Département Santé des Plantes et Environnement

To cope with environmental fluctuations and to prevent invasion by pathogens, plant metabolism must be flexible and dynamic. Active oxygen species, whose formation is accelerated under stress conditions, must be rapidly processed if oxidative damage is to be averted. The lifetime of active oxygen species within the cellular environment is determined by the antioxidative system, which provides crucial protection against oxidative damage. The antioxidative system comprises numerous enzymes and compounds of low molecular weight. While research into the former has benefited greatly from advances in molecular technology, the pathways by which the latter are synthesized have received comparatively little attention. The present review emphasizes the roles of ascorbate and glutathione in plant metabolism and stress tolerance. We provide a detailed account of current knowledge of the biosynthesis, compartmentation, and transport of these two important antioxidants, with emphasis on the unique insights and advances gained by molecular exploration.

It is generally accepted that the functional compartmentalization of eukaryotic cells is reflected by the differential occurrence of proteins in their compartments. The location and physiological function of a protein are closely related; local information of a protein is thus crucial to understanding its role in biological processes. The visualization of proteins residing on intracellular structures by fluorescence microscopy has become a routine approach in cell biology and is increasingly used to assess their colocalization with well-characterized markers. However, image-analysis methods for colocalization studies are a field of contention and enigma. We have therefore undertaken to review the most currently used colocalization analysis methods, introducing the basic optical concepts important for image acquisition and subsequent analysis. We provide a summary of practical tips for image acquisition and treatment that should precede proper colocalization analysis. Furthermore, we discuss the application and feasibility of colocalization tools for various biological colocalization situations and discuss their respective strengths and weaknesses. We have created a novel toolbox for subcellular colocalization analysis under ImageJ, named JACoP, that integrates current global statistic methods and a novel object-based approach.

Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.

In the present paper, we summarize and further develop recent reseach in the estimation of the variance of sterelogical estimators based on systematic sampling. In particular, it is emphasized that the relevant estimation procedure depends on the sampling density. The validity of the variance estimation is examined in a collection of data sets, obtained by systematic sampling. Practical recommendations are also provided in a separate section.

In this review, recent developments and future prospects of obtaining a better understanding of the regulation of nitrogen use efficiency in the main crop species cultivated in the world are presented. In these crops, an increased knowledge of the regulatory mechanisms controlling plant nitrogen economy is vital for improving nitrogen use efficiency and for reducing excessive input of fertilizers, while maintaining an acceptable yield. Using plants grown under agronomic conditions at low and high nitrogen fertilization regimes, it is now possible to develop whole-plant physiological studies combined with gene, protein, and metabolite profiling to build up a comprehensive picture depicting the different steps of nitrogen uptake, assimilation, and recycling to the final deposition in the seed. A critical overview is provided on how understanding of the physiological and molecular controls of N assimilation under varying environmental conditions in crops has been improved through the use of combined approaches, mainly based on whole-plant physiology, quantitative genetics, and forward and reverse genetics approaches. Current knowledge and prospects for future agronomic development and application for breeding crops adapted to lower fertilizer input are explored, taking into account the world economic and environmental constraints in the next century.

The complete sequence of the Arabidopsis thaliana genome revealed thousands of previously unsuspected genes, many of which cannot be ascribed even putative functions. One of the largest and most enigmatic gene families discovered in this way is characterized by tandem arrays of pentatricopeptide repeats (PPRs). We describe a detailed bioinformatic analysis of 441 members of the Arabidopsis PPR family plus genomic and genetic data on the expression (microarray data), localization (green fluorescent protein and red fluorescent protein fusions), and general function (insertion mutants and RNA binding assays) of many family members. The basic picture that arises from these studies is that PPR proteins play constitutive, often essential roles in mitochondria and chloroplasts, probably via binding to organellar transcripts. These results confirm, but massively extend, the very sparse observations previously obtained from detailed characterization of individual mutants in other organisms.

