A genetically engineered, bioluminescent strain of Xanthomonas campestris pv. campestris (Xcc) was used to study the effectiveness of plant growth-promoting rhizobacteria (PGPR) as disease control agents. Black-rot-susceptible cabbage plants were wound-inoculated with PGPR and wound- or mist-inoculated with bioluminescent Xcc 10 days later. Growth of the bioluminescent strain in the plants was followed over time with a low-light, charge-coupled device camera. Several PGPR strains effectively reduced growth of the bioluminescent pathogen in the plants when bacteria were introduced into the plant by wound. PGPR inoculation was less effective when bioluminescent bacteria were introduced into the plant by mist inoculation. Little effect on symptom reduction was observed.
American species in the genus Castanea are susceptible to chestnut blight, caused by the Asian fungus Cryphonectria parasitica. This disease spread throughout the natural range of the American chestnut and reduced the species from a timber and nut producing tree to an understory shrub. The lesser known member of the genus, the chinkapin, has also been affected by this disease and a conservation plan is needed. Genetic diversity within and between geographic populations of the Allegheny chinkapin was evaluated to provide baseline genetic information pertinent to conservation of the species. Nuts of Allegheny chinkapin trees from populations in Mississippi, Florida, Alabama, Virginia, and Ohio were collected and evaluated for isozyme and RAPD marker polymorphism. The genetic diversity of these populations will be compared with that of Ozark chinkapin and American chestnut populations. Conservation strategies will be discussed.
Fenny Dane and Marten H. Dane
A pathogenic strain of Xanthomonas campestris pv. vesicatoria, causal agent of bacterial spot of tomato (Lycopersicon esculentum Mill.), was genetically engineered to bioluminesce. In planta growth of the bioluminescent strain was similar to that of its parental strain. Movement and growth of the bioluminescent strain in susceptible tomato seedlings after wound inoculation was followed over time with a liquid-N-cooled, charge-coupled device camera. Highly significant differences in bioluminescent bacterial growth were observed in the four tomato cultivars used. Systemic bacterial movement was most pronounced in `Sunny', which showed population development not only at the inoculation sites but also on several sites in the leaves and at the leaf margins. Bacterial bioluminescence levels in `Campbell 28' remained significantly lower than those observed in `Walter' and `Sunny'. The technique offers unique possibilities for studying host-pathogen interactions and bacterial pathogenesis.
Fenny Dane and Joe Shaw
The movement of genetically engineered, bioluminescent, Xanthomonas campestris pv campestris, causal agent of black rot in crucifers, was followed after wound or mist inoculation of susceptible and resistant host plants. Wound inoculation resulted in higher bacterial population levels in susceptible (Perfect Ball) as compared to resistant (Hancock) cabbage plants. More leaves became infected in the susceptible host and peak levels were reached after 4-8 days, but after 10-12 days in the resistant plants. Mist inoculation resulted in high levels of bioluminescent Xcc after 11-15 days in susceptible tissues only, unless entry was gained through damaged tissue of the resistant plant. In the field environment, Xcc was found endophytically after mist inoculation in susceptible cabbage only, but found to grow epiphytically for as long as 2 1/2 months to the same degree on resistant and susceptible host plants. Dispersal of the bacterium was limited and survival in the soil environment found to last for four months.
Fenny Dane and Yuqing Fu
Chestnut blight, caused by the Asian fungus Cryphonectria parasitica, has severely affected chinkapin populations (Castanea pumila), especially those limited to the Ozark mountains (var. ozarkensis). Genetic diversity within and between geographic populations of the Allegheny (var. pumila) and Ozark chinkapin populations was evaluated for development of appropriate conservation strategies. Nuts or dormant buds collected from populations along the range of the species were analyzed using allozymes. A unique allele was detected in populations along the gulf of Mexico. Significant differences in genetic diversity were observed among Allegheny populations, but not among Ozark populations. High levels of genetic identity were detected among widely distributed populations from Florida to Virginia (Allegheny chinkapin populations) and Arkansas (Ozark chinkapin populations).
Fenny Dane and Hongwen Huang
The genetic diversity within and between four geographic populations of the Ozark chinkapin was evaluated and partitioned in order to gain an understanding of the overall genetic diversity and structure of this species, which will be instrumental for its preservation and germplasm enhancement. Nuts of chinkapin trees along the natural range of the species in the Sylamore Ranger District of the Ozark National Forest in Arkansas were collected and evaluated with isozyme and RAPD markers scattered across the genome. Allozyme differences were detected among the geographic populations. Allele frequencies will be determined and subjected to genetic diversity statistics. A conservation plan will be recommended.
