Xanthomonas albilineans and the antipathogenesis
approach to disease control
Robert G Birch
Department of Botany, University of Queensland, Brisbane, Qld 4072, Australia
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| Summary: |
Molecular studies into sugarcane
leaf scald disease, caused by X. albilineans, revealed an unusual
pathogenesis strategy, a new family of antibiotics, an extraordinary
biosynthetic apparatus, and a new approach to disease control in plants
and animals. |
| Taxonomy: |
Bacteria; Proteobacteria; gamma
subdivision; Xanthomonadales; Xanthomonas group; X. albilineans
(Ashby 1929) Dowson 1943. |
| Microbiological properties: |
Gram-negative, slender rod-shaped,
nonsporing, aerobic, motile by a single polar flagellum; producing
slow-growing, pale yellow, nonmucoid colonies in culture; ecologically
obligate plant parasite. |
| Host range: |
Monocotyledonous plants in the
Poaceae family, including Saccharum spp. and other grasses. Causal
agent of sugarcane leaf scald. |
| Disease symptoms: |
Characteristic white leaf stripes
with necrotic zones at leaf margins, extensive chlorosis of emerging
leaves, vascular reddening and cavity formation in invaded stems,
production of side shoots, rapid wilting and death of plants. Prolonged
latent infection can occur, necessitating detection by isolation or
sensitive molecular assays. |
| Pathogenesis: |
Xylem-invading pathogen,
transmitted in cuttings, mechanically, and by wind-blown rain. Produces
albicidin toxins that block prokaryotic DNA replication and plastid
development, causing chlorosis in emerging leaves. Albicidins interfere
with host resistance mechanisms, allowing systemic invasion. Strains vary
in virulence. |
| Agronomic importance and control: |
Sugarcane leaf scald is a
widespread and devastating disease. Eradication is impractical because of
alternative hosts. Measures to reduce inoculum sources and transmission
can reduce losses. Long-term control requires sugarcane varieties with
introgressed resistance, thus limiting gains from breeding. |
| Antipathogenesis approach: |
By understanding key pathogenicity
factors (such as albicidins), it may be possible to develop new control
strategies, including novel resistance genes to rescue susceptible
varieties. |
| Useful web site: |
http://cygnus.tamu.edu/Texlab/Sugarcrops/Sugarcane/sugarc.html |
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Left:Colonies of X. albilineans with different growth rates
are commonly observed on original isolation plates, and are typically
first visible after 4- 8 days
Right: Characteristic white pencil lines and extensive chlorosis of
emerging leaves in the chronic form of sugarcane leaf scald disease, and
side shoots on a more severely diseased plant.
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