J. G. Lafontaine

Bio: J. G. Lafontaine is an academic researcher from Laval University. The author has contributed to research in topics: Nucleolus & Gene. The author has an hindex of 10, co-authored 18 publications receiving 423 citations.

Nuclear genes encoding chloroplast hemoglobins in the unicellular green alga Chlamydomonas eugametos.

Abstract: When the green unicellular alga Chlamydomonas eugametos is grown under light/dark regimes, nuclear genes are periodically activated in response to the changes in light conditions. These genetic responses are dependent upon the activation of genes associated with photosynthesis (LI616 and LI637), nonphotosynthetic photoreceptors (LI410 and LI818) and the biological clock (LI818). We report here that the LI410 and LI637 genes are part of a small gene family encoding hemoglobins (Hbs) related to those from two unicellular eukaryotes, the ciliated protozoa Paramecium caudatum and Tetrahymena pyriformis, and from the cyanobacterium Nostoc commune. Investigations of the intracellular localization of C. eugametos Hbs by means of immunogold electron microscopy indicate that these proteins are predominantly located in the chloroplast, particularly in the pyrenoid and the thylakoid region. To our knowledge, this constitutes the first evidence for the presence of Hbs in chloroplasts. Alignment of the LI637 cDNA nucleotide sequence with its corresponding genomic sequence indicates that the L1637 gene contains three introns, the positions of which are compared with those in the Hb genes of plants, animals and the ciliate P. caudatum. Although the LI637 gene possesses a three-intron/four-exon pattern similar to that of plant leghemoglobin genes, introns are inserted at different positions. Similarly the position of the single intron in the P. caudatum gene differs from the intron sites in the LI637 gene. The latter observations argue against the current view that all eukaryotic Hbs have evolved from a common ancestor having a gene structure identical to that of plant or animal Hbs.

In vivo studies on the nuclear behavior of the arbuscular mycorrhizal fungus Gigaspora rosea grown under axenic conditions

Abstract: The distribution and fate of nuclei of the arbuscular-my-corrhizal fungusGigaspora rosea during late stages of axenic cultures were studied in fixed cultures by transmitted light, conventional and confocal laser scanning microscopy, and in live cultures with two-photon fluorescence microscopy. Mature specimens not yet showing apical septation displayed oval-shaped nuclei localized in lateral positions of the hypha all along the germ-tube length. Beside these, round-shaped nuclei were found to migrate along the central germ-tube core. Some (rare) germ-tube areas, delimited by septa and containing irregularly shaped, much brighter fluorescent nuclei were also found. Specimens that had just initiated the septation process after germ-tube growth arrest displayed round or oval-shaped nuclei in several portions of the germ tubes. These hyphal areas often alternated with other septa-delimited cytoplasmic clusters which contained distorted, brightly fluorescent nuclei. Completely septated specimens mostly lacked nuclei along their germ tubes. However, highly fluorescent chromatin masses appeared within remnants of cytoplasmic material, often compressed between close septa. Our results provide a first clear picture of the in vivo distribution of nuclei along arbuscular mycorrhizal fungal germ tubes issued from resting spores, and suggest that selective areas of their coenocytic hyphae are under specific, single nuclear control. They indicate as well that random autolytic processes occur along senescingG. rosea germ tubes, probably as a consequence of the absence of a host root signal for mycorrhizal formation. Finally, the data presented here allow us to envisage the fate of nuclei released by the germinating spore after nonsymbiotic fungal growth arrest.

Cytoplasmic phenols and polysaccharides in ectomycorrhizal and non‐mycorrhizal short roots of pine

Abstract: SUMMARY Pinus strobus seedlings were cultivated in growth pouches and inoculated with Pisolithus tinctorius. Semi-thin sections of mycorrhizal and uninoculated short roots and long roots were stained with toluidine blue and periodic acid Schiff's reagent for phenolic and polysaccharidic intracytoplasmic substances respectively. No obvious difference in intracytoplasmic contents of phenolic substances was observed between ectomycorrhizal and uninoculated short roots. However, the phenolic content of the cortical cells of the long roots was much lower than that of both mycorrhizal and uninoculated short roots. Starch grains were not observed in cortical cells of either mycorrhizal or uninoculated short roots but were abundant both in the cortex and the stele of long roots and in the stele of short roots. The absence of starch synthesis in the cortical cells of potentially ectomycorrhizal short roots should be taken into account when explaining the accumulation of soluble sugars as a prerequisite for ectomycorrhizal formation.

A Proline-, Threonine-, and Glycine-Rich Protein Down-Regulated by Drought Is Localized in the Cell Wall of Xylem Elements

Abstract: A cDNA clone encoding a proline-, threonine-, and glycine-rich protein (PTGRP) was isolated from a wild tomato species (Lycopersicon chilense) (L.X. Yu, H. Chamberland, J.G. Lafontain, Z. Tabaeizadeh [1996] Genome 39: 1185–1193). Northern-blot analysis and in situ hybridization studies revealed that PTGRP is down-regulated by drought stress. The level of the mRNA in leaves and stems of 8-d drought-stressed plants decreased 5- to 10-fold compared with that in regularly watered plants. The mRNA re-accumulated when drought-stressed plants were rewatered. Antibodies raised against a glutathione S-transferase/PTGRP fusion protein were used to elucidate the subcellular localization of the protein by immunogold labeling. In regularly watered L. chilense plants, PTGRP protein was found to be localized in xylem pit membranes and disintegrated primary walls. Examination of sections from drought-stressed plants revealed a significant decrease in the levels of labeling. In these samples, only a few scattered gold particles were detected in the same areas. In the leaf tissues of plants that had been rewatered for 3 d following an 8-d drought stress, the labeling pattern was similar to that of the regularly watered plants. To our knowledge, PTGRP is the first drought-regulated protein that has been precisely localized in the cell wall.

