| Virtually all terrestrial plants are imperative participants of a mycorrhizal association. The ectomycorrhizas are beneficial fungal associations with temperate tree roots. Ectomycorrhizal fungi improve survivability, pathogen resistance, drought tolerance and provide mobilized mineral nutrient in exchange for fixed carbon. At the outset of the symbiosis, there is an exchange of signals between the plant root and fungus. In response to the perception of the plant root, there is a directional shift in fungal cell physiology as early as four hours into the interaction. During the interaction between Laccaria bicolor and its host Pinus resinosa (red pine) we have observed upregulation of metabolic genes like Lb-MS that codes for a malate synthase and Lb-AAT that codes for an acetylCoA acetyltransferase. The former is an enzyme of the glyoxyate pathway that allows the organism to achieve growth while assimilating two-carbon substrates. AcetylCoA acetyltransferase is the terminal thiolase of β-oxidation that generates acetylCoA. AcetylCoA is a substrate to several pathways. The upregulation of Lb-AAT suggests degradation of stored lipid and the upregulation of malate synthase suggests its channeling towards biosynthesis. A very stringent temporal regulation of both these genes is observed through the first 24 hours of interaction. While an immediate, directional shift is evident at the start of symbiosis, a stringent feedback-based and temporal regulation is also observed.; Regulation of expression of Lb-MS and Lb-AAT at pre-infection and in the presence of an alternate carbon source, other than glucose has been studied. Glucose effects on gene expression have been examined. Recombinant proteins of both Lb-MS and Lb-AAT have been produced and the gene structure of Lb-AATg has been elucidated. Based on our studies, we propose a model depicting the directionality of pre-infection metabolism towards recycling of cell components, breakdown of reserve food and input toward biosynthesis of cell components. It appears that amino acid biosynthesis for proteins, sterol synthesis for cell membrane and signaling, chitin synthesis and mannitol synthesis are among the proposed destinations for the products of β-oxidation and the glyoxylate pathway at the pre-infection stage. The expression of Lb-MS and Lb-AAT in conjunction with several other metabolic genes regulated through the pre-infection stage of symbiosis further supports the basis for our model. |
