Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Dr. Krzysztof Szczyglowski

Abstract

Nitrogen is the most abundant element in our atmosphere, yet has become increasingly limited in agricultural lands. Legume plants offer a possible solution to this problem due to their innate ability to symbiotically interact with nitrogen-fixing bacteria called rhizobia. In particular, a histidine kinase cytokinin receptor from the model legume Lotus japonicus (LHK1) has been clearly placed at the core of these interactions. Loss-of-function mutants in LHK1 fail to initiate timely cortical cell divisions in response to abundant bacterial infection, and gain-of-function mutations in the same locus cause L. japonicus plants to form spontaneous nodules in the absence of rhizobia, thus indicating that this receptor is required and sufficient for nodule organogenesis. However, nodulation events do still occur in lhk1-1 mutants. Therefore, this study has sought to address how nodule organogenesis persists in the lhk1-1 mutant background; is this achieved through cytokinin-independent signaling or perhaps redundancy in function with other members of the LHK family? To that end, the present study has identified three new Lhk loci from L. japonicus (described herein as Lhk1A, Lhk2, and Lhk3) and provides a detailed characterization of their roles during the NFS. Furthermore, we highlight the pivotal role of LHK1 signalling during the NFS, but also clearly indicate that the role of this receptor is not entirely unique. Indeed, other LHK family members share promoter localization profiles with LHK1 and can complement for loss-of-function mutations in LHK1. The results have allowed for the refinement of current models involving the cytokinin signalling network, which highlight a possible role for other receptors during LHK1-independent signalling events in the root cortex.

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