Pierre-Mathieu Charest

Taxonomic study on nonpathogenic streptomycetes isolated from common scab lesions on potato tubers.

Numerical analysis was carried out to compare sixteen nonpathogenic actinomycetes isolated from common scab lesions on potato tubers with Streptomyces scabiei type strain as well as with other streptomycete groups. These isolates were divided into two classes according to their level of similarity with S. scabiei. Isolates resembling S. scabiei were associated with S. griseoruber or with S. violaceusniger while isolates exhibiting less than 61% of similarity with S. scabiei were phenotypically related to S. albidoflavus or to S. atroolivaceus. Sequence of the 16S rRNA gene of each isolate was obtained and compared against the GenBank nucleotide database. No significant match could be established between the sequences of two potato isolates and the ones available in the GenBank database. The other isolates were closely related with S. setonii (S. griseus), S. mirabilis, S. fimbriatus, S. violaceoruber, S. melanosporofaciens and S. thermocarboxydus.

Influence of exposure time on the distribution of cadmium within the cladoceran Ceriodaphnia dubia

Abstract The internal distribution of a trace metal can be useful in determining in which body organ toxic effects are most likely to occur and the animal function most likely to be affected. In addition, because a metal’s distribution within an organism is likely to influence the rate and efficiency with which it is transferred to a higher trophic level, laboratory measurements of trophic transfer could be influenced by exposure duration. We compared the internal distribution of cadmium (Cd) in a cladoceran crustacean (Ceriodaphnia dubia) destined as food for a predatory insect (Chaoborus) after either a 1 day or a lifetime of exposure of prey to the trace metal. Prey were exposed to 112+109Cd in both water (10 nM) and their food (algae), as might occur in nature. The internal distribution of 109Cd in the cladoceran was determined by whole-animal autoradiography. Both the amount and the tissue distribution of Cd in prey were the same after the short and the long term exposures, suggesting that metal accumulation parameters measured after short-term metal exposures can be valid for this animal. Cadmium was mainly accumulated in diverticula of the anterior midgut, a region reported to be responsible for nutrient absorption. We hypothesize that Cd is accumulated in the diverticula because of their purported role as sites of calcium uptake.

Effect of exposure regime on the internal distribution of cadmium in Chironomus staegeri larvae (Insecta, Diptera)

Although insects are routinely used to test the acute toxicity of pollutants in water alone, little information exists on the influence of exposure regime on the accumulation of contaminants in these animals. We exposed fourth instar Chironomus staegeri larvae to 10 nM Cd in microcosms under two exposure regimes: a water only short-term (5 days) exposure, or a water and sediment long-term (140 days) exposure. Microautoradiography of whole animals demonstrated that the digestive tract was the principal organ of Cd accumulation under both exposure regimes. The majority of accumulated Cd was confined to a small section of the insects midgut, suggesting uptake in this region. Our results suggest that the Cd exposure regime of Chironomus larvae would not alter the body organ in which Cd would exert its toxicity.

Overexpression of MID2 suppresses the profilin‐deficient phenotype of yeast cells

Profilin‐deficient Saccharomyces cerevisiae cells show abnormal growth, actin localization, chitin deposition, bud formation and cytokinesis. Previous studies have also revealed a synthetic lethality between pfy1 and late secretory mutants, suggesting a role for profilin in intracellular transport. In this work, we document further the secretion defect associated with the pfy1Δ mutant. Electron microscopic observations reveal an accumulation of glycoproteins in the bud and in the mother cell. The MATa, pfy1Δ cells mate as well as wild‐type cells, while the mating efficiency of MATα, pfy1Δ cells is reduced. Pulse‐chase experiments demonstrate an accumulation of the 19 kDa α‐factor precursor and delayed secretion of the mature α‐factor. The TGN protein Kex2p is the principal enzyme responsible for the endoproteolytic cleavage of the α‐factor precursor. An immunofluorescence detection of Kex2p shows an altered localization in pfy1Δ cells. Instead of a discrete punctate distribution, the enzyme is dispersed throughout the cytoplasm. A high‐copy‐number plasmid containing MID2, which encodes a potential transmembrane protein involved in cell cycle control, suppresses the abnormal growth, actin distribution, α‐factor maturation and the accumulation of intracellular membranous structures in pfy1Δ cells.

A Review of Ultrastructural and Ultracytochemical Studies of Infection Processes in Some Plant Wilt Diseases: The Opaque Matter Extensively Involved, Its Links with Pathogen Elements, Insights into Its Nature

The present review is based on results of several years of investigation on the pathogenesis of plant wilt diseases using for one of the main aspects, several labeling techniques at the ultrastructural level: autoradiography with (3)H-thymidine; gold-tagged lectins, enzymes, polyclonal, and monoclonal antibodies. Thus, opaque matter (OM), labeling for DNA with the appropriate markers, can be abundant in and associated with host tissue reactions and pronounced alterations (particularly in newly differentiated xylem). The presence of structures (called P-elements) in OM, paralleled by a similar occurrence in nuclei of both the host and pathogen cells, is considered as being also indicative of a cytoplasmic nature for this OM and most likely originating from fungal elements.