Distribution of Pseudomonas sp. populations in relation to maize root location and growth stage

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anglais. The distribution of Pseudomonas sp. populations of the maize rhizosphere in relation to root location and growth stage was investigated using an agar plate method on TSA medium and on a selective medium S1, and a molecular approach on the 16S rDNA followed by restriction enzyme analysis ARDRA. Maize was cultivated on soil for 15 and 30 days. The root system was divided into three parts: the root tip, the root hair and the ramification zones. After 15 days of plant growth, Pseudomonas sp. were only localized on the root tips and on the roots close to the plant stem. No Pseudomonas were detected in the maize root hair zone. After Day 30, Pseudomonas sp. were recorded on the root tips, but also on the medium part of the root system. After amplification and restriction with the endonuclease HinfI, the 212 bacteria isolated from the different root parts were distributed into 5 major ARDRA ribotypes. The evolution of these 5 groups was analyzed. At Day 15, the 5 major ARDRA ribotypes are well represented, whereas all the isolates are gathered into only 3 major ribotypes at Day 30. The ribotypes ARDRA 1 and ARDRA 3 at 15 days persist and the number of strains belonging to these ARDRA ribotypes increases until Day 30. In contrast, the number of strains belonging to ribotype ARDRA 2 decreases and ribotype ARDRA 4 disappears. Ribotype ARDRA 1 is present in all root sections (except in the root hair zone at Day 15) and during all the growth stages. The diversity of the isolates at Day 30 appeared to be lower than at the first growth stage in all root zones. These shifts along the root system were related to a variation of the nature and the quantity of maize exudates along the root system and during growth.

français. Distribution des populations de Pseudomonas en relation avec la zone racinaire et le stade de développement du maïs. Nous avons déterminé le nombre total de bactéries cultivables sur milieu TSA ainsi que la densité des Pseudomonas sur milieu sélectif S1, ceci dans différents compartiments racinaires de maïs et en sol nu, à deux dates de récolte. Il en ressort que les populations de Pseudomonas ont une densité plus élevée dans la rhizosphère de maïs que dans le sol nu. De plus, ces densités augmentent fortement entre 15 et 30 jours, au niveau de la zone d'élongation et des poils absorbants. Les différentes souches isolées nous ont permis de constituer un souchier de 212 souches caractérisées par la technique ARDRA (amplification de la région 16S de l'ADNr et restriction enzymatique par l'enzyme HinfI). Les souches analysées se répartissent en 5 groupes ARDRA. La structure des populations de Pseudomonas sp. a été modifiée selon les zones racinaires étudiées et au cours du temps.