The root and stem-nodulating bacteria of the genera Bradyrhizobium, Allorhizobium, Sinorhizobium, Mesorhizobium, and Rhizobium are prime targets for genome analysis due to their relatively large genomes, environmental elasticity, diverse growth modes, and their ability to establish nitrogen-fixing symbioses which enhance plant growth [1].

Several Bradyrhizobium - related species have the unusual capacity to form nodules on stems of aquatic legumes from the Aeschynomene genus [2]. This monophyletic rhizobial clade is of special interest because of its unique capacity for photosynthesis [3-5]. They synthesize photosynthetic pigments including both bacteriochlorophyll a and carotenoids, and contain photosynthetic reaction centers like those of the purple nonsulfur photosynthetic bacteria [4,5]. Photosynthetic activity plays a key role in symbiosis by furnishing energy, which can be used for biological nitrogen fixation [4,5]. The regulatory mechanisms for the formation of this photosystem are very different from those described in purple bacteria, and include a bacteriophytochrome, which, as a function of the ambient light, triggers or fails to trigger the expression of photosynthetic genes [5]. This original mechanism of regulation seems especially well adapted for promoting specific photosynthetic activity during symbiosis with the stem [5].

Photosynthetic bradyrhizobial strains may harbor vast biotechnological potential. They are closely related to the highly versatile, xenobiotic-degrading phototrophic bacterium Rhodopseudomonas palustris , a purple bacterium whose genome has been sequenced by the DOE Joint Genome Institute [6], possibly suggesting their ability to degrade certain environmental contaminants. Furthermore, some photosynthetic bradyrhizobial strains produce the carotenoid canthaxanthine, which is used in the agro-alimentary, pharmaceutic and cosmetologic industries for its coloring and photo-protective properties [7].

The photosynthetic bradyrhizobium sp. BTAi1 (Bacteria, 8.2 Mbp genome), isolated from stem nodules of Aeschynomene indica , was the first photosynthetic bradyrhizobial strain identified [2]. The BTAi1 genome is currently being sequenced by the DOE Joint Genome Institute with three principal objectives: (1) Enhance understanding of phototrophic rhizobial genomics, specifically in regard to their carbon-, and nitrogen-fixing capacities. (2) Identify genetic pathways responsible for desirable biotechnological applications and (3) understand the evolution of symbiotic and photosynthetic systems in rhizobia through comparative genome analysis. This third objective will be achieved by comparing the Bradyrhizobium sp. Btai1 genome to that of the photosynthetic Bradyrhizobium sp. ORS278, currently being sequenced by The Centre National de Séquençage (Genoscope) (http://www.genoscope.cns.fr/externe/English/Projets/Projet_MA/organisme_MA.html ). These two strains are 68% identical in their amino acid sequence, and it is therefore assumed that comparative genomics will allow us to significantly enhance understanding of their unique photosynthetic, carbon- and nitrogen-fixing mechanisms.

References:

1. van Rhijn, P., and Vanderleyden, J. (1995) The Rhizobium-plant symbiosis. Microbiol Rev. 59:124-142.
2. Eaglesham, A. R. J., and A. A. Szalay. 1983. Aerial stem nodules on Aeschynomene spp. Plant Sci. Lett. 29:265-272.
3. Molouba, F., Lorquin, J., Willems, A., Hoste, B., Giraud, E., Dreyfus, B., Gillis, M., de Lajudie, P. and Masson-Boivin., C. 1999. Photosynthetic bradyrhizobia from Aeschynomene spp. are specific to stem-nodulated species and form a separate 16S ribosomal DNA restriction fragment length polymorphism group. Appl. Environ. Microbiol. 65:3084-3094.
4. Giraud, E., and Fleischman, D. (2004) Nitrogen-fixing symbiosis between photosynthetic bacteria and legumes. Photosynth Res . 82:115-130.
5. Fleischman, D., and Kramer, D. (1998) Photosynthetic rhizobia. Biochim. Biophys. Acta . 1364:17-36.
6. Larimer, F.W., Chain, P., Hauser, L., Lamerdin, J., Malfatti, S., Do, L., Land, M.L., Pelletier, D.A., Beatty, J.T., Lang, A.S., Tabita, F.R., Gibson, J.L., Hanson, T.E., Bobst, C., Torres, J.L., Peres, C., Harrison, F.H., Gibson, J., Harwood, C.S. (2004) Complete genome sequence of the metabolically versatile photosynthetic bacterium Rhodopseudomonas palustris. Nat Biotechnol. 22: 55-61.
7. Lorquin, J., F. Molouba, and B. L. Dreyfus. 1997. Identification of the carotenoid canthaxanthin from photosynthetic Bradyrhizobium strains. Appl. Environ. Microbiol. 63:1151-1154.

Comments/Questions
@2001-2008 The Regents of the University of California