The diversity of rhizobia associated with introduced and native Acacia species in Algeria was investigated from soil samples collected across seven districts distributed in arid and semi-arid zones. The in vitro tolerances of rhizobial strains to NaCl and high temperature in pure culture varied greatly regardless of their geographical and host plant origins but were not correlated with the corresponding edaphoclimatic characteristics of the sampling sites, as clearly demonstrated by principal component analysis. Based on 16S rRNA gene sequence comparisons, the 48 new strains isolated were ranked into 10 phylogenetic groups representing five bacterial genera, namely, Ensifer, Mesorhizobium, Rhizobium, Bradyrhizobium, and Ochrobactrum. Acacia saligna, an introduced species, appeared as the most promiscuous host because it was efficiently nodulated with the widest diversity of rhizobia taxa including both fast-growing ones, Rhizobium, Ensifer, and Mesorhizobium, and slow-growing Bradyrhizobium. The five other Acacia species studied were associated with fast-growing bacterial taxa exclusively. No difference in efficiency was found between bacterial taxa isolated from a given Acacia species. The tolerances of strains to salinity and temperature remains to be tested in symbiosis with their host plants to select the most adapted Acacia sp.-LNB taxa associations for further revegetation programs.

Plant Growth Promoting Bacteria (PGPBs) are a strong ally for sustainable agriculture. They offer an interesting alternative to chemical fertilizers and pesticides. Many microorganisms have been widely documented for their PGPR traits, but actinobacterial microbes which have been increasingly documented only these two past decades for their ability to promote plant growth. Their action on plant health and yield could be either direct, indirect or both. This review will cover articles that have been published on Actinobacteria PGP traits, highlighting the involved mechanisms to reveal their strong potential as microbial fertilizers. Possible strategies to encourage Actinobacteria use as bioinoculants are also discussed.

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A total of 51 bacterial strains were isolated from root nodules of Scorpiurus muricatus sampled from 6 regions of western Algeria. Strain diversity was assessed by rep-PCR amplification fingerprinting, which grouped the isolates into 28 different clusters. Partial nucleotide sequencing of the 16S rRNA gene and BLAST analysis revealed that root nodules of S. muricatus were colonized by different species close to Rhizobium vignae, Rhizobium radiobacter, Rhizobium leguminosarum, Phyllobacterium ifriqiyense, Phyllobacterium endophyticum, Starkeya sp., and Pseudomonas sp. However, none of these strains was able to form nodules on its host plant; even nodC was present in a single strain (SMT8a). The inoculation test showed a great improvement in the growth of inoculated plants compared with noninoculated control plants. A significant amount of indole acetic acid was produced by some strains, but only 2 strains could solubilize phosphate. In this report we described for the first time the diversity of bacteria isolated from root nodules of S. muricatus growing in different regions in western Algeria and demonstrated their potential use in promoting plant growth.

Five species of Acacia (Acacia ehrenbergiana Hayne, A. nilotica (L.) Delile, A. seyal Delile, A. tortilis (Forssk.) Hayne and A. laeta Delile) indigenous to Tamanrasset (Algeria) were investigated for their nodulation status and nodular endophytic diversity. A. ehrenbergiana showed the highest nodulation ability across the different sites in this region, indicating the widespread occurrence of compatible rhizobia in the soils. Altogether 81 strains were purified. Among this endophytic strain collection, only four bacterial endophytes nodulated their respective host plants. On the basis of partial 16S rDNA sequencing, they were affiliated to Ensifer sp., Ensifer teranga, Mesorhizobium sp. and Rhizobium sp. Among the 79 non-symbiotic endophytes, 24 representative strains on the basis of PCR-RFLP profile obtained with MSPI enzyme digestion were characterized. They belonged to nine genera, namely: Paenibacillus, Ochrobactrum, Stenotrophomonas, Pseudomonas, Microbacterium, Rhizobium, Agrobacterium, Brevibacillus and Advenella. The isolated nodular endophytes in this study revealed a strong tolerance profile to salinity and high temperature. Principal component analysis confirmed that no correlation was found between bacterial tolerance to a maximum temperature of growth and soil depth of sampling. This tolerance profile was distributed over the three levels of soil depth sampling: 20, 40 and 60 cm. On the other hand, there was no relationship between in vitro tolerances of rhizobial strains to NaCl and high temperature and corresponding edaphoclimatic characteristics of the sampling sites. This study is a contribution to nodular bacterial diversity knowledge of desert African Acacia species growing in preserved ecosystems.