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Leguminous plants thrive under nitrogen-limited soil conditions because of their ability to symbiotically interact with nitrogen-fixing bacteria, known as rhizobia. In the presence of compatible strains of rhizobia, they develop specialized symbiotic organs, called root nodules, which host the bacteria and provide the appropriate conditions for symbiotic nitrogen fixation to occur. The plant hormone cytokinin is the key endogenous trigger for the inception of root nodule organogenesis. In the model legume Lotus japonicus, analysis of the cytokinin receptor gene Lotus histidine kinase 1 (Lhk1) showed that it is required and also sufficient for the initiation of nodule organogenesis. However, mutant plants carrying loss-of-function lhk1alleles still form a limited number of nodules while being hyper-infected by their symbiotic partner, Mesorhizobium loti. Here, I show that L. japonicus contains a small family of four cytokinin receptor genes, all of which respond to M. loti infection. Within the root cortex, LHK1 performs an essential role but also works partially redundantly with LHK1A and LHK3 to mediate cell divisions for nodule primordium formation. LHK1 is also shown to be critical in maintaining the homeostasis of M. loti entry into L. japonicus roots. The LHK1-dependent signaling is required to promote the development of infection threads within the root cortex but also to stimulate a negative feedback mechanism that restricts subsequent infections at the root epidermis. I have proposed and tested a hypothesis whereby the increased cytokinin activity in the root epidermis, as mediated by LHK1, enhances ethylene production, which in turn inhibits subsequent M. loti infections. My thesis work identifies a family of seven ACS genes that mediate ethylene biosynthesis in L. japonicus and shows that ACS1 and ACS2 are potential targets of LHK1-dependent regulation during symbiosis. The results of this thesis work contribute to the understanding of the molecular mechanisms for hormonal, cytokinin-ethylene dependent, regulation of nitrogen-fixing symbiosis. In broader terms, this adds to knowledge of the principles that govern the beneficial plant-microbe interactions while bolstering the long-term goal of exploiting bio-fertilization through nutrient acquiring symbioses as a means of mitigating the negative environmental impacts associated with the use of industrial fertilizers in agriculture. |
