Susceptible Clones Research Articles

Leaf phenolics of three willow clones differing in resistance to Melampsora rust infection

Leaf phenolic composition in three Salix myrsinifolia Salisb. clones (V8, V45, and V43), inoculated with Melampsora rust, was analyzed to detect local rust‐induced alterations during different stages of infection (2, 7, and 21 days after inoculation [DAI]). Phenolic levels and percentage of uredial area varied significantly between clones. In the most resistant clone,V8, the levels of some phenolic compounds were lower in rust‐infected plants than in control plants at the initial stages of rust infection, suggesting a rapid response of phenolic metabolism to rust attack. Moreover, the clone V8 contained the highest constitutive (+)‐catechin level. In clone V45, rust infection caused the most pronounced increase in the levels of individual phenolics at 7 DAI. This increase may have been effective in retarding the subsequent spread and development of rust. In the most susceptible clone V43, rust‐induced phenolic responses were less pronounced and delayed. The results suggest that in specific willow‐rust interactions, constitutive levels of phenolics, as well as induced phenolic responses, may contribute to the expression of rust resistance. In general, rust‐induced alterations in willow phenolic levels are highly specific to genotype and compound and vary depending on the stage of rust development.

Penetration, development and emigration of juveniles of the nematode Meloidogyne arenaria in Myrobalan plum (Prunus cerasifera) clones bearing the Ma resistance genes

Penetration, development and emigration of M. arenaria in the roots of three Myrobalan plum (Prunus cerasifera) clones genetically characterized for their resistance to root-knot nematodes (RKN) were studied during the 10 (penetration) and 15 (emigration) days following the date of inoculation (D) of 2500 juveniles (J2s) per plant into the soil. Miniaturized tests were conducted on the two resistant clones P.2175 ( Ma1 gene) and P.1079 (Ma2 gene) and the susceptible clone P.2032 (recessive for both genes), obtained from micropropagated plantlets and grown in mini-containers under controlled conditions at 25 C in a growth chamber. For penetration and development studies, nematodes in the roots were recovered by the acid fuchsin-lactophenol staining technique. Equivalent numbers of J2s were recovered in all the clones at D C 1 and DC 2. Subsequently, the numbers increased rapidly in P.2032 and were significantly different from those in P.1079 and P.2175 that remained at a low level. No swollen larvae were observed in the resistant clones. In P.2032, the first swollen larvae were observed at D C 4, the first females were observed at D C 12, whereas the first females with attached egg sacs and the first new-generation J2s were obtained between DC 21 and DC 28. Our data suggest that the resistance phenomenon does not act on the very early nematode penetration but acts later by preventing feeding-site induction and development into the third-stage. For emigration studies, plants in which J2s had been allowed to penetrate for two days (from D to D C 2) were washed free of soil, repotted and then, after various periods of growth, soil-free roots were placed under a mistifier to evaluate the numbers of emigrating individuals. Emigration of J2s from the roots occured mainly from D C 2 to D C 4 in all the genotypes and was very limited from D C 4t o D C10. There was no significant differences in the number of emigrated juveniles between the resistant and susceptible clones, indicating that emigration cannot explain the difference in the numbers of nematodes recovered in the roots.

Late blight resistant potato germplasm release AWN86514-2

Potato breeding selection AWN86514-2, Solanum tuberosum gp tuberosum, is being released as germplasm that is highly resistant to prevalent North American strains of Phytophthora infestans. This selection has been tested under field conditions in Mount Vernon, Washington (P. infestans US11 and US8 with complex virulence pathotypes), as well as Corvallis, Oregon, and eight other locations in North America (predominantly P. infestans US8) between 1994 and 1997. Both foliage and tubers show partial resistance. Although AWN86514-2 is pollen sterile, it can be successfully used as a female parent. An average of 34% of the progeny from crosses between AWN86514-2 and four susceptible clones were resistant to late blight when tested at Toluca, Mexico, in 1996. AWN86514-2 also has high resistance to Verticillium wilt and potato virus Y. AWN86514-2 is late maturing, with medium yields of smooth, longoblong, buffskinned tubers. Specific gravity is high and french fry color from 7 C (45 F) storage is excellent. The male parent of AWN86514-2 was Ranger Russet, a dual purpose french fry and fresh market variety, and the female parent was KSA195-96, a selection made at Aberdeen, Idaho, from Polish germplasm received as true seed from the Polish Plant Breeding and Acclimatization Institute. Possible sources of the late blight resistance in this clone include S. acaule, S. demissum, S. phureja, S. simiplicifolium, S. stoloniferum, and S. tuberosum gp andigena which are in the lineage of KSA195-96. This germplasm was developed and released by USDA-ARS in cooperation with the Agricultural Experiment Stations of Idaho, Oregon, and Washington.

