Free access
Issue
Agronomie
Volume 10, Number 2, 1990
Page(s) 93 - 102
DOI http://dx.doi.org/10.1051/agro:19900201
Agronomie 10 (1990) 93-102
DOI: 10.1051/agro:19900201

Étude en microscopie électronique à balayage de l'installation de deux souches de Xanthomonas campestris pv manihotis sur feuilles de vitroplants de manioc

V. Verdiera, J. Schmitb and M. Lemattreb

a  ORSTOM, Laboratoire de phytopathologie, BP 181, Brazzaville, Congo
b  INRA, Station de pathologie végétale, Route de St-Cyr, 78026 Versailles Cedex, France

Résumé - L'installation de 2 souches de Xanthomonas campestris pv manihotis, agressive (XCM17) et non agressive (XCM4), est suivie conjointement par numération et par microscopie électronique à balayage, sur des vitroplants de manioc. Les observations réalisées en microscopie électronique à balayage confirment la dynamique de population des 2 souches et permettent aussi de localiser les bactéries à la surface des tissus. Immédiatement après inoculation, l'aspect et la répartition des bactéries à la surface du limbe sont identiques quelle que soit la souche. Six jours après inoculation, la souche non agressive (XCM4) qui ne se multiplie pas ou peu à la surface du limbe n'entraîne pas d'altération des tissus foliaires. L'observation décèle la présence d'un voile recouvrant les cellules bactériennes. Dans les mêmes conditions, la souche agressive (XCM17) se multiplie rapidement ; on note la présence d'amas bactériens à proximité des stomates et les tissus présentent des altérations importantes. La méthode d'inoculation mise au point sur vitroplants de manioc permet de reproduire les symptômes caractéristiques de la maladie et de retrouver le comportement attendu des variétés utilisées. La culture in vitro fournit un matériel commode pour l'étude par différentes techniques des premières étapes de l'infection.


Abstract - Scanning electron microscopic study of the establishment of 2 strains of Xanthomonas campestris pv manihotis on in vitro microcuttings of cassava. The establishment of 2 strains of Xanthomonas compestris pv manihotis (XCM 17: aggressive; XCM 4: non-aggressive) on vitroplants of cassava was monitored by counting and by scanning electron microscopy (SEM). Each strain differed in its establishment following leaf inoculation. Aggressive strain XCM 17 multiplied rapidly, whereas non aggressive strain XCM 4 maintained its initial population level (fig 1). Moreover, only the aggressive strain was detected at increasing levels in leaf tissues 4 days after inoculation (fig 2). SEM examinations confirmed the population dynamic patterns of the 2 strains. In addition, the localization of bacterial cells and their spreading on the leaf surface were more accurately defined. Two methods were used to process samples: examination in fresh state with a "cryoscan" device and after glutaraldehyde-OsO 4 fixation followed by critical point drying. Both methods maintained structural organization of samples, but "cryoscan" gave better preservation of plant tissues (figs 3 and 4), than critical point drying (figs 5 and 6). The reverse was observed for bacterial cells (figs 7 and 8). Comparing both methods, thus helped to avoid major misinterpretations. Immediately after inoculation, the appearance and the distribution of bacteria were similar for both strains (figs 9 and 10). Within 6 days the non-aggressive strain (XCM 4) which multiplied slightly or not at all, did not lead to visible alterations in leaf tissues (figs 17a and b). SEM revealed bacterial cells trapped under a thin layer of unknown composition (figs 16a and b). Under similar conditions, the aggressive strain (XCM 17) increased rapidly (figs 11 and 12). Masses of bacteria were visible near stomata (figs 13, 14, 15) and well-marked lesions developed on leaves (figs 18a and b; 19a and b). In agreement with other studies, our results suggest the relative importance of colonization of stomata and their surroundings in aerial bacterial infection. Here a correlation was found between strain aggressivity and colonization of cassava leaves. Adopting the inoculation method on vitroplants as proposed here, we were able to observe the typical symptoms of the disease and the expected behaviour of a variety of cassava. In vitro culture provides a material suitable for studying several steps of host-pathogen interactions by different experimental techniques.


Key words: Manihot esculenta / cassava bacterial blight / tissue culture / epiphytic phase

Mots clés : Manihot esculenta / bactériose vasculaire / culture de tissus / phase épiphyte