The regulation of the legume symbiosis with nitrogen-fixing bacteria

Thesis

Leguminous plants form a symbiotic relationship with nitrogen-fixing bacteria called rhizobia. These rhizobia are housed within specialised root structures called nodules. Inside these nodules the rhizobia differentiate into nitrogen-fixing bacteroids and convert atmospheric N2 to ammonia which is provided to the host plant in return for photosynthetically derived carbon in the form of malate.

Rhizobia expend large amounts of energy on fixation, therefore there is a fitness benefit to fixing less nitrogen and using the energy for replication. This could create a selection pressure for the rhizobia to ‘cheat’. However, it has been shown previously that legumes conditionally sanction nodules containing cheating strains. I have shown that these punishments cause a drop in the number of bacteroids and premature senescence of the whole nodule. Sometimes nodules are inhabited by multiple strains of varying fixation effectiveness. I have demonstrated that the host plant is unable to differentiate between these strains and instead sanctions at the whole nodule level.

Avenues do exist for ‘cheating’ bacteria to thrive. I showed that the severity of sanctioning is dependent on the ratio of cheats to non-cheats. Therefore, if a cheat occupies most nodules they can evade sanctions. I also showed that the application of sanctions is based on a global comparison of nodule effectiveness and that host plants can differentiate between small variations in effectiveness. 

Finally, I investigated the mechanism that regulates sanctioning and through RNA sequencing discovered several candidate regulators. Follow up experiments suggested that the plant hormone ABA may be a regulator.

These findings improve our understanding of how legumes regulate their symbiotic relationships with a variety of strains. They also provide new insight into the mechanism controlling this regulation. This knowledge is fundamentally important for the exploitation of this symbiosis in reducing reliance on artificial fertilisers.

Authors: Underwood, T

• Alternative title: Crime & Punishment: the regulation of the legume symbiosis with nitrogen-fixing bacteria
• DOI: 10.5287/ora-w4zae7o94
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English