Although it had been suspected that bacterial photosynthesis played a role in this highly particular nitrogen-fixing symbiosis, clear evidence of this had never been presented.

The work of the IRD (Institut de recherche pour le développement, ex-ORSTOM) Research Unit "Tropical and Mediterranean Symbioses" at Montpellier, France, has recently achieved the first demonstration of the process.

The results, important for fundamental science, published recently in the Proceedings of the National Academy of Sciences (PNAS), also open up promising prospects for improving rice field productivity.

The soils in tropical regions are often poor in nitrogen, an element essential for their fertility. Some vegetation -- leguminous plants, for example, can draw on and fix gaseous nitrogen present in the atmosphere thanks to the association with the bacteria Rhizobia.

For most leguminous plants, this nitrogen-fixing symbiosis is accomplished in nodules induced by the bacteria on the roots and, much more rarely, on the stems.

The stems of the leguminous plant of the genus Aeschynomene, which grow in waterlogged ground or along river banks, have the special characteristic of bearing nodules caused by bacteria of the Bradyrhizobium genus.

Although several studies have shown that the bacterial strains attached to the stems were photosynthetic, none had up to now determined the role of the bacterial photosynthesis in this highly specific case of photosynthesis, and its contribution to plant growth.

The IRD research unit set about the task of demonstrating these processes using the tools of molecular biology, gene mapping and mutant construction.

The researchers first isolated and sequenced the four genes coding for the major proteins of the bacteria's photosynthetic apparatus. Subsequently, by genetic transformation, they created a photosynthetic mutant of Bradyrhizobium whose photosynthetic unit, or reaction center, was deactivated.

They inoculated the mutant strain into roots and stems of Aeschynomene sensitiva. These photosynthesis-negative plants were still capable of inducing nodules and fixing nitrogen, but on stems, this ability was severely reduced. The number of stem nodules had halved and so had their nitrogen-fixing capacity; the plant's growth had been retarded to a similar degree.

This experiment demonstrates clearly and for the first time that bacterial photosynthesis plays an essential role in nodulation and nitrogen fixation.

How does this bacterial photosynthetic activity function in the processes of nitrogen-fixing symbiosis?

The scientists involved put forward two hypotheses:

Firstly, the Bradyrhizobium bacteria, thanks to their ability to photosynthesize, could survive at the stem surface devoid of nutrient elements. This absence would be counterbalanced by photosynthesis which provides the chemical and biochemical energy necessary for the bacteria's growth.

In addition, this energy would be used by Rhizobium to infect the cells in the stem and fix nitrogen in the nodule thus generated. The studies have shown in stem nodules an extremely strong expression of the genes which govern the photosynthetic apparatus.

Secondly, the researchers do not rule out that the energy the bacterial photosynthesis produces represents a saving for the leguminous plant in terms of energy it must supply for symbiosis to function. The plant can use its own photosynthates and the resulting energy for its growth.

Sequencing of the genes that control Bradyrhizobium photosynthetic activity has furthermore allowed their phylogenetic history to be traced. It appears that this activity is an ancestral character of all the Bradyrhizobium bacteria, one which has been lost by most soil-dwelling Bradyrhizobium species which infect only the roots because it lost its utility in an environment devoid of light.

Nevertheless, the property has been conserved by species which attach themselves to stems of Aeschynomene because it constituted a selective advantage.

Beyond the interest of the results for fundamental science, promising prospects have been opened up for improving the productivity of rice cultivation. Owing to the symbiosis with the Bradyrhizobium, the Aeschynomene species which grow among the rice plantations in West Africa constitute a natural plant fertilizer. On decomposition, these leguminous aquatic plants fertilize the rice-field soil by way of their input of nitrogen compounds.

Recent research work conducted as part of a doctoral thesis has also shown that these photosynthetic bacteria are capable of generating massive colonies on the roots ("floating" nodal roots exposed to the light) of a wild rice, Oryza breviligulata, without forming a nodule. These bacteria were inoculated into several crop species (O. sativa, O. glaberrima) in the greenhouse and, in conjunction with the Guinea Institute of Agronomic research (IRAG), in a mangrove rice field in that country.

The inoculation turned out to be highly beneficial to rice growth. Inoculated plantations showed a more developed rooting system (55% more in length than in control plantations) and wider stems (by 25%). These results should lead to some important applications both in Guinea and in other tropical rice farming regions.

31-Jan-2001