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Applied and Environmental Microbiology, December 2000, p. 5437-5447, Vol. 66, No. 12
Laboratoire des Symbioses Tropicales et
Méditerranéennes, IRD, INRA, AGRO-M, CIRAD, TA10/J, Campus
International de Baillarguet, 34398 Montpellier cedex 5, France1; Laboratoire de Microbiologie,
ISRA/IRD/UCAD, Dakar, Sénégal2;
and Laboratorium voor Microbiologie, Universiteit Gent, Ghent,
Belgium3
Received 6 April 2000/Accepted 31 August 2000
We investigated the presence of endophytic rhizobia within the
roots of the wetland wild rice Oryza breviligulata, which
is the ancestor of the African cultivated rice Oryza
glaberrima. This primitive rice species grows in the same wetland
sites as Aeschynomene sensitiva, an aquatic stem-nodulated
legume associated with photosynthetic strains of
Bradyrhizobium. Twenty endophytic and aquatic isolates were
obtained at three different sites in West Africa (Senegal and Guinea)
from nodal roots of O. breviligulata and surrounding water
by using A. sensitiva as a trap legume. Most endophytic and
aquatic isolates were photosynthetic and belonged to the same
phylogenetic Bradyrhizobium/Blastobacter subgroup as the
typical photosynthetic Bradyrhizobium strains previously isolated from Aeschynomene stem nodules. Nitrogen-fixing
activity, measured by acetylene reduction, was detected in rice plants
inoculated with endophytic isolates. A 20% increase in the shoot
growth and grain yield of O. breviligulata grown in a
greenhouse was also observed upon inoculation with one endophytic
strain and one Aeschynomene photosynthetic strain. The
photosynthetic Bradyrhizobium sp. strain ORS278 extensively
colonized the root surface, followed by intercellular, and rarely
intracellular, bacterial invasion of the rice roots, which
was determined with a lacZ-tagged mutant of ORS278. The discovery that photosynthetic Bradyrhizobium strains,
which are usually known to induce nitrogen-fixing nodules on stems of
the legume Aeschynomene, are also natural
true endophytes of the primitive rice O. breviligulata
could significantly enhance cultivated rice production.
0099-2240/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Photosynthetic Bradyrhizobia Are Natural Endophytes
of the African Wild Rice Oryza breviligulata
*
Corresponding author. Mailing address: Laboratoire des
Symbioses Tropicales et Méditerranéennes, TA10/J, Campus de
Baillarguet, 34398 Montpellier cedex 5, France. Phone: 334 67 59 38 82. Fax: 334 67 59 38 02. E-mail: dreyfus{at}mpl.ird.fr.
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