Some leguminous species of the genus Aeschynomene are specifically stem-nodulated by photosynthetic bradyrhizobia. To study the effect of bacterial photosynthesis during symbiosis, we generated a photosynthesis-negative mutant of the Bradyrhizobium sp. strain ORS278 symbiont of Aeschynomene sensitiva. The presence of a functional photosynthetic unit in bacteroids and the high expression of the photosynthetic genes observed in stem nodules demonstrate that the bacteria are photosynthetically active during stem symbiosis. Stem inoculation by the photosynthetic mutant gave a 50% decrease in stem-nodule number, which reduced nitrogen fixation activity and plant growth in the same proportion. These results indicate an important role of bacterial photosynthesis in the efficiency of stem nodulation.R hizobia interact symbiotically with leguminous plants by inducing nitrogen-fixing nodules on the roots. Some plants of the genus Aeschynomene, encountered in waterlogged soils or riverbanks, have the peculiar property of forming stem nodules. This very unusual behavior among leguminous plants is shared only with a few species of the genera Sesbania, Neptunia, and Discolobium (1). Rhizobia isolated from Aeschynomene stem nodules exhibit a property uncommon among other rhizobia of developing a photosynthetic system (2, 3). This was first demonstrated in the case of a strain isolated from Aeschynomene indica stem nodules and designated BTAi 1 (2). In ex planta culture, BTAi 1 required a photoperiod for production and accumulation of bacteriochlorophyll a and exhibited a photoheterotrophic and strictly aerobic photosynthesis (4, 5). In this respect, the photosynthetic activity of the bacteriochlorophyll-containing rhizobia most closely resembles that of the aerobic anoxygenic photosynthetic bacteria (6, 7).Since the discovery of BTAi 1, several other rhizobia isolated from Aeschynomene stem nodules have been reported to produce bacteriochlorophyll a (8 -13). These photosynthetic Aeschynomene rhizobia were shown to form a separate subbranch of the Bradyrhizobium rRNA lineage, distinct from Bradyrhizobium japonicum and Bradyrhizobium elkanii (13). All photosynthetic strains were shown to nodulate stem-nodulating Aeschynomene species specifically (13), indicating that the photosynthetic character of these rhizobia could play an important role during this unusual symbiosis. The demonstration by Evans et al. (4) that illumination of the stem nodules of A. indica by far-red light consistently accelerated acetylene reduction and the report of Fleischman and Kramer (3) on the presence of bacteriochlorophyll a in bacteroids isolated from stem nodules of Aeschynomene aspera provide further evidence for the importance of the photochemical activity of the endophytes in the stem nodules. However, no genetic study has determined the real contribution of the bacterial photosynthesis in the symbiotic interaction and its impact on plant growth.In purple bacteria, most of the genes required for the formation of the photosystem are clustered in...