Plant Physiol, July 2000, Vol. 123, pp. 845-852
Mtsym6, a Gene Conditioning
Sinorhizobium Strain-Specific Nitrogen Fixation in
Medicago truncatula1
Leïla
Tirichine,
Françoise
de Billy, and
Thierry
Huguet*
Laboratoire de Biologie Moléculaire des Relations
Plantes-Microorganismes, Centre National de la Recherche
Scientifique-Institut National de la Recherche Agronomique, BP 27, Auzeville, 31326 Castanet-Tolosan cedex, France
The availability of a wide range of independent lines for the
annual medic Medicago truncatula led us to search for
natural variants in the symbiotic association with Sinorhizobium
meliloti. Two homozygous lines, Jemalong 6 and DZA315.16,
originating from an Australian cultivar and a natural Algerian
population, respectively, were inoculated with two wild-type strains of
S. meliloti, RCR2011 and A145. Both plant lines formed
nitrogen-fixing (effective) nodules with the RCR2011 strain. However,
the A145 strain revealed a nitrogen fixation polymorphism, establishing
an effective symbiosis (Nod+Fix+) with
DZA315.16, whereas only small, white, non-nitrogen fixing nodules
(Nod+Fix
) were elicited on Jemalong 6. Cytological studies demonstrated that these non-fixing nodules are
encircled by an endodermis at late stages of development, with no
visible meristem, and contain hypertrophied and autofluorescent
infection threads, suggesting the induction of plant defense reactions.
The non-fixing phenotype is independent of growth conditions and
determined by a single recessive allele (Mtsym6), which
is located on linkage group 8.
1
L.T. is supported by a Bourse du Gouvernement
Français fellowship from the French Centre National des
uvres Universitaires et Sociales and the Ministère de
l'Enseignement Supérieur et de la Recherche.
*
Corresponding author; e-mail thuguet{at}toulouse.inra.fr; fax
33-0-5-61-28-50-61.
© 2000 American Society of Plant Physiologists
