Identification of two early acting plant genes in the mycorrhizal symbiosis

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.

Legumes form mutualistic symbiotic interactions with nitrogen fixing rhizobial bacteria and with arbuscular mycorrhizal fungi. These symbioses play crucial roles in sustainable agriculture, allowing improvement of soil fertility without artificial fertiliser application, growth of crop plants in nutrient poor environments and greatly benefiting the water use efficiency of crops. A conserved signalling pathway in legumes is required for the establishment of both the rhizobial and mycorrhizal symbioses. This is surprising since the developmental processes and gene expression profiles are quite different between these two symbioses, and hence specificity must be maintained. In support of this are nodulation specific receptor-like kinases that function upstream and nodulation specific transcriptional regulators downstream of the common signalling pathway. It is highly likely that there are mycorrhizal-specific components upstream and downstream of the common signalling pathway and there is evidence for additional signalling pathway(s) in the plant with specific role(s) in the mycorrhizal symbiosis. We have recently identified two loci that are specifically required for the mycorrhizal symbiosis. Mutations in these genes show a very early block in the mycorrhizal symbiosis: there is no sustained interaction between the fungus and the plant root. As far as we are able we have excluded a role for these loci in the production of a plant signal recognised by the fungus. Hence these loci are likely involved in the perception of a fungal signal(s) by the plant. In this proposal we will test this hypothesis as well as cloning these two genes. This work will provide crucial insights into the establishment of the mycorrhizal symbiosis and will provide an essential platform for defining the mechanisms underlying the specific recognition of the mycorrhizal fungus and appropriate induction of the developmental processes required for the accommodation of the fungal partner.