytokinins (CKs) increased whereas IAA remained constant, evoking an increase in the CK/IAA ratio in the buds. Moreover, defoliation resulted in a tremendous increase in the concentrations of aromatic amines (AAs): tyramine (TYR), phenethylamine (PEA) and an as yet unidentified compound. Although the total aliphatic monoamine and polyamine (PA) concentration remained constant, putrescine (PUT) and spermidine (SPD) concentrations in the axillary buds decreased, whereas the concentration of spermine (SPM) increased. Similar changes in PAs and AAs could be observed in the buds of untransformed SR1 plants after decapitation, whereas defoliation without removal of the apex had no effect. This is the first report on the possible involvement of PAs and AAs in apical dominance.

Response to mild water stress in transgenic Pssu-ipt tobacco

The response of antioxidant enzymes to cyclic drought was studied in control non-transformed tobacco (Nicotiana tabacum L. cv. Petit Havana SR1) and two types of transgenic Pssu-ipt tobacco (grafted on wild rootstock and poorly rooted progeny of F1 generation) grown under different conditions of irradiation (greenhouse, referred as high light, versus growth chamber, referred as low light). Water stress cycles started with plants at two contrasting developmental stages, i.e., at the stage of vegetative growth (young) and at the onset of flowering (old). Drought reduced the growth of SR1 plants compared with transgenic ones, particularly, when treatment started in earlier stage of plant development. Relative leaf water content was significantly lower (below 70%) in all transgenic grafts and plants compared with the wild type, irrespective of age, drought, and growth conditions. The response of antioxidant enzymes was significantly dependent on plant type and plant age; nevertheless, growth conditions and water stress also affected enzyme activities. Contrary to non-transgenic tobacco, where about half of glutathione reductase activity was found in older plants, both transgenic types exhibited unchanged activities throughout plant development and stress treatment. No differences were found in catalase activity, although the growth in the greenhouse caused a moderate increase in all older plants. In contrast to non-transgenic and Pssu-ipt rooted plants, peroxidase activities (ascorbate, guaiacol, and syringaldazine peroxidase) in older Pssu-ipt grafts were up to four times higher, irrespective of growth and stress, nevertheless, the effect seemed to be age-dependent. Superoxide dismutase (SOD) activity was affected particularly by plant age but also by growth conditions. Unlike in older plants, water stress caused an increase of SOD activities in all younger plants. The differences observed in activities of enzymes of intermediary metabolism (i.e., malic enzyme and glucose-6-phosphate dehydrogenase) revealed that transgenic grafts probably compensated differently for a decrease of ATP and NADPH than control and transgenic rooted plants under stress.

Low-oxygen stress and water deficit induce cytosolic pyruvate orthophosphate dikinase (PPDK) expression in roots of rice, a C3 plant

Pyruvate orthophosphate dikinase (PPDK) is known for its role in C4 photosynthesis but has no established function in C3 plants. Abscisic acid, PEG and submergence were found to markedly induce a protein of about 97 kDa, identified by microsequencing as PPDK, in rice roots (C3). The rice genome was found to contain two ppdk loci, osppdka and osppdkb. We isolated osppdka cDNA, which encodes a cytosolic rice PPDK isoform of 96.6 kDa, that corresponded to the ABA-induced protein from roots. Western blot analysis showed a PPDK induction in roots of rice seedlings during gradual drying, cold, high salt and mannitol treatment, indicating a water deficit response. PPDK was also induced in the roots and sheath of submerged rice seedlings, and in etiolated rice seedlings exposed to an oxygen-free N2 atmosphere, which indicated a low-oxygen stress response. None of the stress treatments induced PPDK protein accumulation in the lamina of green rice seedlings. Ppdk transcripts were found to accumulate in roots of submerged seedlings, concomitant with the induction of alcohol dehydrogenase 1. Low-oxygen stress triggered an increase in PPDK activity in roots and etiolated rice seedlings, accompanied by increases in phosphoenolpyruvate carboxylase and malate dehydrogenase activities. The results indicate that cytosolic PPDK is involved in a metabolic response to water deficit and low-oxygen stress in rice, an anoxia-tolerant species.

Reinvestigation of the chlorophyll distribution among the chlorophyll-proteins and chlorophyll-protein complexes of Hordeum vulgare L

Solubilization of barley (Hordeum vulgare L.) thylakoid membranes with sodium dodecylsulphate plus sodium deoxycholate with or without Triton X-100 and subsequent fractionation in the polyacrylamide gel electrophoresis system described in this paper resulted: (1) in the resolution of the chlorophyll-proteins and chlorophyll-protein complexes commonly known as CP1a, CP1, LHCP(1), LHCP(2), CPa and LHCP(3); (2) in the highly increased stability of CP1 and CP1a, as judged by their chlorophyll content, (3) at the expense of the free pigment concentration (4) which could be reduced to a negligible amount. Some 40% of the total chlorophyll contained in the mature higher plant thylakoid membrane is associated with CP1 and CP1a and as already suggested before [19] no significant amount of free chlorophyll occurs in vivo.

Reinvestigation of the chlorophyll distribution among the chlorophyll-proteins and chlorophyll-protein complexes of Hordeum vulgare L

Solubilization of barley (Hordeum vulgare L.) thylakoid membranes with sodium dodecylsulphate plus sodium deoxycholate with or without Triton X-100 and subsequent fractionation in the polyacrylamide gel electrophoresis system described in this paper resulted: (1) in the resolution of the chlorophyll-proteins and chlorophyll-protein complexes commonly known as CP1a, CP1, LHCP(1), LHCP(2), CPa and LHCP(3); (2) in the highly increased stability of CP1 and CP1a, as judged by their chlorophyll content, (3) at the expense of the free pigment concentration (4) which could be reduced to a negligible amount. Some 40% of the total chlorophyll contained in the mature higher plant thylakoid membrane is associated with CP1 and CP1 a and as already suggested before [19] no significant amount of free chlorophyll occurs in vivo.