Inheritance of resistance to potato y viruses in Phaseolus vulgaris L

Summary Resistance to watermelon mosaic virus-2 in Phaseolus vulgaris L. is conferred by two distinct dominant alleles at independent loci. Based on segregation data one locus is designated Wmv, the other, Hsw. The dominant allele Wmv from cv. Great Northern 1140 prevents systemic spread of the virus but viral replication occurs in inoculated tissue. In contrast, Hsw confers both local and systemic resistance to WMV-2 below 30C. At higher temperatures, plants that carry this allele in the absence of modifying or epistatic factors develop systemic veinal necrosis upon inoculation with the virus that results in rapid death. Patho-type specificity has not been demonstrated for either allele; both factors confer resistance to every isolate tested. A temperature-sensitive shift in epistasis is apparent between dominant alleles at these loci. Because Hsw is very tightly linked if not identical to the following genes for hypersensitivity to potyviruses I, (bean common mosaic virus), Bcm, (blackeye cowpea mosaic virus), Cam, (cowpea aphid-borne mosaic virus) and Hss (soybean mosaic virus), parental, reciprocal dihybrid F₁ populations, and selected F₃ families were inoculated with each of these viruses and held at 35 C. F₁ populations developed vascular necrosis completely or primarily limited to inoculated tissue, while F₃ families from WMV-2-susceptible segregates were uniformly susceptible to these viruses. The relationship between Hsw, Wmv and other genes for potyvirus resistance suggest patterns in the evolution of resistance and viral pathogenicity. Characterization of the resistance spectrum associated with each factor provides an additional criterion to distinguish genes for plant virus resistance.     

Variation in feeding behaviour of Nilaparvata lugens on resistant and susceptible rice varieties

The feeding behaviour of Nilaparavata lugens was monitored on three rice varieties showing different levels of resistance in the Philippines, using a video-assisted observation method. N. lugens made more frequent, shorter probes on the moderately resistant IR46 and resistant IR62 rice varieties than on the susceptible IR22. Honeydew production was significantly lower on the resistant varieties though insect weight gains in 24 h were similar on IR46 and IR22, both being significantly greater than on the highly resistant variety.Population development, growth index and damage ratings were low on IR62 indicating antibiosis and/or non preference. When IR46 plants were infested as seedlings population increase, growth index and damage ratings were similar to those on the susceptible IR22. When infested at a later stage of plant growth the damage rating showed a moderate level of resistance though some population development was maintained, indicating antibiosis and tolerance. N. lugens started probing less frequently after surface exploration on both resistant varieties than on IR22 suggesting the presence of a resistance factor associated with the surface waxes of these varieties.

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Résumé Le comportement alimentaire de Nilaparvata lugens sur variétés de riz, sensible (IR22), partiellement résistante (IR46) et fortement résistante (IR62), a été contrôlé avec une méthode associant la vidéo à l'observation. N. lugens faisait des piqûres plus fréquentes et plus brèves sur IR46 et IR62, que sur la variété sensible. La production de miellat était significativement plus faible sur les variétés résistantes, bien que les gains de poids des insectes aient été les mêmes en 24 h sur IR46 et IR22, les deux étant significativement supérieurs à celui sur IR62.La croissance de la population, l'indice de croissance et le taux de dégâts étaient tous plus faibles sur IR62, ce qui révèle une antibiose et/ou une absence de préférence. Quand la contamination des IR46 a au lieu au stade semis, la croissance de population, l'indice de croissance et le taux de dégâts étaient semblables à ceux de la variété sensible IR22. Quand la contamination avait lieu à un stade ultérieur, le laux de dégâts révélait un niveau modéré de résistance bien qu'une certaine croissance de population se soit maintenue, ce qui révèle antibiose et tolérance.Après exploration de la surface des feuilles des deux variétés résistantes, N. lugens sondait moins fréquemment que sur IR22, ce qui laisse présumer un facteur de résistance associé aux cires superficielles de ces variétés.

     

Further investigation of the primary mechanism of benzimidazole resistance in Haemonchus contortus

The binding of the [³H] benzimidazole carbamates (BZCs)—albendazole (ABZ), oxibendazole (OBZ), parbendazole (PBZ), mebendazole (MBZ), fenbendazole (FBZ) and oxfendazole (OFZ)—to tubulin from three ecologically-related isolates of adult Haemonchus contortus has been examined. The extent of binding of each BZC was inversely proportional to the known resistance status of the isolate. Biochemically, the change in the formation of the BZC-tubulin complex was due to a reduction in the amount of drug bound to resistant tubulin, with no significant change in the association constant of the complex. The resistance factors derived from the binding data support the hypothesis that the complex is ligand-dependent, with the aryl-substituted BZCs—MBZ, OFZ and FBZ—demonstrating lower resistance factors than those of the alkyl-substituted BZCs—ABZ, OBZ and PBZ. Examination of the slope derived from plots of binding against protein concentration demonstrated that the failure of resistant or partially resistant isolates to bind was due to either a decrease in the number of binding sites or, more likely, to reduced stability of the BZC-tubulin complex rendering it unstable to charcoal extraction.     

Ribosomal protein S12 as a site for streptomycin resistance in Nicotiana chloroplasts

Summary A streptomycin resistant Nicotiana plastome mutant, X/str ^R6, was subjected to molecular analysis. In this mutant, a single nucleotide transition, C » T, in the chloroplast gene for ribosomal protein S12 alters codon 90 from proline to serine while the nucleotide sequence of the chloroplast 16 S rRNA gene is identical to that of the wild type. Mutant X/str ^R6 thus differs from several previously reported streptomycin resistant chloroplast mutants which are altered in the gene for 16 S rRNA.     