Effect of Nikkomycin Z, a chitin-synthase inhibitor, on hyphal growth and cell wall structure of two arbuscular-mycorrhizal fungi

Abstract: Different concentrations of Nikkomycin Z, a competitive inhibitor of chitin-synthase, were applied to the arbuscular-mycorrhizal fungi (AMF)Gigaspora margarita andGlomus intraradices under in vitro conditions. These two fungi are known to differ in the structure and composition of their cell wall. The two AMF were able to grow in the presence of a higher concentration of this antibiotic than so far reported for other fungi. Fluorescence, electron microscopic and cytochemical studies showed that the blocking of the fungal chitin-synthase activity induces alterations in hyphal morphology, a reduction in fungal wall thickness, and several other changes in the hyphal wall structure and organization. The possible role of chitin-synthase in the hyphal growth and morphogenesis of these symbiotic fungi and its putative regulation by the host plant during the symbiosis are discussed.

The structure and synthesis of the fungal cell wall.

Abstract: The fungal cell wall is a dynamic structure that protects the cell from changes in osmotic pressure and other environmental stresses, while allowing the fungal cell to interact with its environment. The structure and biosynthesis of a fungal cell wall is unique to the fungi, and is therefore an excellent target for the development of anti-fungal drugs. The structure of the fungal cell wall and the drugs that target its biosynthesis are reviewed. Based on studies in a number of fungi, the cell wall has been shown to be primarily composed of chitin, glucans, mannans and glycoproteins. The biosynthesis of the various components of the fungal cell wall and the importance of the components in the formation of a functional cell wall, as revealed through mutational analyses, are discussed. There is strong evidence that the chitin, glucans and glycoproteins are covalently cross-linked together and that the cross-linking is a dynamic process that occurs extracellularly.

Phenols in the plant and in man. The potential for possible nutritional enhancement of the diet by modifying the phenols content or profile

Abstract: There is growing recognition that many phenolic secondary metabolites present in foodstuffs may possibly exert beneficial effects on human health. This may to some degree be mediated via antioxidant actions, but a range of more specific pharmacological effects have also been proposed. Given this background, there may be favourable consequences for the general health of Western populations as a result of optimising the phenolic content of the diet. This paper reviews what is known of the function of phenolics both in the plant and in man. It also describes current understanding of the biosynthesis of phenolics in plants, with emphasis on where potential controlling steps may exist. Finally, advances in identification and isolation of the genes coding for phenolic biosynthetic enzymes or regulatory proteins are also summarised. Taken together, this information provides a basis for attempts to modify and optimise the phenolic content of food crops, using either conventional plant breeding along with manipulation of agronomic practices, or else the more targeted approaches of modern molecular biology. © 2000 Society of Chemical Industry

Pathways and regulation of sulfur metabolism revealed through molecular and genetic studies

Abstract: Sulfur is essential for life. Its oxidation state is in constant flux as it circulates through the global sulfur cycle. Plants play a key role in the cycle since they are primary producers of organic sulfur compounds. They are able to couple photosynthesis to the reduction of sulfate, assimilation into cysteine, and further metabolism into methionine, glutathione, and many other compounds. The activity of the sulfur assimilation pathway responds dynamically to changes in sulfur supply and to environmental conditions that alter the need for reduced sulfur. Molecular genetic analysis has allowed many of the enzymes and regulatory mechanisms involved in the process to be defined. This review focuses on recent advances in the field of plant sulfur metabolism. It also emphasizes areas about which little is known, including transport and recycling/degradation of sulfur compounds.

The diversity and coevolution of rubisco, plastids, pyrenoids, and chloroplast-based co2-concentrating mechanisms in algae

Abstract: Algae have adopted two primary strategies to maximize the performance of Rubisco in photosynthetic CO2 fixation. This has included either the development of a CO2-concentrating mechanism (CCM), bas...

The role of trace metals in photosynthetic electron transport in O2-evolving organisms

Abstract: Iron is the quantitatively most important trace metal involved in thylakoid reactions of all oxygenic organisms since linear (= non-cyclic) electron flow from H2O to NADP+ involves PS II (2–3 Fe), cytochrome b6-f (5 Fe), PS I (12 Fe), and ferredoxin (2 Fe); (replaceable by metal-free flavodoxin in certain cyanobacteria and algae under iron deficiency). Cytochrome c6 (1 Fe) is the only redox catalyst linking the cytochrome b6-f complex to PS I in most algae; in many cyanobacteria and Chlorophyta cytochrome c6 and the copper-containing plastocyanin are alternatives, with the availability of iron and copper regulating their relative expression, while higher plants only have plastocyanin. Iron, copper and zinc occur in enzymes that remove active oxygen species and that are in part bound to the thylakoid membrane. These enzymes are ascorbate peroxidase (Fe) and iron-(cyanobacteria, and most al gae) and copper-zinc- (some algae; higher plants) superoxide dismutase. Iron-containing NAD(P)H-PQ oxidoreductase in thylakoids of cyanobacteria and many eukaryotes may be involved in cyclic electron transport around PS I and in chlororespiration. Manganese is second to iron in its quantitative role in the thylakoids, with four Mn (and 1 Ca) per PS II involved in O2 evolution. The roles of the transition metals in redox catalysts can in broad terms be related to their redox chemistry and to their availability to organisms at the time when the pathways evolved. The quantitative roles of these trace metals varies genotypically (e.g. the greater need for iron in thylakoid reactions of cyanobacteria and rhodophytes than in other O2-evolvers as a result of their lower PS II:PS I ratio) and phenotypically (e.g. as a result of variations in PS II:PS I ratio with the spectral quality of incident radiation).