A comparative study of Phytophthora meadii isolates from rubber (Hevea brasiliensis) plantations in Sri Lanka

Thirty-three Phytophthora meadii isolates were obtained from different Hevea brasiliensisclones grown in different climatic regions in Sri Lanka. Growth, morphology and pathogenicity of the isolates were compared. Growth and pathogenicity levels varied among the isolates. Some isolates obtained from moderately susceptible rubber clones were highly pathogenic, compared to isolates obtained from resistant clones. Highly pathogenic isolates produced a higher number of sporangia on agar at 27 ± 2°C. It was impossible to group isolates according to the clone or the region from where they were obtained.

Selection for a moderately insecticide resistant clone of Myzus persicae (Hemiptera: Aphididae) on sugarbeet in the absence of pesticides

AbstractHighly resistant and extremely resistant clones (R2 and R3) of Myzus persicae (Sulzer) exhibited a significantly lower mortality and developed a deleterious dark stomach deposit less frequently on sugarbeet than a susceptible (S) clone. After allowing equal populations of four resistant clones to develop together on sugarbeet for four weeks, approximately half of the aphids recovered were from the moderately resistant (R1) clone, suggesting that this clone had a significantly greater reproductive rate than the others. There was no significant difference in the propensity of any clone to produce alates on sugarbeet, although none of the clones produced very many. It is suggested that, even in the absence of insecticides on sugarbeet, resistant clones may become dominant due to their higher fecundity (R1 clones) and lower mortality (R2 and R3 clones) than susceptible clones.

Within–and between–population variation for resistance of Daphnia magna to the bacterial endoparasite Pasteuria ramosa

Genetic variation among hosts for resistance to parasites is an important assumption underlying evolutionary theory of host and parasite evolution. Using the castrating bacterial parasite Pasteuria ramosa and its cladoceran host Daphnia magna, we examined both within- and between-population genetic variation for resistance. First, we tested hosts from four populations for genetic variation for resistance to three parasite isolates. Allozyme analysis revealed significant host population divergence and that genetic distance corresponds to geographic distance. Host and parasite fitness components showed strong genetic differences between parasite isolates for host population by parasite interactions and for clones within populations, whereas host population effects were significant for only a few traits. In a second experiment we tested explicitly for within-population differences in variation for resistance by challenging nine host clones from a single population with four different parasite spore doses. Strong clone and dose effects were evident. More susceptible clones also suffered higher costs once infected. The results indicate that within-population variation for resistance is high relative to between-population variation. We speculate that P. ramosa adapts to individual host clones rather than to its host population.

Morphological and anatomical characterization of six jojoba clones at saline and non-saline sites

Various jojoba ( Simmondsia chinensis (Link) Schneider) clones have been demonstrated to differ in their susceptibility to salinity in northern Chile. Our objective was to characterize the anatomical and morphological response of six of these clones when grown under saline and non-saline conditions. Two of these clones are resistant to soil salinity (4.11.32 and Mirov), based on previous field evaluations, and four are susceptible (5.1, AT1487, 2.1 and 4.8). For each clone, three 2-year-old plants were planted at three locations in Chile. The Las Cardas Experimental Station is non-saline (soil EC=2 dS m −1), and the Quebrada de Camarones and Villa Frontera sites are saline (soil EC=38 and 63 dS m −1, respectively). Significant differences were found among clones at each site for several morphological and anatomical parameters studied, including number and size of stomata, density of trichomes, leaf size, branching characteristics and stem diameter. In general, plants grown under high salt levels did not show much difference from those grown under non-saline conditions for most of the morphological and anatomical parameters measured. Only leaf and cuticle thickness showed a high tendency to increase under saline conditions. The two resistant clones did not show consistent differences from the susceptible clones. However, the most salt-resistant clone (4.11.32) had the lowest stomata and trichome density and the largest stomatal size.

