The Fodder Legume Chamaecytisus albidus Establishes Functional Symbiosis with Different Bradyrhizobial Symbiovars in Morocco

The native distribution of a common legume shrub is limited by the range of its nitrogen-fixing mutualist

Plant-microbe mutualisms, such as the legume-rhizobium symbiosis, are influenced by the geographical distributions of both partners. However, limitations on the native range of legumes, resulting from the absence of a compatible mutualist, have rarely been explored. We used a combination of a large-scale field survey and controlled experiments to determine the realized niche of Calicotome villosa, an abundant and widespread legume shrub. Soil type was a major factor affecting the distribution and abundance of C. villosa. In addition, we found a large region within its range in which neither C. villosa nor Bradyrhizobium, the bacterial genus that associates with it, were present. Seedlings grown in soil from this region failed to nodulate and were deficient in nitrogen. Inoculation of this soil with Bradyrhizobium isolated from root nodules of C. villosa resulted in the formation of nodules and higher growth rate, leaf N and shoot biomass compared with un-inoculated plants. We present evidence for the exclusion of a legume from parts of its native range by the absence of a compatible mutualist. This result highlights the importance of the co-distribution of both the host plant and its mutualist when attempting to understand present and future geographical distributions of legumes.

Characterization of drought stress-mitigating Rhizobium from faba bean (Vicia faba L.) in the Chinese Qinghai-Tibet Plateau

Rhizobium-driven symbiotic nitrogen-fixation in legumes not only benefits the growth but also enhances the stress tolerance of plants. Isolating and characterizing efficient, drought-tolerant rhizobia is a central goal for improving crop yields in arid regions. Here, we phylogenetically and biochemically characterized a novel strain of Rhizobium ('QHCD11') sampled from the root nodules of faba beans growing in an arid agricultural area in Qinghai-Tibet. We further tested the drought tolerance of the strain as well as of 'Qingcan 14' faba bean seedlings inoculated with it. Biochemical characterization involved bromothymol blue (BTB) tests, carbon metabolic profiling (Biolog GENIII), DNA-DNA hybridization (dDDH) assays, average nucleotide identity (ANI) analyses, and 16S rRNA sequencing. The result indicated that strain 'QHCD11' likely belongs to the Rhizobium indicum species. Drought stress tolerance was assessed by exposure to polyethylene glycol (PEG-6000) at concentrations of 0, 10, 15, and 20%. Increasing concentrations of PEG-6000 tended to result in decreased growth of 'QHCD11', although the strain performed better at 20% PEG 6000 than at 15%. Inoculation of drought-stressed faba bean seedlings with strain 'QHCD11' improved root vitality, chlorophyll content, antioxidant enzyme activities, and plant height. We suggest that inoculation of faba beans with 'QHCD11' is an environmentally sound strategy for mitigating crop drought stress in arid and semi-arid regions. In addition, the results presents here will benefit future studies into faba bean-rhizobia symbioses under drought stress.