Identification of two early acting plant genes in the mycorrhizal symbiosis

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Carbon, nitrogen and phosphorus are the three major ingredients for proteins and DNA and as such these three elements are essential for life. Mammals acquire these nutrients through eating plants or other animals. Plants on the other hand must acquire these nutrients from the environment around them. Plants have developed a number of beneficial interactions with micro-organisms that facilitate the uptake of carbon, nitrogen and phosphorus. Photosynthesis in plants, that utilises the suns energy to convert carbon dioxide in the atmosphere to sugars, is the result of an ancient interaction with photosynthetic bacteria. This interaction is so advanced that the bacteria are no longer able to exist in a free-living state, but have been converted into compartments within the plant cell called chloroplasts. In addition, most plants form an interaction with mycorrhizal fungi that invade the plant root and grow through the soil and help the plant take up nutrients particularly phosphates. Furthermore, a selection of plant species, particularly legumes (peas and beans), have evolved an interaction with nitrogen fixing bacteria that convert atmospheric nitrogen into a form readily available to the plant. The mycorrhizal fungi and nitrogen fixing bacterial interactions are beneficial to both the plant and the micro-organism: the plant receives the nutrients nitrogen and phosphate in exchange for sugars generated through photosynthesis. As such the plant can be described as a merchant that trades carbon from an ancient interaction with photosynthetic bacteria for nitrogen and phosphates from interactions with mycorrhizal fungi and nitrogen fixing bacteria. The co-evolution of the plant and the microbe has created complex interactions and in a number of cases the organisms can be entirely dependent upon the interaction for survival: for instance mycorrhizal fungi are unable to live in the absence of the plant host. In order to establish these interactions there is extensive molecular communication between the plant and the micro-organism. Plants release chemical signals into the soil that are recognised by the appropriate fungi and bacteria. In turn the bacteria and fungi release chemical signals to the plant that induce the plant to activate the developmental processes required to accommodate the micro-organisms. There is a single molecular pathway in the plant that is required to recognise both the fungal and bacterial signals. This is surprising since the developmental processes that are induced in the plant by nitrogen fixing bacteria are very different to the developmental processes induced by the mycorrhizal fungi. Hence, despite this commonality in the early signalling pathways the plant must still be able to discriminate between the fungus and the bacteria. In support of this is the fact that there are a number of genes with specific roles in the bacterial interaction that provide specific inputs and outputs of the common signalling pathway. We have recently identified two genes that are only required for the fungal interaction. In this proposal we will test whether these represent fungal specific inputs into the common signalling pathway and in addition we will decipher the gene identities to better understand how the plant is able to perceive the presence of the mycorrhizal fungus. This work will help us understand how this important interaction is established and will aid us in understanding how the plant is able to discriminate between mycorrhizal fungi and nitrogen fixing bacteria.