Access to hematin: the basis of chloroquine resistance.

The saturable uptake of chloroquine by parasites of Plasmodium falciparum has been attributed to specific carrier-mediated transport of chloroquine. It is suggested that chloroquine is transported in exchange for protons by the parasite membrane Na+/H+ exchanger [J Biol Chem 272:2652-2658 (1997)]. Once inside the parasite, it is proposed that chloroquine inhibits the polymerization of hematin, allowing this toxic hemoglobin metabolite to accumulate and kill the cell [Pharmacol Ther 57:203-235 (1993)]. To date, the contribution of these proposed mechanisms to the uptake and antimalarial activity of chloroquine has not been assessed. Using sodium-free medium, we demonstrate that chloroquine is not directly exchanged for protons by the plasmodial Na+/H+ exchanger. Furthermore, we show that saturable chloroquine uptake at equilibrium is due solely to the binding of chloroquine to hematin rather than active uptake: using Ro 40-4388, a potent and specific inhibitor of hemoglobin digestion and, by implication, hematin release, we demonstrate a concentration-dependent reduction in the number of chloroquine binding sites. An equal number of chloroquine binding sites are found in both resistant and susceptible clones, but the apparent affinity of chloroquine binding is found to correlate with drug activity (r2 = 0.93, p < 0.0001). This completely accounts for both the reduced drug accumulation and activity observed in resistant clones and the "reversal" of resistance produced by verapamil. The data presented here reconcile most of the available biochemical data from studies of the mode of action of chloroquine and the mechanism of chloroquine resistance. We show that the activity of chloroquine and amodiaquine is directly dependent on the saturable binding of the drugs to hematin and that the inhibition of hematin polymerization may be secondary to this binding. The chloroquine-resistance mechanism regulates the access of chloroquine to hematin. Our model is consistent with a resistance mechanism that acts specifically at the food vacuole to alter the binding of chloroquine to hematin rather than changing the active transport of chloroquine across the parasite plasma membrane.

Acquisition, carriage, and transmission of pneumococci with decreased antibiotic susceptibility in young children attending a day care facility in southern Israel.

The prevalence and transmission of antimicrobial drug-resistant pneumococci was studied in 48 children attending a day care facility in southern Israel. Nasopharyngeal cultures were obtained every 2 weeks for 10 months, and antibiotic susceptibility of isolates was determined by disk diffusion and E-test. Relatedness of isolates was investigated by capsular typing, ribotyping, and arbitrarily primed polymerase chain reaction. Pneumococci were recovered during 362 (63%) of 573 fortnights, and 219 (60%) of these isolates showed decreased susceptibility to at least one drug; 154 (43%) were intermediately susceptible to penicillin and 51 (14%) were multiresistant. Combining the different typing methods showed that a limited number of clones circulated in the facility. Clones exhibiting decreased antibiotic susceptibility (especially 23F, intermediately susceptible to penicillin and resistant to trimethoprim-sulfamethoxazole, and multiresistant 6B) were more frequently isolated and persisted longer than did fully susceptible clones. By multivariate analysis, carriage of organisms with decreased antibiotic susceptibility was associated with young age, female sex, winter season, and exposure to antimicrobial drugs during the previous month.

Human immunodeficiency virus type 1 (HIV-1) infection and transcytosis activity of a HIV-1 susceptible clone from HeLa cell.

