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Specific Flavonoids Promote Intercellular Root Colonization of Arabidopsis thaliana by Azorhizobium caulinodans ORS571

July 1997 , Volume 10 , Number  5
Pages  560 - 570

Clare Gough , 1 Christine Galera , 1 Jacques Vasse , 1 Gordon Webster , 1 Edward C. Cocking , 2 and Jean Dénarié 1

1Laboratoire de Biologie Moléculaire des Relations Plantes-Microorganismes, INRA-CNRS, BP27, 31326 Castanet-Tolosan, Cedex, France; 2Plant Genetic Manipulation Group, Department of Life Science, University of Nottingham, Nottingham NG7 2RD, England

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Accepted 20 March 1997.

The ability of Azorhizobium caulinodans ORS571 and other diazotrophic bacteria to internally colonize roots of Arabidopsis thaliana has been studied. Strains tagged with lacZ or gusA reporter genes were used, and the principal colonization sites were found to be the points of emergence of lateral roots, lateral root cracks (LRCs). High frequencies of colonization were found; 63 to 100% of plants were colonized by ORS571, and 100% of plants were colonized by Herbaspirillum seropedicae. After LRCs were colonized, bacteria moved into intercellular spaces between the cortical and endodermal cell layers. Specific flavonoids, naringenin and daidzein, at 5 × 10-5 M, significantly promoted colonization by ORS571. By using a nodC and a nodD mutant of ORS571, it was shown that neither Nod factors nor NodD are involved in colonization or flavonoid stimulation of colonization. Flavonoids did not appear to be stimulating LRC colonization by their activity as nutritional factors. LRC and intercellular colonization by H. seropedicae was stimulated by naringenin and daidzein at the same concentration that stimulated colonization by ORS571.

Additional keywords: nitrogen fixation, nod genes, rhizobia.

© 1997 The American Phytopathological Society