Lupin is a high-protein legume crop that grows in a wide range of edaphoclimatic conditions where other crops are not viable. Its unique seed nutrient profile can promote health benefits, and it has been proposed as a phytoremediation plant. Most rhizobia nodulating Lupinus species belong to the genus Bradyrhizobium, comprising strains that are phylogenetically related to B. cytisi, B. hipponenese, B. rifense, B. iriomotense/B. stylosanthis, B. diazoefficiens, B. japonicum, B. canariense/B. lupini, and B. retamae/B. valentinum. Lupins are also nodulated by fast-growing bacteria within the genera Microvirga, Ochrobactrum, Devosia, Phyllobacterium, Agrobacterium, Rhizobium, and Neorhizobium. Phylogenetic analyses of the nod and nif genes, involved in microbial colonization and symbiotic nitrogen fixation, respectively, suggest that fast-growing lupin-nodulating bacteria have acquired their symbiotic genes from rhizobial genera other than Bradyrhizobium. Horizontal transfer represents a key mechanism allowing lupin to form symbioses with bacteria that were previously considered as non-symbiotic or unable to nodulate lupin, which might favor lupin’s adaptation to specific habitats. The characterization of yet-unstudied Lupinus species, including microsymbiont whole genome analyses, will most likely expand and modify the current lupin microsymbiont taxonomy, and provide additional knowledge that might help to further increase lupin’s adaptability to marginal soils and climates.

Thirty-two bacterial isolates were obtained from root nodules of Lupinus angustifolius growing in Northern Tunisia. Phylogenetic analyses based on recA and gyrB partial gene sequences grouped the strains into six clusters: four clusters belonged to the genus Bradyrhizobium (22 isolates), one to Microvirga (8 isolates) and one to Devosia (2 isolates), a genus that has not been previously reported to nodulate lupin. Representative strains of each group were further characterized. Multi-Locus Sequence Analysis (MLSA) based on recA and glnII gene sequences separated the strains within the genus Bradyrhizobium into four divergent clusters related to B. canariense, B. liaoningense, B. lupini, and B. algeriense, respectively. The latter might constitute a new Bradyrhizobium species. The strains in the Microvirga cluster showed high identity with M. tunisiensis. The Devosia isolates might also represent a new species within this genus. An additional phylogenetic analysis based on the symbiotic gene nodC affiliated the strains to symbiovars genistearum, mediterranense, and to a possibly new symbiovar. These results altogether contributed to the existing knowledge on the genetic diversity of lupin-nodulating microsymbionts and revealed a likely new, fast-growing, salt-tolerant rhizobial species within the genus Devosia as a potentially useful inoculant in agricultural practices or landscape restoration.

Nodule endophytes and associated bacteria are non-symbiotic bacteria that colonize legume nodules. They accompany nodulating rhizobia and can form beneficial associations, as some of them are plant growth-promoting rhizobacteria (PGPR) that are able to promote germination and plant growth and increase tolerance to biotic and abiotic stress. White lupin (Lupinus albus) is a legume crop that is gaining relevance as a suitable alternative to soybean as a plant protein source. Eleven nodule-associated bacteria were isolated from white lupin nodules grown in a Tunisian soil. They belonged to the genera Rhizobium, Ensifer, Pseudomonas and Bacillus. Their plant growth-promoting (PGP) and enzymatic activities were tested in vitro. Strains Pseudomonas sp., L1 and L12, displayed most PGP activities tested, and were selected for in planta assays. Inoculation with strains L1 or L12 increased seed germination and had the same positive effects on all plant growth parameters as did inoculation with symbiotic Bradyrhizobium canariense, with no significant differences among treatments. Inoculation with efficient nitrogen-fixing rhizobia must compete with rhizobia present in the soil that sometimes nodulate efficiently but fix nitrogen poorly, leading to a low response to inoculation. In such cases, inoculation with highly effective PGPR might represent a feasible alternative to boost crop productivity.