Abstract: Understanding the mechanisms by which legumes are infected by symbiotic, nitrogen-fixing bacteria (rhizobia) may prove crucial in the pursuit of bio-engineering nitrogen-fixation in current food crops, and as a result, reducing reliance on nitrogen fertilisers. In my project I have examined the roles and interplay of the plant signalling factors, flavonoids, reactive oxygen species (ROS), and cytokinin in establishment of symbiotic infection of rhizobia in the roots of the model legume Medicago truncatula. Specifically, I have examined the spatiotemporal response of flavonoids in Medicago roots during infection, studied the role of flavonoids in the production of infection necessary ROS within root hairs, and investigated the role of cytokinin perception in the establishment and progression of infection.

Infection of Medicago roots is driven by the production of ‘infection threads’ by the plant. Infection threads are infection structures that are unique to nitrogen-fixing symbioses, and critical to our understanding of the establishment of the symbiosis in legumes. To facilitate the study of infection, I first developed a new method of imaging infection threads via fluorescent labelling and confocal microscopy, that has allowed high resolution imaging of their structure in 3D cell space.

Flavonoids are plant metabolites that have specialised functions in the establishment of the legume/rhizobia symbiosis. Through tissue isolation and mass-spectrometry analysis, we have performed spatiotemporal mapping of flavonoids within Medicago roots at three key stages of infection, and shown that the profile and response of flavonoids to infection is distinct between different root tissues.

ROS are produced in root hairs during infection and are believed to be necessary for infection thread formation. I have shown that a specific flavonoid, methoxychalcone, is required to produce ROS, and in combination with another flavonoid identified in the same tissue by the mapping above, can rescue infection in roots where all other flavonoids are reduced.

Finally, I have investigated an aberrant infection phenotype in Medicago cre1-1 where the nodulation necessary cytokinin receptor CRE1 is silenced. Through this I have developed a new model for cytokinin activity in infection whereby cytokinin signalling via CRE1 is required to control cell cycle stalling that is necessary for infection thread progression.

Biography: I completed my Bachelor of Science (Honours) in plant cell biology with a focus on developing methods in microscopy to facilitate the study of transfer cell wall ingrowths in Arabidopsis under A/Prof David Collings and A/Prof David McCurdy at Newcastle University in 2018. In the summer of 2017/2018 I visited ANU to complete a summer research project in the Mathesius lab, where I developed a deep fascination in the legume/rhizobia nitrogen fixing symbiosis. I returned to the Mathesius group in 2019 to commence a PhD continuing the study of symbiotic infection I had begun in the summer project, and incorporating the love of microscopy and cell biology I had developed in my honours research. As my PhD comes to a close I will be continuing at ANU, where I have been fortunate enough to secure a role as a life science microscopy specialist at the Centre for Advanced Microscopy (CAM).