Mustapha Trabelsi

Rhizobium gallicum as an efficient symbiont for bean cultivation

Rhizobia are soil bacteria that fix atmospheric nitrogen in symbiosis with legumes in specialized organs called nodules. The legumes thus acquire the autonomy to grow in nitrogen-deficient soils. When nitrogen fixation by indigenous rhizobia is limited, field inoculation with efficient and competitive strains is an economically feasible way to increase production. When the inoculant is made from native strains of rhizobia the success of inoculation should be increased, since local strains are better adapted than commercial inoculants. Here, a Rhizobium gallicum strain, 8a3, previously selected for its competitiveness and symbiotic effectiveness with common bean under laboratory conditions, was tested in field trials in Tunisia. The experiments were conducted in six fields using three common bean cultivars. The majority of the fields showed a low density of the native rhizobia and inefficient nodulation by Sinorhizobium meliloti, a known symbiont of Medicago. Our results show that inoculation with R. gallicum strain 8a3 induced an increase in nodule numbers accompanied by a more than twofold increase in shoot dry yield. Monitoring of the nodulation occupancy through the fingerprinting of the repetitive extragenic palindromic sequences (REP-PCR) showed that strain 8a3 was competitive even in the soil showing a high population density of indigenous R. gallicum, and occupied more than 40% of the nodules. Moreover, in vitro antibiosis assays indicated that strain 8a3 produces antimicrobial activity on agar medium against indigenous common bean rhizobia, including the inefficient strains of S. meliloti. These results point out the benefits that could be achieved by selecting efficient and competitive strains among natural populations of rhizobia.

Co-inoculation with Glomus intraradices and Rhizobium tropici CIAT899 increases P use efficiency for N2 fixation in the common bean (Phaseolus vulgaris L.) under P deficiency in hydroaeroponic culture

Common bean (Phaseolus vulgaris L.) genotypes CocoT and Flamingo were inoculated with Rhizobium tropici CIAT899 and Glomus intraradices (Schenck & Smith) and grown under sufficient versus deficient phosphorus supply for comparing the effects of double inoculation on growth, nodulation, mycorrhization of the roots, phosphorus use efficiency and total nitrogen. Although the double inoculation induced a significant increase in all parameters whatever the phosphorus supply in comparison to control, significant differences were found among genotypes and treatments. Nevertheless, the highest phosphorus use efficiency and plant total nitrogen were found under P deficiency in combination with arbuscular mycorrhizal fungi. It is concluded that inoculation with rhizobia and arbuscular mycorrhizal fungi could improve symbiotic nitrogen fixation even under phosphorus deficiency.

Comparison between the reference Rhizobium tropici CIAT899 and the native Rhizobium etli 12a3 for some nitrogen fixation parameters in common bean (Phaseolus vulgaris L.) under water stress

Drought stress is one of the most important factors limiting plant growth; in particular, water is regarded as a major problem in symbiotic nitrogen fixation for legume growth. The adverse effects of water stress on the rhizobial symbiosis of common bean genotypes CocoT and Flamingo were investigated in field conditions. Common bean plants were inoculated with rhizobia strains: Rhizobium tropici CIAT899 or Rhizobium etli 12a3, and grown under field conditions, plants were spray irrigated until the first flower was observed on set. Irrigation was stopped for 20 days in plants submitted to water stress. The enzyme-linked immunosorbent assay (ELISA) technique was used with polyclonal antibody to assess the occupancy in nodule of the rhizobia used for inoculation. Under water stress, a significant decrease in nodule population, nodules biomass, shoot content of nitrogen, growth and chlorophyll with all symbioses, whereas proline accumulation increased. The interaction between symbioses and water stress for all parameters was significant. Overall, 12a3 was more efficient than CIAT899 for Flamingo, though not for CocoT. It is concluded that symbiosis between Flamingo and 12a3 has potential for application in field under water stress in desert soil.

Flamingo is a new common bean ( Phaseolus vulgaris L.) genotype with tolerance of symbiotic nitrogen fixation to moderate salinity

Salinity is one of the major environmental threats to agriculture. In particular, soil salinization is a major limiting factor of symbiotic nitrogen fixation for legume growth. The adverse effects of salinity on the rhizobial symbiosis of common bean genotypes CocoT and Flamingo and the subsequent photosynthesis and plant-growth were investigated in glasshouse. Common bean plants were grown in sterilized sand and irrigated with salt concentrations of 2, 4, 6, 8 and 10 gl-1, respectively and inoculated with rhizobial strains of Rhizobium tropici CIAT899 or Rhizobium etli 12a3. Nodule number and biomass, shoot content of nitrogen and chlorophyll were decreased by increasing salinity with all used symbiosis, whereas, the proline accumulation increased. The interaction between symbioses and salt levels for all parameters was highly significant. Overall, Flamingo inoculated with R. etli had the highest plant growth and nitrogen fixation under salinity. It is concluded that Flamingo has potential for application in field conditions exposed to soil salinization or irrigated with saline water, unless native rhizobial would interact negatively. Key words: Legume, mediterranean area, rhizobia, salinity.