Plant-microbe interactions: Understanding the signal loop for improved plant productivity

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A plant is always associated with a diverse and carefully orchestrated community of microbes - the phytomicrobiome. Many of these microbes provide beneficial services to the associated plant. In the case of legumes, nitrogen fixing rhizobia are able to infect root tissues, leading to the differentiation of root nodules (in some legumes determinate nodules and in others indeterminate), where modified rhizobial cells take nitrogen from the air and supply it to the plant. Biological nitrogen fixation through the legume-rhizobia symbiosis provides an extremely important low-input and environmentally friendly access to nitrogen, otherwise supplied as chemical fertilizer. It is becoming clear that other bacteria in the phytomicrobiome are able to assist in the establishment and/or function of the nitrogen fixing symbiosis. These facilitator strains area able to promote both establishment and function of the legume nitrogen fixing symbiosis. In this work, we will evaluate the ability of two potential facilitator strains, both isolated by our laboratory, to enhance the nodulation, nitrogen fixing capacity and growth of Canada's two most important legume crops, soybean and pea. These two plants are of particular interest because the represent the two main forms of legume nodulation, these being determinate (soybean) and indeterminate (pea) nodules. In the relationship between rhizobia and legumes the behaviour of the rhizobial bacterium is regulated by materials from the plant roots, and it is becoming clear that many root-associated microbes are similarly regulated by the associate plants. We will evaluate the effects of root exudates, and materials contained within them, to affect the beneficial aspects of the facilitator strains. We will also determine the long-term presence and affects of the facilitator strains within nodules, and their ability to alter plant protein expression, metabolism and physiology. Finally, using macro- and micro-CT scanning technologies (CT: computed tomography), to determine the effects on root branching and rate of nodule initiation and subsequent development. Enhanced nodulation and nitrogen fixation will allow reduced reliance on fertilizer nitrogen in Canadian crop production systems. This is expected to substantially reduce the energy inputs into crop production, ground and surface water pollution (nitrate), and the greenhouse gas emissions associated with crop production (nitrous oxide derived from nitrogen fertilizer). Soybean and pea are the two most widely produced legumes in Canada and Canada is the world's largest producer of dry pea so that this research will provide direct benefit to Canada through the agriculture sector. This proposed research will make Canada a world leader in the use of facilitator strains technologies to enhance sustainable crop production, leading to technology development and global commercialization of Canadian technologies.