In order to clarify the transmission process of human immunodeficiency virus type 1 (HIV-1) through the epithelial cell barrier, HeLa cells susceptible and non-susceptible to HIV-1 were cloned and designated as P6 HeLa and N7 HeLa cells, respectively. P6 HeLa cells could be infected with the LAI strain of HIV-1 and mediated HIV-1 transcytosis. In contrast, N7 HeLa cells exhibited neither HIV-1 infection nor transcytosis. CD4 and galactosylceramide as the receptors for HIV-1 were not detected on P6 HeLa cells, although an anti-CD4 monoclonal antibody (mAb) blocked HIV-1 infection. Since HIV-1-infected P6 HeLa cells exhibited no fusion and survived, we speculated that the P6 HeLa cells expressed molecules other than CD4 which facilitated HIV-1 infection. Two mAbs (A-14 ITK and C57 a9-9) which inhibited the HIV-1 infection of P6 HeLa cells were generated. Each mAb recognized distinct molecule(s) as shown by Western blotting. Transcytosis by the P6 HeLa cells was inhibited by C57 a9-9 but not by A-14 ITK or anti-CD4 mAb. Both infection and transcytosis may be responsible for HIV-1 transmission through epithelial cells in a complex manner. Although infection and transcytosis occurred via different mechanisms, the molecule(s) recognized by C57 a9-9 mAb may be associated with both processes.

Insecticide Resistance in a Novel Laboratory-Produced Greenbug (Homoptera: Aphididae) Clone

To assess the potential development of new resistant greenbug genotypes, a greenbug clone with over-expressed esterase activity in 2 different esterases (identified by polyacrylamide gel electrophoresis) was created through laboratory breeding between pattern 1 (R1) and pattern 2 (R2) field-derived insecticide-resistant clones. This clone was tested for insecticide resistance to 2 insecticides. The laboratory created clone was designated pattern 3 (R3) and has not been discovered in the field. The R3 clone was compared with susceptible (S), R1, and R2 greenbug clones using an insecticide residue vial bioassay. The R1, R2, and R3 clones were significantly more resistant to parathion than the S clone. The LD50 values for the R1, R2, and R3 clones were 45.5-, 213.2-, and 134.0-fold (respectively) higher than the LD50 of the S clone. The R2 and R3 clones were found to be significantly more resistant to parathion than the R1 clone, but the differences between the R2 and R3 clones were not significant. The respective LD50 values for the R1, R2, and R3 clones were 1.64-, 1.73-, and 1.91-fold greater than the S clone when tested against carbofuran. However, these differences were not found to be significant, indicating little cross resistance to this carbamate insecticide. Control measures effective against R2 insects should be effective against R3 insects if they occur in the field.

Characterization of Venturia inaequalis pathogenicity on leaf discs of apple trees

Characterization of pathogenicity on whole plants is required to study host-pathogen interactions between Malus × domestica and Venturia inaequalis. We studied the reliability of an in vitro test of pathogenicity on leaf discs. Three strains of V. inaequalis (races 1, 6 and an English race) were inoculated in vitro onto a range of 16 Malus sp. clones including susceptible and resistant clones. The results were compared to those previously obtained in vivo. Resistant clones contained the main major known genes, i.e. Va, Vb, Vbj, Vf, Vg, Vm and Vr. Scab severity and the sporulation of the fungus were assessed 21 days after the inoculation date. The results indicated that it was possible to reproduce incompatible and compatible situations in vitro. A null severity corresponded to the avirulence of the strain for the clone considered. The resistance given by the Vb, Vbj, Vf, Vg, Vm and Vr genes were expressed in vitro. Only the clone carrying the Va gene and inoculated with the race 6 strain presented a compatible situation which was inconsistent with the observations on the whole plant. Improving this test will facilitate studies on the pathogenicity of V. inaequalis populations in relation to resistance genes of the host expressed in vitro as well as its genetic determinism.

Ligation of cell surface CD4 inhibits activation-induced death of human T lymphocytes at the level of Fas ligand expression.