Fenny Dane and Joe Shaw
Growth of genetically engineered, bioluminescent, Xanthomonas campestris pv. campestris (Xcc), causal agent of black rot of crucifera, was followed in cabbage plants after the plants were prior inoculated with Xanthomonas campestris pv. vesicatoria (Xcv) or a nonvirulent strain of Xcc. Wound inoculation with Xcv induced a hypersensitive response and restricted the bioluminescent bacteria-host interaction if prior inoculation was carried out one day before challenge inoculation. Mist inoculation with Xcv was effective in restricting Xcc when the time period between inoculation end challenge was 6 days. In field studies, however, mist inoculation with Xcv or a nonvirulent strain of Xcc, one week before challenge inoculation with bioluminescent Xcc, did not significantly effect the growth and persistence of biolum inescent Xcc. The biolum inescent strain overwintered endophytically in cabbage and could be detected for many months throughout the vegetative period of the host.
John F. Murphy and Fenny Dane
The watermelon (Citrullus lanatus var. lanatus) ‘AU-Performance’ was developed for resistance to multiple fungal pathogens and the plant virus, zucchini yellow mosaic virus (ZYMV). A greenhouse-based evaluation was carried out to determine the response of ‘AU-Performance’ to inoculation with three important cucurbit (Cucurbitaceae) viruses in the genus Potyvirus: papaya ringspot virus (PRSV), watermelon mosaic virus (WMV), and ZYMV. The evaluation included the resistant parent (PI595203), the susceptible parent (‘AU-Producer’), and varieties AU-Allsweet and Charleston Gray. Each of the three viruses systemically infected ‘AU-Performance’ with 100% infection and development of characteristic systemic symptoms. The susceptible parent (‘AU-Producer’), ‘AU-Allsweet’, and ‘Charleston Gray’ responded similarly with 100% infection and systemic symptoms. In contrast, the resistant parent (PI595203) was resistant to WMV and ZYMV; however, PRSV-inoculated plants developed a systemic infection with accompanied symptoms and high levels of PRSV accumulation in noninoculated leaves. PI595203 was shown in previous studies to be resistant to PRSV. We show in this report that under greenhouse conditions and application of virus by mechanical inoculation, ‘AU-Performance’ was not resistant to infection by the three potyviruses.
J.D. Norton and Fenny Dane
The American or Allegeny chinquapin (Castanea pumila) is native to the same area of the United States as the American chestnut (C. dentata) from Florida to Canada and westward to Arkansas. The high-quality nuts are an excellent source of food for wildlife and humans. Resistance to chestnut blight (Cryphnuectria parasitica) was discovered in seedlings in virgin forest at Elgin Air Force Base, Fla., with observations of plants for 35 years. A recurrent selection breeding program was established at Auburn Univ. to improve the blight resistance, precocity, dwarfism, pest resistance, cold hardiness, yield, and quality. A number of seedlings appear to be very promising selections for improvement of the American chinquapin. Since there is little information available regarding hereditability of certain traits in perennial tree species, results of breeding at Auburn Univ. should provide us with guidance for further improvement of the American chinquapin.
Young-Seok Kwon and Fenny Dane
Watermelon (Citrullus lanatus Thumb. Matsum. and Nakai) flower petals usually are yellow, but in watermelon line Kw-695, light-green flowers were detected. To study the inheritance of light-green flower color, Kw-695 plants were crossed with yellow-flowered Korean cultures `SS-4' and `Dalgona'. The resulting F1, F2, and reciprocal backcross generations were analyzed for flower color. Segregation ratios in the F2 and backcross to Kw-695 were 3 yellow: 1 light green and 1 yellow: 1 light green, respectively. Backcross generations to the yellow-flowered parents showed yellow flowers only. These results indicate that inheritance of the light-green flower character in Kw-695 is governed by a single recessive gene. We propose the gf gene symbol for the green flower trait. Kw-695 plants have large vines with large, light-green leaves. The plants are andromonoecious, have large, oval, bright yellow-green fruit with irregular dark-green stripes, bright yellow-orange, inedible flesh with very low sugar content (about 3.2 °Brix), and light-yellow seeds. The trait should be useful as a marker in watermelon breeding programs. Linkages between this trait and other genetic markers in watermelon will be investigated.