ABSTRACT Free radicals and other active derivatives of oxygen are inevitable by‐products of biological redox reactions. Reduced oxygen species, such as hydrogen peroxide, the superoxide radical anion and hydroxyl radicals, inactivate enzymes and damage important cellular components. In addition, singlet oxygen, produced via formation of triplet state chlorophyll, is highly destructive. This oxygen species initiates lipid peroxidation, and produces lipid peroxy radicals and lipid hydroperoxides that are also very reactive. The increased production of toxic oxygen derivatives is considered to be a universal or common feature of stress conditions. Plants and other organisms have evolved a wide range of mechanisms to contend with this problem. The antioxidant defence system of the plant comprises a variety of antioxidant molecules and enzymes. Considerable interest has been focused on the ascorbate‐glutathione cycle because it has a central role in protecting the chloroplasts and other cellular compartments from oxidative damage. It is clear that the capacity and activity of the antioxidative defence systems are important in limiting photo‐oxidative damage and in destroying active oxygen species that are produced in excess of those normally required for signal transduction or metabolism. In our studies on this system, we became aware that the answers to many unresolved questions concerning the nature and regulation of the antioxidative defence system could not be obtained easily by either a purely physiological or purely biochemical approach. Transgenic plants offered us a means by which to achieve a more complete understanding of the roles of the enzymes involved in protection against stress of many types: environmental and man‐made. The ability to engineer plants which express introduced genes at high levels provides an opportunity to manipulate the levels of these enzymes, and hence metabolism in vivo. Studies on transformed plants expressing increased activities of single enzymes of the antioxidative defence system indicate that it is possible to confer a degree of tolerence to stress by this means. However, attempts to increase stress resistance by simply increasing the activity of one of the antioxidant enzymes have not always been successful presumably because of the need for a balanced interaction of protective enzymes. The study of these transformed plants has allowed a more complete understanding of the roles of individual enzymes in metabolism. Protection against oxidative stress has become a feasible objective through the application of molecular genetic techniques in conjunction with a biochemical and physiological approach.

The TRANSPARENT TESTA8 (TT8) locus is involved in the regulation of flavonoid biosynthesis in Arabidopsis. The tt8-3 allele was isolated from a T-DNA-mutagenized Arabidopsis collection and found to be tagged by an integrative molecule, thus permitting the cloning and sequencing of the TT8 gene. TT8 identity was confirmed by complementation of tt8-3 and sequence analysis of an additional allele. The TT8 gene encodes a protein that displays a basic helix-loop-helix at its C terminus and represents an Arabidopsis ortholog of the maize R transcription factors. The TT8 transcript is present in developing siliques and in young seedlings. The TT8 protein is required for normal expression of two flavonoid late biosynthetic genes, namely, DIHYDROFLAVONOL 4-REDUCTASE (DFR) and BANYULS (BAN), in Arabidopsis siliques. Interestingly, TRANSPARENT TESTA GLABRA1 (TTG1) and TT2 genes also control the expression of DFR and BAN genes. Our results suggest that the TT8, TTG1, and TT2 proteins may interact to control flavonoid metabolism in the Arabidopsis seed coat.

The Arabidopsis thaliana hypocotyl is widely used to study the effects of light and plant growth factors on cell elongation. To provide a framework for the molecular-genetic analysis of cell elongation in this organ, here we describe, at the cellular level, its morphology and growth and identify a number of characteristic, developmental differences between light-grown and dark-grown hypocotyls. First, in the light epidermal cells show a characteristic differentiation that is not observed in the dark. Second, elongation growth of this organ does not involve significant cortical or epidermal cell divisions. However, endoreduplication occurs, as revealed by the presence of 4C and 8C nuclei. In addition, 16C nuclei were found specifically in dark-grown seedlings. Third, in the dark epidermal cells elongate along a steep, acropetal spatial and temporal gradient along the hypocotyl. In contrast, in the light all epidermal cells elongated continuously during the entire growth period. These morphological and physiological differences, in combination with previously reported genetic data (T. Desnos, V. Orbovic, C. Bellini, J. Kronenberger, M. Caboche, J. Traas, H. Höfte [1996] Development 122: 683-693), illustrate that light does not simply inhibit hypocotyl growth in a cell-autonomous fashion, but that the observed growth response to light is a part of an integrated developmental change throughout the elongating organ.

