Pesticides are intensively used to protect crops from pests. However, pesticides are known to increase pest resistance and affect the environment and non-target organisms, including humans, which points to the need for sustainable alternatives. Cereal weevils Sitophilus spp. are one of the main cereal pests. Their capacity to thrive on a cereal-exclusive diet relies on their mutualistic association with the intracellular symbiotic bacterium (endosymbiont) Sodalis pierantonius, which provides them with amino acids, vitamins and cofactors that are scarce in the grains. Instead of targeting the insect itself, and because weevils rely on this symbiosis to survive, a new, specific and sustainable control strategy could be to target the nutritional functioning that has evolved between associated partners in these long-term successful relationships. Metabolic exchanges between the cereal weevils and S. pierantonius are at the core of this symbiosis. Both partners have evolved towards a metabolic complementarity and dependency on each other: the bacteria benefit from carbohydrates that insects process from the grain; in return, bacteria produce and provide to the host metabolites that complement its diet. While the importance and nature of the metabolic exchanges between symbiotic partners is well documented, how the nutrients are exchanged between partners remains an open question. The metabolic integration between Sitophilus weevils and Sodalis takes place into dedicated host cells, the bacteriocytes, which house the endosymbionts. To date, the mechanisms by which bacteriocytes are turned into highly specialized cross-kingdom ‘metabolic factories’ remain unclear, despite the fact that this adaptation is a key element in bacteria-insect partnership. The FOCuS project aims at deciphering the functional and ultrastructural organization of these ‘cross-kingdom metabolic factories’, thanks to recent advances in forefront imaging and molecular biology techniques. Work package 1 will address how bacteriocyte subcellular organization allows for the intensive metabolic exchanges required in an efficient nutritional endosymbiosis. We will analyze the three-dimensional ultrastructure of bacteriocyte and endosymbiont, with a focus on the cell membranous networks and the localization of transporters and exchanged metabolites. Work package 2 will investigate how bacteriocytes differentiate into fully functional specialized cells, and how the bacteria participate in this process. The impact of host nutrition on these structures will also be analyzed. Cellular and developmental biology approaches will be used to better characterize the differentiation process. Microdissection and Dual RNAseq will be conducted to uncover the host and endosymbiont interactome (paired transcriptomes) during bacteriocyte differentiation. The key genetic elements of this interactome will be functionally studied by using complementary tools, including RNA interference. Work package 3 will analyze whether and how the bacteriocyte structural organization is impacted by changes in the host diet. We expect with FOCuS to identify novel target mechanisms to disturb either bacteriocyte differentiation or function and, consequently, impact host or endosymbiont fitness as a novel strategy to control crop pests.