Evaluation of rhizobia to maximise nitrogen fixation in the annual pasture legume Scorpiurus muricatus

Kit A Burns

doi:10.60867/00000146

Productivity in southern Australian farming systems is constrained by a lack of well adapted annual pasture legumes. Scorpiurus muricatus, an annual pasture legume from the Mediterranean Basin, has the potential to fill this niche, but its successful adoption depends on identifying rhizobial strains capable of forming effective N₂-fixing symbioses under Australian conditions. Before this study, the identity, diversity, symbiotic effectiveness, and host range of S. muricatus microsymbionts were poorly resolved. This thesis addressed these uncertainties by characterising the diversity, symbiotic effectiveness, and host range of S. muricatus microsymbionts. The ability of commercial inoculant strains and resident soil bacteria to nodulate this host was investigated, and candidate inoculant strains were evaluated under field conditions. Thirty-nine S. muricatus-nodulating strains isolated from soils in Australia, Croatia, Israel, Morocco, and Sardinia were investigated. Core genome analysis identified 36 strains as Mesorhizobium and three as Bradyrhizobium. The Mesorhizobium strains were highly diverse, spanning 14 species groups, including three recognised species and 11 uncharacterised groups. Despite this genomic diversity, all S. muricatus-nodulating Mesorhizobium clustered within a single symbiosis gene phylogenetic clade that also included strains associated with Biserrula pelecinus and Lotus spp., indicating that the S. muricatus symbiosis formed part of a broader symbiotic group rather than a distinct host-specific lineage. Consistent with the symbiosis gene phylogeny, host range studies showed that S. muricatus-nodulating Mesorhizobium also nodulated hosts associated with that broader symbiotic group, including B. pelecinus, Lotus corniculatus, and L. ornithopodioides. Assessment of S. muricatus specificity showed that the Australian commercial Mesorhizobium inoculants for L. corniculatus and L. ornithopodioides nodulated S. muricatus and fixed N₂. Together, these findings indicate that the S. muricatus symbiosis falls within a broader Biserrula–Lotus symbiotic group. S. muricatus formed effective N₂-fixing symbioses with diverse Mesorhizobium strains, whereas Bradyrhizobium strains, including Australian commercial inoculants and strains isolated from Western Australian soils, nodulated this host but did not fix N₂. Four highly effective Mesorhizobium strains, WSM1343, WSM1386, WSM4821, and WSM4842, were identified as candidate inoculants for field evaluation. Field trials at two sites in Western Australia showed that inoculation established effective symbioses under field conditions, despite the presence of resident soil Bradyrhizobium capable of nodulating S. muricatus. WSM1386 outperformed the other three strains at both sites, with superior nodulation and N₂ fixation, resulting in greater shoot N content. This thesis defines the N₂-fixing symbiosis between S. muricatus and its microsymbionts, placing the S. muricatus-nodulating Mesorhizobium strains within a broader Biserrula–Lotus symbiotic group. Importantly, it identified Mesorhizobium sp. WSM1386 as a promising inoculant candidate for field application, strengthening the basis for the introduction of S. muricatus into southern Australian farming systems, and providing a basis for evaluating S. muricatus in other regions where it may have value as a pasture legume.

Evaluation of rhizobia to maximise nitrogen fixation in the annual pasture legume Scorpiurus muricatus

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