Morphological and molecular modifications induced by different carbohydrate sources in Tuber borchii

During the life cycle of mycorrhizal fungi, morphological, genetic and metabolic modifications are induced in the fungus and its symbiotic partner. These changes are influenced by environmental factors: light, gravity, oxygen, temperature, soil type, nutrients, root exudates and the presence of particular bacterial and perhaps fungal and viral populations in the mycorrhizosphere. To determine whether different carbohydrates lead to cell-signalling events and morphofunctional changes in cultured Tuber borchii mycelia, the expression level of genes involved in morphological modifications was investigated using a macroarray technique and real-time RT-PCR. The morphological study showed an increased growth of Tuber mycelia in glucose, while the hyphae were thinner and less branched in sucrose and maltose. This was accompanied by an upregulation of the genes involved in the general cell metabolism, detoxification processes, hyphal growth and cytoskeleton organization. Since glucose is also present in root exudates, the increased expression of these genes might support the hypothesis that glucose can act as a signal for the fungus to indicate the presence of the plant, and to trigger the complex symbiotic process. These mechanisms can lead to morphological modifications, including increased branching of the root which is necessary for the fungus to establish the symbiosis.