Percentage Of Resistant Plants Research Articles

Spread of introgressed insect-resistance genes in wild populations of Brassica juncea: a simulated in-vivo approach

Introgression between transgenic, insect-resistant crops and their wild relatives could lead to a progressive increase of the frequency of resistant plants in wild populations. However, few studies help predict the impact on the population dynamics. To simulate the performance of introgressed insect-resistant plants of wild Brassica juncea, independently from the interspecific hybridization cost, healthy plants were cultivated in pure and mixed stands with damaged plants through cutting leaves in field experiments over two field seasons. As expected, resistant (healthy) plants held a competitive advantage when in competition with susceptible (damaged) plants. Individual biomass and seed production of both types of plants decreased as the percentage of resistant plants increased, so that the relative advantage of resistant plants increased. The combined effects of defoliation and competition on the individual performance of B. juncea were additive. Replacement series experiments confirmed this trend but did not show different seed output in pure stand of susceptible versus resistant plots. The total vegetative and reproductive production of mixed populations was not significantly different of that of pure populations. These results suggest that if a transgene for insect-resistance were to colonize wild populations, high herbivory of susceptible plant and low resource availability would facilitate the spread of resistant individuals. However, at the population level, the shift from an insect-susceptible to a predominantly resistant population would not result in exacerbated habitat colonization.

Evolution of a herbicide-resistant population of Alopecurus myosuroides Huds. in a long-term cropping system experiment

Due to heavy reliance on herbicides and a lack of cultural control measures, herbicide-resistant populations of Alopecurus myosuroides Huds. (blackgrass) appeared in the early 1990s in winter cereal rotations in France and in Europe. The aim of the present study was to analyse the effects of different cropping systems on an aryloxy-phenoxypropionate herbicide-resistant population in a field trial. Two crop rotations, one consisting exclusively of winter crops and another including spring crops, were assessed over a six-year period. The rotations were combined with different cultural practices including mouldboard ploughing, delayed sowing, and efficient herbicides for controlling resistant plants. A. myosuroides densities decreased in all the cropping systems, but the weed management was most effective when herbicides were combined with non-chemical practices. Rotation with an alternation of spring and winter crops was the most efficient solution against A. myosuroides. Moreover, during the six years, the percentage of resistant plants in different crop rotations was estimated independently of the cropping systems. This proportion did not vary during the six years of the experiment, suggesting that the resistance gene persisted, despite the removal of selection pressure by the aryloxy-phenoxypropionate herbicides.

New sources of resistance for stripe rust in barley

Eight spring barley accessions from the gene bank in Gatersleben, Germany, and 10 cultivars were tested for stripe rust resistance. Tests were performed at the seedling stage in the growth chamber and as adult plants in the field. All accessions and six cultivars were scored as resistant against race 24 under all test conditions, with very few plants as exceptions, while the susceptible control cultivars ‘Karat’ and ‘Certina’, and four other cultivars were attacked in all cases. Differences between accessions and between cultivars were detected after infection with isolates from ‘Trumpf’ and ‘Bigo’ (seedling tests only). Infection structures within seedling leaves without pustules and for the first time within leaves of adult plants from the field were analysed by fluorescence microscopy. With this method additional genetic Differences in the resistance reaction could be detected which could not to be seen in the resistance test. Crosses between the accessions and the susceptible cultivar ‘Karat’ led to segregating F2 progenies. The percentage of resistant plants varied between the accessions. This also indicates a different genetic basis of resistance in the accessions. The infection structures observed by fluorescence microscopy stopped earlier in leaves of the two accessions HOR 8979 and HOR 8991 than in leaves of other accessions in all the tests. These accessions were the only ones with more than 50% resistant plants in all F2 tests. In general, the accessions from the gene bank can be used as new resistance sources against stripe rust.

Inverted-repeat transgenic maize plants resistant to sugarcane mosaic virus

RNA silencing is a post-transcriptional gene-silencing phenomenon induced by double-stranded RNA (dsRNA). In an attempt to generate dsRNA-mediated transgenic maize plants resistant to sugarcane mosaic virus (SCMV), we cloned SCMV NIb gene-specific sequences and inserted it into the binary vector p3301 in the sense and antisense orientations (named SCMVirNIb), which could produce RNAs capable of duplex formation in plant cells. Maize immature embryos were co-cultured with Agrobacterium carrying two vectors, one marker-free vector harboring the SCMVirNIb and one vector harboring bar gene as the selective marker. Resistant calli were recovered by selection on medium containing Biolaphos. Among the regenerated plantlets from resistant calli, 14 plants have been certified to contain SCMVirNIb by PCR amplification and DNA dot blot. T1 plants derived from the 14 plants were challenged in a greenhouse with SCMV inoculums and the percentages of resistant plants in 11 T1 lines were higher than 60%. One plant in the T1 line was found to carry SCMVirNIb without bar gene by PCR assay.

