[Anublo bacteriano de la yuca : patogenicidad, resistencia e implicaciones en el desarrollo de lineas resistentes].
1995
Cooper R.M. | Deshappriya N. | Clarkson J.M. | Henshaw G.G.
Xanthomonas campestris pv. manihotis (Xcm) mainly invades xylem vessels and is present in advance of symptoms. Initial parasitism appears to be biotrophic because leaf necrosis is slow, electrolyte leakage increases only after 2 days and host suspension-cultured cells are killed far more slowly (3-6 days) than by necrotrophs (within 1 day). Three bacterial products are potential determinants of pathogenicity. Copious extracellular polysaccharide (xanthan) is evident ultrastructurally in vessels and probably causes the characteristic water stress and possibly suppression of host defences. Pectate lyase is produced as a single form of isoelectric point 9, kills cassava cells at high dilution (1/1000 culture fluids) and reproduces leaf symptoms; it is highly induced by cassava cell walls. A toxin has been detected in planta by others, but production of this methionine metaboline (MTPA) requires at least 50 microgramo ml(-1) of this amino acid and free methionine in cassava leaves is 0.1 microgramo/gramo fresh wt. Resistance of certain field-resistant lines (MNGA1, M Ven 77) to virulent isolates from Niger and Brazil appeared incomplete as although there was a delay of 2-4 days in leaf symptoms and 1 d in electrolyte loss compared with susceptible lines (e.g., M Col 113, M Col 22), by 12 days symptoms and bacterial growth were not significantly different. Resistance remained stable only with a low inoculum of 10(2) cells ml or less. This may be effective against the low numbers spread by rain splash but is likely to be overcome by potentially high inoculum on propagation tools. Co-culture of embryogenic cassava cells and Xcm has potential for regenerating resistant plants from cells surviving challenge with Xcm or its products and may facilitate development of cultivars with more effective disease resistance.
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