Date of Award


Degree Type


Degree Name

Doctor of Philosophy




As the world population approaches seven billion and is predicted to reach nine billion by the year 2040 it is essential to improve our agricultural methods in order to meet the growing demand for food. The challenge is to increase the yield without negatively affecting the environment.

A new approach that would make use of beneficial plant-microbe interactions should be considered. One of these interactions is the establishment of nitrogen-fixing symbiosis between plants and soil bacterium, commonly known as Rhizobium. However the ability to interact symbiotically with Rhizobium is almost completely restricted to leguminous plants. Therefore, understanding how the legume-rhizobium symbiosis is established might allow us to improve or engineer new N2 acquiring plant-microbe associations.

In recent years, we witnessed many breakthrough discoveries that improved our understanding of these interactions; however, significant gaps in our knowledge of this important biological process still remain. The research objective of my thesis has been, therefore, to enhance our understanding of the mechanisms governing the development of nitrogen-fixing root nodule symbiosis in a model legume, Lotusjaponicus.

The key findings of this thesis are as follows: (1) the identification and characterization of many symbiosis-relevant loci in L. japonicus-, (2) discovery of an alternative mechanism for successful rhizobial colonization of legume roots; (3) the molecular cloning of a gene that is required for root hairs development in L. japonicus; this is in relation to the function of root hair as the primary sites for the initial physical contact and entry of the compatible nitrogen-fixing bacteria inside the host-plant root and last; (4) discovery of a key signaling element that is necessary and sufficient for nodule organogenesis. This breakthrough finding demonstrated that perception of the plant hormone cytokinin is crucial for development of the symbiotic root nodule.



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