Resistance to Grapevine leafroll associated virus-2 is conferred by post-transcriptional gene silencing in transgenic Nicotiana benthamiana

Grapevine leafroll-associated virus-2 (GLRaV-2) is an important component of the leafroll disease complex in grapevine. We have previously sequenced the GLRaV-2 genome and identified the coat protein (CP) gene. The objective of this study is to test the concept of pathogen-derived resistance against a closterovirus associated with grapevine leafroll disease. Because GLRaV-2 is capable of infecting Nicotiana benthamiana, we decided to test the concept on this herbaceous host. Thirty-seven T(0) transgenic N. benthamiana plants expressing the GLRaV-2 CP gene were regenerated following Agrobacterium-mediated transformation. Disease resistance was evaluated in greenhouse-grown T(1) and T(2) plants by mechanical inoculation with GLRaV-2. Although all the inoculated non-transgenic plants showed symptoms 2-4 weeks post inoculation, various numbers of transgenic plants (16-100%) in 14 of 20 T(1) lines tested were not infected. In these resistant plants, GLRaV-2 was not detectable by enzyme linked immunosorbent assay. Although virus resistance was confirmed in T(2) progenies, the percentage of resistant plants was generally lower (0-63%) than that of the corresponding T(1) lines (0-100%). Northern blot and nuclear run-off results showed that virus resistance in the transgenic plants was consistently associated with the low level of transgene RNA transcript suggesting a post-transcriptional gene silencing. The success of pathogen-derived resistance to GLRaV-2 in transgenic N. benthamiana plants represents the first step towards eventual control of the leafroll disease in grapevines using this strategy.

Resitência de Eucalyptus globulus e Eucalyptus nitens à ferrugem (Puccinia psidii)

Avaliou-se a resistência das espécies de Eucalyptus globulus e Eucalyptus nitens inoculadas com um isolado uredinospórico monopustular de Puccinia psidii origininário de plantio de Eucalypstus grandis (UFV-2) em Itapetininga, SP. A avaliação foi realizada aos 12 dias após a inoculação, e quantificou-se a doença por meio de uma escala de notas com quatro classes de severidade da doença (S0, S1, S2 e S3). Em média, aproximadamente 60% das plantas de E. globulus e 50% de E. nitens foram resistentes a P. psidii. A variabilidade intra-específica nos materiais estudados indica ser possível a clonagem de genótipos resistentes para plantio comercial ou para uso em programas de melhoramento genético.

The Effect of Different Levels of Beet Cyst Nematodes ( Heterodera schachtii ) and Beet Necrotic Yellow Vein Virus (BNYVV) on Single and Double Resistant Sugar Beet Cultivars

Beet cyst nematode-resistant cultivars, which were introduced recently, originated from the homozygous inbred line B883. This translocation product was unstable and the transmission of resistance when crossed with a susceptible cultivar did not exceed 94%. Tests with the resistant cultivars in climate cabinets showed a wide variety of resistance against Heterodera schachtii and beet necrotic yellow vein virus (BNYVV), expressed as average numbers of infective units per plant and percentages of resistant plants. In a series of field trials at different levels of infection of H. schachtii, their multiplication rates on all resistant cultivars depended on the initial density, which was caused by the presence of small numbers of susceptible plants. Since tolerance to wilting was also incorporated in B883, reasonable yields were obtained in the presence of H. schachtii. However, at increasing initial densities of H. schachtii, yields decreased considerably, since penetrating juveniles cause a hypersensitivity reaction in resistant plants. Based upon the results of three series of field trials, it was concluded that resistant cultivars should preferably be applied at population densities between 500 and 2000 eggs and juveniles of H. schachtii per 100ml of soil. Cultivars with double resistance against H. schachtii and BNYVV behaved like those with H. schachtii resistance in soils infected with beet cyst nematodes, but not with BNYVV. In soils with a combined infection of H. schachtii and BNYVV double resistant cultivars were far superior to single resistant ones, since damage caused by BNYVV was far more serious than damage caused by H. schachtii. No substantial interaction between soil pathogens nor types of resistance could be detected.

