Host regulation of the legume-rhizobia symbiosis

Leguminous plants host nitrogen-fixing bacteria, known as rhizobia, in specialised root nodules, providing them with carbon in return for nitrogen. However, plants face an investment dilemma, as they usually house multiple strains of rhizobia that can vary in their fixation ability. This thesis characterises the host plant response to poorly fixing strains and compares this across nodule types. While plants are known to sanction non-fixing strains, non-fixers are rare in field settings, while intermediate fixers are common. We showed that pea (Pisum sativum) plants tolerated an otherwise isogenic intermediate fixer only when a better strain was unavailable. Nodules containing the intermediate fixer were large and healthy when the only alternative was a non-fixer. When a more effective strain was available, carbon transport to the intermediate fixer was decreased, nodules were smaller, with reduced nitrogen fixation, and contained fewer viable rhizobia. Flow cytometry showed that sanctioning hinders nodule development, as sanctioned nodules have fewer specialised nitrogen-fixing bacteroids. Furthermore, limited evidence of sanctioning was observed within mixed nodules, containing two strains, suggesting that poorly fixing strains may not be able to evade sanctioning by sharing a nodule with a better strain. This indicates that legumes can make conditional decisions at a per-nodule level, and possibly even within nodules, giving them remarkable control over their symbiotic partners. We compared hosts with indeterminate (pea, Pisum sativum) and determinate (bean, Phaseolus vulgaris) nodules, to show that for strains differing only in nitrogen-fixation, the mechanism of host regulation was sanctioning (reducing resource allocation), rather than partner choice (forming fewer nodules). This opens avenues for further work investigating sanctioning mechanisms in both indeterminate and determinate nodules. Furthermore, indeterminate fixing nodules were largest when rare, showing an even finer level of host regulation. These findings improve the understanding of how host plants manage their interactions with multiple symbiotic partners, which is of great importance when developing more effective rhizobia as alternatives to nitrogenous fertilisers.