The emergence of insecticide resistance in the mosquito poses a serious threat to the efficacy of many malaria control programs. We have searched the Anopheles gambiae genome for members of the three major enzyme families- the carboxylesterases, glutathione transferases, and cytochrome P450s-that are primarily responsible for metabolic resistance to insecticides. A comparative genomic analysis with Drosophila melanogaster reveals that a considerable expansion of these supergene families has occurred in the mosquito. Low gene orthology and little chromosomal synteny paradoxically contrast the easily identified orthologous groups of genes presumably seeded by common ancestors. In A. gambiae, the independent expansion of paralogous genes is mainly a consequence of the formation of clusters among locally duplicated genes. These expansions may reflect the functional diversification of supergene families consistent with major differences in the life history and ecology of these organisms. These data provide a basis for identifying the resistance-associated enzymes within these families. This will enable the resistance status of mosquitoes, flies, and possibly other holometabolous insects to be monitored. The analyses also provide the means for identifying previously unknown molecules involved in fundamental biological processes such as development.

Shoot branching is inhibited by auxin transported down the stem from the shoot apex. Auxin does not accumulate in inhibited buds and so must act indirectly. We show that mutations in the MAX4 gene of Arabidopsis result in increased and auxin-resistant bud growth. Increased branching in max4 shoots is restored to wild type by grafting to wild-type rootstocks, suggesting that MAX4 is required to produce a mobile branch-inhibiting signal, acting downstream of auxin. A similar role has been proposed for the pea gene, RMS1. Accordingly, MAX4 and RMS1 were found to encode orthologous, auxin-inducible members of the polyene dioxygenase family.

Crucial roles in sulphur metabolism and plant defence have been described in recent years for the tripeptide thiol glutathione. In spite of this, the metabolism of glutathione and its response to stress conditions remained only partly understood. In many plants, one of the major difficulties in studying the control of glutathione synthesis is the low extractable activities of the enzymes involved. Consequently, several groups have exploited transformation technology using genes for the enzymes of glutathione synthesis or reduction. This approach has allowed the production of plants with systematically enhanced levels of glutathione (up to 4-fold higher than untransformed controls) and has permitted numerous insights into the control of glutathione synthesis or reduction state and its interaction with other areas of primary or defensive metabolism.

To enhance our understanding of the genetic basis of nitrogen use efficiency in maize (Zea mays), we have developed a quantitative genetic approach by associating metabolic functions and agronomic traits to DNA markers. In this study, leaves of vegetative recombinant inbred lines of maize, already assessed for their agronomic performance, were analyzed for physiological traits such as nitrate content, nitrate reductase (NR), and glutamine synthetase (GS) activities. A significant genotypic variation was found for these traits and a positive correlation was observed between nitrate content, GS activity and yield, and its components. NR activity, on the other hand, was negatively correlated. These results suggest that increased productivity in maize genotypes was due to their ability to accumulate nitrate in their leaves during vegetative growth and to efficiently remobilize this stored nitrogen during grain filling. Quantitative trait loci (QTL) for various agronomic and physiological traits were searched for and located on the genetic map of maize. Coincidences of QTL for yield and its components with genes encoding cytosolic GS and the corresponding enzyme activity were detected. In particular, it appears that the GS locus on chromosome 5 is a good candidate gene that can, at least partially, explain variations in yield or kernel weight. Because at this locus coincidences of QTLs for grain yield, GS, NR activity, and nitrate content were also observed, we hypothesize that leaf nitrate accumulation and the reactions catalyzed by NR and GS are coregulated and represent key elements controlling nitrogen use efficiency in maize.