Cross-linking of cell surface CD4 molecules by anti-CD4 mAb or HIV-1 gp120/anti-gp120 Ab primes resting T lymphocytes for activation-induced cell death (AICD) triggered via the CD3/TCR complex. In striking contrast, we demonstrate here that preincubation of activated human CD4+ T cells with anti-CD4 mAb consistently inhibited AICD triggered via anti-CD3 mAb or Staphylococcus aureus enterotoxin A superantigen. Inhibition of AICD of CD4+ T cell clones was also observed with F(ab')2, but not with Fab, of anti-CD4 mAb. Moreover, soluble HIV-1 gp120, but not rIL-16, inhibited AICD stimulated by S. aureus enterotoxin A. In susceptible clones, CD4 ligation prevented the up-regulation of Fas ligand mRNA and cell surface expression in response to anti-CD3 mAb or superantigen stimulation. CD3/TCR-dependent protein tyrosine phosphorylation and cytokine production were also prevented by preceding CD4 ligation. The inhibition of AICD due to the prevention of Fas ligand upregulation reveals a novel immunoregulatory consequence of CD4 ligation that might play a role in HIV infection and in the therapeutic application of anti-CD4 mAb.

First Report of Black Sigatoka in Bolivia

Black Sigatoka, caused by Mycosphaerella fijiensis, is the most important disease of banana worldwide (1). It affects cultivars of the Cavendish subgroup that are used for export and important, locally consumed cooking and dessert bananas and plantains, reducing yields by 50% or more. Black Sigatoka first appeared in the Western Hemisphere in 1972 in Honduras, and has spread to all other countries in Central America (1980), Mexico (1980), and the following islands in the Caribbean: Cuba (1992), Hispanola (Dominican Republic) (1996), and Jamaica (1994). In South America, the disease has spread to Colombia (1981), Ecuador (1986), Venezuela (1992), and Peru (1994) (1). In June 1996, symptoms of the disease were observed in the San Carlos area in the western Chapare region of Bolivia. During surveys conducted in March and June 1997, several Cavendish clones, Dulce Cajita (Pisang mas), Guineo (Silk), Morado (Red), and Platano (French and Horn plantain) were affected. In each of eight major banana-producing areas in the region, disease incidence and severity were recorded at several representative sites on Cavendish cultivars, which were the most widely spread and susceptible clones in the region. Disease incidence was 100% in all areas from San Carlos to Ingavi B, 30 km to the east, and disease severity, rated as the youngest leaf spotted (YLS), ranged from means of 4.5 to 8 in the same areas. The disease was less common or rare in the Valle Sajta area, 60 km east of San Carlos, the reserve of the Yuqui indigenous group at the confluence of the Rio Chimore and Rio Useuta, 15 km northeast of San Carlos, and the southernmost settlements of the Yuracare indigenous group on the Rio Chapare, 20 km north of Ingavi B (incidences = 0 to 50%). Symptoms began as brown streaks on the abaxial leaf surface, 1 to 3 mm in length, and became visible on the adaxial surface and enlarged to wet, dark brown streaks, 1 to 2 × 10 to 20 mm, with chlorotic haloes. Ultimately, large portions of the leaf became blackened and watersoaked. The presence of the disease in the San Carlos, Ingavi B, and Senda B areas and the Yuqui reserve was confirmed after microscopic examination of the anamorph, Paracercospora fijiensis, on affected leaf tissue: scars were present on the base of conidia, and only simple conidiophores were found (2). This is the first report of black Sigatoka in Bolivia, and represents the southernmost extent of the disease on the South American continent. High rainfall in western portions of the Chapare (4 to 7 meters per year) makes it unlikely that the disease could be controlled effectively or economically in the region with fungicides. We believe this is the closest approach of the disease to Brazil (ca. 700 km), the last major banana-producing country in which black Sigatoka has not been reported. Moreover, the outbreaks in the northern Chapare are thought to be the first across the colonist frontier to indigenous Amazonian populations that rely on plantains and bananas as staple foods.