Field Reactions of Segregating Populations of Interspecific Hybrids of Lycopersicon Species to Natural Infection by Xanthomonas campestris pv. vesicatoria (Doidge) Dye

The progenies arising from crosses between cultivated tomatoes and a wild relative were screened with their parents for resistance to bacterial leaf spot disease. The aim is to develop a tolerant variety that is well adapted to humid forest ecology where the disease is endemic. The results obtained showed that the cultivated varieties, Roma and Nsukka local, were highly susceptible with disease incidence of 100 per cent. The wild relative on the other hand showed a high level of resistance with a reduced disease incidence. The reactions of the backcrosses and F2 offspring were not impressive as negligible percentage of resistant plants were identified in the F2 . Plants with resistance were recovered in the F3 generation from crosses between the wild and L. esculentum indicating the possibility of transfer of gene(s) for resistance of leaf spot from the wild to the cultivated tomatoes. The increase in the number of resistant plants in this generation is an indication that genes for resistance to leaf spot can be accumulated over generations thereby increasing the selection potentials for resistance to the pathogen in higher generations.

Genetics of resistance to alfalfa mosaic virus in red clover

Virus diseases are known to reduce the yield and persistence of stands of red clover (Trifolium pratense L.). Alfalfa mosaic virus (AMV) is one of the causes of mosaic in red clover and may also cause mottling and leaf distortion. The goals of this study were to identify genotypes resistant to AMV within commercial red clover cultivars and to determine the inheritance of this resistance. Cultivars and progenies were screened for resistance by means of mechanical inoculations. Plants showing no symptoms after three successive inoculations were assayed for the presence of the virus using ELISA (enzyme-linked immunosorbent assay). Resistant plants were detected at a frequency of just over 1% in the five cultivars screened. Crosses among and between resistant (R) and susceptible (S) plants were carried out over two successive cycles of selection. In cycle 1, R × R crosses produced a higher frequency of resistant plants (19.8%) in the progenies than R × S crosses (10.9%). S × S crosses produced no resistant progeny. For crosses of resistant plants from cycle 1 progeny, (R × R) × (R × R) produced 47.8%, (R × R) × (R × S) produced 31.4% and (R × S) × (R × S) produced 29.3% resistant cycle 2 progeny. Crosses of resistant and susceptible cycle 1 plants resulted in resistance frequencies in cycle 2 which were not significantly (P < 0.05) greater than cycle 1. When only resistant plants were selected for further crossing, the percentage of resistant plants increased from 1.3 to 47.8 over the two cycles of selection. It was concluded that resistance to AMV in red clover is under polygenic control and that it should be relatively simple to select a highly resistant population through recurrent selection. Key words: Red clover, Trifolium pratense L., virus disease, alfalfa mosaic virus, recurrent selection

Oligogenic inheritance of resistance to zucchini yellow mosaic virus in melons

The melon (Cucumis melo) plant introduction PI 414723 was observed to segregate for resistance after inoculation with a highly virulent non-aphid-transmissible strain of zucchini yellow mosaic potyvirus (ZYMV-NAT). By self-pollination of selected resistant plants, the percentage of resistant plants increased in successive generations until nearly 100% in the S7 progeny. A reciprocal cross was made between a resistant S5 plant of PI 414723 and a plant of the susceptible cultivar Dulce. Parental-, filial-, and backcross- generation plants were inoculated with ZYMV-NAT. One-half of the F1 plants, regardless of the direction of the cross, were resistant, indicating that the PI 414723 individual used for crossing was heterozygous. The F2 generation obtained by self-pollination of resistant F1 plants segregated to resistant and susceptible in accordance with a 27:37 ratio, indicating that, in this cross, three complementary dominant genes are needed for resistance to be expressed. The resistance to ZYMV carried by PI 414723 is oligogenic with the number of genes observed to segregate in crossing depending on the genotype of the susceptible parent.

Evaluation of Capsicum Germplasm for Sources of Resistance to Rhizoctonia solani

A reliable screening method to detect Rhizoctonia solani Kuhn resistance in chiles (Capsicum annuum L.) was developed using infested corn (Zea mays Bonaf.) kernels as inoculum. The most aggressive New Mexican isolate of R. solani (PWB-25) was used to screen 74 Capsicum accessions for resistance to root rot caused by the fungus. The accessions differed in resistance, with disease ratings ranging from 2.9 to 8.6 on a 0 (no disease) to 9 (seedling dead) scale. The percentage of resistant plants, those in the interaction phenotype index class 0, 1, 2, and 3, ranged from 2.4% to 77.1%. Nineteen accessions representing four species had ≥50% resistant individuals and would be useful in breeding programs.