The roles of two cytosolic maize glutamine synthetase isoenzymes (GS1), products of the Gln1-3 and Gln1-4 genes, were investigated by examining the impact of knockout mutations on kernel yield. In the gln1-3 and gln1-4 single mutants and the gln1-3 gln1-4 double mutant, GS mRNA expression was impaired, resulting in reduced GS1 protein and activity. The gln1-4 phenotype displayed reduced kernel size and gln1-3 reduced kernel number, with both phenotypes displayed in gln1-3 gln1-4. However, at maturity, shoot biomass production was not modified in either the single mutants or double mutants, suggesting a specific impact on grain production in both mutants. Asn increased in the leaves of the mutants during grain filling, indicating that it probably accumulates to circumvent ammonium buildup resulting from lower GS1 activity. Phloem sap analysis revealed that unlike Gln, Asn is not efficiently transported to developing kernels, apparently causing reduced kernel production. When Gln1-3 was overexpressed constitutively in leaves, kernel number increased by 30%, providing further evidence that GS1-3 plays a major role in kernel yield. Cytoimmunochemistry and in situ hybridization revealed that GS1-3 is present in mesophyll cells, whereas GS1-4 is specifically localized in the bundle sheath cells. The two GS1 isoenzymes play nonredundant roles with respect to their tissue-specific localization.

The plant hormone abscisic acid (ABA) triggers production of reactive oxygen species (ROS) in guard cells via the AtrbohD and AtrbohF NADPH oxidases, leading to stomatal closure. The ABA-activated SnRK2 protein kinase open stomata 1 (OST1) (SRK2E/SnRK2.6) acts upstream of ROS in guard cell ABA signaling. Here, we report that OST1 phosphorylates Ser13 and Ser174 on AtrbohF. In addition, substitution of Ser174 to Ala results in a approximately 40% reduction in the phosphorylation of AtrbohF by OST1. We also show that OST1 physically interacts with AtrbohF. These results provide biochemical evidence suggesting that OST1 regulates AtrbohF activity.

Abstract To investigate the role of stored and neosynthesized mRNAs in seed germination, we examined the effect of α-amanitin, a transcriptional inhibitor targeting RNA polymerase II, on the germination of nondormant Arabidopsis seeds. We used transparent testa mutants, of which seed coat is highly permeable, to better ascertain that the drug can reach the embryo during seed imbibition. Even with the most permeable mutant (tt2-1), germination (radicle protrusion) occurred in the absence of transcription, while subsequent seedling growth was blocked. In contrast, germination was abolished in the presence of the translational inhibitor cycloheximide. Taken together, the results highlight the role of stored proteins and mRNAs for germination in Arabidopsis and show that in this species the potential for germination is largely programmed during the seed maturation process. The α-amanitin-resistant germination exhibited characteristic features. First, this germination was strongly slowed down, indicating that de novo transcription normally allows the synthesis of factor(s) activating the germination rate. Second, the sensitivity of germination to gibberellic acid was reduced 15-fold, confirming the role of this phytohormone in germination. Third, de novo synthesis of enzymes involved in reserve mobilization and resumption of metabolic activity was repressed, thus accounting for the failure in seedling establishment. Fourth, germinating seeds can recapitulate at least part of the seed maturation program, being capable of using mRNAs stored during development. Thus, commitment to germination and plant growth requires transcription of genes allowing the imbibed seed to discriminate between mRNAs to be utilized in germination and those to be destroyed.

Glutamate (Glu) metabolism and amino acid translocation were investigated in the young and old leaves of tobacco (Nicotiana tabacum L. cv Xanthi) using [15N]ammonium and [2-15N]Glu tracers. Regardless of leaf age, [15N]ammonium assimilation occurred via glutamine synthetase (GS; EC 6.1.1.3) and Glu synthase (ferredoxin [Fd]-GOGAT; EC 1.4.7.1; NADH-GOGAT; EC 1.4.1.14), both in the light and darkness, and it did not depend on Glu dehydrogenase (GDH; EC 1.4.1.2). The [15N]ammonium and ammonium accumulation patterns support the role of GDH in the deamination of [2-15N]Glu to provide 2-oxoglutarate and [15N]ammonium. In the dark, excess [15N]ammonium was incorporated into asparagine that served as an additional detoxification molecule. The constant Glu levels in the phloem sap suggested that Glu was continuously synthesized and supplied into the phloem regardless of leaf age. Further study using transgenic tobacco lines, harboring the promoter of the GLU1 gene (encoding Arabidopsis [Arabidopsis thaliana] Fd-GOGAT) fused to a GUS reporter gene, revealed that the expression of Fd-GOGAT remained higher in young leaves compared to old leaves, and higher in the veins compared to the mesophyll. Confocal laser-scanning microscopy localized the Fd-GOGAT protein to the phloem companion cells-sieve element complex in the leaf veins. The results are consistent with a role of Fd-GOGAT in supplying Glu for the synthesis and transport of amino acids. Taken together, the data provide evidence that the GS-GOGAT pathway and GDH play distinct roles in the source-sink nitrogen cycle of tobacco leaves.

