Efficient energy use has constrained the evolution of nervous systems. However, it is unresolved whether energy metabolism may resultantly regulate major brain functions. Our observation that Drosophila flies double their sucrose intake at an early stage of long-term memory formation initiated the investigation of how energy metabolism intervenes in this process. Cellular-resolution imaging of energy metabolism reveals a concurrent elevation of energy consumption in neurons of the mushroom body, the fly's major memory centre. Strikingly, upregulation of mushroom body energy flux is both necessary and sufficient to drive long-term memory formation. This effect is triggered by a specific pair of dopaminergic neurons afferent to the mushroom bodies, via the D5-like DAMB dopamine receptor. Hence, dopamine signalling mediates an energy switch in the mushroom body that controls long-term memory encoding. Our data thus point to an instructional role for energy flux in the execution of demanding higher brain functions.

Summary Some forest understorey plants recover carbon (C) not only from their own photosynthesis, but also from mycorrhizal fungi colonizing their roots. How these mixotrophic plants use the resources obtained from mycorrhizal and photosynthetic sources remains unknown. We investigated C sources and allocation in mixotrophic perennial orchids from the genus Epipactis. Based on the assumption that fungal biomass has high δ13C and N content, while photosynthetic biomass has lower δ13C and N content, we indirectly estimated the respective contributions of these two resources to various organs, at various times over the growth season. Fully heterotrophic and fully autotrophic plants from the same sites were used as references for δ13C and N content of biomass purely issuing from fungi and photosynthesis, respectively. In four investigated populations, the biomass shifted from fully heterotrophic in young spring shoots to 80–100% autotrophic in leaves and fruits at fruiting time, suggesting that photosynthesis supported mostly fruiting costs. In addition, fungal colonization decreased in roots over this period. Based on δ13C and N content, below‐ground organs and young spring shoots from green (mixotrophic) individuals and spontaneous achlorophyllous variants (fully heterotrophic) displayed similar fungal C contributions. Similar fungal contributions were also found in shoots of individuals that were either sprouting (and thus partially photosynthetic) or dormant (and thus fully heterotrophic) in the previous years. Therefore, fungal C supported mostly young spring shoots and below‐ground organs. Although experimentally shaded plants had decreased contributions of photosynthetic C in shoots, experimentally defoliated plants showed no increase in fungal C contribution as compared with non‐defoliated controls. Strikingly, these defoliated plants maintained the same seed production: they likely compensated defoliation by increasing stem and fruit photosynthesis. Synthesis. We propose a falsifiable model of C resource allocation in mixotrophic orchids, where mycorrhizal fungi mostly support below‐ground organs and survival, while photosynthesis mostly supports above‐ground sexual reproduction, but not below‐ground reserves. We discuss how this allocation pattern, where seed production depends on photosynthesis, complicates the evolutionary route to full heterotrophy in mixotrophic orchids.

Brain function relies almost solely on glucose as an energy substrate. The main model of brain metabolism proposes that glucose is taken up and converted into lactate by astrocytes to fuel the energy-demanding neuronal activity underlying plasticity and memory. Whether direct neuronal glucose uptake is required for memory formation remains elusive. We uncover, in Drosophila, a mechanism of glucose shuttling to neurons from cortex glia, an exclusively perisomatic glial subtype, upon formation of olfactory long-term memory (LTM). In vivo imaging reveals that, downstream of cholinergic activation of cortex glia, autocrine insulin signaling increases glucose concentration in glia. Glucose is then transferred from glia to the neuronal somata in the olfactory memory center to fuel the pentose phosphate pathway and allow LTM formation. In contrast, our results indicate that the increase in neuronal glucose metabolism, although crucial for LTM formation, is not routed to glycolysis.