This paper summarizes a multinational collaborative project to search for natural, intimate associations between rhizobia and rice (Oryza sativa L.), assess their impact on plant growth, and exploit those combinations that can enhance grain yield with less dependence on inputs of nitrogen (N) fertilizer. Diverse, indigenous populations of Rhizobium leguminosarum bv. trifolii (the clover root-nodule endosymbiont) intimately colonize rice roots in the Egyptian Nile delta where this cereal has been rotated successfully with berseem clover (Trifolium alexandrinum L.) since antiquity. Laboratory and greenhouse studies have shown with certain rhizobial strain-rice variety combinations that the association promotes root and shoot growth thereby significantly improving seedling vigour that carries over to significant increases in grain yield at maturity. Three field inoculation trials in the Nile delta indicated that a few strain-variety combinations significantly increased rice grain yield, agronomic fertilizer N-use efficiency and harvest index. The benefits of this association leading to greater production of vegetative and reproductive biomass more likely involve rhizobial modulation of the plant's root architecture for more efficient acquisition of certain soil nutrients [e.g. N, phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca), zinc (Zn), sodium (Na) and molybdenum (Mo)] rather than biological N 2 fixation. Inoculation increased total protein quantity per hectare in field-grown grain, thereby increasing its nutritional value without altering the ratios of nutritionally important proteins. Studies using a selected rhizobial strain (E11)

Rhizobia are the common bacterial symbionts that form nitrogen-fixing root nodules in legumes. However, recently other bacteria have been shown to nodulate and fix nitrogen symbiotically with these plants. Neptunia natans is an aquatic legume indigenous to tropical and subtropical regions and in African soils is nodulated by Allorhizobium undicola. This legume develops an unusual root-nodule symbiosis on floating stems in aquatic environments through a unique infection process. Here, we analyzed the low-molecular-weight RNA and 16S ribosomal DNA (rDNA) sequence of the same fast-growing isolates from India that were previously used to define the developmental morphology of the unique infection process in this symbiosis with N. natans and found that they are phylogenetically located in the genus Devosia, not Allorhizobium or Rhizobium. The 16S rDNA sequences of these two Neptunia-nodulating Devosia strains differ from the only species currently described in that genus, Devosia riboflavina. From the same isolated colonies, we also located their nodD and nifH genes involved in nodulation and nitrogen fixation on a plasmid of approximately 170 kb. Sequence analysis showed that their nodD and nifH genes are most closely related to nodD and nifH of Rhizobium tropici, suggesting that this newly described Neptunia-nodulating Devosia species may have acquired these symbiotic genes by horizontal transfer.Neptunia natans (L.f.) Druce is an aquatic legume native to several continents of the humid tropics and is used for both human consumption and as green manure for rice cultivation in Asiatic countries. This legume is unusual in that it normally develops buoyant floating stems that grow profusely on the surface of freshwater ponds, and in this aquatic environment it develops many stem-associated nitrogen-fixing nodules.The developmental morphology of the infection process leading to formation of nitrogen-fixing nodules in N. natans has been examined under strict gnotobiotically controlled conditions, and several unique aspects of this specialized, aquatic plant-bacterium symbiosis distinguish it from other legume root-nodule symbioses (22). After colonizing the root and floating stem surfaces, the aquatic bacterium symbiont enters the primary root cortex and stem interior through natural wounds caused by splitting of the epidermis and emergence of young lateral and adventitious roots, respectively, and then stimulates early development of nodules in the cortex at the base of these roots primordia, but not in the stem itself (22). Following crack entry through the nodule periphery, the bacteria penetrate internal nodule host cells; induce formation of bona fide tubular infection threads that disseminate them further intracellularly; and then release the bacteria into infection droplets, where they multiply. The endosymbiotic bacteria transform into nitrogen-fixing bacteroids within symbiosomes that eventually become filled with an unusual fibrillar matrix towards the end of their active nitrogen-fixing cycle in the aquatic e...

The biofertilization of crops with plant-growth-promoting microorganisms is currently considered as a healthy alternative to chemical fertilization. However, only microorganisms safe for humans can be used as biofertilizers, particularly in vegetables that are raw consumed, in order to avoid sanitary problems derived from the presence of pathogenic bacteria in the final products. In the present work we showed that Rhizobium strains colonize the roots of tomato and pepper plants promoting their growth in different production stages increasing yield and quality of seedlings and fruits. Our results confirmed those obtained in cereals and alimentary oil producing plants extending the number of non-legumes susceptible to be biofertilized with rhizobia to those whose fruits are raw consumed. This is a relevant conclusion since safety of rhizobia for human health has been demonstrated after several decades of legume inoculation ensuring that they are optimal bacteria for biofertilization.

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