Graduate Thesis Or Dissertation
 

Occurrence, transmission, and properties of strains of cucumber mosaic virus that are seed-transmitted in Phaseolus vulgaris

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  • A survey of major bean (Phaseolus vulgaris) breeding programs and seed producing areas in Idaho, Washington, and Oregon was conducted to determine the occurrence of cucumber mosaic virus (CMV), recently reported to be seedborne in beans. Some 259 plants, sampled from breeding lines of diverse germplasm, were found by enzyme-linked immunosorbent assay (ELISA), bioassay, or gel double-diffusion serology to be free of seedborne CMV. CMV was, however, detected in 1 of 32 Plant Introduction (PI) bean accessions tested by the same methods. Seed-transmission rates of selected isolates (isolates will hereafter refer to those reportedly seedborne and one not reported to be seedborne in beans) were determined under field and greenhouse conditions. An isolate from PI No. 271998 (originating in Spain) was found to be seed-transmitted in up to 49% of the progeny of infected 'Top-crop' bean plants. Seed-transmission depended on early inoculation of plants, i.e., less than four weeks after planting. Rates of transmission under our conditions differed from values reported by others. Most notably, seed-transmission of CMV-F, reported at a rate of 54% in seeds of 'Toperop' (61), was found in our studies to be 3.3% in this cultivar. CMV was distributed uniformly throughout seedlings arising from infected seed but relative virus concentration was most consistently higher near the growing point. Symptoms were mild or masked in plants arising from infected seed. Cucumber (Cucumis sativus 'Lemon') and tobacco (Nicotiana tabacum 'Xanthi') plants infected with CMV were detected by gel serology using standard procedures. To obtain gel serological reactions with infected bean or cowpea tissue, a 0.5 M citrate buffer, pH 6.5 with 0.1% sodium thioglycollate and 0.1% Triton X-100, was employed. CMV was readily detected in infected plant tissues by ELISA. Purified CMV was detected by gel serology at concentrations of 0.05-0.1 mg/ml and by ELISA at concentrations of 6 ng/ml. Isolates were compared for differences in host range, infectivity, serological relatedness, electrophoretic mobility of protein and nucleic acid species, and presence of minor or satellite RNA components. No differences among isolates were found in any of the above properties, except that CMV-Pg seemed to produce lower yields in beans and tobacco and that its major RNA components appeared to fragment into unique electrophoretic species. Otherwise, the greater seed-transmission rate of CMV-Pg was the principal distinction among these isolates. The nucleic acid components of CMV-Pg (seedborne) and CMV-Le (nonseedborne) were separated by sucrose density gradient centrifugation. Pseudorecombinants were prepared among RNA components of these isolates for continuing studies of the genomic determinants of CMV seed-transmissibility in beans.
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