Host Control of Intracellular Bacteria Survival in the Nitrogen-fixing Symbiosis

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For the growth of plants the availability of nitrogen is frequently the limiting factor. While most crops rely on nitrogen fertilizer, much of it chemically synthesized, legumes have the remarkable ability to provide their own nitrogen nutrition by entering a symbiotic relationship with nitrogen-fixing bacteria, converting otherwise inert nitrogen gas in the air into ammonia. Because synthesizing chemical nitrogen fertilizers consumes massive amounts of fossil fuel and fertilizer runoff causes environmental degradation, harnessing the full potential of nitrogen-fixing symbiosis can contribute to a sustainable future. This project aims to understand at the molecular level how legumes have the capacity, absent in other crops, to engage nitrogen-fixing bacteria in a productive symbiosis. It will identify the function of key plant genes necessary to allow the bacteria to work in concert with the host. Discovering the mechanism of the nitrogen-fixing symbiosis will have broad beneficial impacts on the society, which will be communicated at multiple levels to a scientifically literate public. Plants and animals engage in complex and elaborate interactions with microbial partners, some detrimental and others beneficial to the host. During the symbiosis between legume and nitrogen-fixing rhizobia, individual bacteria are ingested by the host and converted to membrane-bound nitrogen-fixing organelles. Recent advances in the field showed that this intracellular compartment is sustained by a specialized protein secretory system from the host, which delivers key proteins to modulate the host- microbe interface. This research project centers on understanding the mechanisms by which host secretory proteins interact with the microbe to ensure the survival of the bacteria during the intracellular stage of the symbiosis. The findings will significantly advance our understanding of the molecular dialog between hosts and their microbial partners.