DEVELOPING ENHANCED PEPPER GERMPLASM RESISTANT TO VERTICILLIUM WILT

A Verticillium wilt-resistant population of pepper (Capsicum) is being developed from the USDA P.I. 215699 accession. Three selfing cycles with disease screening and selection have been done. The disease screen was conducted at a soil temperature of 25 ± 1°C and at an inoculum level of 2000 microsclerotia per gram of soil. Sixty days after inoculation, individual plants were scored using an interaction phenotype scale (IP), ranging from 1 to 9, where 1= no aerial symptoms, and 9=death. Plant shoot height was also measured. Even after three selfing cycles, P.I. 215699 segregates for Verticillium wilt resistance. The percentage of resistant plants at S3, is 75%. Another cycle of selection (S4) was done and the progeny are being tested. The S4 progeny data, interaction phenotype, shoot height, and percentage of resistant plants, will be analyzed by using standard analysis of variance followed by appropriate means separation. An analysis of correlation for shoot height by interaction phenotype variables will also be conducted.

Effects of Natural Selection in Segregating Soybean Populations Exposed to Phytophthora Rot or Soybean Cyst Nematodes1

A study was conducted to determine the effects of natural selection in soybean [Glycine max (L.) Merr.] populations segregating for reaction to phytophthora rot (caused by Phytophthora megasperma Drechs. var. sojae Hildeb.) and soybean cyst nematode (SCN) (Heterodera glycines Ichinohe). Plantings of an F2 population of ‘Forrest’ ✕ ‘Tracy’ and 10 backcross F3 families of Forrest (2) ✕ Tracy were made at Stoneville, MS, where phytophthora rot caused injury to susceptible genotypes and at Ames Plantation in west Tennessee where SCN race 3 was present. The parent cultivar, Forrest, is resistant to SCN race 3 and moderately resistant to phytophthora rot, and Tracy is susceptible to SCN and resistant to phytophthora rot, races 1 through 9. The populations were advanced in bulk at each location for four generations.Natural selection was not effective in increasing the percentage of plants having major genes for resistance to phytophthora rot in Stoneville plantings where susceptible cultivars show injury from the disease. The failure of natural selection to be effective was attributed to the presence of field resistant plants in the segregating population which competed effectively with plants having major genes for resistance.Segregating populations grown on SCN infested soil showed a distinct increase in percentage of resistant plants as compared with populations grown on non‐infested soil. On the basis of three recessive genes and 1 dominant gene being necessary for resistance to SCN race 3, 5.5% of the population would be expected to be resistant after the F5 generation. The population of Forrest ✕ Tracy grown on non‐infested soil had 5% resistant plants after the F5 generation, while the population advanced on SCN race 3 infested soil had 40% resistant plants. Backcross families of Forrest (2) ✕ Tracy also had a distinctly higher percentage of SCN resistant plants when grown on infested soil than when grown on non‐infested soil.

Field Tests of Alfalfa Selected for Resistance to Potato Leafhopper in the Greenhouse21

Seedlings of 10 alfalfa entries selected for resistance or susceptibility to Empoasca fabae (Harris) in the greenhouse and unselected seedlings of the same entries were transplanted in the field and evaluated visually (1-9 ratings) for yellowing on 2 dates. The percentages of resistant plants from the 3 Groups differed little on the 2 dates, indicating that the frequency of field-resistant plants was not increased by selection for resistance in the greenhouse. The entries differed significantly in resistance to yellowing.

Selection and utilization of solanum phureja clones with high resistance to different strains of pseudomonas solanacearum

Over 250 accessions ofSolanum phureja from Colombia and 300S. phureja—S. tuberosum hybrids were screened in the greenhouse for resistance toPseudomonas solanacearum (isolate K-60). Each plant was inoculated by stem puncture at the pre-bud stage and then held at a constant 28 C for 15 days. Seventeen clones with different levels of resistance to K-60 were inoculated with nine additional isolates representing all three races of the bacterium. The reactions of individual clones varied from resistance to almost all isolates, resistance to only one isolate, or complete susceptibility. Crosses amongS. phureja clones and betweenS. phureja and 24-chromosomeS. tuberosum stocks gave progeny with various percentages of resistant plants. Clones with resistance equal to that inS. phureja were found in theS. phureja × S. tuberosum crosses. Tests conducted in growth chambers showed that resistance was expressed more frequently at high light intensity (2000 ft-c) than at low intensities (1300 ft-c) at 24 C and 28 C. Resistance appears to be relatively simply inherited and the prospects for transferring a high level of resistance into cultivated forms are promising.

Greenbug Resistance in Rye12

Twenty varieties of rye, Secale cereale L. exhibited a wide range (0—48%) of seedling survival following infestation with Schizaphis graminum (Rondani). The percentage of resistant plants among different seed sources of the same variety usually varied greatly except for ‘Caribou.’ Recurrent selection within Caribou increased its resistance from 39% to 96%. Resistance to the B biotype of greenbugs in Caribou appeared to be inherited as a single dominant gene. Wheat X Caribou Selection F1 plants were as resistant as their rye parents.