Plants constantly sense the changes in their environment; when mineral elements are scarce, they often allocate a greater proportion of their biomass to the root system. This acclimatory response is a consequence of metabolic changes in the shoot and an adjustment of carbohydrate transport to the root. It has long been known that deficiencies of essential macronutrients (nitrogen, phosphorus, potassium and magnesium) result in an accumulation of carbohydrates in leaves and roots, and modify the shoot-to-root biomass ratio. Here, we present an update on the effects of mineral deficiencies on the expression of genes involved in primary metabolism in the shoot, the evidence for increased carbohydrate concentrations and altered biomass allocation between shoot and root, and the consequences of these changes on the growth and morphology of the plant root system.