Obtaining multi-biofortified plant crops could help alleviate micronutrient malnutrition. Vitamin B12 is the only micronutrient that plants are unable to synthesize. As many people in low-income countries do not have access to the more expensive foods of animal origin, and a shift toward plant-based dietary habits is gaining popularity in high-income countries, vitamin B12 deficiency is becoming more and more prevalent in the global population. The development of B12-biofortified plant foods is therefore a highly relevant challenge within the plant biofortification field. The main goal of this PhD project is to generate a metabolically engineered rice that produces hydrogenobyrinic acid (HBA), the first stable intermediate in the vitamin B12 biosynthetic pathway. The introduction of a completely new heterologous metabolic pathway into an organism will cause physiological adaptations, which will be closely monitored. In particular, we will investigate the metabolism of tetrapyrroles, with which B12 shares the initial biosynthetic steps. This will include quantitative variations in related metabolites, together with analysis of gene expression patterns. Furthermore, important physiological and agronomic parameters will be evaluated, as this will attest the fitness of the obtained transgenic plants.