Activation-Induced T Cell Death: Resistance or Susceptibility Correlate with Cell Surface Fas Ligand Expression and T Helper Phenotype

Activated T cells undergo apoptosis when the Fas-antigen (Apo-1, CD95) is ligated by Fas ligand molecules (FasL) or agonistic anti-Fas antibodies. Restimulation of T lymphocytes via the TCR/CD3 complex induces activation-induced cell death (AICD). AICD and Fas-induced cell death are causally related since TCR-induced AICD at least in part depends on Fas/FasL interactions. Thus, restimulation of T cells leads to FasL gene transcription and surface expression. Membrane-bound or secreted FasL molecules then bind to Fas receptors on the same cell or on a neighbor cell to trigger the death signaling cascade. We have compared Fas-mediated apoptosis and AICD in a panel of human T cell clones. While all clones were killed by anti-Fas mAb, several clones were resistant to AICD triggered by anti-TCR/CD3 mAb or superantigen. The pattern of TCR-induced protein tyrosine phosphorylation was comparable in AICD-resistant and -susceptible clones, as was the induction of FasL mRNA. However, significant differences were observed at the level of FasL surface expression which was induced in AICD-susceptible but not in AICD-resistant clones. Cytokine profiles of CD3-stimulated clone cells support the recent observations that AICD sensitivity is restricted to the Th1 subset. However, AICD-resistance is not only associated with the classical Th2 phenotype.

Outbreak of Sugarcane Mosaic in Commercial Sugarcane Cultivar CP 72-2086, in Florida.

In May 1996, an outbreak of sugarcane mosaic virus (SCMV) was detected in Florida affecting a major commercial sugarcane (Saccharum spp. hybrid) cultivar, CP 72-2086. Identification of SCMV was confirmed by enzyme-linked immunosorbent assay (ELISA) with SCMV antiserum (ATCC no. PVAS 115), and by reverse transcription(RT)-PCR (1,2). The strain of SCMV was identified as E with an RT-PCR-based restriction fragment length polymorphism analysis (1). Although SCMV strain E has been detected in Florida for many decades, it had been confined primarily to S. officinarum clones, to occasional susceptible sugarcane clones in breeding programs, and to various grasses throughout the sugarcane production area. CP 72-2086 plus seven other sugarcane cultivars and 10 sorghum cultivars reacted similarly to inoculation with the isolate of SCMV found recently on CP 72-2086 and an isolate of SCMV collected in 1986. CP 72-2086 was released in 1982 and currently is the second most widely grown cultivar, constituting 18.0% of the commercial sugarcane area in Florida. At the epicenter of infection, located 11 km southeast of Canal Point, over 50% of CP 72-2086 plants had SCMV symptoms. The incidence of mosaic decreased rapidly away from the epicenter. No SCMV was observed in the western area of the sugarcane production area, west of South Bay, FL.

Deme formation in gall‐making aphids Adelges japonicus (Homoptera: Adelgidae)

AbstractThe deme formation hypothesis in herbivorous insects states that herbivores differentiate genetically into small demes, each specialized in the idiosyncratic traits of individual plants. We examined this hypothesis in gall‐forming aphids, Adelges japonicus, by reciprocal transfers of aphids among three susceptible clones of Picea jezoensis standing in proximity. Performance of aphids transferred to their natal clone was compared with that of aphids transferred to other novel clones. ANCOVA showed no significant donor (source of aphids) by receptor (recipient of aphids) interactions for both total aphid performance on the host tree over a life cycle and its three subdivisions (q1,q2 and q3). In addition, the total performance on the natal clone was lowest among those in the three clones, which was explicitly against the prediction from the local adaptation. In contrast, there were significant receptor effects on q1 (fecundity of gallicolae multiplied by survivorship of fundatrices while feeding on needles) and q3 (fecundity of fundatrices multiplied by survivorship of gallicolae before emergence from the galls), suggesting genetic differences in resistance among clones. The two‐fold difference in q3 among clones was due to different gall size, which was highly correlated with the number of gallicolae that emerged from the gall. The two‐fold difference in q1 seemed to be caused by different survivorship of larval fundatrices while feeding on needles.

Molecular epidemiology of antibiotic resistance of Salmonella enteritidis during a 7-year period in Greece.

