Legumes play an integral role in increasing agricultural productivity, particularly in low input agricultural systems in Australia, due to their ability to form nitrogen fixing symbioses with a group of soil bacteria called rhizobia. In medium-to-low rainfall areas of southern Australia, productivity is constrained by a lack of suitable annual pasture legumes. Scorpiurus muricatus is an annual pasture legume from the Mediterranean basin, with potential to fill this niche. All agricultural legumes in Australia are exotic and do not form effective symbioses with indigenous rhizobia, so a highly effective inoculant strain for S. muricatus needs to be identified. There is very little information on the diversity, effectiveness, and host-range of S. muricatus microsymbionts. Additionally, understanding the promiscuity of S. muricatus is essential as inoculants for other legumes and pre-existing soil bacteria have the potential to reduce S. muricatus yield if they fix N2 suboptimally on this host.
A total of 39 authenticated strains were isolated from soils from Australia (5), Croatia (2), Israel (11), Morocco (4) and Sardinia (17). Core genome analysis identified several potential novel Mesorhizobium species. Symbiotic effectiveness testing of 36 strains inoculated on S. muricatus showed 35 strains produced equivalent mean shoot dry weight (SDW) to the reference strain WSM1386 and one strain produced significantly less. The most effective strain WSM4842, produced 149% of the SDW of WSM1386 warranting further investigation into WSM4842 as a potential inoculant.
S. muricatus promiscuity was investigated by inoculating plants with Australian commercial Mesorhizobium and Bradyrhizobium inoculants. Mesorhizobium inoculants for Lotus spp. readily nodulated and fixed N2 on S. muricatus while Bradyrhizobium inoculants for Lotus pedunculatus, Lupinus spp. and Ornithopus spp. formed few ineffective nodules. Furthermore, S. muricatus is nodulated by resident Bradyrhizobium with initial testing suggesting they fix N2 ineffectively. Future experiments will quantify effectiveness and saprophytic competence of effective Mesorhizobium strains in field conditions and in sites with a history of cultivation with other legumes nodulated by Mesorhizobium spp.