California Agricultural Research Priorities Pierce's Disease (2004) / Chapter Skim
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4 Plant-Pathogen Interaction
4 Plant-Pathogen Interaction
Pages 88-104
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From page 88... ...
In practice, the most widely used and effective measures for controlling xylem-localized plant pathogenic bacteria are exclusion, (as in the use of uninfected propagative materials for control of the bacterial wilt pathogen Ralstonia solanacearum, or host plant disease resistance conferred, for example, by the use of major genes for resistance to control bacterial blight of rice caused by Xanthomonas oryzae pv. oryzae.
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From page 89... ...
and the microarrays based on the sequence are facilitating the identification of genes that could lead to effective management strategies. In fact, many genes have been implicated or eliminated from consideration based on comparison of the Xf genome sequences with other pathogenic bacteria (Van Sluys et al., 2002)
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From page 90... ...
However, the genome sequence does contain sequences related to fimbrial and pili genes; there are at least three Type 4 fimbriae gene clusters in XfCVC and XfPD. Some of the genes associated with Type 4 pili, such as fimT-, pilZ-, and pilA-like genes, are present in more
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From page 91... ...
. It is not known whether products of those genes are involved in movement or adhesion of Xf within plant xylem vessels or the insect foregut Virulence Regulation How are virulence genes regulated?
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From page 92... ...
. TOWARD HOST PLANT RESISTANCE It is not well understood how the Xfplant interaction results in disease symptoms, but in general, pathogens that target the xylem induce water stress in the host plant by increasing resistance to water flow (Tyree and Sperry, 1989; Zimmermann, 1983)
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From page 93... ...
Plant defense responses to wilt pathogens are common to defenses observed for other types of diseases. Those induced responses involve biosynthesis of structural barriers (including cell wall, callose, and lignin biosynthesis or modification)
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From page 94... ...
For example, the use of resistant varieties has stabilized rice production in areas where bacterial blight, caused by the vascular bacterial pathogen Xanthomonas oryzae pv. oryzae, is endemic (Mew, 1987)
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From page 95... ...
Once a source of resistance is identified, it can be introgressed into agronomic varieties of a crop species through recurrent selection. However, the introduction of traits into grapevines, particularly wine grapes, is complicated by several factors (Mullins et al., 1992)
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From page 96... ...
vinifera is unacceptable in the global wine market because of the historical stigma attached to interspecific hybrid wine grapes and their subsequent prohibition in Europe. Because the wine industry relies on a few select and very old cultivars for commercial production, classical breeding programs have not developed many new varieties that are commercially successful, and thus have not had a significant effect on grapevine improvement.
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From page 97... ...
Public perception is particularly relevant to California's current sociological landscape, which is not receptive to the use of genetically modified plants. Although not insurmountable, the successful commercialization of grapevine varieties improved through transgenic technologies will depend on the resolution of those areas of difficulty.
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From page 98... ...
Improvements in tissue transformation systems and in the ability to regenerate plants from transformed tissue have made transgenic technology increasingly feasible, although the availability of genes of known function that could be introduced to target desired effects is limited. In the long term, however, transgenic technology could hold promise for improving resistance to Xf (Category 2)
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From page 99... ...
. Existing chemical control methods or compounds have not provided effective control of Xf or any other vascular bacterial pathogen.
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From page 100... ...
Manipulation of Alternative Hosts Many pathogens infect large numbers of hosts and, in some situations, those alternative hosts are important sources of inoculum for the crop species. The host plant range of Xf is broad, including plants from at least 28 families (for review, see Hopkins and Purcell, 2002)
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From page 101... ...
Although Xf has a broad host range, there are Xf strains that exhibit considerable host species specificity. Strains that cause oleander leaf scorch will not cause disease on grapevines, and strains that cause PD will not cause disease on oleander (Purcell et al., 1999)
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From page 102... ...
However there are major gaps in our knowledge about the complex interactions of the environment, pathogen, vector, host, and alternative host plants. They include the need for research to identify the importance of various alternative hosts to epidemic potential, the genetic potential for the pathogen to adapt to other hosts, the abilities of the new vector -- the glassy-winged sharpshooter -- to transfer the pathogen between and among various hosts, and the environmental consequences of manipulating particular alternative hosts.
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From page 103... ...
Evaluations of nonpathogenic, naturally occurring bacterial endophytes of grapevine to control PD are in progress. Because Xf develops small colonies even in unplugged vessels and can spread to newly formed xylem tissues over different growing seasons, a biological-control agent would need to establish a long-term endophytic relationship with the host and continuously colonize the newly formed vessels to curtail establishment of pathogenic strains of Xf.
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From page 104... ...
Finally, use of the approach could produce unintended effects on other microbes, particularly beneficial microbes or naturally occurring endophytic populations in nontarget insects and plants. Because projects that have examined biological control of bacterial vascular pathogens, particularly of perennial crops, generally have shown limited success, the committee views them as Category 4 studies.
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