A significant increase in the frequency of isolation of Salmonella enteritidis has been observed during recent years in Greece, parallelled by an increasing rate of resistance of this organism to antibiotics. A substantial proportion of ampicillin- and doxycycline-resistant isolates exhibited cross-resistance to drugs of other classes, such as sulfonamides and streptomycin. Isolates of human origin were overall less resistant than those of animal or food-feed origin. Indeed, strains associated with animal infections were characterized by the highest rates of resistance to several antibiotics. These phenotypic data were correlated with genotypic information concerning two distinct populations: isolates from all sources that were resistant only to ampicillin, the drug toward which resistance rates were highest, and a control group of sensitive isolates. Ampicillin resistance was due to a 34-MDa conjugative plasmid. DNA fingerprinting by macrorestriction of genomic DNA revealed two types, A and B, common to both ampicillin-resistant and -sensitive strains, with 80 to 90% of strains being of type A. However, a third type, C, was specific for the sensitive population, representing 17% of those strains. Therefore, although the majority of resistant isolates were genetically related to sensitive ones, there existed a susceptible clone which had not acquired any resistance traits.

Scopoletin production and degradation in relation to resistance of Hevea brasiliensis to Corynespora cassiicola

Inoculation of leaves of resistant and susceptible Hevea brasiliensis clones with Corynespora cassiicola induced foliar necrosis and biosynthesis of scopoletin (Scp), considered as a Hevea phytoalexin. Foliar symptoms appeared, as soon as 24 h after infection, and precede Scp accumulation in inoculum droplets (peak: 48 h after inoculation). Scp concentration was 5-fold higher in the susceptible than in the resistant clone 48 h after infection. Nevertheless, a fungitoxic effect of Scp on spore germination and on mycelium growth was shown in bioassays, but the efficient concentrations were greater than those found with another foliar pathogenic fungus of rubber tree such as Microcyclus ulei and Colletotrichum gloeosporioides. Bioassay showed that the low Scp fungitoxicity to C. cassiicola could also be related to the ability of fungus to detoxify Scp. Isoelectric focusing analysis of peroxidase activity in Hevea infected leaflets has shown an increase in acidic and basic isoperoxidases that are able to use Scp as substrate. The rapid increase of Scpoxidase activity (as soon as 16 h after inoculation) was higher in the resistant than in the susceptible clone. In vitro testing of Scp peroxidation products has shown a toxic effect on C. cassiicola (conidia germination and mycelium growth), but not significantly higher than Scp. It appears that the level of Scp accumulation was a balance between its synthesis and its degradation by the pathogen or/and by foliar Scp-peroxidases. In conclusion, these results lead to the fact that Scp cannot be considered as a major defense mechanism of Hevea towards C. cassiicola disease. (Resume d'auteur)

Short and long distance spread of potato leafroll luteovirus: effects of host genes and transgenes conferring resistance to virus accumulation in potato.

Potato leafroll luteovirus (PLRV) movement through phloem of PLRV-resistant potato clones was examined in experiments in which stem pieces were grafted either between infected rootstocks and virus-free susceptible scions or between infected scions and virus-free susceptible rootstocks. These test plants permitted either upwards or downwards virus movement into the susceptible tissue. Resistant potato clones had either host gene-mediated resistance (H-MR) or transgene-mediated resistance (T-NR, conferred by transformation with the PLRV coat protein gene) to PLRV accumulation. The rate of PLRV movement was similar whether stem tissue was taken from H-MR, T-MR or susceptible potato clones. Virus movement through two graft unions began around 7 days after grafting and was generally complete by about 14 to 16 days. Virus movement occurred soon after acquiring functional phloem continuity across grafts as demonstrated by tracing with 6(5)-carboxyfluorescein, a phloem-mobile dye. Most of the delay in virus detection after grafting probably resulted from the time necessary to develop new phloem strands across graft unions; subsequent movement of PLRV was rapid suggesting a passive process. PLRV infection was largely excluded from external phloem bundles in stem tissue of clones with either H-MR or T-MR. This trait was less pronounced as tissue aged. The mechanism limiting PLRV invasion of external phloem bundles of the T-MR clones appears to be similar to that operating in the H-MR clones. Results are discussed in the context of a proposed model of PLRV movement.