UNLABELLED: SUMMARY Taxonomy: Eukaryota; Metazoa; Nematoda; Chromadorea; order Tylenchida; Tylenchoidea; Heteroderidae; genus Meloidogyne. Physical properties: Microscopic-non-segmented worms. Meloidogyne species can reproduce by apomixis, facultative meiotic parthenogenesis or obligate mitotic parthenogenesis. Obligate biotrophic parasites inducing the re-differentiation of plant cells into specialized feeding cells. Hosts: Meloidogyne spp. can infest more than 3000 plant species including vegetables, fruit trees, cereals and ornamental flowers. SYMPTOMS: Root swellings called galls. Alteration of the root vascular system. Disease control: Cultural control, chemical control, resistant cultivars. Agronomic importance: Major threat to agriculture in temperate and tropical regions.

To study the genetic variability and the genetic basis of nitrogen (N) use efficiency in maize, a set of recombinant inbred lines crossed with a tester was studied at low input (N-) and high input (N+) for grain yield and its components, grain protein content, and post-anthesis nitrogen uptake and remobilization. Other physiological traits, such as nitrate content, nitrate reductase, glutamine synthetase (GS), and glutamate dehydrogenase activities were studied at the level of the lines. Genotypexnitrogen (GxN) interaction was significant for yield and explained by variation in kernel number. In N-, N-uptake, the nitrogen nutrition index, and GS activity in the vegetative stage were positively correlated with grain yield, whereas leaf senescence was negatively correlated. Whatever N-input, post-anthesis N-uptake was highly negatively related to N-remobilization. As a whole, genetic variability was expressed differently in N+ and N-. This was confirmed by the detection of QTLs. More QTLs were detected in N+ than in N- for traits of vegetative development, N-uptake, and grain yield and its components, whereas it was the reverse for grain protein content and N-utilization efficiency. Several coincidences between genes encoding for enzymes of N metabolism and QTLs for the traits studied were observed. In particular, coincidences in three chromosome regions of QTLs for yield and N-remobilization, QTLs for GS activity and a gene encoding cytosolic GS were observed. This may have a physiological meaning. The GS locus on chromosome 5 appears to be a good candidate gene which can, at least partially, explain the variation in nitrogen use efficiency.

How do we keep an object in mind? Based on evidence from animal electrophysiology and human brain-imaging techniques, it is commonly held that short-term memory relies on sustained activity in a network distributed over sensory and prefrontal cortices. How does neural firing persist in such a distributed network in the absence of visual input? Hebb's influential but so far unproved proposal, developed more than 50 years ago, is that sustained activation in short-term memory networks is maintained by reverberating activity in neuronal loops. We hypothesized that synchronized oscillatory activity, proposed to provide a dynamic link between distributed areas, could not only coordinate activity in the network but also establish reentrant loops in the system to enable both sustained firing and temporal coincidence of inputs. We show in human intracranial recordings that limited regions of extrastriate visual areas, separated by several centimeters, become synchronized in an oscillatory mode during the rehearsal of an object in visual short-term memory. Synchrony occurs specifically in the beta range (15-25 Hz) and disappears in a control condition. These findings thus confirm experimentally the hypothesis of a functional role of synchronized oscillatory activity in the coordination of distributed neural activity in humans, and support Hebb's popular but unproved concept of short-term memory maintenance by reentrant activity within the activated network.