The role of the glyoxylate cycle in the symbiotic fungus Tuber borchii: expression analysis and subcellular localization.

Expression profiles of isocitrate lyase (TbICL), malate synthase (TbMLS) and fructose-1,6-bisphosphatase (TbFBP) from the mycorrhizal ascomycete Tuber borchii were investigated by real-time RT-PCR in fruiting bodies at different stages of maturation. In addition, a time course experiment was set up to determine how the transcription profile of TbICL, TbMLS and TbFBP in axenic-grown mycelia is affected by different carbon sources. The transcript levels of the three genes in the fruiting bodies were all much higher than those measured in the vegetative stage. The investigation on axenic-grown mycelia revealed that the main positive regulator of TbICL and TbMLS gene expression is the availability of acetate and ethanol, while oleic acid is a too complex substrate for the limited degradative capacities of T. borchii. Immunolabelling on axenic-grown mycelia showed a co-localization of TbICL and the peroxisomal marker protein FOX2. This result demonstrated that in T. borchii ICL is compartmentalized in peroxisomes. The high induction of TbICL, TbMLS and TbFBP transcription and the translocation of lipids in fruiting bodies let us hypothesize that glyoxylate cycle and gluconeogenesis are key metabolic pathways in the recycling of existing cell material and the channelling towards the biosynthesis of new cell components during the maturation of fruiting bodies.