Into the root: the cytokinin-dependent regulation of rhizobial infection

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Leguminous plants, such as garden pea and soybean, have a mechanism for sustainable growth through “self-fertilization” resulting from their interactions with nitrogen-fixing soil bacteria, commonly known as rhizobia. This symbiotic liaison renders the host plant capable of tapping into the reservoir of nitrogen stored in the Earth's atmosphere, making its growth independent from soil nitrogen. The high efficacy of nitrogen-fixing symbiosis is determined primarily by the legumes' ability to host rhizobia within intracellular root compartments, ensuring reliable nutrition for both partners. Current knowledge suggests that this useful ability could be extended to other plants, including major cereal crops. If successful, this would lead to a new generation of crops that are less dependent on industrial fertilizers, thereby making agriculture more efficient and environmentally-friendly. In order to reach this goal, several important obstacles need to be overcome. Further advancement in the knowledge of the plant mechanisms that select for and promote the accommodation of rhizobia is critical. The proposed research aims at contributing to this knowledge by investigating a role of the plant hormone cytokinin in these processes. Cytokinins are known to regulate soil nutrient acquisition by plants and we and other have shown that they also play a central role during nitrogen-fixing root nodule symbiosis. Using genetics and genomics tools available for the model legume, Lotus japonicus, we will further define how cytokinins regulate the predisposition of legume roots to symbiotic infection. Acquiring this knowledge is crucial to any rational consideration of transferring the capacity for nitrogen-fixing symbiosis to other crops, such as nitrogen-greedy cereals. The same research may also contribute to advancement of methods for the effective selection of microbial communities (microbiomes) which, like those in the human gut, provide many additional useful services, including defence against pathogens and resistance to abiotic stress.