Abstract- The rhizobium-legume symbiosis that occurs within root nodules provides a crucial way for legumes to acquire nitrogen under nitrogen limited conditions. Legumes tightly regulate nodule number to balance the cost of supporting rhizobial proliferation inside nodules with the benefits of nitrogen fixation. C-TERMINALLY ENCODED PEPTIDEs (CEPs) are 15 amino acid peptides are root-to-shoot signals that are made under low N conditions. CEPs, which play roles in the inhibition of lateral root number under nitrogen limited conditions, nitrogen demand signalling, as well as the allocation of carbon and nitrogen to strong sinks, also positively control root nodule number in Medicago. My thesis investigated the interaction of CEP7 with its putative receptor, COMPACT ROOT ARCHITECTURE2 (CRA2). Quantitative qRT-PCR, genetic, and physiological assays show that the induction of CEP7 depends on rhizobia and intact Sym pathway rather than low nitrogen, and CEP7 positively regulates rhizobial infection and nodulation. Using mass spectrometry, I identify the mature in vivo structure of the predominant peptide product of CEP7 and showed that it promotes nodule number without affecting lateral root number. I termed this peptide SymCEP7, since it is the only peptide discovered so far that affects nodule number without affecting lateral root number. I demonstrated that SymCEP7’s hydroxylation pattern is crucial for its symbiosis specific biological activity. Confocal imaging showed that FITC-labelled CEP7 binds to shoot vascular tissue in wild type but not in the cra2 mutant, which support CRA2 being the receptor for CEP7. Grafting and split root studies show that CEP7 increases nodule number systemically through CRA2 in shoots. Physiological and confocal approaches show that SymCEP7 applied to shoots is more potent and acts more rapidly to control nodule number than CEP7 applied to roots. I also demonstrate that CEP7 is phylogenetically and functionally conserved in other legume species. These findings identify a unique and specific role for CEP7-CRA2 in the regulation of symbiotic nodule number, which may be conserved across legume species.

Biography - I have been working at RSB since the second year of my undergraduate degree. My research has largely focused on root system architecture and symbiotic interactions and understanding how plants adapt their growth to nutrient limitation. I feel really lucky to have gotten to work with so many talented scientists across multiple fields over the course of my time at ANU.