Molecular Plant Pathology – Pathogen Profiles

Xanthomonas albilineans and the antipathogenesis approach to disease control

Robert G Birch
Department of Botany, University of Queensland, Brisbane, Qld 4072, Australia

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; Xanthomonasgroup; 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.