Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium–legume symbiosis

doi:10.1073/pnas.0600912103

Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium–legume symbiosis

*Institut des Sciences du Végétal, Centre National de la Recherche Scientifique, Unité Propre de Recherche 2355, Avenue de la Terrasse Bātiment 23, 91198 Gif-sur-Yvette Cedex, France; and
^‡Unité Mixte de Recherche 6061, Bâtiment 13, Centre National de la Recherche Scientifique, Université de Rennes I, Faculté de Médecine, 2 Avenue du Pr. Léon Bernard, 35043 Rennes Cedex, France

Communicated by Marc C. E. Van Montagu, Ghent University, Ghent, Belgium, February 3, 2006

Abstract

Symbiosis between legumes and Rhizobium bacteria leads to the formation of root nodules where bacteria in the infected plant cells are converted into nitrogen-fixing bacteroids. Nodules with a persistent meristem are indeterminate, whereas nodules without meristem are determinate. The symbiotic plant cells in both nodule types are polyploid because of several cycles of endoreduplication (genome replication without mitosis and cytokinesis) and grow consequently to extreme sizes. Here we demonstrate that differentiation of bacteroids in indeterminate nodules of Medicago and related legumes from the galegoid clade shows remarkable similarity to host cell differentiation. During bacteroid maturation, repeated DNA replication without cytokinesis results in extensive amplification of the entire bacterial genome and elongation of bacteria. This finding reveals a positive correlation in prokaryotes between DNA content and cell size, similar to that in eukaryotes. These polyploid bacteroids are metabolically functional but display increased membrane permeability and are nonviable, because they lose their ability to resume growth. In contrast, bacteroids in determinate nodules of the nongalegoid legumes lotus and bean are comparable to free-living bacteria in their genomic DNA content, cell size, and viability. Using recombinant Rhizobium strains nodulating both legume types, we show that bacteroid differentiation is controlled by the host plant. Plant factors present in nodules of galegoid legumes but absent from nodules of nongalegoid legumes block bacterial cell division and trigger endoreduplication cycles, thereby forcing the endosymbionts toward a terminally differentiated state. Hence, Medicago and related legumes have evolved a mechanism to dominate the symbiosis.

Footnotes

^§To whom correspondence should be addressed. E-mail: eva.kondorosi{at}isv.cnrs-gif.fr
↵ ^†Present address: Department of Chemistry and BioScience, Faculty of Sciences, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan.
Author contributions: P.M., F.B.-H., F.G., and E.K. designed research; P.M., T.U., B.A., G.E., A.C., O.C., and A.-E.M. performed research; P.M., F.B.-H., F.G., A.K., and E.K. analyzed data; and P.M. and E.K. wrote the paper.
Conflict of interest statement: No conflicts declared.
Abbreviations:

CGH,

comparative genomic hybridization;

CTC,

5-cyano-2,3-di-4-tolyl tetrazolium chloride;

DAPI,

4′,6-diamidino-2-phenylindole;

PI,

propidium iodide.