Moncef Mrabet

Salt-tolerant rhizobia isolated from a Tunisian oasis that are highly effective for symbiotic N2-fixation with Phaseolus vulgaris constitute a novel biovar (bv. mediterranense) of Sinorhizobium meliloti

Nodulation of common bean was explored in six oases in the south of Tunisia. Nineteen isolates were characterized by PCR–RFLP of 16S rDNA. Three species of rhizobia were identified, Rhizobium etli, Rhizobium gallicum and Sinorhizobium meliloti. The diversity of the symbiotic genes was then assessed by PCR–RFLP of nodC and nifH genes. The majority of the symbiotic genotypes were conserved between oases and other soils of the north of the country. Sinorhizobia isolated from bean were then compared with isolates from Medicago truncatula plants grown in the oases soils. All the nodC types except for nodC type p that was specific to common bean isolates were shared by both hosts. The four isolates with nodC type p induced N2-fixing effective nodules on common bean but did not nodulate M. truncatula and Medicago sativa. The phylogenetic analysis of nifH and nodC genes showed that these isolates carry symbiotic genes different from those previously characterized among Medicago and bean symbionts, but closely related to those of S. fredii Spanish and Tunisian isolates effective in symbiosis with common bean but unable to nodulate soybean. The creation of a novel biovar shared by S. meliloti and S. fredii, bv. mediterranense, was proposed.

Cu-tolerant Sinorhizobium meliloti strain is beneficial for growth, Cu accumulation, and mineral uptake of alfalfa plants grown in Cu excess

In this work, we have compared the physiological responses of alfalfa plants inoculated with either Sinorhizobium meliloti strain S412 (Cu-tolerant) or S112 (Cu-sensitive) in the presence or absence of 0.5 mM of CuSO4. The addition of copper (Cu) introduced a decrease of nodule number and their dry weight (DW) in both symbioses. The interaction established with the Cu-sensitive strain is more affected by Cu than that with the tolerant one. In fact, plants inoculated with the sensitive strain revealed a decrease of shoot and root DW, larger than that found in plants inoculated with the tolerant strain. However, under copper supply, Medicago sativa with the Cu-tolerant strain did not show any significant changes in both shoot and root biomass production. Under Cu excess, high levels of Cu were detected in different parts of the plant in the two symbioses and a high translocation of Cu to aerial parts was shown with the strain S412. Plants with S412 were able to accumulate large quantities of calcium (Ca) in their roots and nodules. While Ca content decreased drastically in shoot at 0.5 mM of Cu treatment. Moreover, nodulation with S412 allowed plants to maintain high levels of magnesium (Mg) in all tissues and high iron (Fe) levels in nodules. Results suggest that this symbiotic pair could be used in phytostabilization of Cu-contaminated soils.

Diversity and geographic distribution of fungal strains infecting field-grown common bean (Phaseolus vulgaris L.) in Tunisia

A collection of 103 fungal strains was established from infected common bean plants (Phaseolus vulgaris L.) field-grown in three geographic regions from Tunisia and known for their long history in bean culture; Boucharray, Chatt-Mariem, and Metline. The fungal strain collection was established from common bean root and aerial parts. The pathogenicity test carried out on germinated seedlings showed that among the fungal collection, 41% of fungal strains were assigned to be highly pathogenic. In fact, serious cases of seedling damping-off, as well as a significant reduction in root and shoot biomass in cv. Coco blanc were noticed (up to 90% biomass reduction) considering fungal strains from the three prospected localities. The identification of fungal isolates belonging to this high pathogenicity class, based on the internal transcribed spacer region (ITS), showed a wide generic and specific diversity among common bean pathogenic fungi in Tunisia. Fusarium spp. strains were dominant and represented 67% of the characterized fungal collection. Fungal genera including Alternaria (22%), Rhizoctonia (4%), Ascomycota (4%), Macrophomina (10%) and Phoma (4%) were also reported. The highest richness levels were found in the Chatt-Mariem and Boucharray regions, showing the highest generic and interspecific diversity. In this work, we revealed also a variability in the abundance and geographic distribution of fungal species between the three prospected regions. Fungal strains infecting common bean in Metline were represented exclusively by Fusarium oxysporum. However, the genus Fusarium represented about 66% of fungal strains recovered from Boucharray, and only 20% from Chatt-Mariem. The genus Alternaria represented 11% and 40% of total fungal isolates in Boucharray and Chatt-Mariem, respectively, and was isolated only from the foliar parts of diseased common bean plants. The present work represents an important database that should be considered for surveying common bean fungal diseases.