A simple procedure for the isolation of streptomycin resistant plants in Solanaceae

A system has been developed for rapid selection of streptomycin resistant mutants, as adventitious shoots arising from explants of several Solanaceous species. Efficient mutagenesis was achieved by incubating shoot culture-derived leaf strips with 1 or 5 mM nitroso-methylurea, for 90 or 120 min. In Nicotiana tabacum and Lycopersicon peruvianum these treatments resulted in white or variegated adventitious shoots from up to 3.5% of explants placed on medium promoting shoot regeneration. Chlorophyll deficiencies were only observed very rarely in Solanum nigrum. Streptomycin resistant shoots were obtained from leaf explants placed on medium containing 500 mg l^-1 streptomycin sulphate, under which conditions explants are bleached and adventitious shoot development suppressed. Green adventitious s shoots appeared at a frequency dependent both on the mutagenic treatment and on the species. The best response was with S. nigrum where >70% of the explants produced streptomycin resistant shoots, most of which retained their resistance on subsequent testing. Maternal inheritance of streptomycin resistance has been confirmed for several N. tabacum and S. nigrum mutants, and there is also evidence for paternal transmission in the latter species. The procedure has been successfully extended to other species, including N. sylvestris and N. plumbaginifolia, and also to obtain spectinomycin resistant mutants.Communicated by R. Hagemann     

A locus involved in kanamycin, chloramphenicol and L-serine resistance is located in the bglY-galU region of the Escherichia coli K12 chromosome

Summary Spontaneous mutants of Escherichia coli K12 displaying an increased level of the kanamycin resistance conferred by plasmid pGR71 were selected. Several mutants obtained in this way apparently carry large chromosomal deletions extending into galU and/or bglY (27 min). This positive selection of deletions allowed detection of a new locus located between galU and bglY. Deletions of this locus are responsible for increased resistance to kanamycin (Irk), decreased resistance to l-serine in minimal medium (Drs) and decreased resistance to chloramphenicol (Drc) when a cat gene is present in the bacteria.     

Influence ofGlycine max nodulation on the persistence in soil of a genetically markedBradyrhizobium japonicum strain

Since competition with indigenous strains limits nodule occupancy by bacteria applied to seeds, the ecology of Bradyrhizobium inoculum strains used for soybean is of concern. A genetically marked strain,B. japonicum I-110 ARS, was directly enumerated from soil on selective medium. A clear long-term positive influence of even limitedGlycine max nodulation was shown by comparisons of population densities obtained with or without plant removal prior to nodule senescence in the first year and with an incompatible as well as a compatible soybean variety after 5 years.     

Nitrite reduction and carbohydrate metabolism in plastids purified from roots of Pisum sativum L.

Nitrate reduction in roots and shoots and exchange of reduced N between organs were quantitatively estimated in intact 13-d-old seedlings of two-row barley (Hordeum vulgare L. cv. Daisengold) using the ¹⁵N-incorporation model (A. Gojon et al. (1986) Plant Physiol. 82, 254–260), except that NH ₊ ⁴ was replaced by NO _- ² . N-depleted seedlings were exposed to media containing both nitrate (1.8 mM) and nitrite (0.2 mM) under a light-dark cycle of 12:12 h at 20°C; the media contained different amounts of ¹⁵N labeling. Experiments were started either immediately after the beginning (expt. 1) or immediately prior to the end (expt. 2) of the light period, and plants were sampled subsequently at each light-dark transition throughout 36 h. The plants effectively utilized ¹⁵NO _- ³ and accumulated it as reduced ¹⁵N, predominantly in the shoots. Accumulation of reduced ¹⁵N in both experiments was nearly the same at the end of the experiment but the accumulation pattern in roots and shoots during each 12-h period differed greatly depending on time and the light conditions. In expt. 1, the roots accounted for 31% (light), 58% (dark), and 9% (light) of nitrate reduction by the whole plants, while in expt. 2 the contributions of the root were 82% (dark), 20% (light), and 29% (dark), during each of the three 12-h periods. Xylem transport of nitrate drastically decreased in the dark, but that of reduced N rather increased. The downward translocation of reduced ¹⁵N increased while nitrate reduction in the root decreased, whereas upward translocation decreased while nitrate reduction in the shoot increased. We conclude that the cycling of reduced N through the plant is important for N feeding of each organ, and that the transport system of reduced N by way of xylem and phloem, as well as nitrate reduction by root and shoot, can be modulated in response to the relative magnitude of reduced-N demands by the root and shoot, with the one or the other predominating under different circumstances.Symbols Anl accumulation of reduced ¹⁵N from ¹⁵NO _- ³ in ¹⁴NO _- ³ -fed roots of divided root system - Ar accumulation in root of reduced ¹⁵N from ¹⁵NO _- ³ - As accumulation in shoot of reduced ¹⁵N from ¹⁵NO _- ³ - Rr ¹⁵NO _- ³ reduction in root - Rs ¹⁵NO _- ³ reduction in shoot - Tp translocation to root of shoot-reduced ¹⁵N from ¹⁵NO _- ³ in phloem - Tx translocation to shoot of root-reduced ¹⁵N from ¹⁵NO _- ³ in xylem